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New Phytol. (1998), 140, 599–624
Tansley Review No. 102
Plant hybridization
B L O R E N H. R I E S E B E R G    S H A N N A E. C A R N E Y
Dept of Biology, Indiana University, Bloomington, IN 47405, USA
(Received 19 March 1998 ; accepted 18 July 1998)

I.
II.
III.
IV.
V.
Summary
Introduction
Concepts and terminology
Historical background
Studies of experimental hybrids
1. Isolating mechanisms
2. Prezygotic barriers
(a) Gametic barriers to hybridization
3. Postzygotic barriers
(a) Chromosomal rearrangements
(b) Genic sterility or inviability
4. Hybrid vigour
5. Introgression
6. Hybrid speciation
Experimental manipulations of natural
hybrid populations
1. Hybrid-zone formation
2. Pollinator-mediated selection
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3. Habitat selection
VI. The biology of different classes of hybrids
1. Character expression
(a) Morphological characters
(b) Chemical characters
(c) Molecular characters
2. The fitness of different classes of
hybrids
(a) The importance of variance
(b) Estimating hybrid fitness
3. Interactions with parasites and
herbivores
4. Patterns of mating
(a) Outcrossing rate
(b) Hybridization frequency
(c) Mate choice
VII. Conclusions and future research
Acknowledgements
References
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Most studies of plant hybridization are concerned with documenting its occurrence in different plant groups.
Although these descriptive, historical studies are important, the majority of recent advances in our understanding
of the process of hybridization are derived from a growing body of experimental microevolutionary studies.
Analyses of artificially synthesized hybrids in the laboratory or glasshouse have demonstrated the importance of
gametic selection as a prezygotic isolating barrier ; the complex genetic basis of hybrid sterility, inviability and
breakdown ; and the critical role of fertility selection in hybrid speciation. Experimental manipulations of natural
hybrid zones have provided critical information that cannot be obtained in the glasshouse, such as the
evolutionary conditions under which hybrid zones are formed and the effects of habitat and pollinator-mediated
selection on hybrid-zone structure and dynamics. Experimental studies also have contributed to a better
understanding of the biology of different classes of hybrids. Analyses of morphological character expression, for
example, have revealed transgressive segregation in the majority of later-generation hybrids. Other studies have
documented a high degree of variability in fitness among different hybrid genotypes and the rapid response of such
fitness to selection – evidence that hybridization need not be an evolutionary dead end. However, a full accounting
of the role of hybridization in adaptive evolution and speciation will probably require the integration of
experimental and historical approaches.
Key words : Hybridization, introgression, reproductive isolation, speciation.
.            
In the conclusions from his 1979 benchmark volume
on hybridization, Levin predicted that most major
advances in understanding hybridization would
‘ come from experimental analyses (of hybridization
E-mail : lriesebe!indiana.edu
phenomena), and manipulations of natural hybrid
swarms or hybrid zones, and a very thorough analysis
of the biology of first-generation, advanced generation, and backcross hybrids relative to the parental taxa under a range of environments ’. He also
argued that major advances were unlikely to ‘ come
from further documentation of hybridization in the
fashion of our time ’.
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L. H. Rieseberg and S. E. Carney
Although many studies continue to document
hybridization phenomena, the past two decades have
seen a large increase in the kinds of experimental,
microevolutionary studies that Levin predicted
would lead to advances in our knowledge of the
process and implications of hybridization in natural
populations. Here, we discuss the three kinds of
studies Levin thought would be important : studies
of experimental hybrids ; studies that experimentally
manipulate natural hybrid populations ; and studies
that describe the biology of different generational
classes of hybrids. As with Levin’s volume, the focus
of the review is restricted to homoploid hybrids.
These recent studies have, as predicted, led to major
new discoveries and advances.
 .                      
Hybridization can have several different meanings
for evolutionary biologists. The term ‘ hybrid ’ can
be restricted to organisms formed by cross-fertilization between individuals of different species. Alternatively, hybrids can be defined more broadly as the
offspring between individuals from populations
‘ which are distinguishable on the basis of one or
more heritable characters ’ (Harrison, 1990). Similarly, introgression can be defined narrowly as the
movement of genes between species mediated by
backcrossing or more broadly defined as the transfer
of genes between genetically distinguishable populations. We prefer the broader definitions of
hybridization and introgression, as they provide
greater flexibility in usage. Nonetheless, here our
focus will be on hybridization and introgression
between species.
A focus on interspecific hybrids requires consideration of species concepts. Unfortunately, the
term species has a wide variety of definitions that
range from concepts based on the ability to interbreed to those based on common descent. Perhaps
the most widely accepted of these is Mayr’s (1963)
biological species concept : ‘ species are groups of
interbreeding natural populations which are reproductively isolated from all other such groups ’. This
concept is useful for studies of hybridization and
speciation, but it would deny species status of
hybridizing taxa if applied stringently. Thus, here
we will refer to biological species as groups of
interbreeding populations that are ‘ genetically isolated ’ rather than ‘ reproductively isolated ’ from
other such groups. This may seem to be a trivial
distinction, but it is clear that most hybrid zones
serve as an effective barrier to interspecific genetic
exchange, even if local introgression is extensive.
  .                    
Hybridization has been important to humans since
the Neolithic era when the domestication and
breeding of plants and animals began (Zirkle, 1935).
However, at least in plants, hybridization appears to
have been inadvertent rather than intentional, because of a lack of understanding of plant sexuality
and pollen function. Hand pollination of dioecious
date palms in the Old World and of monoecious
maize in the New World are exceptions to this
general rule, but this knowledge does not appear to
have been transferred to other species. As a result,
knowledge and study of plant hybridization lagged
behind that of animals until the 16th century.
The modern history of plant hybridization was
initiated by Camerarius, who in 1694 speculated that
it might be possible to fertilize a female plant of one
species with pollen from a male plant of another
species (Zirkle, 1935). The first written reference to
spontaneous plant hybridization, found in a letter
written by Cotton Mather in 1716, describes naturally occurring crosses between Indian and yellow
corn and between gourds and squash that were
planted together (Zirkle, 1935). However, it appears
that the first intentional artificial hybrid was generated by Thomas Fairchild in a cross between
carnation (Dianthus caryophyllus) and sweet William
(Dianthus barbatus). Other authors have ascribed the
first artificial hybrid to Linnaeus’ experiments on
Tragopogon in 1759, but it is clear that Fairchild’s
hybrid was generated almost 50 yr earlier (Zirkle,
1935).
Rigorous scientific study of plant hybridization
began with the publication of Ko$ lreuter’s hybridization experiments in 1761 (Roberts, 1929). Ko$ lreuter made several discoveries about hybridization
that have endured the test of time. First, he
demonstrated that hybrids from interspecific crosses
are sometimes sterile or ‘ botanical mules ’. As a
result, Ko$ lreuter concluded that hybrid plants are
produced only with difficulty and are unlikely to
occur in nature without human intervention or
disturbance of the habitat. This is the first explicit
reference to the importance of ecological factors,
particularly disturbance, in mediating hybridization.
Second, Ko$ lreuter showed that hybrids are usually
intermediate morphologically relative to their parents. Third, he discovered that if later-generation
hybrids are produced, they tend to revert back to the
parental forms. This discovery refuted an earlier
suggestion by Linnaeus that hybrids were constant
or true-breeding and represented new species
(Linne! , 1760).
In the latter part of the 18th century and the early
19th century hybridization techniques were widely
applied to plant breeding, a focus that continues
today. Early botanical hybridizers were also interested in the validity of hybrid sterility as a species
criterion (Roberts, 1929). This work was accompanied by increasing numbers of reports of spontaneous hybrids between wild plant species.
The implications of these early hybridization
studies were summarized by Focke (1881). He noted
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Plant hybridization
that some taxonomic groups hybridize readily,
whereas others do not. These taxonomic correlates
of hybridization have been quantified in a recent
survey of five biosystematic flora (Ellstrand, Whitkus
& Rieseberg, 1996). Focke also suggested that plants
with zygomorphic flowers were more likely to
hybridize than plants with actinomorphic flowers –
a tendency that is still recognized (Stebbins, 1957).
Finally, Focke hypothesized that natural hybridization is most likely when flowers from one of the
parental species are in a quantitative minority. Biased
ratios of parental species flowers could be caused by
the rarity of one of the parental species or by
variation between species in flowering phenology.
Recent studies of Louisiana iris hybrids by Arnold
and co-workers (e.g. Arnold, Hamrick & Bennett,
1993 ; Carney, Cruzan & Arnold, 1994) confirm
Focke’s hypothesis and suggest that interspecific
pollen competition is the mechanism that largely
prevents hybrid production when flowers of both
parental species are abundant.
Although there was little discussion of an evolutionary role for hybridization during this period,
there was speculation concerning the possible origin
of new species via hybridization. For example,
Naudin (1863) suggested that hybrid characters may
become fixed in later generations and that this might
facilitate species formation. This idea was expanded
by Kerner (1894–1895), who discussed the possible
role habitat might play in mediating the establishment of hybrid species. Kerner realized that
although hybrids were formed frequently in nature,
their successful establishment required suitable open
habitat that was free of the parental species. This was
a significant contribution because ecological factors
play an important role in current models of hybrid
speciation (e.g. Templeton, 1981).
In the early 20th century, three key discoveries
laid the foundation of modern evolutionary studies
of hybridization. The first discovery was by Winge$
(1917), who showed that new and constant hybrid
species could be derived instantaneously by the
duplication of a hybrid’s chromosome complement
(i.e. allopolyploidy). This hypothesis was quickly
confirmed experimentally in a variety of plant
species, and allopolyploidy is now recognized to be a
prominent mechanism of speciation in flowering
plants and ferns (Soltis & Soltis, 1993 ; Leitch &
Bennett, 1997).
A second important discovery resulted from the
work of Mu$ ntzing (1930) on homoploid hybrid or
‘ recombinational ’ speciation. He postulated that the
sorting of chromosomal rearrangements in latergeneration hybrids could, by chance, lead to the
formation of new population systems that were
homozygous for a unique combination of chromosomal sterility factors. The new hybrid population
would be fertile, stable, and at the same ploidal level
as its parents, yet partially reproductively isolated
601
from both parental species because of a chromosomal
sterility barrier. Although early authors focused on
chromosomal rearrangements (e.g. Mu$ ntzing, 1930 ;
Grant, 1958), it is clear that the sorting of genic
sterility factors should generate similar results.
Thus, current models incorporate both genic and
chromosomal sterility factors. Modern contributions
to the study of this process, termed recombinational
speciation by Grant (1958), include rigorous experimental and theoretical tests of the model, as well
as the gradual accumulation of well-documented
case studies from nature (Rieseberg, 1997).
A third key advance resulted from studies of
natural hybrid populations by Anderson and coworkers (Anderson, 1936 ; Anderson & Hubricht,
1938). Anderson suggested that products of interspecific hybridization, particularly those resulting
from backcrossing or introgression, might be favoured by selection and thus contribute to adaptive
evolution within populations. Over the past two
decades, molecular markers have been used to
document introgression in many groups of plants,
but its adaptive significance remains poorly understood (Rieseberg & Wendel, 1993 ; Arnold, 1997).
Although these advances provided the conceptual
foundations for most recent studies of hybridization,
scientific interest has expanded considerably to
include theoretical modelling of the structure and
maintenance of hybrid zones, the detection and
consequences of hybridization in phylogenetic reconstruction, the interactions of hybrids with pathogens and herbivores, the potential extinction of
rare species through hybridization, and the role of
hybridization in mediating the escape of genetically
engineered genes. The experimental, microevolutionary studies advocated by Levin (1979) have
informed each of these areas.
 .                            
Natural hybridization phenomena can often be
replicated in the glasshouse, allowing the process of
hybridization and introgression to occur under
controlled conditions. Artificial hybridization experiments provide a means for investigating genetic
isolating mechanisms and for testing models of
hybrid speciation and introgression.
1. Isolating mechanisms
Reproductive isolating mechanisms are generally
divided into two categories based on whether they
act before or after fertilization. Mechanisms that act
to prevent mating or fertilization are referred to as
prezygotic, whereas those that act to reduce the
viability or fertility of the hybrid zygote or latergeneration hybrid offspring are referred to as
postzygotic. Hybridization experiments have been
used to study both kinds of mechanisms, although
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L. H. Rieseberg and S. E. Carney
the emphasis has been on mechanisms that act after
mating, because these are more easily studied under
greenhouse conditions.
100
Expected
IH × IF
(a) Gametic barriers to hybridization. Probably the
most detailed studies of gametic barriers to hybridization have been in the Louisiana irises. Studies in
this group of naturally hybridizing species have
focused on pollen competition or conspecific pollen
advantage, including measurements of pollen-tube
lengths in conspecific and heterospecific styles and
examination of hybrid seed production following
different types of controlled pollinations. Measurements of Iris fulva and I. hexagona pollen-tube
lengths 3n5 h after pure conspecific and heterospecific
pollinations revealed that heterospecific pollen tubes
grew at least as well as conspecific pollen tubes in the
initial stages of growth (Carney et al., 1994). This
result suggests that relative pollen-tube growth rates
are unlikely to act as a barrier to hybridization in
these species. However, only pure parental progeny
were obtained from controlled crosses of I. fulva and
I. hexagona with 1 : 1 mixtures of pollen from the two
species (Arnold et al., 1993). These experimental
crosses were later repeated as part of a larger crossing
study, in which 1 : 9, 1 : 3, 1 : 1, 3 : 1 and 9 : 1 ratios of
pollen from the two species were used in addition to
pure conspecific and heterospecific crosses (Carney
et al., 1994). Although some hybrid seeds were
formed from the crosses using mixed pollen loads,
there were significantly fewer than would be expected if fertilization was random (Fig. 1). Similar
results were obtained from crosses involving I. fulva
Hybrid seeds (%)
Prezygotic isolating mechanisms in plants include
habitat, temporal and ethological barriers, as well as
gametic competition or incompatibility. The role of
the first three isolating mechanisms in mediating
hybridization are most easily studied under natural
conditions or in experimentally manipulated hybrid
populations (e.g. Campbell, Waser & MelendezAckerman, 1997 ; Emms & Arnold, 1997 ; Nagy,
1997 a, b). Thus, the few artificial hybridization
experiments that investigate habitat and temporal
isolation have largely focused on the genetic basis of
these traits (e.g. Macnair & Christie, 1983 ; Macnair,
Smith & Cumbes, 1993 ; Schat, Vooijs & Kuiper,
1996 ; Kuittinen, Sillanpa$ a$ & Savolainen, 1997).
Glasshouse experiments offer little for studying
how hybridization affects pollinator preferences,
although substantial information has been gathered
on the genetic basis of floral traits that affect these
preferences (e.g. Macnair & Cumbes, 1989 ; Bradshaw et al., 1995 ; Kuittinen et al., 1997). By contrast,
artificial hybridization studies have yielded extensive
data on how gametic competition and incompatibility serve as isolating mechanisms.
IF × IH
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2. Prezygotic barriers
IB × IF
IF × IB
50
25
0
0
25
50
75
100
Interspecific pollen (%)
Figure 1. Percentage of hybrid seeds formed from
pollinations of Iris hexagona (IH), I. fulva (IF) and I.
brevicaulis (IB) with mixtures of conspecific and heterospecific pollen. For each cross listed, the first species is the
maternal species and the second is the competing pollen
type. The ‘ expected ’ line shows the percentage of hybrid
seeds that would be formed assuming equal fertilization by
pollen grains present on the stigma. For 0 % and 100 %
heterospecific pollen, the observed percentage of hybrid
seeds did not differ significantly from that expected, but
significant differences were seen for all other values
(IBiIF, P 0n05 ; all other crosses, P 0n005). Adapted
from Carney et al., 1994 and Emms et al., 1996.
and a third species, I. brevicaulis (Emms, Hodges &
Arnold, 1996). Iris fulva and I. brevicaulis pollen
tubes did not differ significantly in length in I.
brevicaulis flowers, but after 3 h of growth in I. fulva,
conspecific pollen tubes were significantly longer
than heterospecific tubes, suggesting that pollen
competition is a stronger barrier in I. fulva flowers
than in I. brevicaulis flowers. When 1 : 1 mixtures of
I. fulva and I. brevicaulis pollen were used to
pollinate flowers of each of the parental species, a
significantly larger fraction of seeds was conspecific
than heterospecific (Fig. 1). Although these studies
suggest that faster growth of conspecific pollen tubes
might be acting as a barrier to hybridization, its
importance relative to postzygotic factors could not
be determined.
Sequential pollination studies were used to investigate further the role of relative pollen-tube
growth rates in isolating the pairs of iris species
described (Carney, Hodges & Arnold, 1996 ; Carney
& Arnold, 1997). If pollen-tube growth factors
isolate species, the fraction of hybrid seed produced
should vary with the length of time between stylar
applications of heterospecific and conspecific pollen.
No differences would be expected if postzygotic
mechanisms, such as hybrid seed abortion, are
responsible for reduced hybrid seed set. For both
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Plant hybridization
Iris sp. pairs, increasing frequencies of hybrid seed
were produced with increasing ‘ head starts ’ afforded
to heterospecific pollen, confirming that conspecific
pollen-tube growth advantage and\or heterospecific
attrition (or prefertilization pollen-tube growth failure) play an important role in isolating the Louisiana
irises.
Similar studies have been performed in other
naturally hybridizing plant species. Rieseberg, Desrochers & Youn (1995 a) pollinated Helianthus
annuus and H. petiolaris with mixed pollen loads and
obtained significantly fewer hybrid progeny than
expected. In addition, no hybrids were formed when
H. annuus pollen was given a 15- or 30-min head
start over H. petiolaris pollen in H. petiolaris styles.
However, conspecific and heterospecific pollen-tube
lengths did not differ significantly in flowers of either
species. Although pollen-tube measurements did not
accurately predict relative fertilization success, these
experiments suggest that gametic barriers to hybridization also exist between these species.
Smith (1968, 1970) was able to correlate the
relatedness of Haplopappus species with the strength
of their reproductive barriers using sequential pollinations. Specifically, he investigated the temporal
advantage necessary to equalize the competitive
ability of pollen for each species pair (i.e. to produce
50 % hybrid seed).
Chen & Gibson (1972) studied pollen germination,
pollen-tube growth and fertilization in conspecific
and heterospecific crosses of Trifolium repens with T.
nigrescens, T. occidentale, T. hybridum, T. ambiguum
and T. uniflorum. Pollen germination frequency was
lower in heterospecific crosses than in conspecific
crosses. Heterospecific pollen tubes often grew abnormally, with swelling, coiling and bursting in the
styles. Pollen-tube growth rate and fertilization
frequency were found to be correlated with relatedness of the pollen donor species to T. repens.
An interesting discovery of many studies of prezygotic barriers to hybridization has been that reproductive isolation is often asymmetrical. For all
Haplopappus sp. pairs, the direction of the cross has
been shown to affect the pollination interval needed
to produce 50 % hybrid seeds (Smith, 1970). Similarly, in the Louisiana irises, I. fulva is the father of
hybrids far more frequently than it is the mother
(Carney et al., 1994, 1996 ; Emms et al., 1996 ;
Hodges, Burke & Arnold, 1996 ; Fig. 1), and H.
petiolaris sired many more hybrid seeds than did
H. annuus following pollinations with mixed pollen
loads (Rieseberg et al., 1995 a).
Other studies have demonstrated a correlation
between self- and hetero-incompatibility (Lewis &
Crowe, 1958). In general, heterospecific incompatibility between a self-incompatible and selfcompatible species is unidirectional ; only the selfcompatible parent is able to produce hybrid offspring.
603
Although pollen competition has been studied as a
barrier to hybridization in several naturally hybridizing plants, studies of gametic barriers in commercially important plant species have been more
prevalent. These studies have generally been undertaken to learn what barriers exist so that they can be
overcome and the desired crosses can be performed
(e.g. Sanyal, 1958 ; Gore et al., 1990 ; Beharav &
Cohen, 1995 ; Marcella! n & Camadro, 1996). Other
investigations have sought to estimate the risk of
escape of transgenes into wild populations via
cropiweed hybridization (e.g. Lefol, Fluery &
Darmency, 1996).
3. Postzygotic barriers
Much of the biosystematic work in the early 1900s
focused on postzygotic isolating barriers. Part of this
work was motivated by a desire to understand
speciation. An even greater motivation was the
prevailing belief that the strength of postmating
barriers was proportional to the degree of relatedness
among species. The use of crossing studies to
investigate evolutionary relationships was extended
to the analysis of chromosome pairing relationships.
This approach assumes that the degree of chromosome pairing is proportional to the level of overall
genomic divergence – an assumption now largely
invalidated by the discovery of genes that control
chromosome pairing (Jackson, 1985). Nevertheless,
numerous crossing studies have been performed,
and substantial evidence has been gathered on the
kinds and strength of postmating reproductive
barriers in both closely and distantly related plants
species (Heiser, 1949 ; Stebbins, 1957 ; Levin, 1979 ;
Grant, 1981 ; Jackson, 1985). Common postmating
barriers include hybrid weakness or inviability,
hybrid sterility and hybrid breakdown in which first
generation (F ) hybrids are robust and fertile, but
"
later generation hybrids are weak or inviable.
An important early observation was the high
degree of variability in viability and fertility observed
in both first- and later-generation hybrids (Stebbins,
1959). This is perhaps best exemplified by F hybrids
"
of Primula elatioriP. vulgaris, which vary dramatically in vigour, ranging from significant positive to significant negative heterosis (Valentine,
1947).The variation is even more extreme between
different interspecific combinations, with some combinations generating inviable or sterile hybrids, and
others producing hybrids that are robust and fertile
and show no evidence of breakdown in later hybrid
generations (Gillett, 1972). All gradations in fertility
and viability have been observed between these
extremes.
Extensive variability in viability and fertility is
also observed within and between hybrid generations
from the same interspecific cross. Variability levels
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604
L. H. Rieseberg and S. E. Carney
tend to be greatest in F and first back-cross (BC )
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"
generations. Successive hybrid generations are characterized by an increase in fertility and viability and
a reduction in variation as a result of natural selection
(e.g. Grant, 1966 a). The large variability observed
in the viability and fertility of natural hybrids and
the rapid response of these characteristics to selection
were interpreted correctly by Anderson (1949),
Stebbins (1957), Grant (1981) and others to indicate
that hybridization need not be a ‘ blind alley ’ of
evolution.
(a) Chromosomal rearrangements. Over the past few
decades, considerable progress has been made toward understanding the genetic basis of hybrid
inviability, sterility and breakdown. In crosses
between chromosomally divergent species, sterility
is typically attributed to the effects of chromosomal
rearrangements on meiotic pairing. However, this
assumption has been questioned recently because
individuals heterozygous for chromosomal rearrangements often show little meiotic impairment
or loss of fertility (Sites & Moritz, 1987 ; Coyne et
al., 1993). These authors have suggested that genic
factors may explain much of the loss of fertility
typically attributed to chromosomal rearrangements.
Unfortunately, it has been difficult until recently to
distinguish unambiguously between chromosomal
and genic effects.
Two approaches have been employed successfully
to separate the effects of chromosomal rearrangements and genic factors on sterility in interspecific
crosses. One approach involves genetic mapping of
quantitative trait loci (QTLs) for sterility. An early
example involved the analysis of hybrids between
two lentil species, Lens culinaris and L. ervoides
(Tadmor, Zamir & Ladizinsky, 1987). The two
species appear to differ by a single translocation, and
it was thought that the translocation was responsible
for reduced fertility in F hybrids. To test this
"
hypothesis, Tadmore et al. (1987) used a segregating
F population between the two species to generate a
#
map based on 18 isozyme markers. Correlations
between four of the isozyme markers and quadrivalent formation in meiosis allowed precise identification of the translocation end-points. All plants
with pollen viability
65 % were heterozygous
for the translocation, whereas plants with pollen
viability 85 % were homozygous for the translocation. Thus, the chromosomal translocation does
appear to represent the primary postmating reproductive barrier between these two species.
A similar study was conducted recently in Helianthus. Most species in the genus appear to differ by
one or more chromosomal translocations, and these
chromosomal rearrangements are generally well
correlated with the pollen viability of F hybrids
"
(Chandler, Jan & Beard, 1986). To provide a
quantitative estimate of the influence of chrom-
osomal rearrangements on pollen viability, Quillet et
al. (1995) analysed the segregation of 48 genetic
markers in BC progeny of an interspecific hybrid
"
between H. argophyllus and the common sunflower,
H. annuus. Helianthus argophyllus is the sister species
of H. annuus (Rieseberg, 1991), and cytogenetic
analyses indicate that the two species differ by two
reciprocal translocations (Chandler et al., 1986). As
predicted by cytogenetic studies, a wide range of
variability in pollen viability was observed in the
mapping family (27–93 %). Over 80 % of this
variation was explained by three genetic intervals
located on linkage groups 1, 2 and 3, respectively.
Analyses of meiosis in the backcross hybrids revealed
that meiotic abnormalities were also tightly correlated with the markers, indicating that chromosomal rearrangements are largely responsible for
reducing fertility in hybrids between these species.
A second approach that has been employed
successfully to distinguish between chromosomal
and genic effects involves analysis of introgression
patterns across the sterility barrier. If the chromosomal rearrangements contribute to reduced hybrid
fitness, then linkage blocks carrying these rearrangements will be selected against in introgressed
progeny. An example of this approach comes from
the analysis of introgression lines between two
sunflower species, H. annuus and H. petiolaris
(Rieseberg, Linder & Seiler, 1995 b, Rieseberg et al.,
1996 a). Comparative genetic mapping studies have
allowed the identification of 10 chromosomes that
differ in gene order between the two species. The
remaining seven chromosomes appear to be collinear. Analysis of the distribution of interspecific
genetic material in the introgression lines revealed
that introgression is significantly reduced in the
rearranged chromosomes as compared with the
collinear chromosomes. These data support the view
that chromosomal rearrangements do provide significant barriers to gene exchange, particularly
within the rearranged linkage groups. These results
also suggest that species genomes are often differentially permeable to introgression, where certain
portions of the genome are open to the incorporation
of alien alleles, but introgression is restricted in other
parts of the genome.
(b) Genic sterility or inviability. Substantial efforts
have also been made in understanding the mechanistic basis of genic sterility or inviability in
hybrids. The most widely accepted model was first
proposed by Dobzhansky (1937). In this ‘ standard
model ’, a gene from one species interacts negatively
with a gene from another species, causing some
degree of inviability or sterility. Wu & Palopoli
(1994) argue that the most plausible interpretation of
this model is that the hybrid sterility\inviability
gene acts like a mutation whose deleterious effects
are suppressed by another gene in the source species ;
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Plant hybridization
however, when placed in the genetic background of
another species, the deleterious effects of the sterility\inviability gene are expressed.
A somewhat different model for the evolution of
hybrid inviability\sterility is that a much larger
number of diverging loci interact negatively in a
hybrid genetic background, and that these weak
interactions act cumulatively to cause inviability or
sterility (Wu & Palopoli, 1994). Others have suggested that meiotic drive plays a key role in the
evolution of postmating reproductive isolation (e.g.
Frank, 1991).
Data that support the standard model come from
the many studies of plant hybrids in which one or
two genes appear to have major effects on hybrid
sterility or inviability. Examples include barley
(Wiebe, 1934), cowpea (Saunders, 1952), Crepis
(Hollingshead, 1930), cotton (Gerstel, 1954), Melilotus (Sano & Kita, 1978), Mimulus (Macnair &
Christie, 1983 ; Christie & Macnair, 1984), rice (Oka,
1974 ; Wan et al., 1996 ; Li et al., 1997) and wheat
(Hermson, 1963). However, these observations do
not rule out the possibility that many additional
genes may affect inviability and sterility in these
species as well. Indeed, detailed studies of the
genetic basis of hybrid sterility and hybrid breakdown between subspecies of rice suggest that several
mechanisms are involved (Li et al., 1997). These
mechanisms include a cytoplasmic gene that causes
both male and female sterility ; and interactions
between a pair of complementary genes that lead to
greatly reduced fertility. Both of these mechanisms
fit the standard model. In addition, Li et al. (1997)
found that recombination between differentiated
supergenes represents a major source of sterility.
Map comparisons suggest that these regions may
contain inversion polymorphisms, and that sterility
may be caused by crossing over between cryptic
structural rearrangements (cytologically undetectable chromosomal aberrations) (Stebbins, 1958).
Thus, chromosomal mutations might also play an
important role in the evolution of hybrid sterility in
rice.
Li et al. (1997) also provide evidence that hybrid
breakdown in rice largely fits the polygenic model
and results from the uncoupling of coadapted
subspecific gene complexes by recombination. In
later-generation hybrids, semisterility appears to be
caused largely by incompatibility interactions between many loci, and hybrid weakness appears to
result from the break-up of coadapted gene complexes that affect fitness traits such as heading age
and floret number per panicle. The presence of these
coadapted gene complexes in rice has long been
suspected as a result of observations that intersubspecific hybrids tend to revert quickly back to
one of the parental types in successive hybrid
generations. The complex genetic basis of postmating reproductive isolation in rice accords well
605
with studies of Drosophila (Wu & Palopoli, 1994)
that indicate that sterility and breakdown in fly
hybrids involve many genes and higher-order epistatic interactions.
A complex genetic basis for postmating reproductive isolation in many species of plants is also
suggested by introgression mapping studies. For
instance, the presence of genomic intervals where
introgression is reduced or absent is often reported
in map-based studies of introgression between crop
plants and their wild relatives (Jena, Khush &
Kochert, 1992 ; Williams et al., 1993 ; Garcia, Stalker
& Kochert, 1995 ; Wang, Dong & Paterson, 1995 ;
McGrath, Wielgus & Helgeson, 1996 ; Fulton,
Nelson & Tanksley, 1997). Presumably, many of
these genomic regions harbour genes that contribute
to reproductive isolation. Likewise, strong segregation distortion is often observed in interspecific
crosses, suggesting that many genes are negatively
selected in hybrids. For example, Zamir & Tadmor
(1986) report segregation distortion at 54 % of loci
from interspecific crosses of lentil, pepper and
tomato, compared with only 13 % in intraspecific
crosses.
These results suggest that both the standard model
and the polygenic model are necessary to account for
the behaviour of first- and later-generation hybrids.
Meiotic drive might also play an important role, but
there are few data that directly test the meiotic-drive
hypothesis.
4. Hybrid vigour
Interspecific hybrids are highly variable in fertility
and vigour. However, one general rule is that F
"
hybrids, particularly between geographic races or
closely related species, tend to exceed their parents
in vegetative vigour or robustness (Grant, 1975).
This phenomenon – hybrid vigour (heterosis) – is
often used to maximize yields in crop plants.
Heterosis has major implications for evolutionary
biology and at least partly explains the success of
allopolyploid species and many clonal hybrid lineages (e.g. Huskins, 1931 ; Grootjans, Allersma &
Kik, 1987). It may also contribute to the successful
establishment of introgressive hybrid races or hybrid
species, but this argument is less convincing because
hybrid vigour is more difficult to maintain in
segregating hybrid generations.
Although heterosis is a likely contributor to the
evolutionary success of hybrids, its genetic basis is
still poorly understood. Possible models for the
evolution of heterosis are listed in Mitchell-Olds
(1995) and include : dominance (the masking of
deleterious recessives) ; overdominance (single locus
heterosis) ; and epistasis (enhanced performance of
traits derived from different lineages). It is difficult
to distinguish between these models using classical
quantitative genetic approaches because the effects
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606
L. H. Rieseberg and S. E. Carney
of individual loci cannot be distinguished. The
models have been tested in a few marker-based
quantitative genetic studies, but the data are too few
to permit generalizations.
The first study of this type was conducted in maize
by Stuber et al. (1992). Of nine QTLs affecting
yield, eight showed significant overdominance. A
reanalysis of the data by Cockerham & Zeng (1996)
suggests that the apparent overdominance observed
may represent an example of pseudo-overdominance
caused by the presence of several linked QTLs.
Cockerham & Zeng argue that the data are most
consistent with the model of dominance of favourable genes, but admit that epistasis could also play an
important role and that overdominance could not
be ruled out. There is convincing evidence for the
overdominance model in Arabidopsis, in which
Mitchell-Olds (1995) identified a QTL that resulted
in a 50 % increase in viability in heterozygotes
relative to homozygotes. Mitchell-Olds argues that
overdominance will be most important in partially
inbred species because major deleterious recessives
are likely to be rare and recessives with minor effects
are likely to be purged from the population by
inbreeding.
5. Introgression
Most experimental studies of introgression have
focused on the best methods of moving important
agronomic traits from a wild species to a cultivated
relative. To move a gene across a reproductive
barrier, the allele must recombine into a new genetic
background before it is eliminated by selection
against the alleles with which it is initially associated
(Barton & Hewitt, 1985). As a result, successful
introgression, whether in the glasshouse or in the
wild, will depend in part on the genetic architecture
of the reproductive barriers. If many genes contribute to hybrid unfitness, then much of the genome
may be resistant to introgression because of linkage
(Whittemore & Schaal, 1991 ; Rieseberg & Wendel,
1993), particularly if recombination rates are low.
This problem may be exacerbated if the genes
interact epistatically or are ‘ co-adapted ’ (Harlan,
1936 ; Carson, 1975). However, if reproductive
barriers are under simple genetic control, then most
of the genome should be permeable to introgression.
Only those traits tightly linked to sterility or
inviability genes will be difficult to introgress.
These predictions have largely been confirmed by
experimental crossing programmes. In interspecific
backcrosses involving divergent parental species, the
donor parent genome is often eliminated much more
rapidly than would be predicted under neutral
conditions. For example, Stephens (1949) noted that
in backcrosses between Gossypium spp., the donor
parent genotype is selectively eliminated, regardless
of the direction of the backcrosses. Similar obser-
vations have been made for species hybrids in
Antirrhinum (Mather, 1947), Hordeum (Koba, Handa
& Shimada, 1991), Helianthus (Rieseberg et al.,
1995 a), Melilotus (Baenziger & Greenshields, 1958),
Lycopersicon (Rick, 1963), Zea (Mangelsdorf, 1958),
Nicotiana (Neelam & Narayah, 1994), Oryza (Mao
et al., 1995 ; Harushima et al., 1996) and Phaseolus
(Wall, 1968).
Skewed segregation ratios in hybrids now appear
to be the rule rather than the exception. For example,
Zamir & Tadmor (1986) report segregation distortion in 54 % of loci from interspecific crosses of
Lenz, Capsicum and Lycopersicon, compared with
only 13 % in intraspecific crosses. In Helianthus,
segregation distortion has been observed at 7–13 %
of loci in intraspecific mapping populations (Rieseberg et al., 1993 ; Berry et al., 1995 ; Gentzbittel et
al., 1995) compared with 23–90 % of loci in interspecific crosses (Quillet et al. 1995 ; Rieseberg et al.,
1995 b, 1996 a). Not only are distorted ratios prevalent, but they can also be extreme. For example,
segregation ratios that were skewed 12 : 1 in favour of
‘ wild ’ alleles have been reported in crosses between
cultivated pearl millet (Pennisetum glaucum) and one
of its wild relatives (P. violaceum) (Liu et al., 1996).
An overall result of these skewed segregation ratios is
that hybrid progeny receive more alleles from one
parent than would be expected under Mendelian
rules of segregation and thus resemble that parent
more closely than Mendelian rules would predict.
Although most deviating ratios observed in species
backcrosses have favoured the genes of the recurrent
parent, there have been several exceptions to this
general rule. For example, the white lint gene of the
donor parent was favoured over brown lint alleles of
the recipient parent in backcrosses from G. barbadense into G. hirsutum (Stephens, 1949). Likewise,
5 % of H. petiolaris markers introgressed at significantly higher than predicted rates into an H. annuus
genetic background (Rieseberg et al., 1995 b). This is
a relatively small fraction, however, when compared
with the 85 % that introgressed at significantly lower
than expected rates (Rieseberg et al., 1995 b). Finally,
Wang et al. (1995) noted that the same G. hirsutum
chromosome fragments were maintained in independently generated G. barbadense introgression
lines. It is not clear whether these loci or chromosomal fragments are selectively favoured in the
recurrent parent or whether they represent examples
of ‘ self genes ’ – genes that enhance the success of
gametes they inhabit even if they pose a significant
fitness cost during the diploid phase of the life cycle
(Haldane, 1932).
Although this discussion focuses on factors that
directly contribute to reproductive isolation, genome-wide introgression can also be reduced by lower
recombination rates, because the alien chromosomal
blocks must recombine into a new genetic background for introgression to occur. Not surprisingly,
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Plant hybridization
recombination rates appear to be reduced in crosses
between genetically divergent taxa such as maize and
teosinte (Doebley & Stec, 1993), rice spp. (Causse
et al., 1994), sunflower spp. (Quillet et al., 1995)
and tomato spp. (Paterson et al., 1988 ; Miller &
Tanksley, 1990). Recombination is often completely
eliminated in somatic hybrids involving species that
are too divergent to allow successful sexual crosses
(e.g. Parokenney et al., 1994).
Experimental studies have also been used to assess
the efficiency of different mating designs for moving
alleles across species barriers. From a theoretical
standpoint, it appears clear that mating designs that
enhance recombination rates will be most effective
in facilitating gene transfer (Hanson, 1959 a, b ;
Stephens, 1961 ; Wall, 1970). Unfortunately, recurrent backcrossing, the mating design typically
employed for breeding purposes and invoked for
natural introgression scenarios, has been demonstrated theoretically to be an extremely inefficient
mechanism for the break-up of parental linkage
blocks (Hanson, 1959 a, b). This problem is exacerbated for chromosomes with short map lengths,
where the disruption of parental linkage blocks will
be even slower because of lower recombination rates
(Hanson, 1959 a). Hanson (1959 b) also noted that
chromosomal structural differences greatly reduce
effective recombination rates and map lengths. Thus,
he recommended that mating designs that enhance
recombination between the parental genomes be
employed in these types of situations to ensure the
disruption of parental linkage groups. Selfing and
sib-mating represent two such systems, because
these mating systems result in twice the recombination found in the simple backcross method
(Wall, 1970 ; Liu et al., 1996).
A second requirement for the successful introgression of alleles is the maintenance of reasonable
levels of fertility and viability in hybrid or backcross
populations. Unfortunately, populations resulting
from either selfing or sib-mating will generally
exhibit lower levels of fertility than those resulting
from recurrent backcrossing (Wall, 1970). In particular, selfing tends to result in high proportions of
‘ subvital ’ plants because of hybrid breakdown
(Stebbins, 1950 ; Stephens, 1950). Given these
considerations, Wall (1970) suggested that there is
an optimal level of recombination between genetically or chromosomally divergent populations. If
recombination is too low, no introgression occurs,
whereas if it is too high it may severely reduce overall
viability and fertility in the resulting population.
Thus, Wall (1970) argued that mating designs
employing one or more generations of sib-mating
interspersed with backcrossing will be more effective
than backcrossing alone for moving alleles across
linkage groups where effective recombination rates
are low, such as in chromosomally divergent linkages. Alternatively, Haghighi & Ascher (1988) have
607
proposed the use of congruency backcrossing, in
which backcrosses in the direction of one parent are
alternated with backcrosses in the direction of the
other parent. This approach quickly leads to the
formation of fertile hybrids that can be used as a
bridge for gene flow between widely divergent
species.
Experimental studies have largely confirmed the
utility of these approaches. Substantial increases in
heterospecific recombination have been observed in
Arabidopsis (Liu et al., 1996), maize (Horner, 1968)
and Phaseolus (Wall, 1968, 1970) using sib-mating
and selfing breeding designs. Rieseberg et al. (1996 a)
compared the effectiveness of three different mating
designs on the movement of genes across a species
barrier in Helianthus. Mating designs that employed
sib-mating early in the hybridization process resulted in a close to twofold increase in the total
length of introgressed fragments per individual.
Haghighi & Ascher (1988) were able to generate
fertile intermediate hybrids between Phaseolus vulgaris and P. acutifolius by congruity backcrossing,
suggesting that this approach may be particularly
useful in divergent species crosses.
Two conclusions can be drawn from these studies.
First, the genetic architecture of the reproductive
isolation plays a critical role in controlling patterns
of introgression. Thus, the successful movement of
an allele across a species barrier will depend both on
overall genome architecture and on the genomic
location and linkage relationships of the allele in
question. Second, recurrent backcrossing is unlikely
to be the mating design of choice for breeding
programmes, and successful introgression in breeding populations and in natural hybrid zones probably
requires a more diverse history of matings.
6. Hybrid speciation
This section discusses several studies that both
exemplify the utility of hybrid speciation experiments and suggest the most fruitful directions for
future study ; see Rieseberg (1997) for a detailed
review of experimental studies of hybrid speciation.
The most widely accepted model for homoploid
hybrid speciation is the recombinational model
already described. Briefly, the sorting of parental
chromosomal and genic sterility factors in hybrid
populations can, under appropriate conditions, lead
to the formation of a hybrid neospecies that is
homozygous for some combination of parental
sterility factors. The new hybrid lineage would be
fertile, but at least partially intersterile with both
parental species. Factors that appear to play a critical
role in recombinational speciation include : strong
natural selection for the most fertile or viable hybrid
segregants (Templeton, 1981 ; McCarthy, Asmussen
& Anderson, 1995) ; rapid chromosomal evolution
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608
L. H. Rieseberg and S. E. Carney
(Shaw, Wilkinson & Coates, 1983 ; Rieseberg, Van
Fossen & Desrochers, 1995 c) ; and the availability of
habitat suitable for the establishment of hybrid
neospecies (Templeton, 1981 ; Arnold, 1997).
This discussion has assumed that the establishment of the hybrid neospecies will occur in sympatry
with both parents. Although hybrid speciation must
be initiated in sympatry, Charlesworth (1995) argues
that this mode is most likely when ‘ a group of hybrid
plants colonize a new locality and are by chance
spatially or ecologically isolated from the parental
species ’. Thus, hybrid founder events might be
viewed as foci of speciation. The possibility that a
hybrid derivative might be stabilized in parapatry or
allopatry should not be seen as minimizing the
importance of the development of reproductive
barriers. As the hybrid derivative becomes established and expands its geographical distribution, it
probably will come back in contact with its parents.
Presumably, the existence of reproductive barriers
will allow it to survive the challenge of sympatry.
Hybrid speciation experiments are useful because
they can test the feasibility of the recombinational
model, as well as provide insights regarding the
evolutionary conditions under which speciation is
most likely. The first rigorous experimental study of
hybrid speciation was conducted by Stebbins (1957),
who crossed microspecies of two grass genera,
Elymus and Sitantion. Most F individuals were
"
sterile and could not produce seed, but four plants
produced a small number of seeds. In three cases,
the F seeds were not useful because the offspring
"
had either recovered the morphology of their
maternal parent, undergone polyploidization, or
were sterile. However, a single seed from the fourth
F appeared to result from a backcross toward E.
"
glaucus. This plant had a seed fertility of 30 % and
was selfed for two generations. The resulting
progeny were vigorous and had normal seed fertility
(88–100 %). Moreover, crosses with the original
E. glaucus parent indicated almost complete reproductive isolation ; pollen fertility in the progeny of
these crosses was 0–3 %. These experiments not only
verified the feasibility of the recombinational speciation model, but also indicated that the origin of
homoploid hybrid species is likely to involve backcrosses when the F hybrids are highly sterile.
"
Backcross progeny are typically more easily generated and more fertile than self- or sib-crosses in
early hybrid generations.
A series of elegant studies involving hybrids of
Gilia malioriG. modocensis also provide experimental validation of the recombinational model (Grant,
1966 a, b). The two species are selfing annual
tetraploids with a relatively high chromosome number (2n l 36). The F hybrids are semi-sterile with
"
pollen and seed fertility of 2 and 0n007 %, respectively. Abnormal meiotic pairing suggests that
this reduction in fertility is caused by chromosomal
structural differences between the parental genomes.
To generate fertile and meiotically normal hybrid
lines, the most fertile and viable plants were
artificially selected from each generation, thus augmenting natural selection on the same traits. Although early-generation plants were weak and partially sterile, vigour and fertility improved rapidly.
By the F or F generation, full vigour, normal
)
*
chromosomal pairing and full fertility had been
recovered in three hybrid lineages or branches.
Branch I and branch III each possessed a new
combination of morphological and cytogenetic features (Grant, 1966 a), whereas branch II reverted
largely to the G. modocenis parent both morphologically and in terms of crossability (Grant,
1966 b). As in the case of Elymus, the two recombinant Gilia lineages were isolated strongly from
their parents (4–18 % pollen fertility). This is
concordant with theoretical expectations that the
strength of genetic isolation between hybrid derivatives and their parents should be correlated
strongly with barrier strength between the parents
themselves (Grant, 1954).
In the Elymus and Gilia studies, the experimentally synthesized hybrid lineages could not be
compared with natural hybrid species from the same
parental combinations, making it difficult to evaluate
how closely the glasshouse experiments replicated
the natural speciation process. However, Rieseberg
et al. (1996 b) recently performed a similar experiment using the wild sunflower species H. annuus,
H. anomalus and H. petiolaris, except that the
experimental sunflower hybrid lineages were directly
compared to H. anomalus, a wild species that also
originated from H. annuusiH. petiolaris. To facilitate these comparisons, mapped molecular
markers were used to assess precisely the genomic
composition of the experimental hybrid lineages and
to compare their genomes to H. anomalus.
As was observed for Elymus and Gilia, the three
experimental sunflower hybrid lineages recovered
fertility in a small number of generations (Fig. 2).
Comparison of the genomic composition of the
natural (H. anomalus) and synthetic hybrid lineages
revealed that all three synthetic hybrid lineages had
converged to nearly identical gene combinations,
and that this set of gene combinations was statistically concordant with that of H. anomalus (Fig.
3). Concordance in genomic composition between
the synthetic and natural hybrid lineages suggests
that deterministic forces such as selection, rather
than stochastic forces, largely govern the formation
of ‘ recombinational ’ species. Because the synthetic
hybrid lineages were generated in the greenhouse,
fertility selection probably played a greater role than
ecological selection in shaping hybrid genomic
composition. Congruence in genomic composition
also implies that the genomic structure and composition of hybrid species may essentially be fixed
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Plant hybridization
609
100
90
Pollen fertility (%)
80
70
60
50
40
30
Lineage I
20
Lineage II
10
Lineage III
0
1
0
3
2
Generation
4
5
Figure 2. Mean pollen fertility in three synthetic hybrid
lineages between Helianthus annuus and H. petiolaris :
lineage I, P-F -BC -BC -F -F ; lineage II, P-F -F -BC "
"
# # $
" #
"
BC -F ; and lineage III, P-F -F -F -BC -BC . For each
# $
" # $
"
#
line, 100 pollen grains from each of 20 plants per
generation were tested by viability staining. Standard
error bars are shown.
135
225
296
A4
261
tr1
215
226
254
R
T
rp1
D4
487
Q
199
313
B6
443
227
nf 73
239
103
Synthesized H. anomalus
hybrids
S
295
A15
H. petiolaris markers or
linkage blocks
H. annuus markers or
linkage blocks
471
40 cM
Figure 3. Genomic composition of ancient and experimental hybrid sunflower lineages for selected linkage
groups. Letters at the left of each linkage group designate
linkage blocks in the ancient hybrid, Helianthus anomalus,
and indicate their relationship to homologous linkages in
the parental species, H. annuus and H. petiolaris. The
distribution of parental markers within the H. anomalus
genome is indicated by gray or black bars within linkage
groups, whereas bars at the left of each linkage group
indicate the distribution of parental genomic regions in the
synthesized hybrids. Adapted from Rieseberg et al., 1996 b.
after a small number of generations of hybridization
and remain relatively static thereafter.
Although congruence between the genomes of the
synthetic hybrid lineages and H. anomalus is quite
high (rs l 0n68 ; P 0n0001), substantial differences
remain. Because the synthetic hybrid lineages were
exposed to fertility selection alone, it is possible that
the observed differences in genomic composition are
a result of chromosomal segments that affect morphological or ecological traits rather than fertility.
Maps are currently being generated for H. deserticola
and H. paradoxus, two other species that appear to
have originated in the wild following hybridization
between H. annuus and H. petiolaris (Rieseberg,
1991). Interspecific gene combinations shared by the
three natural hybrid species and the synthetic hybrid
lineages should be attributable to fertility selection,
whereas those exclusive to the natural hybrid species
will provide evidence for habitat selection.
Although substantial progress has been made in
studying homoploid hybrid speciation, much remains to be understood. A major gap in our
knowledge relates to the origin of hybrid species that
are isolated from their parents by premating barriers.
Empirical data indicate that species have arisen in
this manner (e.g. Arnold, Hamrick & Bennett, 1990,
Wang et al., 1990 ; Arnold et al., 1993 ; Wang &
Szmidt, 1994 ; Sang, Crawford & Stuessy, 1995 ;
Wolfe, Xiang & Kephart, 1997), but experimental
and theoretical studies have focused on the strict
recombinational model, which involves the sorting
of genic and chromosomal sterility factors. Because
hybrid speciation is both reticulate and rapid, it is
particularly amenable to experimental manipulation
and replication. Thus, it should be feasible experimentally to synthesize new homoploid hybrid species
that are isolated by premating barriers only. The
design of these experimental studies could be
improved by theoretical studies that identify parameters critical to this mode.
.                           
  
By manipulating naturally hybridizing species in the
field, scientists have been able to study a number of
habitat-dependent phenomena of crucial importance
to our understanding of hybridization and its
ramifications. These studies have provided insights
into the requirements for the formation and establishment of hybrids and the effects of habitat and
pollinator-mediated selection on hybrid-zone structure and dynamics.
There have been relatively few studies in which
hybrid zones are experimentally manipulated. It is
possible that this is because scientists are wary about
interfering with the natural processes occurring
within hybrid zones. However, these experiments
provide crucial data involving the formation, main-
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610
L. H. Rieseberg and S. E. Carney
tenance and evolutionary outcome of hybrid zones
that cannot be obtained in any other way. For
example, reciprocal transplant experiments are
the only truly accurate method for obtaining
environment-dependent fitness data of parental
and hybrid individuals. More experimental manipulations of hybrid zones are needed. Perhaps scientists
should focus their efforts on groups where a number
of independent hybrid zones exist. Thus, disturbance of individual hybrid zones for scientific study
would not eliminate the existence of ‘ pristine ’ hybrid
zones. Additionally, in some instances, experiments
can be engineered to minimize interference with
natural processes in the hybrid zone. For example, in
the case of reciprocal transplants, anthers of transplanted individuals can be removed to prevent the
introduction of foreign pollen, and plants can be
harvested before their seeds are dispersed into the
natural population.
1. Hybrid-zone formation
Although many hybrid zones have been described in
plants, little is known about how these zones form.
Experimental manipulation is an excellent tool for
investigating the early stages of hybrid-zone formation, as has been illustrated by studies of Louisiana irises.
Genetic marker surveys in several Louisiana iris
hybrid zones failed to identify F hybrids (Arnold
"
et al., 1990 ; Arnold, Buckner & Robinson, 1991 ;
Arnold, 1993 b), leading (Arnold, 1993 b) to the
suggestion that F hybrid formation was rare. To
"
investigate the rate of hybridization in nature, the
initial stages of hybrid-zone formation were simulated by introducing a block of I. hexagona rhizomes
into a natural population of I. fulva (Arnold,
Hamrick & Bennett 1993). For each species, a subset
of fruits derived from flowers that were open on days
when both species were blooming was collected, and
a total of 710 seeds were genotyped. Only seven
seeds, 1 %, were hybrids (Arnold et al., 1993), and
all of them had I. hexagona mothers. This bias could
be because of differences in the strength of reproductive barriers in the two species (Emms, Hodges
& Arnold, 1996) or because approximately three
times as many I. fulva flowers were produced as
I. hexagona flowers. This would lead to pollen loads
consisting mostly of heterospecific pollen being
deposited on I. hexagona stigmas. Arnold and
Hamrick continued to monitor the population for
3 yr, and 5000 seeds were genotyped with the
percentage of hybrid seeds remaining below 1 %
(Hamrick & Arnold, unpublished). The results of
this study support the hypothesis that F hybrids
"
rarely form in the Louisiana irises.
A hypothesis that F formation is rate might seem
"
unlikely because of the large number of advancedgeneration hybrid irises found in nature. However,
this conflict is resolved if later-generation hybrids
are formed more easily than F hybrids. Hodges,
"
Burke and Arnold (1996) investigated how a hybrid
zone might be formed once an initial hybridization
event has occurred by introducing F hybrids
"
produced in the glasshouse into the population
already described. Seeds from F , I. fulva and I.
"
hexagona plants were genotyped using two diagnostic
alozyme loci. A maximum likelihood programme
was used to estimate the frequency of seeds from
hybrid mothers that were F , Bf and Bh hybrids (Bf
#
and Bh, backcrossed to I. fulva or to I. hexagona,
respectively). Five of the 68 fruits collected from
parental plants contained hybrids. The three fruits
from I. hexagona had seed genotypes consistent with
Bh hybrids, and the two from I. fulva had a mixture
of I. fulva and Bf seeds. F plants located near the
"
I. hexagona plot produced 95 % Bh seeds and 5 %
Bf seeds. Those near I. fulva plants produced 90 %
F seeds and 10 % Bh seeds, as estimated by the
#
maximum likelihood model. In I. hexagona, backcross formation was 10 times more likely than the
formation of F hybrids and, in I. fulva, backcrosses
"
were formed 60 times more frequently than F
"
hybrids. The introduced F hybrids also contributed
"
to the formation of backcrossed and F hybrids.
#
Thus, once an F hybrid is formed and established in
"
nature, it can lead to the formation of advancedgeneration hybrids. However, the initial formation
of F hybrids may act as a bottleneck in the formation
"
of hybrid zones in this group.
2. Pollinator-mediated selection
Observations and experiments involving pollinator
preferences and visitation in hybrid zones supply
information on interspecific gene flow and the
strength and mode of selection on reproductive
traits. Patterns of pollinator visitation in animalpollinated plants may limit gene flow among those
plants (e.g. through positive assortative mating).
Interspecific pollinator foraging, and thus interspecific pollen transfer, may occur only rarely.
However, in hybrid zones, hybrids can act as a
bridge for gene flow between the parental species if
they share pollinators. Alternatively, hybrids could
potentially have pollination syndromes that differ
from either parent, isolating them from the parental
species.
Pollinators are a major source of selection on floral
traits in angiosperms, since appropriate pollinator
visitation greatly affects reproductive success (e.g.
Waser & Price, 1983 ; Stanton, Snow & Handel,
1986 ; Nilsson, 1988 ; Campbell et al., 1991, Campbell, Waser & Price, 1996). Pollinator choice is
determined by the presence of rewards (e.g. nectar,
pollen, or scents), and a variety of floral traits can be
used as cues to the value of the reward in a given
plant. Flower colour, size, or shape characters may
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Plant hybridization
be associated with larger volumes or higher quality
rewards. Additionally, the ease of obtaining the
reward in question will affect a pollinator’s choice of
flowers. For example, hummingbirds are known to
prefer flowers with wider corolla tubes, which enable
them to obtain nectar more easily, than flowers with
narrow corolla tubes, which restrict their entry
(Waser, 1983 ; Campbell et al., 1991, 1996). When
pollinators associate specific floral traits with increased rewards, they are more likely to forage on
plants with those traits, increasing the reproductive
success and thus fitness of individuals that possess
the traits in question. The most common pollinator
of a given plant taxon will impose the strongest
selection on its floral characters.
Campbell et al. (1997) investigated pollinatormediated selection in an Ipomopsis aggregatai
I. tenuituba hybrid zone. The most frequent pollinators of these species and their hybrids are broadtailed and rufus hummingbirds and, more rarely,
hawkmoths. Ipomopsis aggregata has red, trumpetshaped flowers characteristic of hummingbird-pollinated species, while I. tenuituba has longer, narrower corollas that range from white to pink or pale
violet. The latter is more characteristic of hawkmoth
pollination. The species are separated altitudinally,
with hybrid zones forming in intermediate altitudes.
To observe pollinator visitation, Campbell et al.
(1997) used artificial arrays of pure parental and
hybrid plants as well as a mixed array at a lowaltitude site within the range of I. aggregata ; mixed
arrays within the hybrid zone ; and natural parental
and hybrid populations. Corolla length, corolla
width and flower colour were chosen for study
because they influence pollinator visitation and vary
across the hybrid zone. Selection differentials and
gradients were calculated for the three focal traits
using pollinator visitation rates as a fitness component. As expected, hummingbirds were found to
prefer the red, wide corollas of I. aggregata plants to
hybrids and I. tenuituba. These pollinators appear
to impose directional selection for I. aggregata-like
flowers in the hybrid zone. Hawkmoths were only
present at the study site in statistically relevant
numbers during 1 yr of the 3-yr study (and a total of
3 yr out of the last 20). However, when present, they
preferred plants with narrow corolla tubes, resulting
in disruptive selection on floral traits during those
years.
A recent study of pollinator-mediated selection
has focused on the Louisiana irises (Emms & Arnold,
unpublished). Mixed experimental arrays of Iris
fulva, I. hexagona and their F hybrids were observed
"
in a population of pure I. hexagona and at the I. fulva
site into which I. hexagona and F hybrids were
"
introduced. Pollinators of these species include rubythroated hummingbirds and queen and worker
bumble bees. Pollinator preference was measured as
approach frequency, the percentage of approaches
611
that led to a legitimate visit (acceptance rate), and the
total number of legitimate visits to each flower type
(overall performance). Hummingbirds were the most
common pollinator at the I. fulva site, and they
preferred I. fulva flowers. At the I. hexagona site,
queen bumble bees were the most common pollinator, and they preferred I. hexagona and hybrids
over I. fulva. Worker bees, the least common
pollinator at both sites, preferred F flowers. Ap"
proach and acceptance rates were used to infer the
types of cues used by pollinators in selecting flowers
for visitation. Preferences of queen bees appeared to
be a result of differences in long-distance attractiveness of flowers, because they made more approaches to I. hexagona and hybrid flowers than to
I. fulva, but their acceptance rates did not differ.
Worker bees approached the three flower types with
equal frequency, but accepted few I. hexagona
flowers, suggesting that they were rejecting them
based on close-range cues. Hummingbirds seemed
to use a mixture of short- and long-range cues,
because they exhibited variation in approach and
acceptance rates at both sites.
The following were measured and compared
among the species and their hybrids : petal, anther,
nectary and stigma length ; anther exsertion ; nectar
volume ; and nectar concentration. For most characters, I. fulva is smaller than I. hexagona with F
"
hybrids intermediate. Nectary length, nectar volume
and nectar concentration did not differ significantly
among the three flower types, except that F hybrids
"
had significantly more-concentrated nectar. Therefore, the authors suggest that differences in floral
preferences are most likely related to foraging
efficiency on flowers of different sizes. Colour, the
most obvious long-distance cue, is probably used to
indicate differences in efficiency. Close-range cues
may be based on an assessment of rewards prior to
acceptance or on scent marks left by previous
visitors.
To infer the effect of pollinator preferences and
movement patterns on hybridization frequencies,
Emms and Arnold (unpublished) analysed intertaxon patterns of hummingbird and queen bumble
bee movements. They compared expected movement frequencies based on overall pollinator preferences with observed movements. Combining data
from both pollinator types revealed a pattern of overrepresentation of parental–F movements and an
"
under-representation of heterospecific and F –F
" "
movements. These data agree with the results of the
study by Hodges et al. (1996), which showed that
once an F is established, the formation of back"
crossed individuals is much more frequent than the
initial hybridization event.
These manipulative studies have provided insights
into the selection pressures exerted by pollinators in
hybrid zones. Because pollinator-mediated selection
acts on floral characters, it has implications for the
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612
L. H. Rieseberg and S. E. Carney
strength of reproductive barriers and speciation.
Additionally, gene flow is determined by pollinator
movement patterns in animal-pollinated plants.
Thus, observing pollinator movements provides
information about the fitness of different phenotypes
in hybrid zones.
3. Habitat selection
Because plants are so intimately associated with their
environment, the importance of ecological factors in
hybrid establishment and mating success is paramount. As discussed earlier, Ko$ lreuter was the first
to recognize the role of ecology, noting the importance of habitat disturbance in creating opportunities for spontaneous hybridization. This observation formed the basis for Anderson’s (1948)
classic paper on hybridization of the habitat. Anderson’s argument was simple. Habitat disturbance
creates an array of different habitats that are best
exploited by the extraordinary diversity of genotypes
created by hybridization. Disturbance also leads to
the breakdown of premating reproductive barriers,
thus increasing hybridization frequency.
Anderson’s conclusions appear to be corroborated
by empirical evidence. Hybridization is strongly
associated with disturbance. Disturbance often provides corridors for the movement of species and
leads to sympatry and hybridization between otherwise allopatric species (Levin et al., 1996). When
disturbance is reduced, the number of hybrids at the
site declines (Heiser, 1979).
Other evidence for the importance of habitat
includes the genotype–habitat associations often
reported for hybrid swarms (Cruzan & Arnold, 1993,
1994) and the mosaic nature of most plant hybrid
zones (Rieseberg & Ellstrand, 1993). However, these
correlative studies are problematic because each
taxon need not be found in the habitat in which it is
most fit. For example, one taxon could be outcompeted from its optimal habitat and forced to live
in a less ideal habitat. Another possibility is that
correlations between genotypes and environments
could be explained by history (Barton & Hewitt,
1985).
Reciprocal transplant experiments are extremely
useful for identifying the ecological preferences of
plants, and they have been used to assess the effects
of varied environments on fitness for nearly sixty
years (e.g., Clausen, Keck & Hiesey, 1940, 1948 ;
Bradshaw, 1960 ; Briggs, 1962 ; Barton, 1980 ; Antonovics & Primack, 1982 ; Potts, 1985 ; Helenurm,
1998). This experimental approach allows the comparison of fitness of taxa or adaptation of genotypes
to a number of habitats, but it has been used
infrequently in the study of hybrid zones.
Wang et al. (1997) used a reciprocal transplant
study to determine whether endogenous or exogenous (i.e. habitat-independent or -dependent)
selection stabilizes a big sagebrush hybrid zone
(Artemesia tridentata ssp. tridentataiA. tridentata
ssp. vaseyana). After planting seeds and 1-yr-old
seedlings of each subspecies and their hybrids into
the native habit of each taxon, Wang et al., 1997
assessed survivorship, size and reproduction. Each
taxon did significantly better than the others in its
native habitat, demonstrating that selection on big
sagebrush is habitat-dependent, and that each taxon
occurs in the habitat in which it is most fit. Similarly,
Levin & Schmidt (1985) transplanted seeds of Phlox
drummondii ssp. drummondii, P. drummondii ssp.
mcallisteri and their hybrids into the habitats of each
and monitored germination, survivorship, fecundity
and finite rate of increase. In contrast to the previous
study, they found no significant differences between
plant types in the different habitats. Therefore, it is
unlikely that extrinsic selection maintains the Phlox
hybrid zone ; it is probably maintained by restricted
gene flow.
Emms & Arnold (1997) used rhizomes rather than
seeds of Iris fulva, I. hexagona and their F and F
"
#
hybrids in their reciprocal transplant study, because
previous experiments involving transplantation of
seeds into natural populations were largely unsuccessful (M. Arnold, pers. comm.). The rhizomes
were transplanted into pure parental and hybrid
sites, and survival, growth and clonal and sexual
reproduction were studied. Hybrid leaf production
exceeded that of the parental species at all sites, and
hybrid rhizome production was greater than that of
both parents in all but one hybrid site. Hybrids were
intermediate in the percentage of flowering plants.
Fitness measures in this study seem to be affected
largely by habitat-independent hybrid vigour. In
addition, there are problems with estimating fitness
of long-lived perennial plants from data obtained in
one or a few years.
These reciprocal transplant experiments have
increased our understanding of the role of habitat
selection in the genetic structure and maintenance of
plant hybrid zones. However, more studies of this
type are needed before generalizations are possible.
 .                              

Much of what we know about the biology of different
classes of hybrids comes from the study of experimental hybrids and experimental manipulations
of hybrid zones already described. However, substantial knowledge of hybrid behaviour has also
come from studies in which natural hybrid zones
themselves served as the experimental system. This
approach has been particularly useful for studying
the response or resistance of different kinds of
hybrids to pathogens or herbivores, as well as for
studying the mating behaviour of different hybrid
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Plant hybridization
classes. In these studies, hybrid genotypes have been
classified using morphological or molecular markers
or a combination of both. Our discussion of the
biology of different classes of hybrids attempts to
incorporate information from all three kinds of
studies.
1. Character expression
The expression of morphological, chemical, and
molecular characters in different classes of hybrids
has been summarized by Rieseberg & Ellstrand
(1993). This discussion highlights the most interesting results from this summary and discusses both
the implications of these data for identifying hybrids
and the possible creative role of hybridization in the
origin of evolutionary novelties.
(a) Morphological characters. Rieseberg & Ellstrand
(1993) compiled a list of 46 studies that report
morphological character expression in hybrids. The
list included 32 examples of character expression in
F hybrids, nine examples from later-generation
"
hybrids, and four examples of homoploid hybrid
speciation. For each hybrid, the number of intermediate, parental and extreme or transgressive
characters was determined. The studies analysed
were not parallel in terms of their treatment of
hybrids. For example, several of the data sets were
taken from cladistic studies, where quantitative
variation may be partitioned into discrete classes.
Thus, partially intermediate character states might
have been scored as parental because of the lack of an
intermediate state for that character (e.g. McDade,
1990). In other instances, means and\or ranges of
values were given for hybrids rather than absolute
values. Quantitative traits were emphasized by some
studies, particularly those interested in hybrid
identification or morphological genetics, whereas
other studies, particularly phylogenetic ones, tended
to emphasize qualitative characters. Finally, some
studies employed floral characters only, others
emphasized vegetative characters, and still others
reported on both floral and vegetative characters.
Given these caveats, Rieseberg & Ellstrand (1993)
recommended that the results from this compilation
be interpreted with caution.
Nonetheless, analysis of these studies revealed
several surprising tendencies. First, F hybrids were
"
shown to be a mosaic of both parental (45n2 %) and
intermediate (44n7 %) morphological characters
rather than just intermediate ones. A possible
explanation for the high proportion of parental
characters expressed in hybrids is that many morphological traits that differentiate closely related
species display dominant inheritance patterns (Hilu,
1983 ; Gottlieb, 1984). Thus, the expression of
parental or intermediate character states in hybrids
will depend on the nature of the genetic control of a
613
particular character, as well as interactions with the
environment.
A second important finding from this survey was
the high frequency of transgressive or novel characters observed in hybrids. Over 10 % of morphological characters in F hybrids were transgressive,
"
and over 30 % were transgressive in later-generation
hybrids. The expression of transgressive characters
was not restricted to a few interspecific combinations, but rather seems to be a predictable feature
of most first- (64 %) and later-generation (89 %)
hybrids.
Several explanations have been offered to account
for the expression of novel or transgressive characters
in hybrids, including : an increased mutation rate in
hybrids ; the complementary action of new combinations of normal alleles ; the placement of unexpressed (or expressed) alleles in a new genetic
background (epistasis), as has been suggested to
explain novel floral pigmentation in Clarkia gracilis
(Gottlieb & Ford, 1988) ; the fixation of recessive
alleles present in the heterozygous form in the
parents (dominance) ; reduced developmental stability (Wagner, 1962 ; Levin, 1970 ; Grant, 1975) ; and
simple heterosis (overdominance). However, we are
aware of only one study that provides a definitive
genetic basis for transgressive segregation. DeVicente & Tanksley (1993) performed a QTL
analysis of eight transgressive characters in an
interspecific cross. The major cause of transgression
was the complementary action of genes from the two
parental species. No evidence was observed for
epistasis, but this might be because of the low power
of the data analysis method used. However, overdominance was implicated as a secondary cause of
transgression. Surprisingly, the more similar the
phenotype of the parents, the more likely transgressive segregation was to be observed at that trait.
The high frequency of transgressive segregation
supports the view of hybridization as a source of
variability upon which selection can act. From a
systematic perspective, however, the unpredictability of hybrid character expression diminishes the
utility of morphological characters for hybrid identification and suggests that some traits may be more
predictive of hybridity than others.
(b) Chemical characters. Tabulation of results from
24 studies of secondary compound expression in
hybrids from 22 plant genera revealed that the
majority of compounds were expressed additively in
both F hybrids (67n7 %) and hybrid species (53n6 %).
"
The lack of complete complementation for chemical
compounds in some taxa seems to result from
differences in genetic control, whereas in other cases
it appears to result from polymorphism of the
parental loci that control the biosynthesis of secondary compounds (Harborne & Turner, 1984).
Thus, a parent heterozygous for the controlling
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614
L. H. Rieseberg and S. E. Carney
locus in question may yield progeny with or without
that particular compound. Alternatively, the interaction of two disparate genomes in the hybrid may
disrupt biosynthesis, resulting in the loss of compounds.
Other deviations from a strictly complementary
pattern of character expression involve the production of novel compounds in many plant hybrids.
Indeed, novel compounds were observed in F
"
hybrids of 11 of the 24 genera listed (46 %).
However, these compounds were relatively infrequent compared with the total number of compounds assayed in the hybrids and their parents
(about 4 %). As might be predicted, the frequency
of novel compounds increased in later-generation
hybrids (7n8 %) and hybrid species (17n9 %).
In almost all cases, novel compounds produced in
hybrids can be explained relatively easily from a
genetic standpoint. One explanation is that the
enzymes necessary for the formation of the compounds already exist in one of the parents, but are
not expressed until the parental genomes are combined in the hybrids. Alternatively, the new compounds require the additive effects of both parental
sets of enzymes on the same basic chemical skeleton
(Stace, 1975).
Initial interests in chemical character expression
focused on their utility as markers for identifying
hybrids. However, with the advent of more powerful
and efficient macromolecular tools, interest in secondary compounds in hybrids has shifted to their
role in herbivore and pathogen defence. For example, the loss of parental compounds may partly
explain the observations that many hybrid zones act
as pest sinks. It is also possible that the production of
novel secondary chemicals in hybrids might stimulate the evolution of pathogen or herbivore genotypes
that are tolerant of the new compounds.
(c) Molecular characters. The expression of molecular characters in hybrids is inherently less
interesting than that of morphological or chemical
characters because in most instances genotypes (i.e.
DNA polymorphisms) are directly assayed and few
epigenetic effects have to be considered. Molecular
markers generally follow Mendelian laws of inheritance, although biases are often observed because
of selection.
There are some exceptions to these general rules.
Null alleles are observed infrequently in isozyme
assays and in PCR-mediated assays of DNA markers
such as microsatellites (Gottlieb, 1981 ; Pemberton et
al., 1995). Hybridization has also been reported to
generate novel isozyme alleles (Woodruff, 1989), but
this result may simply be an artifact of sampling
error. Restriction fragment patterns are known to
change because of different patterns of methylation
in hybrids (Jablonka & Lamb, 1995 ; Song et al.,
1995), and genes introduced through sexual hybrid-
ization or genetic engineering are often inactivated
(Wallace & Landbridge, 1971 ; Martin-Tanguy et
al., 1996). Perhaps the most serious problems arise
from analyses of repetitive sequences in which
concerted evolution can lead to the replacement or
loss of alleles from one of the parental species
(Arnold, Contreras & Shaw, 1988). Other problems
relate to spurious band formation or artifactual
variation in marker systems that employ arbitrary
primers (Riedy, Hamilton & Aquadro, 1992), but
these should be no more common in hybrids than in
intraspecific progenies.
2. The fitness of different classes of hybrids
Any discussion of the role of hybridization in
evolution must address the issue of hybrid fitness
(Arnold & Hodges, 1995). If hybrids were uniformly
less fit than the parental species, the role of
hybridization in adaptive evolution would be minimal : species would be unlikely to merge as a result of
hybridization ; speciation by reinforcement would
increase in likelihood ; tension (Barton & Hewitt,
1985) and mosaic (Harrison, 1986) hybrid-zone
models would be validated ; and outbreeding depression would be a greater threat to hybridizing rare
species than genetic assimilation. However, if hybrids were uniformly more fit than either parental
species, regardless of ecology, stable hybrid zones
would not exist, sympatric or parapatric speciation
would not occur and species merging would be the
primary outcome of hybridization events.
The true situation is much more complex. Hybrid
genotypes are highly heterogeneous with regard to
fitness, both within and between generations. As
discussed earlier, ecological factors, particularly
habitat may influence fitness relationships. Moreover, estimates of lifetime fitness, which are crucial
to discussions of this topic, are difficult to obtain,
particularly for long-lived organisms. Thus, sweeping conclusions about hybrid fitness are difficult to
make, but some generalizations are possible.
(a) The importance of variance. In discussing hybrid
fitness, it is important to distinguish between average
fitness of a genealogical class of hybrids and the
fitnesses of particular genotypes. The average viability and fertility of early hybrid generations (e.g.
F and F hybrids) is predicted to be lower than that
#
$
of the parental species because of the break-up of
adaptive gene combinations (Dobzhansky, 1937).
This is generally what is found, particularly for
species with strong postmating reproductive barriers. Well-characterized examples include Gilia
(Grant, 1966 a), Helianthus (Heiser, 1947), Layia
(Clausen, 1951), Oryza (Li et al., 1997) and
Zauschneria (Clausen, 1951). This makes sense,
because hybridizing species would merge if the
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Plant hybridization
average fitness of the early-generation hybrids was
greater than that of the parents. The fact that most
plant hybrid zones are limited in extent also implies
that early-generation hybrids are on average less fit
than their parents (Coyne, 1996), at least in parental
habitats. Although several recent studies have described the replacement of populations of rare taxa
by hybrid swarms (Brochmann, 1984 ; Rieseberg &
Gerber, 1995 ; Levin et al., 1996), this appears to be
because of genetic swamping by a numerically larger
congener rather than by higher average hybrid
fitness.
Low average fitness of a particular class or classes
of hybrids does not rule out the possibility that
certain hybrid genotypes may be as fit or more fit
than either parental species, particularly latergeneration hybrid segregates. Substantial empirical
evidence supports this hypothesis. First, heterosis is
commonly observed in hybrids and has often been
shown to result in increased vigour and fecundity.
Although the effects of heterosis are often partly
masked by disharmonious interspecific genomic
interactions in early-generation hybrids, strong
fertility and viability selection will favour the
elimination of negative gene combinations and the
maintenance of heterosis. Thus, after just a few
generations of selection, fertile hybrid genotypes can
be generated that sometimes outperform both parental species. Second, studies that describe fertility,
viability, or other fitness parameters in hybrids
almost invariably report the presence of a small
fraction of hybrid genotypes that are as fit or fitter
than parental individuals, even if the hybrids on
average exhibit reduced fitness (Heiser, 1947 ; Valentine, 1947 ; Grant, 1966 a). Third, significant
genotype–habitat associations are often reported for
hybrid swarms (Stebbins & Daly, 1961 ; Potts &
Reid, 1985 ; Cruzan & Arnold, 1993, 1994 ; Arnold,
1997). Presumably, this indicates that a selective
advantage accrues for certain hybrid genotypes when
found in favourable habitats, although these correlations could also result from historical factors
(Barton & Hewitt, 1985).
The preceding discussion has assumed that average hybrid fitness in early hybrid generations is
less than that of parental species – an assumption
that has substantial empirical support. However,
there are several examples for which the average
fitness of a particular class or classes of hybrids
appears to be equivalent or to exceed that of the
parents, at least for those fitness parameters measured. For example, Artemisia hybrids were more
developmentally stable and had higher seed germination and growth rates than either parent
(Freeman et al., 1995 ; Graham, Freeman & McArthur, 1995 ; Wang et al., 1997) ; Iris (Emms &
Arnold, 1997) and Oryza (Langevin, Clay & Grace,
1990) hybrids exhibited higher vegetative growth
rates than either parental species ; and no differences
615
in fitness were reported between hybrid and parental
individuals in a Phlox hybrid zone (Levin & Schmidt,
1985). Unfortunately, none of these studies measured lifetime fitness, so whether they represent
valid exceptions to the general rule of reduced
average hybrid fitness is unclear. Vegetative growth
rates are particularly problematic as a general
indicator of hybrid fitness in annuals or short-lived
perennials, since hybrid vigour is often observed in
even highly sterile plants. In clonal plants such as the
Louisiana irises or in long-lived organisms, vegetative growth rates become a much more critical
component of hybrid fitness.
These results should not be viewed as a challenge
to modern speciation theory (Dobzhansky, 1937).
First, in the absence of lifetime fitnesses, the fitness
measurements reviewed here must be seen as
preliminary, albeit exciting. Second, enhanced hybrid fitness should be plausible if postmating
isolating barriers are weak and the hybrids occupy a
novel habitat (Endler, 1977 ; Moore, 1977), or if
environmental conditions change (Anderson, 1948 ;
Grant & Grant, 1993 ; Arnold, 1997). These conditions are common in hybridizing plant species, so
rank-order fitness estimates that occasionally favour
hybrids should be expected (Rieseberg, 1997).
(b) Estimating hybrid fitness. An important caveat in
all of these discussions relates to the reliability or
relevance of most of the fitness estimates provided in
the literature to date. One problem is that it is difficult
to obtain lifetime fitness estimates for long-lived organisms. Another issue relates to the pooling of data
from heterogeneous hybrid genotypes. It is easy to
see how pooling data from several hybrid generations
could lead to erroneous conclusions (Arnold &
Hodges, 1995). However, even pooling individuals
from the same hybrid class can lead to faulty
conclusions because the variance in fitness is probably more critical than the mean. Another problem is
that most experimental studies of hybrid fitness have
been restricted to F or F hybrids or backcrosses,
"
#
yet the fitness of stabilized hybrid segregants is
of greater importance in terms of predicting the
evolutionary consequences of hybridization. Finally,
very few measurements of hybrid fitness have been
conducted under natural conditions (Levin et al.,
1996), even though fitness relationships among plant
hybrids appear to be habitat dependent.
Conducting experiments that correct these flaws
will be difficult or impossible in long-lived plants,
but should be feasible in annuals or short-lived
perennials. A possible alternative to classic transplant experiments would be the use of molecularmarker-based parentage studies. By tracking all
genotypes in a hybrid population over multiple
generations with a large number of molecular
markers, it should be possible to estimate parental
success or true fitness of all genotypes in a popu-
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616
L. H. Rieseberg and S. E. Carney
lation. Long-term experiments over several generations will be needed to determine what fraction of
observed fitness differences among different genotypic categories is caused by year-to-year environmental variation. These data will be most informative if combined with detailed habitat surveys so that
interactions between genotype and habitat can be
assessed. Data from experiments such as those just
described will probably be required fully to understand hybrid fitness and to make reliable inferences
regarding the role of hybridization in adaptive
evolution and speciation.
3. Interactions with parasites and herbivores
Over the past decade, considerable interest has
developed concerning the interactions of hybrids
with herbivores and parasites. This area of study was
initiated by Sage et al. (1986), who reported higher
levels of nematode and cestode parasites in hybrid
mice than in parental species individuals. The high
parasite loads found in the hybrids were attributed to
the disruption of genetically based resistance mechanisms. This was followed by a report of higher
aphid herbivore loads in cottonwood hybrids (Whitham, 1989). Whitham suggested that susceptible
hybrids might act as a sink for herbivore pests,
perhaps even drawing herbivores away from the
more resistant parental individuals. However, it soon
became apparent that the response of herbivores and
pathogens to hybrids could vary considerably and
was partly dependent on the hybrid generation or
even hybrid genotype being analysed (Boecklen &
Spellenberg, 1990 ; Paige, Capman & Jennetten,
1991 ; Aguilar & Boecklen, 1992 ; Fritz, NicholsOrians & Brunsfeld, 1994 ; Whitham, Morrow &
Potts, 1994). For example, Boecklen & Spellenberg
(1990) found that oak hybrids supported lower
densities of herbivores than did parental individuals,
possibly suggesting that the hybrids exceed pure
parental individuals in resistance. Another study
revealed that the response of different herbivores or
pathogens to the same hybrids could be strikingly
different (e.g. Fritz et al., 1994). This led to the
formation of a more explicit theoretical framework
for studying the interactions of hybrids and their
pests (Aguilar & Boecklen, 1992 ; Frit et al., 1994 ;
Strauss, 1994), as well as to a recent review that
summarized data from different studies on levels of
herbivory and parasitism in hybrid zones (Strauss,
1994).
In a survey of 19 studies comprising 17 hybrid
zones and a wide variety of herbivores, Strauss
(1994) reported the following : 32 cases in which
hybrids exhibited greater abundance of herbivores
or parasites than either parental species ; 24 cases in
which hybrids displayed intermediate levels of pest
abundance ; six cases in which pest loads were
reduced in the hybrids relative to either parental
species ; and 27 cases in which the hybrids did not
differ significantly from either parent in pest load.
Although these studies were not parallel in terms of
methodology or the kinds of hybrid genotypes
analysed, the summary does suggest that the response of herbivores to hybrids will be hard to
predict across hybrid zones and that increased
susceptibility is probably more likely than increased
resistance. Studies published after the Strauss (1994)
review have largely confirmed these conclusions (e.g.
Hanhima$ ki, Senn & Haukioja, 1994 ; Morrow et al.,
1994 ; Whitham et al., 1994 ; Christensen, Whitham
& Keim, 1995 ; Gange, 1995 ; Fritz et al., 1996 ;
Gaylor, Preszler & Boecklin, 1996).
Both genetic and ecological hypotheses have been
advanced to explain different resistance patterns in
hybrids. Fritz et al. (1994) have described a series of
explicit hypotheses based on inheritance of resistance
in F hybrids : the additive hypothesis, the domi"
nance hypothesis, the hybrid-susceptibility hypothesis, the hybrid-resistance hypothesis and the no
differences hypothesis. If F hybrids exhibit re"
sistance levels intermediate with those of the parental
species, this would suggest that hybrid resistance is
caused by additive inheritance of the resistance trait
from both parents. If hybrid resistance is similar to
that of one parent but not of the other, this might
imply dominant inheritance of resistance traits. If
hybrids are less resistant than either parent, perhaps
because of the disruption of resistance mechanisms,
the hybrid-susceptibility hypothesis would be supported. Alternatively, heterosis in hybrids could lead
to increased resistance – a result that would support
the hybrid-resistance hypothesis. Finally, hybrids
may be variable in pest resistance and not differ
significantly from their parent.
Studies of the inheritance and expression of
secondary compounds that contribute to resistance
support a genetic mechanism for at least some of the
differences in pest abundance on hybrids. As noted
earlier, a slim majority of secondary compounds are
inherited in an additive or complementary manner
(Rieseberg & Ellstrand, 1993 ; Orians & Fritz, 1995),
which is concordant with the large number of cases
in which pest abundance appears to be intermediate
between the two parental species. In many other
cases, dominant inheritance of defensive chemicals
has been reported (e.g. Huesing et al., 1989 ; Levy &
Milo, 1991), which may explain the many instances
in which hybrid resistance is similar to that of one
parent but differs significantly from that of the other.
The many examples of hybrid susceptibility are
more difficult to explain, because most hybrids do
not appear to differ drastically from their parents in
the expression of secondary compounds.
Although the patterns of pest abundance on hybrid
and parental individuals can be attributed to variation in the inheritance of resistance traits, ecological
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Plant hybridization
factors may play a role as well. For example, Floate,
Whitham & Keim (1994) demonstrated that the
increase of beetles in a popular hybrid zone resulted
in part from the extended time that young leaves
were available in hybrid zones relative to their
availability in pure populations. In a similar vein,
Paige & Capman (1993) showed that both genotype
and tree location were required to explain differences
in aphid survivorship patterns in the same poplar
hybrid zone. Thus, both ecological and genetic
factors appear to play an important role in determining pathogen and herbivore loads in natural
hybrid zones.
Another confounding factor in these studies has
been the varied response of herbivores to different
hybrid genotypes from the same and different hybrid
generations. This has placed a premium on correct
hybrid identification, precipitating a debate concerning the most appropriate tools for classifying
hybrids. Reliance on morphological characters has
been criticized because of their plasticity (Paige &
Capman, 1993) and unpredictable expression (Rieseberg & Ellstrand, 1993). Although the expression of
molecular markers is largely unaffected by environmental factors, hybrid and parental genotypic classes
often differ minimally in terms of expected marker
proportions. Thus, extremely large numbers of
molecular markers may be required to distinguish
between them. A more fundamental difficulty relates
to the potential for selection to bias marker proportions in hybrids, thus leading to faulty genealogical
assignments (Rieseberg et al., 1995 b, Rieseberg &
Linder, 1999). As a result, Rieseberg & Linder
(1999) recommended that hybrids be classified
according to genetic relatedness or genetic admixture
rather than genealogical category.
Although understanding of the inheritance and
evolution of resistance in hybrid zones is still in its
infancy, several possible consequences of these
interactions have been suggested. For example,
Whitham, Morrow & Potts (1991) and Whitham &
Maschinski (1996) have proposed that hybrid zones
might serve as important reservoirs of herbivore and
pathogen biodiversity and, in some cases, as a bridge
for host shifts. The host shift could lead to the
formation of a new herbivore species or simply
provide an avenue for movement of insect or
pathogen pests across a species barrier. Whitham
(1989) has also pointed out that if hybrid zones serve
as evolutionary sinks for pests, they might be
important in limiting the evolution of virulence.
From a plant perspective, hybrid resistance may
contribute either to hybrid breakdown if resistance is
disrupted or to heterosis if resistance is enhanced.
Finally, hybrid zones provide an excellent ecological
experimental situation for studying the genetic basis
of resistance in natural populations and for understanding the ecological and evolutionary aspects of
plant–pest interactions (Fritz et al., 1994).
617
4. Patterns of mating
One of the important factors that affects the outcome
of hybridization is the pattern of mating in hybrid
populations. Key parameters include outcrossing
rate, hybridization frequency and mate choice by
different classes of hybrids. Although it is clear why
hybridization frequency would be important, outcrossing rate and mate choice are equally critical.
Theoretical studies indicate, for example, that hybrid
speciation will be facilitated by selfing (McCarthy
et al., 1995). Selfing will also lead to lower rates of
hybridization. Differences in mate choice can lead to
patterns of differential introgression, and either
strengthen or weaken reproductive barriers (e.g.
Hodges et al., 1996). Surprisingly, there are few
studies of mating patterns in natural hybrid zones or
experimentally manipulated populations (notable
exceptions include Vickery, 1990 ; Hodges et al.,
1996 ; Bacilieri et al., 1996 ; Rieseberg, Baird &
Desrochers, 1998). Instead, most reports are restricted to static descriptions of population genetic
structure.
(a) Outcrossing rate. Outcrossing rates in hybrid
populations are assumed to be similar to those of
pure parental populations in the absence of information to the contrary. However, this line of
reasoning creates a paradox when applied to hybrid
speciation. Theoretical studies indicate that rates of
hybrid speciation should increase with selfing, yet
most natural homoploid hybrid species appear to
have an outcrossing mating system. A recent review
found that close to 90 % of 50 proposed examples
of hybrid species were outcrossers (Rieseberg, 1997).
Of these, at least ten have been well-documented
with molecular markers : Encelia virginensis (Allan,
Clark & Rieseberg, 1997) ; H. anomalus, H. deserticola and H. paradoxus (Rieseberg, Carter & Zona,
1990 ; Rieseberg, 1991 ; Rieseberg et al., 1995 c,
1996 b) ; I. nelsonii (Arnold et al., 1990 ; Arnold,
1993 a), Peaonia emodi and Peaonia sp. (Sang et al.,
1995) ; Penstemon clevelandii (Wolfe et al., 1997) ;
Pinus densata (Wang et al., 1990 ; Wang & Szmidt,
1994) and Stephanomeria diegensis (Gallez & Gottlieb, 1982). All of these are outcrossers. This
apparent bias toward outcrossing hybrid species is
particularly striking in Helianthus, in which all three
putative hybrid species and their parents, H. annuus
and H. petiolaris, are obligate outcrossers characterized by a sporophytic self-incompatibility (SI)
system.
Desrochers & Rieseberg (1998) suggested a possible explanation for this puzzle. Perhaps conditions
in hybrid zones favour selfing, even in normally
outcrossing species. For example, in many selfincompatible species, selfing can be induced by
mixed loads of self and heterospecific pollen
(Richards, 1986). This ‘ mentor effect ’ was demon-
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L. H. Rieseberg and S. E. Carney
618
Mean outcrossing rate
1
0·95
0·9
0·85
0·8
0·75
10
30
50
Fertility (%)
70
90
Figure 4. The mean outcrossing rate estimate taken over
each fertility class for maternal plants from the three
hybrid zones between Helianthus annuus and H. petiolaris,
shown with standard errors. Adapted from Rieseberg
et al., 1998).
strated in experimental sunflower hybrids by Desrochers & Rieseberg (1998), and a significant increase
in selfing has been reported in three wild sunflower
hybrid zones (Rieseberg et al., 1998). However, the
increase in selfing was largely restricted to the
parental-like fraction in the population. Outcrossing
rates in the critical hybrid fraction of the population
did not differ significantly from 1n0, suggesting that
mentor effects were unlikely to facilitate hybrid
speciation in wild sunflowers. Similar results have
been reported for self-incompatible oak hybrid
populations in which outcrossing rates approach 1n0
(Bacilieri et al., 1996).
Rieseberg et al. (1998) suggested that hybrids were
less likely to self than parental individuals because of
semisterility and the resulting reduction in the
probability of gamete union (Fig. 4). Semisterility
may inhibit selfing in hybrids of other species as
well, suggesting that selfing may be less important in
hybrid speciation than has been suggested by theory
(Templeton, 1981 ; McCarthy et al., 1995). However,
this cannot be the complete answer, since species
that are primarily differentiated by premating barriers often produce fully fertile hybrids. Possibly, the
advantage of a selfing breeding system for enhancing
the rate of establishment of hybrid species is counterbalanced by lower rates of natural hybridization
among selfing taxa. It is also possible that the high,
observed proportion of outcrossing hybrid species is
simply an artifact of small sample size and the
preponderance of outcrossing species of flowering
plants.
(b) Hybridization frequency. Direct estimates of
hybridization frequencies in natural or manipulated
populations are available from at least seven plant
groups : Brassica (Scott & Wilkinson, 1998) ; Helianthus (Rieseberg et al., 1998) ; Iris (Hodges et al.,
1996 ; Arnold, 1997) ; Mimulus (Vickery, 1990) ;
Quercus (Bacilieri et al., 1996) ; Phlox (D. Levin,
pers. comm.) ; and Senecio (Marshall & Abbott,
1980). Hybridization rates in Mimulus (0 %), Iris
( 1 %), Phlox ( 1 %), Senecio ( 1 %) and Brassica (0n4–1n5 %) are extremely low, whereas those in
Helianthus (4–7n5 %) and Quercus (31n7 %) are much
higher. Arnold (1997) has argued that the major
barrier to interspecific gene flow in Iris is the rarity
of F formation. Once F hybrids are formed,
"
"
extensive introgression is typically observed. By
contrast, sterility and ecological selection, respectively, represent formidable obstacles to introgression after F formation in Helianthus and
"
Quercus.
(c) Mate choice. The pollen competition experiments
described earlier indicate that the progeny produced
by different genotypes in hybrid populations are
unlikely accurately to reflect the genotypic composition of pollen loads. Instead, fertilization success
appears to depend largely on pollen–style interactions, with conspecific pollen tending to be
favoured relative to interspecific pollen.
Analyses of progeny arrays from natural hybrid
populations largely confirm the experimental
studies. In wild sunflower hybrid zones, for example,
parental-like individuals of H. annuus and H.
petiolaris are fertilized largely by intraspecific pollen.
By contrast, pollen from both parental species
successfully fertilizes ovules of semisterile (intermediate) hybrids. However, as hybrids recover
fertility by backcrossing, they become more likely to
be fertilized by pollen of the backcross parent. In
addition, as predicted by pollen competition experiments, hybridization was much more likely with one
of the species as the maternal parent than with the
other.
Strong asymmetric patterns of hybridization similar to those observed in Helianthus have also been
reported for Iris and Quercus. For example, in
hybrid Quercus populations, interspecific crossing
frequencies of 30 % have been reported for
maternal plants of Q. robur, whereas negative values
(presumably 0 %) were obtained for Q. petraea
mothers. Asymmetric patterns of hybridization have
also been inferred from analyses of genetic admixture
in hybrid zones (e.g. Wheeler & Guries, 1987 ; De
Pamphilis & Wyatt, 1990) ; pollen competition
experiments (e.g. Emms et al., 1996) ; and estimates
of hybrid and parental genotype frequencies (e.g.
Keim et al., 1989 ; Paige et al., 1991 ; Nason,
Ellstrand & Arnold, 1992). Asymmetry in hybridization frequencies often appears to lead to unidirectional introgression (e.g. Rieseberg, Choi & Ham,
1991 ; Bacilieri et al., 1996 ; Hodges et al., 1996), but
this is not always the case. For instance, bidirectional
introgression has been reported in some Iris hybrid
zones (Arnold, 1997), and introgression was sym-
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Plant hybridization
metric in three Helianthus hybrid ones analysed by
Rieseberg et al. (1998).
The direction of introgression will also be affected
by the mating patterns of the early-generation
hybrids (Hodges et al., 1996) and by the relative
proportions of parental and hybrid genotypes in the
hybrid swarms (Rieseberg et al., 1991). In Helianthus, F hybrids do not appear to favour the pollen of
"
one parental species over the other, but in hybrid
populations where one species predominates, interspecific gene flow tends to be in the direction of the
minority species (Rieseberg et al., 1991 ; Dorado,
Rieseberg & Arias, 1992).
  .                            
This review represents a validation of the predictions
of Levin (1979) about the kinds of studies that would
produce major advances in our understanding of
hybridization. The experimental studies advocated
by Levin (1979) have revealed the importance of
gametic selection as a reproductive barrier, elucidated the genetic architecture of postmating reproductive barriers, and demonstrated the critical
role of gene interactions and fertility selection in
hybrid speciation. By experimentally manipulating
hybrid zones, students of hybridization have been
able to generate reliable estimates of the frequency of
spontaneous hybridization and the strength of habitat selection – two parameters that are critical to
reliable predictions of the evolutionary or ecological
consequences of hybridization. In addition, a great
deal of information has been compiled concerning
the biology of different classes of hybrids. Morphological character expression and fitness of hybrid
genotypes has been found to be surprisingly difficult
to predict, as has been the response of pathogens and
herbivores to hybrids. By contrast, the mating
behaviour of hybrids appears to be largely predictable, as it seems to be governed in large part by
strong gametic selection.
Although Levin’s predictions are valid, the past
two decades have seen other advances in our
understanding of hybridization that do not fall easily
within Levin’s categories. The most important of
these are as follows :
(1) Molecular phylogenetic studies have revealed
the surprising power of gene trees to detect ancient
hybridization events (Smith & Sytsma, 1990 ; Wendel, Stewart & Rettig, 1991 ; Rieseberg, Whitton &
Linder, 1996). As a result, we now have strong
evidence for reticulate evolution in many plant
lineages, some of which are completely unexpected.
(2) Theoretical studies by Endler (1977), Moore
(1977), Barton & Hewitt (1985), McCarthy et al.
(1995) and Baird (1995), among others, have led to a
greater understanding of the maintenance of hybrid
zones, the genetic architecture of reproductive
619
barriers and the process of hybrid speciation. Theory
has also provided a series of explicit predictions
regarding the evolutionary dynamics and outcomes
of hybridization events and has provided a means for
extracting the maximum information content from
empirical data sets.
(3) Technical developments in molecular biology
have made available a virtually unlimited supply of
molecular markers. The quality of genotypic resolution afforded by these markers has made it
possible to view hybrid zones as natural experiments
and to study mating patterns, dispersal and genetic
architecture in the absence of manipulative experimentation.
(4) Similarly, advances in genetic mapping and
marker-based quantitative genetics have made it
possible to estimate precisely the genomic composition of natural and experimental hybrids and to
determine the genomic location of genes or chromosomal rearrangements involved in reproductive
isolation. Both advances facilitate direct comparisons
between experimental and historical studies of
hybridization.
Future advances are more difficult to predict. The
experimental studies of artificial and natural hybrids
advocated by Levin nearly two decades ago will
probably continue to lead to important new discoveries. Also, the use of genetic mapping approaches to analyse the genomes of ancient hybrids
and to map important quantitative traits will make
valuable contributions. However, the studies that are
likely to prove most useful are those that combine
experimental ecological and historical genetic approaches. Because hybrid species and introgressive
lineages originate rapidly, the majority of genetic
processes associated with them can be accurately
replicated by experimental studies.
Perhaps the best example of this approach to date,
already described, involved a comparison of the
genomic composition of three experimentally generated hybrid lineages with that of a natural hybrid
species that originated from the same two parents
(Rieseberg et al., 1996 b). The striking concordance
in genomic composition between the ancient and
synthetic hybrid lineages attests to the potential
utility of this kind of approach. However, these
experiments were conducted in the glasshouse, and
it is not yet clear how selection under natural
conditions might have affected hybrid genomic
composition. Clearly, it would be instructive to
determine whether a hybrid species could be replicated in the wild simply by allowing natural selection
to take its course. Genomic composition of the end
product could be compared with that of the natural
hybrid species to provide insights into the repeatability of speciation. Knowledge of the genomic
location of genes that control important traits such as
habitat differentiation would further inform these
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620
L. H. Rieseberg and S. E. Carney
kinds of studies. For example, it should be feasible
to determine whether important adaptations arose
via hybridization (i.e. involved genes from
both parental species) or arose via divergent evolution.
Similarly, it would be of interest to reconstruct the
genome of introgressive races. This could be accomplished in the greenhouse using a combination
of fertility and marker-based selection or by natural
selection in field experiments. In both cases, genetic
mapping data could be used to determine how
closely the synthesized introgressive races match
those produced historically in nature. Because introgression has often been thought to provide an avenue
for the transfer of genetic adaptations, it would be of
interest to compare the fitness and ecological amplitude of the synthesized introgressants with the
native species and its introgressive races. Moreover,
by determining the location of important QTLs that
differentiate the hybridizing species, it would be
possible to verify the transmission of these QTLs in
both the natural and synthetic introgressive races, as
well as to test the adaptive significance of individual
QTLs.
In some senses, the proposed use of historical data
to guide experimental studies represents a departure
from the classic interplay of theory and experimentation – the deductive approach championed by
introductory scientific textbooks. However, this is
perhaps inevitable given the historical nature of
evolutionary study. Experimental replication of
historical events probably represents the best way of
estimating the contributions of deterministic and
stochastic forces in evolution. This will not be
possible with experiments based on optimality
theory in model organisms. Of course, the strong
historical component of research proposed does not
rule out an important role for theory in future
studies of hybridization. Clearly, the interpretation
of historical phenomena and the choice of the most
appropriate conditions for replication will require
strong grounding in theory. Moreover, many questions relating to the dynamics and maintenance of
hybrid zones largely lack a historical component.
Thus, experimental manipulations and theoretical
study of hybrid zones will often be highly informative in the absence of historical context.
In conclusion, the future of plant hybridization
studies is extremely exciting given our newly
acquired ability to characterize precisely and\or
reconstruct hybrid genotypes on a chromosome by
chromosome or trait by trait basis. This will provide
a new rigour and precision to field ecological studies
that has not been possible in the past. Experiments
that fully exploit these capabilities should allow
us much more accurately to estimate the role
of hybridization and introgression in both adaptive evolution and in the formation of new
species.
              
We thank Rhonda Rieseberg for careful editing of the
manuscript. The research on hybridizing sunflowers
described here was funded by National Science Foundation (NSF) grants to LHR. SEC was supported by NSF
postdoctoral grant DBI-9750293.

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