Download Key Concepts -- Lecture 17 (BIOSYSTEMATICS 2) Spring 2009 IB

Key Concepts -- Lecture 17 (BIOSYSTEMATICS 2)
Spring 2009
IB 168
Cytogenetics (chromosomal genetics). Differences in chromosome number or
arrangements of chromosomal arms or segments are common in plants. (DNA was not
known to be the genetic material until 1953, whereas chromosomes were associated with
inheritance in 1903, ± simultaneous with rediscovery of Mendel's genetic work)
Crosses are widely used in cytogenetic studies, although are not always necessary.
Major phenomena involving changes in chromosome numbers:
- Polyploidy: Addition of one or more entire set(s) of chromosomes. Common
phenomenon in flowering plants, ferns, and lycophytes (not known in cycads; rare in
conifers; occurs in gnetophytes), both in past and today.
- Allopolyploidy believed to be especially common in plants: Chromosome doubling
following hybridization (as opposed to chromosome doubling not involving
hybridization = autopolyploidy, which also occurs in plants). Results in instantaneous
evolution of a reproductively isolated lineage and a good example of reticulate
evolution. High incidence of polyploidy may be in part due to selection for fixed hybrid
genotypes (allopolyploids will breed true for the hybrid phenotype because of lack of
pairing between chromosomes inherited from the different parental species).
hybridization
Species 1
haploid
gamete
Species 2
haploid
gamete
meiosis I
fails
Hybrid
diploid
sporophyte
Hybrid
diploid
gametophyte
selfing
Hybrid
diploid
gametes
True-breeding
tetraploid
sporophyte
Hybrid
diploid
gametes
Tetraploid sporophyte
Diploid sporophyte
Species 1
ANALYSIS OF GENOMIC
SIMILARITY BETWEEN
TETRAPLOID AND DIPLOID
PLANT
EQUAL NUMBER OF PAIRED AND UNPAIRED
CHROMOSOMES IN TETRAPLOID X DIPLOID
HYBRID IS CONSISTENT WITH THE TWO
SPECIES SHARING ONE GENOME (AND THE
OTHER GENOME CONTRIBUTED FROM
ANOTHER TAXON)
meiosis
hybridization
Diploid
gametophyte
Haploid
gametophyte
1 chr. pair (bivalent)
and
1 unpaired (univalent) chr.
Dysploidy: change in chromosome number by less than a whole set of chromosomes
(gains or losses), often by chromosomal rearrangements (types of macro-mutation) and
loss or gain of a chromosome without significant loss or gain of gene dosage (=
dysploidy; see example on next page).
Chromosome rearrangements involve two simultaneous structural mutations (breakages
and reunions) and therefore are highly unlikely to arise independently in different
lineages; that is, are unlikely to exhibit homoplasy (as phylogenetic studies of such
mutations has confirmed). Consequently, chromosome rearrangements (such as
inversions, translocations, etc.) are excellent phylogenetic characters and have been used
to infer relationships at deep and shallow levels of divergence. Also, chromosome
rearrangements result in loss of interfertility between plants bearing the ancestral versus
descendant arrangements (~50% loss of fertility with each reciprocal translocation, for
example).
loss of
centric
fragment
Unequal reciprocal
translocation of
chromosome arms
(macromutation)
centric
fragment
New arrangement and
reduction in chr. number by
1 without loss of genetic
material
Plant with 2
pairs of chr.
(gametophyte)
hybridization
Taxon with derived
arrangement
(gametophyte)
Taxon with
ancestral
arrangement
(gametophyte)
At meiosis, a chain of 3
chromosomes: diagnostic for
homology between main arms of 2
small chr. and each of arm of
large chr.
Breeding relationships: Early biosystematists believed that presence or absence of
crossability (= ability to generate a viable, but not necessarily fertile, hybrid) and hybrid
vigor, and levels of interfertility (= fertility of hybrids) were good indications of
relationship. For example, two species that cannot hybridize were considered more
distantly related to one another than two species that could hybridize; also, two species
that could form a highly fertile hybrid were considered more closely related than two
species that formed sterile hybrid. These beliefs were based on the assumption that
evolution is gradualistic, with progressive divergence between lineages manifested by
gradually increasing reproductive isolation. These progressive stages, determined by
crossability/interfertility experiments, formed the basis of classification. See Clausen
(1951), figure 76 in handout for example of how crossing data influenced perception of
relationships in Madia and Layia (the figure is essentially a couple of crossing diagrams
laid on their sides and with crossing connections pulled down as evolutionary branches).
Problems:
(1) Exceptions numerous and of all possible types, even in the groups that formed the
basis for these ideas. For example, internal reproductive isolation (loss of interfertility or
crossability) arises well after ecological isolation and evolutionary divergence in many
groups of woody plants (oaks, Ceanothus -- see Nobs 1963 figure on handout, Ribes,
Pinus, etc.). Reproductive isolation by multiple chromosomal rearrangements may arise
before morphological or ecological divergence in some annual plants (for example, in
some members of Holocarpha -- see Clausen 1951 fig. 42 on handout). In short,
internal barriers to gene flow are not necessary prior to evolutionary divergence in plants
and the presence of internal barriers to gene flow between two groups will not necessarily
result in their rapid divergence in morphological and/or ecological features (stabilizing
selection or evolutionary constraints may cause them to remain similar, as noted in the
speciation lecture by Brent Mishler).
(2) Crossability and interfertility are plesiomorphic (ancestral) features and therefore
cannot reliably diagnose clades or lineages; barriers to gene flow can arise rapidly and at
irregular rates in different groups, so non-monophyletic groups are likely to be
recognized using those criteria as the basis for classification.
(3) Circularity in reasoning if you base classification on crossability/interfertility and then
make conclusions about how speciation/diversification has occurred based on breeding
data.
Note: In general, perennials (including woody plants) and annuals appear to differ in the
timing of origin of reproductive barriers, with different perennial lineages often retaining
interfertility long after divergence and different annual lineages often becoming
intersterile or losing crossability before much morphological or ecological change has
occurred. This difference between annuals and perennials may reflect higher overall
rates of molecular evolution in annuals (recently demonstrated). In part, more rapid
evolution of reproductive isolation in annuals (at least with regard to pre-zygotic
barriers) could be attributable to different levels of natural selection acting against
gametic wastage. That is, annuals have one reproductive opportunity before dying; if
they hybridize, they may leave behind only unfit progeny. Perennials, on the other hand,
have multiple reproductive opportunities over a long timeframe and may not experience
such strong selection against the ability to hybridize; if seed set is not the limiting factor
on reproductive success, then producing occasional hybrids may not be deleterious.
Bias: The focus on making hybrids in biosystematic methods may account for the
widespread assumption among biosystematists that if hybrids could be made, then gene
flow or reticulate evolution between those crossable taxa would eventually occur in
nature. That prediction formed the basis for defining genera as groups of species
among which reticulate evolution was possible. Reticulation may be possible, but we
cannot predict the future and, again, crossability may not diagnose an actual lineage.
Chemosystematics - Systematic study of variation in secondary metabolites (often
defensive compounds sequestered in plants -- highly variable, even among close
relatives). First became popular as a means of distinguishing hybrids (look for
additive profile of compounds that combine the two parental profiles, by paper
chromatography, HPLC, etc.). Broad array of aromatic compounds have been studied,
including terpenes, flavonoids, and alkaloids. These methods were not in force until the
early 1960s and have gradually waned as studies revealed extensive homoplasy in
secondary compounds (not good phylogenetic characters at any level of divergence).
Protein (enzyme) electrophoresis -- A set of methods involving electrophoresis
(separating molecules of unlike charge, or of similar charge but different size, in an
electric field, in some type of matrix) to examine the different allelic forms of enzymes,
or duplicated versus unduplicated enzymes. Used in part by systematists to diagnose
hybrids, as with chemosystematic data; used more widely by population
geneticists/biologists to examine variation within and between populations.