Download Changing Patterns of Gene Regulation in the Evolution of Arthropod

AMER. ZOOL., 38:818-828 (1998)
Changing Patterns of Gene Regulation in the
Evolution of Arthropod Morphology1
LISA NAGY 2
Department of Molecular and Cellular Biology, University of Arizona, Tucson, Arizona 85745
SYNOPSIS. What can the comparative study of gene expression patterns during
development contribute to the study of phytogeny? I discuss the basic properties
of gene networks that function in development, using information gleaned from
developmental model systems. Using examples from the analysis of anteroposterior,
dorsoventral and proximodistal axis formation, I outline how the gene networks
that pattern these three axes of development are linked in evolution. Finally, I
discuss the types of analyses necessary to further our understanding of how gene
networks function in regulating the evolution of morphology.
INTRODUCTION
Remarkable similarities have been found
in the gene networks that pattern basic cellular and developmental tasks in diverse
metazoans. Examples from studies in development include similarity of the HOX
genes in anteroposterior patterning (reviewed in McGinnis and Krumlauf, 1992;
Ruddle et al, 1994; Carroll, 1995), similarity in the signaling network that patterns
the dorsoventral axis of insects and vertebrates (reviewed in DeRobertis and Sasai,
1996) and similarity of the cell-signaling
network that functions in proximodistal axis
formation during vertebrate and insect limb
development (reviewed in Shubin et al.,
1997). The similarity of gene networks that
pattern structures in insect and vertebrates
has led to propositions like the homology
of the ventral side of arthropods with the
dorsal side of vertebrates (Arendt and Niibler-Jung, 1994; Peterson, 1995), and the
"deep" homology of insect and vertebrate
limbs (Shubin et al., 1997).
To what extent are the similarities among
gene networks that pattern development informative about phylogeny? Does similarity
in process of subcellular design or developmental patterning imply evolutionary relationship? It is important to remember that
1
From the symposium Evolutionary Relationships
of Metazoan Phyla: Advances, Problems, and Approaches presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 3—7
January 1998, at Boston, Massachusetts.
2
E-mail: [email protected]
we have known for a long time that all animals are built from similar tissue level
building blocks—neurons, muscles, epithelia. Similarly, eukaryotic organisms share
biochemical pathways that perform "housekeeping" functions—transcription, translation, or arrangement of the cytoskeleton.
Should it really come as any surprise that
other types of cell and developmental processes are equally conservative features of
metazoan evolution? Gene networks that
function in development give us another
character, or suite of characters for comparative analysis. By examining changes in developmental gene networks in the context
of already established phylogenies, we can
begin to determine how development
evolves. It remains to be seen whether developmental characters will be useful in and
of themselves for establishing phylogenies
and maybe even more significantly, whether
they will provide insight into morphological transformation.
How do we distinguish between homology and convergence of morphological features patterned by similar gene networks
(see Bolker and Raff, 1996)? For example,
consider the gene network that patterns vertebrate and insect limbs. Should vertebrate
and insect limbs be considered homologous
because they are patterned by similar gene
networks? Or is the similarity an example
of molecular convergence, representing not
an extreme conservation of limb construction throughout metazoa, but merely a consequence of a limited number of molecular
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ARTHROPOD GENE REGULATION
tools an organism has available to change
its form? From a morphological and historical standpoint, vertebrate and insect limbs
have rarely been considered homologs.
There are several clades between insects
and vertebrates that do not have limbs. We
do not know the functions the limb patterning gene networks serve in these intermediate clades, or whether they are even conserved as a network in all intermediates. Although the molecules that function in insect
and vertebrate limb development are similar
in kind, i.e., some of them are TGF-3 class
signaling molecules, they are not in all cases direct orthologs of one another. In addition, the "homology" of a pathway cannot
be solely determined by comparing the sequence similarity of the component parts.
Standards used for determining degree of
homology of a DNA sequence are virtually
irrelevant for determining homology of
gene networks. Differences in gene sequences are interesting, and informative, in
and of themselves to phylogenetics. However, developmental characters, like morphological characters, inherently have "systems" properties—not reflected in sequences.
The challenge for developmental biologists is to connect differences in gene sequence to differences in function. What are
the overall consequences of changes in
gene sequence for the performance of the
gene network as a whole? How do we relate
the uni-dimensional information of gene sequences into the multi-dimensional information of development? The important task
is to ask about the rules of transformation
and focus on the evolution of genes as circuits or networks. We can then ask how
change is enacted within gene networks. If
for example, the gene networks that pattern
insect and vertebrate limbs are convergent,
why do these particular combinations of
genes seem to have an affinity for one another? The long term goal for developmental biologists seeking to provide phylogenetic information is to ask how developmental mechanisms map onto phytogenies
and to ask what this mapping tells us of
evolutionary mechanism.
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What types of questions can the analysis
of developmental gene networks address?
Developmental characters have traditionally been very useful for phylogeny. Consider three classical developmental characters: 1. deuterostomy versus protostomy; 2.
spiral versus radial cleavage; 3. schizocoely
versus enterocoely. In spite of all the changes in our views of metazoan phylogeny over
the years, all three of these, or at least certainly the first two, make rather robust single character diagnostics for metazoan
clades. This may be indicative of the persistence of an early developmental ground
plan since before the Cambrian. Unfortunately, we know nothing about the genetic
basis of these characters. The organisms
that have become model genetic systems,
like the fruit-fly Drosophila or the nematode C. elegans do not fit nicely into the
dichotomous categories of protostome vs.
deuterostome or spiral vs. radial cleavage.
And presently its not exactly clear how one
could uncover the genetic basis of protostomy, or even spiral cleavage. It might be
possible to mutagenize a species with spiral
cleavage, looking for radialized phenotypes, but this might not be a very easy
experiment.
Although genetic analysis of model systems has not been yet been useful for understanding the classical developmental
characters described above, the comparative
study of gene expression patterns has, however, been informative concerning the questions of "How are body plans related to one
another?" and "What are the origins of
segmentation?" For questions of evolution
within phyla, this approach has been useful
for the study of tagmosis in arthropods, the
loss and gain of appendages, and the evolution of pigmentation patterns (Averof and
Akam, 1993, 1995; Carroll et al., 1994;
Warren et al, 1994; Brakefield et al., 1996).
Contributions from the developmental
genetics of model systems to the
understanding of the evolution
of morphology.
Throughout the rest of the paper, I use
examples from arthropod development and
evolution to illustrate some of the ideas that
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LISA NAGY
have emerged from the comparative analysis of developmental gene networks. Different arthropod classes can be distinguished by unique combinations of external
morphological features, including: 1. the
manner in which the body is tagmatized; 2.
the number of head segments; 3. the number of trunk segments; 4. the presence or
absence of an appendage on a segment; and
5. the characteristics of an appendage. What
do we know about the developmental genetics of these features? Genetic analyses in
Drosophila have provided a great deal of
inspiration for imagining what changes in
development might underlie evolutionary
modifications (Goldschmidt, 1940; Lewis,
1978). However, it is a common misconception that mutations in model systems retrace the evolutionary changes within an organism's genome. Drosophila mutations are
unique to the developmental system that
patterns Drosophila. The known genetic bases of evolutionary changes in morphology,
even those of which are similar to those invoked by mutation in Drosophila are not
ascribable to simple mutations that appear
in Drosophila. In addition, many features
that vary in evolution have never appeared
as mutations in Drosophila.
There are a few examples where Drosophila mutations have been characterized
as atavistic, or invoking the pattern of the
ancestor. For example, in homeotic mutations which remove the abdominal HOX
genes from the genome limb primordia appear on every segment, reminiscent of a hypothetical homonomously-segmented arthropod ancestor, more like a present day
centipede or branchiopod crustacean (Lewis, 1978). However, centipedes and crustaceans have the same component of abdominal HOX genes found in Drosophila (Averof and Akam, 1993, 1995; Grenier et al,
1997). Their homonomous segmentation is
not caused by fewer homeotic genes, as
suggested by the Drosophila homeotic mutations.
Another example is that of Drosophila
segmentation. Some mutations add or subtract the number of segments, a common
variation seen in arthropod evolution. The
Drosophila segmentation mutants are gap,
pair-rule or segment-polarity mutants (re-
viewed in Akam, 1987; Ingham, 1988). As
their names imply, the mutations result in
deletions of groups or pairs of segments, or
alter the polarity of a segment. However,
the patterns in which segments are deleted
in these mutations are very different from
the common variations in segment number
seen in arthropod evolution.
Consider also the changes in segment
character that have been elicited through
mutation in Drosophila. A Drosophila appendage can be changed to look like another Drosophila appendage, or something so
perturbed that it can at best be called a
blob-like-thing, but not to look like a lepidopteran or a hymenopteran appendage.
The take home message is that mutagenesis
in model systems does not undo evolution
or reveal, in any direct fashion, the basis of
evolutionary change. We cannot look to
model systems to define the changes we see
in evolution, but we can use them as a baseline of comparison. Through a comparative
approach, we can begin to understand properties of developmental systems. By measuring what stays the same and what changes we can learn about both developmental
possibility and constraint.
What are the characteristic features of
developmental gene networks?
Before beginning a comparative analysis
of the gene networks that pattern the three
developmental axes in arthropods, I first review some of the features of developmental
gene networks that have been gleaned from
50 years of the study of developmental genetics in model organisms.
1. First, development occurs through sequentially-acting cascades of genes. This
initial gene activity results in the subdivision of the embryo into multiple unique domains of gene activity, or compartments of
gene activity. Boundaries between these
compartments of information then generate
new sites of initiation for additional cascades of gene activation (reviewed in
Lawrence and Struhl, 1996).
2. Second, most networks of developmental patterning genes are comprised of
genes with pleiotropic functions. Rather
than having genes with unique functions,
most genes function at multiple times and
ARTHROPOD GENE REGULATION
places. For example, in Drosophila, wingless functions first in segmentation, then in
Malphigian tubule development, then later
in leg and wing development. There are a
few notable exceptions to this generality,
including the maternal-coordinate gene bicoid, that functions at the top of the Drosophila segmentation cascade described below.
3. Third, gene networks can create dissociable units, or "modules." This property
of developmental systems has been shown
both through classical transplantation experiments and modern genetic experiments.
For example, classical experimental embryology revealed that limb development is
dissociable from anteroposterior axis formation in vertebrates (Huxley and De Beer,
1934). This ability of limb development to
be dissociated from anteroposterior axis
formation is even more striking because it
is known that both anteroposterior and limb
patterning utilize many of the same genes.
In addition, there exists a class of genes,
selector genes, which have the capacity to
regulate a cascade of downstream gene activity. Ectopic expression of selector genes
can result in the development of ectopic
structures (Garcia-Bellido, 1975; Haider et
ai, 1995). Alterations in the expression of
these genes could be the initial trigger for
dissociation in developmental processes
(Garcia-Bellido, 1975).
4. Finally, it is apparent that development
is amazingly robust in the face of perturbation. Embryos with up to five extra copies of the maternal coordinate gene bicoid
produce a fine animal. In these flies, the primary maternal gradient that sets the entire
segmentation gene cascade described below
is very abnormal. But the animals recover,
and become normally segmented (Frohnhoffer and Niisslein-Volhard, 1986; Berleth
et ai, 1988; Namba et al, 1997). Embryos
can adapt to an amazing diversity of perturbations (see Gilbert, 1997 for examples).
This property of embryos has many names,
including adaptability, plasticity, regulation,
accommodation, self-adjustment, self-organization, tolerance, buffering, and homeostasis. While we currently have very little
understanding of the genetic mechanisms
responsible for the robustness of develop-
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mental pathways, it is an important property
of developmental systems to keep in mind
when we consider the manner in which developmental systems can be modified.
How do these properties relate to the potential for evolutionary change within a developmental system? One approach to the
study of this problem is to apply the comparative method and examine how a particular gene network changes within a group
of related organisms. The basic method is
to ask, "What are the genetic changes underlying morphological diversity?" Beginning with an integrated functioning circuit
in one organism, what are the possible ways
in which you could modify it to produce a
viable alternative? Are there particular
types of changes that occur frequently and
others not seen at all? I begin with the Drosophila gene networks that build segments
and position appendages and ask how the
circuitry changes in another species. I will
compare these networks with those of the
branchiopod crustacean, Triops longicaudatus. An important goal is to be able do
the same for multiple arthropods as well as
other phyla.
An approach to studying the evolution of
gene networks
I describe what has been learned about
how the primary patterning networks that
function in Drosophila development vary in
relation to two critical features of arthropod
morphology: tagmatization and limb diversification. I begin with a brief and generic
explanation of the gene networks that function in segmentation and appendage development in Drosophila (Fig. 1). The Drosophila embryo is patterned by a series of
gene interactions that subdivide the egg into
increasingly smaller units of positional information. It is not important at this point
to understand the details of all these cascades of gene activity, nor to be totally familiar with all the gene names. The essence
of the process relevant to the present discussion is that the egg begins with anteroposterior and dorsoventral polarity determined by the mother. These initial polarities
are detectable as gradients of information
along the entire length of each respective
egg axis (reviewed in Akam, 1987; Ingham,
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LISA NAGY
Anterior-Posterior Axis
ed in discrete domains of the developing
embryo (reviewed in McGinnis and Krumlauf, 1992). Coincident with this segment
and region-specific specification along the
A/P axis, the D/V axis also becomes further
subdivided. Along the D/V axis each region
gap
Kr
adopts a specific developmental fate: mesoderm, ventral nervous system and aminoserosa (reviewed in Ferguson and Anderhairy
son, 1991).
Thus, the activity of the A/P and D/V
gene networks create stripes of positional
information along both axes of the embryo.
segment
polarity
The boundaries of these stripes are then
used to activate new gene networks that
function to position discrete morphological
structures. Limbs, and other organs such as
homeotic
the salivary glands and trachea develop at
these boundaries. For example, legs develop at the boundary between the lateral
stripe of decapentaplegic and the segmentally reiterated stripes of wingless and engrailed (reviewed in Cohen, 1993). Limb
primordia are marked by a cluster of cells
expressing Distalless, a gene which encodes a homeodomain protein required for
Dorsal-Ventral
FIG. 1. Drosophila axis formation, simplified. Dia- proximodistal limb outgrowth (Cohen et
grammatic representation of some of the gene net- ah, 1989; Fig. 1).
works involved in Drosophila A/P, DA7 and P/D axis
This description is, of course, an overformation. Details are described in the text, bed: bi- simplified version of embryonic patterning.
coicl; Dll: Distal-less; dpp: decapentaplegic; Kr: Kriippel; wg: wingless; zen: zerknullt. Modified after Orenic Indeed, each one of these tiers of gene activity involves a complex series of gene inand Carroll, 1992; Cohen, 1993.
teractions (Fig. 2). How this positional information is translated into discrete units of
1988; St. Johnston and Ntisslein-Volhard, terminal differentiation is not yet as well
1992). These gradients then activate down- understood. However, from this very genstream targets in a concentration dependent eral description we can extract some of the
manner. Along the A/P axis the immediate general characteristics of how the fly emdownstream targets are the gap genes. In- bryo is patterned. Below I outline the reteractions between these large domains of sults of recent comparative studies of these
the gap genes then break the egg axes into pathways from my own and other labs, in
even smaller domains of gene activity: each which we have been asking how these gene
future Drosophila segment is subdivided networks are conserved in evolution.
into anterior and posterior (A/P) compart1. Establishing the conserved compoments. Cells in the posterior compartment nents.—Like the examples outlined in the
express the transcription factor engrailed beginning of this paper, the first observation
{en); cells which lie immediately anterior to the comparative method reveals is that dethe compartment boundary express the se- velopmental gene networks are composed
creted signaling molecule, wingless (re- of remarkably conserved components.
viewed in Martinez-Arias, 1993). As the in- HOX genes are most likely a major synadividual segments become patterned, the pomorphy for metazoans (see Duboule,
homeotic genes, which confer region-spe- 1994). Distalless is also conserved in both
cific segment identity, also become activat- sequence and its apparent function in limb
bic
ARTHROPOD GENE REGULATION
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Secondary
Pair Rule
Genes
FIG. 2. Drosophila segmentation, less simplified. While the Drosophila segmentation cascade is frequently
depicted in the simplified manner shown in Figure 1, the gene networks that pattern segmentation are much
more intricate and interdependent. Most of the genes in the segmentation pathway have multiple regulatory
targets as shown here. They also function at other times and places in development (not shown). This diagram
is adapted from White et al., 1986; Irish et al., 1989; Gerhardt and Kirshner, 1995; and Fujioka et al., 1995.
development (Panganiban et al., 1994). In
fact, it seems to be conserved in a multitude
of what Greg Wray has called "sticky-outies" produced in the animal kingdom (Panganiban et al., 1997). Wingless, engrailed,
hairy, decapentaplegic, nearly all the genes
in the diagram in Figure 1 have been shown
to be conserved at least between Drosophila and mouse, and in many cases are even
much more extensively conserved.
The fact that so many patterning genes
are conserved throughout the metazoa leads
to a view that changes in patterning predominantly involve changes in the regulation between conserved components (King
and Wilson, 1975; Jacob, 1977). We are
currently somewhat limited by the fact that
when we compare developmental gene
pathways across phyla, the focus is on conserved features, because they are easiest to
assay and recognize. But, is everything
conserved? Is all of evolutionary change
the result of changing regulatory interactions between conserved components? It is
very difficult to assess this question as we
have currently identified only a very small
portion of the genes encoding proteins even
within the most-studied model organisms.
One of the most surprising things to come
from the complete sequence of the yeast genome was that less than 20% of the identified protein coding regions represent pre-
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LISA NAGY
viously identified genes (Goffeau et al, clear in the past five years that gene dupli1996). It is likely that as more and more cations persist. In the early vertebrate linecomplete genome sequences become avail- age there is evidence for several total geable, we will get a much better handle on nome duplications. It has been suggested
what percentage of genes are novel and that this increase in gene number may be
unique to particular lineages and what per- responsible for the appearance of unique
centage are conserved. We may even some- morphological innovations seen in the verday be able to measure degree of conser- tebrate lineage (Holland et al., 1994). As
vation between non-coding regions of DNA mentioned above, many of the components
as well.
of the axes patterning gene networks are
Understanding whether different arthro- members of gene families, wingless, decapods share components in patterning gene pentaplegic, paired, and the HOX genes are
networks is important, but does not allow all members of gene families, that have at
us to compare how networks operate di- least one other paralogous gene in the Drorectly. Missing components do not neces- sophila genome. It is presumed these parsarily mean altered networks. Nor do con- alogous genes all arose via gene duplicaserved components necessarily mean con- tion. In some, but not all cases, the dupliserved gene networks. How do we make cated genes have diverged in function.
At what rate do their functions diverge?
the transition from studying individual
genes to gene networks and developmental The HOX gene family is interesting in this
processes? One critical necessity is to be regard. It is presumed that the HOX genes
able to isolate novel components of gene arose through a series of ancestral gene dupathways and to shift the focus from an plications. Based on the HOX genes that
amazement at how similar everything is to have been surveyed in a wide range of spean emphasis on what might underlie cies, the ancestral arthropod cluster is prechange. In addition, we need to be able to dicted to have the same composition of
analyze how conserved components vary in HOX genes found in Drosophila. In my
their regulatory relationships with one an- preliminary survey of Triops HOX genes, I
other. Below I outline two examples of ap- have found more than the expected number
of HOX genes, including a Triops HOX3
proaches to these problems.
2. Finding novel components.—There are ortholog. HOX3 is found within the vertesome notable exceptions of Drosophila pat- brate HOX clusters (Holland et al, 1992),
terning genes that have not yet been found but is absent from Drosophila (Kaufman et
to be conserved over long-evolutionary dis- al, 1990; Falciani et al, 1996). In this potances. For example, the bicoid gene at the sition in the fly HOX cluster there are three
top of the maternal hierarchy (Sander, homeobox containing genes, zen 1, zen 2
1994), has not been found outside the Dip- and bicoid with very divergent homeobox
tera, nor has the wing-patterning gene ves- sequences, which either do not function at
tigial. Are these examples of genes unique all in A/P patterning (zen 1, zen 2), or serve
to Drosophilal What is the source of new a very different role in A/P patterning than
genes like bicoidl There is accumulating all the other HOX genes (bicoid). The Droevidence to suggest that gene duplication sophila homeobox-containing genes zen 1
followed by diversification of gene function and zen 2 function in dorsal-ventral patternis a common source of new genes and gene ing (Rushlow and Levine, 1990). bicoid, as
function (Thomas, 1994; Nowak et al, described above, is a maternal-coordinate
1997; Cooke et al., 1997). When originally gene, that has not yet been identified outproposed as a theoretical mechanism for side the Diptera. By contrast, in chick and
change in evolution, it was argued that gene mouse embryos HOX3 has an expression
duplications could never survive. Because pattern with an anterior expression boundduplicated genes would be redundant, they ary that lies between those of HOX2 and
would necessarily accumulate mutation and H0X4, as would be expected from the fact
drift to eventual loss (Fisher, 1935; Ohno, that the order of the HOX genes along the
1970). However, it has become abundantly chromosome parallels their spatial sequence
ARTHROPOD GENE REGULATION
of expression along the body. These data
imply that the absence of a HOX3 orthologue in Drosophila is a derived condition.
Falciani et al., 1996 cloned HOX3 orthologs from a beetle and a locust and identified non-homeobox sequence motifs
shared between these HOX3 genes and the
Drosophila zen genes. Like the Drosophila
zen genes, these insect HOX3 genes are expressed only in extra-embryonic tissues.
They speculate that the HOX3 genes within
the insects have lost their role as a canonical HOX genes, and acquired a new function. In the Drosophila lineage, the HOX3
gene diverged rapidly, and is currently represented by zen 1 and its recent duplicate,
zen 2.
A comparison of the head segments of
flies and crustaceans indicates that crustaceans have a head segment—the second antennal segment, which is vestigial or absent
in insects. Is it possible that the loss of
HOX3 correlates with the loss of this head
segment? Although entirely speculative at
the moment, the evidence is consistent with
the idea that basally within the arthropods,
HOX3 function became redundant with another HOX gene. Once redundant, HOX3
function may have diverged differentially
between lineages. When analyzed in other
arthropods, other modifications to HOX3
expression and presumed function may be
seen. Thus, while the HOX genes represent
a major metazoan synapomorphy, they may
also provide the genetic resources for the
evolution of new gene function.
How do duplicated genes arrive at new
functions? How do old genes change their
patterns of regulation? On the most general
level, change in gene networks can clearly
be obtained through "tinkering" (Jacob,
1977). But by what means are changes in
regulation accomplished and what types of
changes are used? Are the bulk of the
changes cw-acting regulatory changes in
the promoters of duplicated genes? Or are
changes in the protein coding regions of
genes equally important? Is any change
possible, or are particular types of change
more likely to occur than others? One way
to begin an analysis of how gene networks
evolve is to go beyond comparing individual genes and their expression patterns
825
among species and to ask how gene-regulatory interactions diverge.
3. Making the transition from individual
components to networks.—Three of the
characteristics of gene networks that pattern
developmental systems described above are
1) they are composed of sequentially acting
cascades of genes; 2) they can be dissociable in time; and 3) they are robust in the
face of many kinds of perturbation. If a
gene that acts at the top of a cascade of
gene interactions that regulated a morphological structure can be modified to elicit
its action at a new time or in a novel location, this change would be manifest as a
shift in timing or position of the appearance
of that structure from ancestor to descendent. One of the many examples of a this
type of change in arthropod evolution is the
appearance and/or disappearance of legs in
different body regions between ancestor
and descendant species. For example, adult
insects have a limbless abdomen. In contrast, many other adult arthropods have
limbs on every segment. The comparison of
the circuitry of the segmentation and limb
patterning cascade has provided evidence
for the repeated addition of repressible elements into the gene circuitry (Warren et
al., 1994; Panginiban et al., 1995). Regulation of the morphological appearance of
limbs between crustaceans and insects and
within insect larvae appears to be controlled
via the modification of these repressors.
We know that the abdominal HOX genes
Ultrabithorax, abdominal-A, and Abdominal-B repress limb development in Drosophila. This is achieved at least in part
through their direct repression of the promoter of Distalless a gene required for limb
development (Cohen et al., 1989; Vachon
et al., 1992). Within insects the anterior
boundary of the expression of these abdominal HOX genes correlates with the morphological boundary between thorax and
abdomen. However when the expression of
the HOX genes and their target Distalless
is examined in crustaceans, these two genes
are expressed coincidentally (Panginiban et
al., 1995), which would be impossible if the
crustacean HOX genes repressed the crustacean Distalless gene. Somewhere in the
insect lineage, the HOX genes have ac-
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LISA NAGY
quired the capacity to repress the Distalless 1992; Ruddle et al, 1994; Carroll, 1995;
gene, thereby suppressing abdominal leg DeRobertis and Sasai, 1996; Shubin et al,
development. Thus, a potential genetic 1997). The HOX genes pattern the A/P
decapentaplegic-shortend-gastrulamechanism of morphological change in axis,
evolution is via the addition of an inhibitory tion pattern the D/V axis, and wingless step in an ancestral gene pathway. There is hedghog-decapentaplegic work together to
evidence for the repeated application of this set up the P/D axes. These genes set up a
mechanism within insect evolution. In the core reaction or "module" for each of these
development of larval legs in the lepidop- axes. The genes that function both upstream
teran Precis coenia there is evidence for the and downstream of these core circuits have
activity of an inhibitor of this inhibitor, al- not been found to be as conservative. This
lowing for the evolution of abdominal legs finding forms a molecular basis for the phein the abdomen of a holometabolous insect nomenon that Waddington (1940) called
larva (Warren et al., 1994). Abdominal legs "canalization." Development consists of a
appear repeatedly in the evolution of holo- robust, buffered morphogenetic system.
metabolous insect larvae (reviewed in Nagy Ancient, conserved genetic functions may
and Grbic, 1998). It will be extremely in- be locked into very interdependent, pleioteresting to determine whether larval legs tropic gene networks. Many types of muare regulated by independent mechanisms, tations may have no consequence for the
or whether the gene network is changing in phenotype, as they are masked by the buffering capacity of the system. Gene duplithe same way repeatedly.
cations that provide an initially redundant
SUMMARY
function may provide an avenue of escape
I began with the question "What can the from a multiply interdependent pathway to
comparative study of gene expression con- a novel gene circuit. It will be interesting
tribute to the study of phylogeny?" Be- to see what the next ten years of comparacause it is becoming increasingly clear that tive studies of the genetic basis of develmetazoans share a common molecular tool- opment reveal concerning the interplay
box used during development, can we use among pleiotropy and redundancy and the
this information to either infer phylogenetic evolution of developmental mechanisms.
relationships, or to learn about the interface
ACKNOWLEDGMENTS
of developmental mechanisms and evolutionary mechanisms? If developmental patThe author thanks H. Eisthen, E. Jockterning genes, like the HOX genes, are ex- usch and T. Williams for many helpful sugtremely conserved, they are unlikely to pro- gestions on improving the clarity of this
vide information about evolutionary rela- manuscript.
tionships. On the other hand, if the gene
sequences are conservative, but the gene
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Corresponding Editor: Douglas H. Erwin