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Insect
pigmentation is
one of
the simplest morphological traits in any animal. It is
essentially a two-dimensional pattern of biopolymers deposited by
epidermal cells. Pigmentation does not require morphogenetic
movements or complex cell behavior of any kind. This relative
simplicity provides us with an opportunity to develop a detailed
mechanistic model that relates gene expression to the final phenotypic
output.
The most upstream events in the spatial
patterning of
pigmentation have been elucidated in our previous work. In
particular, every aspect of the adult abdominal pattern is accounted
for by the action of known regulatory genes. At the other end of
the pathway, many of the structural genes involved in the production of
various pigments have also been identified and characterized.
Moreover, the biochemistry of pigment synthesis is sufficiently well
understood that we can predict the functions of many uncharacterized
genes based on their sequence. What remains to be done is to
identify the missing components of this pathway, especially its
intermediate regulatory tiers, and to understand the organization and
properties of the system as a whole.
We are trying to reconstruct the entire
developmental
pathway that controls pigment patterning and synthesis in Drosophila
melanogaster. To accomplish this, we are combining genome-wide
microarray experiments with targeted genetic designs made possible by
the availability of many mutations in both upstream and downstream
tiers of the developmental pathway. Our ultimate goal is to
develop predictive models that explain how positional information is
translated into a morphological phenotype.
Pigmentation is also one of the most rapidly
evolving
traits
in many animals, including Drosophila. Among the >3000 known species
of Drosophila and related genera, an astonishing variety of
pigmentation patterns is observed – from solid colors to stripes to
polka-dots. This diversity is a reflection of the differences in
the underlying developmental pathways that control pigment patterning
and synthesis. We try to identify these differences by comparing the
development of pigmentation among several dozen of Drosophila species
that we keep in the lab. At the same time, we take advantage of
the fact that many Drosophila species can be hybridized to take a more
direct genetic approach, where we try to map and identify the genes
responsible for differences in pigmentation within species or among
closely related species.
One of our main interests is convergent
evolution. We
are all familiar with the pictures of whales and sharks, bats and
birds, and other textbook clichés. But what is the
molecular basis of convergence? Does phenotypic similarity imply
that the genes and developmental pathways responsible for producing
these phenotypes are also similar? Or can superficial resemblance
be produced by entirely different molecular mechanisms? Darker or
lighter pigmentation has evolved many times in different evolutionary
lineages of Drosophila, giving us an opportunity to address this
question.
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Sex comb development.
The sex comb of
Drosophila is
a male-specific array of modified bristles that develops at a precise
position on the first pair of legs from a set of precursor bristles
present in both sexes. This little structure has surprisingly
complicated development, providing us with an excellent model to study
the organization of developmental pathways. It is known from
genetic studies that the developing sex comb integrates a very large
number of upstream regulatory inputs: the sex determination pathway,
the HOX genes, proximo-distal leg patterning genes, and the Hedgehog,
Wnt, Dpp (TGF-b) and Notch signaling pathways. This integration is
presumably achieved through joint regulation of target genes, which
remain unknown. Identification of these genes and analysis of
their regulation will help elucidate how diverse regulatory inputs are
integrated during development, and how abstract spatial information
provided by regulatory genes is interpreted by cells and translated
into a morphogenetic output. As with our other models, we use a
combination of classical developmental genetics and microarray-based
techniques to reconstruct the genetic circuit that controls sex comb
development.
Sex comb is also a very recent
evolutionary innovation. Most Drosophila species do not have sex
combs, although the precursor bristles are always present. Among
the flies that do have it, the size and structure of the sex comb show
dramatic variation, ranging from a pair of simple straight bristles to
seriously over-sexed flies that have over 150 curved teeth arranged in
an enormous spoon-like structure. By comparing sex comb
development among these species, and by using combless species as
outgroups, we hope to reconstruct the history of evolutionary assembly
and modification of a new developmental pathway. At the same
time, we are analyzing microevolutionary variation in this pathway in a
group of closely related Southeast Asian Drosophila species.
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Sexually
dimorphic
development of sensory systems.
Mating
behavior in Drosophila depends on the reception and interpretation of
sexually dimorphic chemical, visual, and auditory cues. For this
reason, many aspects of sensory system development and function are
also expected to be sexually dimorphic. Indeed, we have found a
number of genes whose expression in the sensory organs differs between
males and females. Many of these genes encode olfactory receptors and
odorant-binding proteins, molecules involved in neurotransmission, and
transcription factors that may function in directing sexually dimorphic
differentiation of chemoreceptive neurons and support cells. By
analyzing the function of individual genes and the structure of the
developmental pathway that controls their expression, we hope to
understand the genetic control of sexual behavior. Now, if only
that could be done for humans…
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