Molecular Genetics of Limb Development 
in Drosophila 
Stephen M. Cohen, Ph.D. — Assistant Investigator 
Dr. Cohen is also Assistant Professor in the Department of Cell Biology and in the Institute for Molecular 
Genetics at Baylor College of Medicine. He obtained his Ph.D. degree at Princeton University, working 
with Malcolm Steinberg. He did postdoctoral work with Harvey Lodish at the Whitehead Institute and with 
Herbert Jackie at the Max- Planck- Institut fur Entwicklungsbiologie in Tiibingen and at the University of 
Munich in Germany. 
ONE of the most striking features of early em- 
bryogenesis is the rapidity and reproducibil- 
ity with which the egg organizes the embryonic 
body plan. The process by which cells in the em- 
bryo acquire distinct identities is known as pat- 
tern formation. The concepts are as old as de- 
scriptive embryology, and the basic problems are 
the same in all animal and plant embryos. In 
order to generate an embryo with an integrated, 
coherent body pattern, cells need to adopt devel- 
opmental programs compatible with those of 
their neighbors. We are interested in understand- 
ing how the embryo can orchestrate its genetic 
information to specify the identities of groups of 
cells within the context of a larger body pattern. 
To address these questions, we are analyzing the 
genetic control of limb development in the fruit 
fly Drosophila. 
We would like to understand the genetic and 
molecular mechanisms that the fly embryo uses 
to determine where the limbs will develop with 
respect to the rest of the body pattern. How is the 
position identified? How do cells that occupy the 
correct position become committed to develop 
as limb precursors? Once the limb primordium is 
established, how do cells get the information that 
tells them what part of the limb to make? 
Specification of Limb Cell Identity 
Using classical genetics, we have identified a 
gene named Distal-less that plays an important 
role in limb development. Limbs do not develop 
in embryos that lack Distal-less gene function. 
This observation suggests that Distal-less activity 
may play an essential early role in limb develop- 
ment, but it does not tell us what the nature of 
that role might be. We get an additional hint from 
examining genetically mosaic animals in which 
Distal-less activity is removed from a group (or 
clone) of cells that were already committed to 
develop into limb structures. These cells shift 
their program and develop into body wall. Dis- 
tal-less, then, seems to function as a developmen- 
tal switch that is important for telling a cell that it 
is a limb precursor. Distal-less may also define 
and maintain the identity of the presumptive 
limb cells. 
How might Distal-less afi'ect cell identity? To 
approach an answer in molecular terms, we have 
cloned and characterized the gene. Distal-less is 
a member of a growing family of important regu- 
latory genes that encode a sequence-specific 
DNA-binding motif called the homeodomain. A 
number of Drosophila homeodomain-containing 
genes have been shown to control important de- 
velopmental decisions by regulating the expres- 
sion of other genes. We hope to identify the tran- 
scriptional targets through which Distal-less 
exerts its regulatory functions. 
How Does the Embryo Position Its Limbs? 
Using the Distal-less gene as a molecular 
marker, we have been able to visualize popula- 
tions of limb precursor cells in the embryo well 
before any overt sign of limb development be- 
gins. These small clusters of cells, placed at well- 
defined sites, are called the limb primordia. 
Clearly, to define these points, the embryo must 
use some preexisting source of information, 
which leads to the spatially localized activation 
of Distal-less. A number of observations impli- 
cate the wingless gene, without which (in mu- 
tant embryos) the limb primordia do not form. 
The gene is expressed in a reiterated series of 
well-defined stripes perpendicular to the ante- 
rior-posterior axis of the embryo. These and other 
observations suggest that the ivingless gene pro- 
vides a positional cue by which limb formation is 
oriented. 
If wingless alone sufficed to define the limb 
primordia, we would expect the limbs to begin 
development as stripes of cells expressing Distal- 
less. The limb primordia, however, arise as spots, 
not as stripes. One way to limit the limb primor- 
dia to spots would be to require interaction be- 
tween the stripes of cells expressing wingless 
and an intersecting longitudinal stripe of cells 
providing a different type of information. The 
wingless gene encodes a secreted growth-factor- 
like molecule homologous to the INT-1 onco- 
gene of mammals. A longitudinal stripe of cells 
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