Pattern Formation and Neuronal Cell Recognition in the Drosophila 
Visual System 
Drosophila the proper development of the aduh 
optic gangha, the central nervous system portion 
of the visual system, depends on innervation from 
the eye. In the absence of retinal innervation, 
adult flies entirely lack the first optic ganglion, 
the lamina, which receives direct synaptic input 
from the outer photoreceptor cells Rl-6. 
^' We have found that the birth of lamina neurons 
is controlled by innervation from the developing 
eye. The arrival of photoreceptor axons in the 
brain induces a wave of mitotic activity that pro- 
duces the lamina neurons. These results suggest a 
novel mechanism for matching the number of tar- 
get neurons in the first optic ganglion to the num- 
ber of incoming photoreceptor axons, and they 
explain how developmental synchrony between 
the Drosophila retina and first optic ganglion is 
achieved. We are now using several different ap- 
proaches to elucidate the detailed cellular and 
molecular mechanisms underlying this process. 
While the importance of retinal innervation on 
the development of the adult optic ganglia of 
Drosophila is well documented, little is known 
about retrograde effects of the brain on photore- 
ceptor cells in the compound eye. We have re- 
cently discovered the first evidence for the exis- 
tence of such retrograde effects in the 
Drosophila visual system. Although photorecep- 
tor cells develop normally in the absence of con- 
nections to the optic ganglia, we have found that 
their continued survival requires these connec- 
tions. This situation is reminiscent of trophic in- 
teractions that are commonly found in 
vertebrates. 
Genetic Control of Cell Death 
Apoptosis, the deliberate and orderly removal 
of cells by natural death, is a prominent feature of 
normal development throughout the animal 
kingdom. In many organisms, a large number of 
cells die in the absence of obvious external in- 
sults. For example, in vertebrates neurogenesis 
produces about twice as many neurons as are 
needed in the mature nervous system, and approx- 
imately half of these neurons are eliminated by 
cell death. We are interested in isolating genes 
that are required for the initiation or execution of 
cell death in Drosophila. We have found that the 
ultrastructural characteristics of cell deaths seen 
in the Drosophila embryo are strikingly similar 
to apoptotic deaths described in other systems. 
We have developed techniques utilizing the vital 
dyes acridine orange and nile blue that permit the 
rapid and reliable visualization of apoptotic cells 
in live embryos, and we have used these methods 
to screen for cell death-defective mutants. We 
have identified a complex genetic locus on the 
third chromosome that is required for either the 
commitment to or the execution of a cell death 
program. We have cloned the DNA encompassing 
this locus, and we expect that its molecular char- 
acterization will provide insight into the bio- 
chemical mechanisms underlying cell death in 
Drosophila, and possibly other organisms as 
well. 
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