Pattern Formation and Neuronal Cell Recognition in the Drosophila 
Visual System 
Drosophila the proper development of the adult 
optic ganglia, 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 recently discovered 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 activ- 
ity that produces the lamina neurons. These re- 
sults suggest a novel mechanism for matching the 
number of target neurons in the first optic gan- 
glion to the number 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 approaches to elucidate the de- 
tailed cellular and molecular mechanisms under- 
lying 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 
recently discovered the first evidence for the ex- 
istence of such retrograde effects in the Drosoph- 
ila visual system. Although photoreceptor cells 
develop normally in the absence of connections 
to the optic ganglia, their continued survival re- 
quires these connections. This situation is remi- 
niscent of trophic interactions widely described 
in vertebrates. 
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