Abstract
Dipteran insects such as Drosophila obtain visual information using compound eyes. In Drosophila,these compound eyes are composed of approximately 800 unit eyes called ommatidia. An ommatidium contains eight distinct photoreceptor cells, each of which projects an axon directly to the optic lobe of the brain. This structure contrasts sharply with the mammalian eye, which contains a single lens and a retina with multiple layers of neurons. In spite of these and other substantial differences in the morphological appearance of insect and vertebrate eyes, work in the last several years has revealed common underlying genetic pathways controlling retinal cell fate specification. This discovery is surprising since the eye was considered an extreme case of convergent evolution, evolving independently as many as 40 different times (reviewed in Land and Fernald 1992). Much of the flurry of molecular and genetic data that has accumulated in recent years challenges this notion and suggests divergence from a single, prototypical visual processing unit. Thus, Drosophila has proven to be an excellent model system for identifying new genes that are conserved in vertebrate retinal development. This chapter will mainly be concerned with describing the factors responsible for specification and determination of retinal cell fate in Drosophila.
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