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Color vision and photoreceptor differentiation in Drosophila melanogaster
A fundamental problem common to the development of most sensory systems is the generation of functionally distinct neuronal cell types. The visual system constitutes a unique model to study the generation of cellular diversity within an otherwise homogeneous neuronal population. We use the fly retina to dissect signaling events that regulate the late phase of eye development, in particular those that control the selective expression of different rhodopsin genes in distinct photoreceptor (PR) subtypes. In many cases it has been shown that factors important for the development of the fly retina may also play a role in the vertebrate retina. Thus, in addition to the elucidation of basic developmental processes, our studies will aid the development of tools to fight eye diseases in humans.
In Drosophila color vision is achieved by the differential expression of blue-, green-, and uv-sensitive rhodopsin molecules in R7 and R8 cells. Throughout the retina, color ommatidia fall into two classes, pale and yellow that are defined by the expression of different sets of rhodopsin molecules in the inner PR cells R7 and R8. Pale and yellow ommatidia are distributed randomly in the fly retina with a ratio of 30:70 and results from a stochastic decision in R7 cells, which is then communicated to the underlying R8 cell.
While the genetic pathways regulating the initial steps of the differentiation of inner PRs have been understood in some detail later steps of PR differentiation are widely unknown. We could show that the specification of the yellow ommatidial subtype is induced by the bHLH transcription factor spineless, whereas ommatidia expressing the transcription factor orthodenticle adopt the pale fate. However, the signaling pathway that activates the specific rhodopsin in yellow R7 cells is still not known. Additionally, the molecules and pathways that underlie signaling between R7 and R8 cells, which are responsible for the correlated expression of specific pairs of rhodopsins in pale and yellow ommatidia remain equally elusive. Aiming to identify missing members of this differentiation pathway we have previously performed a genetic screen using enhancer trapping. We will determine the roles of this newly identified genes in late stages of PR differentiation and rhodopsin expression.