Inductive signaling pathways determining paired rhodopsin expression in R7/R8 cells of the Drosophila compound eye

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Pattern formation in the developing compound eye of Drosophila is primarily a result of inductive cell-cell signaling events. A highly interesting but poorly understood pattern in the differentiated compound eye has been recognized almost two decades ago in larger flies (Musca, Calliphora): Two subtypes of the inner photoreceptor cells R7 and R8 are organized in matched pairs randomly distributed across the compound eye. Our and others recent work revealed that these matched pairs correspond to a precisely regulated expression of the rhodopsins of R7 and R8 cells. One cell pair expresses the "odd" rhodopsin combination Rh3 and Rh5, the other cell pair expresses the "even" numbered rhodopsin pair Rh4 and Rh6. This coordinated expression of R8 and R8 cell rhodopsins is proposed to be the result of an induction of Rh5 expression in the R8 cell by the adjacent Rh3-expressing R7 cell ("odd" pair), versus an Rh6-expressing default pathway in the R8 cell of the "even" pair. Neither the components of the postulated inductive signaling pathway from R7 to R8 cells nor the developmental mechanisms which programm the R8 cell towards the Rh6-expressing default state are know. We propose to isolate Drosophila mutants which show defects in the R7/R8 cell pattern as a means to identify the genes and gene products which constitute this novel cell-cell signaling pathway and control the terminal differentiation of R7 and R8 cells in the Drosophila compound eye. The project has special relevance within the field of photoreceptor cell differentiation and is likely to provide insight into the mechanisms of pattern formation in other organisms and organ systems.