Patterning the drosophila retina for detection of color and polarized light

In the Drosophila retina, the R7 and R8 inner photoreceptors integrate light information to detect polarized or color light. In fart, there are three types of ommatidia with a stochastic distribution. One set expresses rhodopsin3 (rh3) in R7 and rh5 in R8, another the rh4-rh6 pair, while the last one expresses rh3 in both R7 and R8. We are studying the mechanisms underlying the exclusive pxpression of rh genes as well as the signaling events between the R7 and R8 cells that allow the coordinate expression of rh. Using genetic situations where there are supernumerary R7 cells, or where there are no R7 cells, we concluded that there is a reciprocal communication between R7 and R8, In fact, we can propose that there is a 'ground state' (rh4/rh6) that does not require interaction between R7 and R8, and an acquired state frh3/rh5) that requires exchange of information between the cells. As control of rh expression is purely transrriptional, an analysis of the rh promoters has allowed us to understand their modes of regulation. Consistent with the model of ground/acquired states, rh4 contains promoter elements that allow its expression in all R7 cells, and an dement that represses its transcription in rh3-expressing cells. To the contrary, rh3 (and rho) is only activated in a subset of ommatidia in response to the activity of the orthodenticle (old) gene, a gene that has been shown to be critical in vortpbrate photoreceptor development. Using genetir approaches, we hope to understand how the diversification of rhodopsin genes and modification of iliir f'xprfssion have evolved to achieve color vision and phototactic behavior.