A NEW RHODOPSIN IN R8 PHOTORECEPTORS OF DROSOPHILA - EVIDENCE FOR COORDINATE EXPRESSION WITH RH3 IN R7 CELLS

The photoreceptor cells of the Drosophila compound eye are precisely o rganized in elementary units called ommatidia, The outer (R1-R6) and i nner (R7, R8) photoreceptors represent two physiologically distinct sy stems with two different projection targets in the brain (for review s ee Hardie, 1985), All cells of the primary system, R1-R6, express the same rhodopsin and are functionally identical, In contrast, the R7 and R8 photoreceptors are different from each other, They occupy anatomic ally precise positions, with R7 on top of R8, In fact, there are sever al classes of R7/R8 pairs, which differ morphologically and functional ly and are characterized by the expression of one of two R7-specific o psins, rh3 or rh4. Here, we describe the identification of a new opsin gene, rhodopsin 5, expressed in one subclass of R8 cells, Interesting ly, this subclass represents R8 cells that are directly underneath the R7 photoreceptors expressing rh3, but are never under those expressin g rh4. These results confirm the existence of two subpopulations of R7 and R8 cells, which coordinate the expression of their respective rh genes, Thus, developmental signaling pathways between R7 and R8 lead t o the exclusive expression of a single rhodopsin gene per cell and to the coordinate expression of another one in the neighboring cell, Cons istent with this, rh5 expression in R8 disappears when R7 cells are ab sent (in sevenless mutant), We propose a model for the concerted evolu tion of opsin genes and the elaboration of the architecture of the ret ina.