Patterning of the R7 and R8 photoreceptor cells of Drosophila: evidence for induced and default cell-fate specification.

Opsin gene expression in the R7 and R8 photoreceptor cells of the Drosophila compound eye is highly coordinated. We have found that the R8 cell specific Rh5 and Rh6 opsins are expressed in non-overlapping sets of R8 cells, in a precise pairwise fashion with Rh3 and Rh4 in the R7 cells of individual ommatidia. Removal of the R7 cells in sevenless, boss or sina mutants, disrupts Rh5 expression and dramatically increases the number of Rh6-expressing R8 cells. This suggests that the expression of Rh5 may be induced by an Rh3-expressing R7 cell, whereas Rh6 expression is most likely a default state of the R8 cell. We found that the paired expression of opsin genes in the R7 and R8 cells occurs in a sevenless and boss independent manner. Furthermore, we found that the generation of both Rh3- and Rh4-expressing R7 cells can occur in the absence of an R8 cell. These results suggest that the specification of opsin expression in the R7 cells may occur autonomously, whereas the R7 photoreceptor cell may be responsible for regulating a binary developmental switch between induced and default cell-fates in the R8 cell.