Expression of Drosophila rhodopsins during photoreceptor cell differentiation: insights into R7 and R8 cell subtype commitment.

The R7 and R8 photoreceptor cells of the Drosophila retina are thought to mediate color discrimination and polarized light detection. This is based on the patterned expression of different visual pigments, rhodopsins, in different photoreceptor cells. In this report, we examined the developmental timing of retinal patterning. There is genetic evidence that over the majority of the eye, patterned expression of opsin genes is regulated by a signal from one subtype of R7 cells to adjacent R8 cells. We examined the onset of expression of the rhodopsin genes to determine the latest time point by which photoreceptor subtype commitment must have occurred. We found that the onset of rhodopsin expression in all photoreceptors of the compound eye occurs during a narrow window from 79% to 84% of pupal development (approximately 8 h), pupal stages P12-P14. Rhodopsin 1 has the earliest onset, followed by Rhodopsins 3, 4, and 5 at approximately the same time, and finally Rhodopsin 6. This sequence mimics the model for how R7 and R8 photoreceptor cells are specified, and defines the timing of photoreceptor cell fate decisions with respect to other events in eye development.