Generating patterned arrays of photoreceptors

Abstract

One of the most fascinating topics in biology is to understand the development of highly differentiated cells such as photoreceptors (PRs). This process involves successive steps, starting with the generation of the eye primordium, recruitment and specification of PRs and finally, expression of the proper rhodopsin, the photopigment that initiates the signaling cascade underlying light input excitation. In this review, we describe the sequential steps that take place in the Drosophila eye, from the initial neuronal specification of PRs through their full maturation, focusing specifically on the transcription factors and signaling pathways involved in controlling the precise expression of different rhodopsins in specialized PRs.

Figure 1

Developmental timetable of the Drosophila retinal mosaic. After the eye-antennal disc is specified by the eye determination genes, PRs are recruited sequentially into an ommatidium (first R8, then R2 and R5, later R3 and R4, and finally R1 and R6). In the late larval and early pupal stages initial cell-fate decisions are made. First, a generic color PR fate is established through the action of sal. Then R7 and R8 cells are distinguished from each other by the specific expression of pros and sens in R7 and R8, respectively. Hth commits inner PRs to the DRA fate; while otd generates the pale fate, ss patterns, the yellow ommatidial subset. Finally, the subset-specific expression of wts and melt in yellow and pale R8 cells, respectively, enforces the decision that has been communicated by the R7 cell.