Positive and negative signaling mechanisms in the regulation of photoreceptor induction in the developingDrosophila retina

An ommatidium of aDrosophila compound eye contains eight photoreceptor cells, R1–R8. The fates of the photoreceptors are determined exclusively by inductive interactions between neuronal precursors in the cell cluster from which the ommatidium is formed. R7 induction has been extensively analysed at the molecular level. Activation of a membrane receptor tyrosine kinase (Sevenless) in the R7 precursor by a ligand (Bride of sevenless) present on the surface of R8 triggers a transduction cascade mediated by Ras, establishing the R7 fate of this cell. Other Sev-expressing cells are prevented from taking on the R7 fate by several different mechanisms. Pokkuri-mediated repression represents one such regulatory mechanism. The positive and negative signaling pathways operating in the fate determination of other photoreceptor cells are also discussed.