Ommatidia in the developing Drosophila eye require and can respond to sevenless for only a restricted period.

The sevenless gene encodes a putative cell surface receptor that is expressed in a highly specific and complex pattern in the developing Drosophila eye. We show here that this pattern is not required for the development of either the R7 cell or any other cell in the retina. By transiently expressing sevenless protein, we show that there is only a brief period during eye development when the sevenless protein is required for the formation of the R7 photoreceptor. Our results indicate that the specificity of the cell-cell signaling mediated by the sevenless protein is not achieved by regulating the distribution of the sevenless protein, but rather must reside in some combination of the temporal and spatial distributions of the sevenless ligand or other components of the sevenless signaling pathway.