Undifferentiated cells in the developing Drosophila eye influence facet assembly and require the Fat facets ubiquitin-specific protease.

The Drosophila compound eye develops by a complex series of cell interactions where multiple positive and inhibitory cues guide cells in each facet into their positions and fates. The results of many genetic and molecular experiments have led to the view that facet assembly is directed by cells within developing ommatidial preclusters. Here fat facets mutants and the cloned fat facets gene were used to show that, in order to limit the number of photoreceptors in a facet to eight, undifferentiated cells surrounding assembling facets send an inhibitory signal to extraneous cells within the facet preclusters. Generation of the inhibitory signal requires the ubiquitin-specific protease encoded by the fat facets gene and is thus regulated by ubiquitin-dependent proteolysis.