senseless repression of rough is required for R8 photoreceptor differentiation in the developing Drosophila eye

An outstanding model to study how neurons differentiate from among a field of equipotent undifferentiated cells is the process of R8 photoreceptor dif ferentiation during Drosophila eye development. We show that in senseless m utant tissue, R8 differentiation fails and the presumptive R8 cell adopts t he R2/R5 fate. We identify senseless repression of rough in R8 as an essent ial mechanism of R8 cell fate determination and demonstrate that misexpress ion of senseless in non-R8 photoreceptors results in repression of rough an d induction of the R8 fate. Surprisingly, there is no loss of ommatidial cl usters in senseless mutant tissue and all outer photoreceptor subtypes can be recruited, suggesting that other photoreceptors can substitute for R8 to initiate recruitment and that R8-specific signaling is not required for ou ter photoreceptor subtype assignment. A genetic model of R8 differentiation is presented.