Tomlinson, A.; Arias, R. A.

The Drosophila R7 photoreceptor is a classic model for understanding how integration of signaling information can provide precise fate directives. It entails extensive interactions between the RTK and Notch signaling pathways, with Notch providing three distinct functions: it both opposes and promotes the general photoreceptor fate, and it determines the specific photoreceptor type. The RTK pathway promotes transcription of phyl - a gene expression critical for photoreceptor specification. We show that Notch activity induces transcription of yan which encodes a transcriptional repressor of phyl. This defines the antagonism between the two pathways, with RTK promoting and Notch opposing phyl transcription. We previously showed that Notch provides Sevenless to the cell to allow RTK pathway hyperactivation to overcome the Notch repression, and we now identify the regulation of Yan activity as the integration site of the RTK and Notch functions. Once the cell is specified as a photoreceptor, the third Notch function then prevents seven-up (svp) transcription. The Svp transcription factor directs the R1/6 photoreceptor fate, and the prevention of its expression ensures the default R7 specification.