The R8-photoreceptor equivalence group in Drosophila: fate choice precedes regulated Delta transcription and is independent of Notch gene dose

It has been suggested that lateral specification of cell fate by Notch signaling depends on feedback on Notch (N) and Delta (Dl) transcription to establish reciprocal distributions of the receptor and its ligand at the protein level. In Drosophila neurogenesis the predicted reciprocal protein distributions have not been observed. Either this model of lateral specification or the description of N and/or Dl protein distributions must be incomplete. We have reexamined R8 photoreceptor specification in the developing eye to resolve this question for this example of lateral specification. N and Dl protein levels were assessed in the cell as a whole and at the cell surface, where these proteins were mostly found at the intercellular cell junctions. Protein levels did not correspond to Notch signaling in wild type. However, Dl transcription and protein levels did correlate with altered N signaling in mutant genotypes. Our findings suggest the difference relates to the speed of lateral specification in vivo. The time required for N signaling to inhibit ato expression was at most 90 min, but changes in the Dl protein distribution in mutant genotypes arose more slowly. N expression was little regulated by N signaling, but protein encoded by the Nts1 allele was temperature-sensitive for appearance at the cell surface. Some aspects of the pattern of Dl protein appeared to be due to endocytosis. We conclude that feedback of N signaling on Dl transcription does occur but is too slow to account for the pattern of R8 specification. Studies of ommatidia mosaic for a Notch duplication, or for the Nts1 allele at semi-restrictive temperatures, found that cells beginning with less N activity were not necessarily predisposed to be selected for R8 differentiation. Our data argue that other signals may be responsible for the pattern of R8 cell fate allocation by N. Potential relevance to other neurogenic regions is discussed.