Signalling by the sevenless protein tyrosine kinase is mimicked by Rasl activation

Abstract

CELL-FATE specification of R7 photoreceptors in the developing Drosophila eye depends on an inductive signal from neighbouring R8 cells. Mutations in three genes, sevenless (sev), bride-of-sevenless (boss) and seven-in-absentia (sina) cause the R7 precursor to become a non-neural cone cell^1–3. The sev gene encodes a receptor protein tyrosine kinase (Sev) localized on the R7 surface, activated by a boss-encoded ligand presented by R8 (refs 4–6). The sina gene encodes a nuclear factor required in R7 (ref. 3). Reduction in the dosage of the Rasl gene impairs Sev-mediated signalling, suggesting that activation of Rasl may be an important consequence of Sev activation⁷. We report here that Rasl activation may account for all of the signalling action of Sev; an activated Rasl^Val12 protein rescues the normal R7 precursor from transformation into a cone cell in sev and boss null mutants and induces the formation of supernumerary R7 cells. Similar activation of the Drosophila Ras2 protein does not produce these effects, demonstrating Ras protein specificity.