EXPERIMENTAL FIGURE 16-20 Genetic studies reveal that activation of Ras induces development of R7 photoreceptors in the Drosophila eye. (a) During larval development of wild-type flies, the R8 cell in each developing ommatidium expresses a cell-surface protein, called Boss, which binds to the Sev RTK on the surface of its neighboring R7 precursor cell. This interaction induces changes in gene expression that result in differentiation of the precursor cell into a functional R7 neuron. (b) In fly embryos with a mutation in the sevenless (sev) gene, R7 precursor cells cannot bind Boss and therefore do not differentiate normally into R7 cells. Rather, the precursor cell enters an alternative developmental pathway and eventually becomes a cone cell. (c) Double-mutant larvae (sev^–/–; Ras^D) express a constitutively active Ras (Ras^D) in the R7 precursor cell, which induces differentiation of R7 precursor cells in the absence of the Boss-mediated signal. This finding shows that activated Ras is sufficient to mediate induction of an R7 cell. See M. A. Simon et al., 1991, Cell 67:701, and M. E. Fortini et al., 1992, Nature 355:559.