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We have isolated a dominant mutation in a gene called Son of sevenless (SOS) that Is an allele-specific suppressor of the sevenkss phenotype. This suppressor function is autonomously required In R7 and is sensitive to the dosage of the SOS and bride of sevenless genes. Loss-of-function alleles of Sos are recessive lethals, but in the eye SOS has a role in R cell development. Mutations in SOS also Interact wlth the Ellipse allele of the Drosophila EGF receptor. We propose a model suggesting that the SOS product is downstream of sevenhss and the EGF receptor, and that the dominant suppression results from the overexpression or increased activity of the gene product.