NATURALLY-OCCURRING GENETIC-VARIATION AFFECTS DROSOPHILA PHOTORECEPTOR DETERMINATION

The signal transduction pathway controlling determination of the ident ity of the R7 photoreceptor in the Drosophila eye is shown to harbor h igh levels of naturally occurring generic variation. The number of ect opic R7 cells induced by the dosage-sensitive Sev(S11.1) transgene tha t encodes a mildly activated form of the Sevenless tyrosine kinase rec eptor is highly sensitive to the wild-type genetic background. Phenoty pes range from complete suppression to massive overproduction of photo receptors that exceeds reported effects of known single gene modifiers , and are to some extent sex-dependent. Signaling from the dominant ca in-of-function Drosophila Epidermal Growth Factor Receptor (DER-Ellips e) mutations is also sensitive to the genetic backgrounds, but there i s no correlation with the effects on Sev(S11.1) This implies that diff erent genes and/or alleles modify the two activated receptor genotypes . The evolutionary significance of the existence of high levels of gen etic variation in the absence of normal phenotypic variation is discus sed.