CHARACTERIZATION OF STAR AND ITS INTERACTIONS WITH SEVENLESS AND EGF RECEPTOR DURING PHOTORECEPTOR CELL-DEVELOPMENT IN DROSOPHILA

Loss-of-function mutations in Star impart a dominant rough eye phenoty pe and, when homozygous, are embryonic lethal with ventrolateral cutic ular defects. We have cloned the Star gene and show that it encodes a novel protein with a putative transmembrane domain. Star transcript is expressed in a dynamic pattern in the embryo including in cells of th e ventral midline. In the larval eye disc, Star is expressed first at the morphogenetic furrow, then in the developing R2, R5 and R8 cells a s well as in the posterior clusters of the disc in additional R cells. Star interacts with Drosophila EGF receptor in the eye and mosaic ana lysis of Star in the larval eye disc reveals that homozygous Star patc hes contain no developing R cells. Taken together with the expression pattern at the morphogenetic furrow, these results demonstrate an earl y role for Star in photoreceptor development. Additionally, loss-of-fu nction mutations in Star act as suppressors of R7 development in a sen sitized genetic background involving the Son of sevenless (Sos) locus, and overexpression of Star enhances R7 development in this genetic ba ckground. Based on the genetic interactions with Sos, we suggest that Star also has a later role in photoreceptor development including the recruitment of the R7 cell through the sevenless pathway.