Several Levels of EGF Receptor Signaling during Photoreceptor Specification in Wild-Type,Ellipse,and Null MutantDrosophila

AbstractDominantEllipsemutant alleles of theDrosophilaEGF receptor homologue (DER) dramatically suppress ommatidium development in the eye and induce ectopic vein development in the wing. Their phenotype suggests a possible role for DER in specifying the founder R8 photoreceptor cells for each ommatidium. Here we analyze the basis ofEllipsemutations and use them to probe the role of DER in eye development. We show thatElpmutations result from a single amino acid substitution in the kinase domain which activates tyrosine kinase activity and MAP kinase activation in tissue culture cells. Transformant studies confirmed that the mutation is hypermorphicin vivo,but the DER function was elevated less than by ectopic expression of the ligand spitz. Ectopic spi promoted photoreceptor differentiation, even in the absence of R8 cells. Pathways downstream of DER activation were assessed to explore the basis of these distinct outcomes.Elpmutations caused overexpression of theNotchtarget gene E(spl) mδ and required function ofNotchto suppress ommatidium formation. TheElpphenotype also depended on the secreted proteinargosand was reverted inElp aosdouble mutants. Complete loss of DER function in clones of null mutant cells led to delay in R8 specification and subsequently to loss of mutant cells. The DER null phenotype was distinct from that of eitherspitzorveinmutants, suggesting that a combination of these or other ligands was required for aspects of DER function. In normal development DER protein was expressed in most retinal cells, but at distinct levels. We used an antibody specific for diphospho-ERK as well as expression of the DER target geneargosto assess the pattern of DER activity, finding highest activity in the intermediate groups of cells in the morphogenetic furrow. However, studies of mutant genotypes suggested that this activity may not be required for normal ommatidium development. Since we saw distinct phenotypic effects of four different levels of DER activity associated with wild-type, null mutant,Elpmutant, or fully activated DER function, we propose that multiple thresholds separate several aspects of DER function. These include activation ofNsignaling to repress R8 specification, turning onargosexpression, and recruiting photoreceptors R1–R7. It is possible that during normal eye development these thresholds are attained by different cells, contributing to the pattern of retinal differentiation