Analysis of the EGF Receptor Homologue in Drosophila: a Thesis

In the first part of this thesis, the expression of the Drosophila homolog (DER) of the vertebrate epidermal growth factor (EGF) receptor was investigated during development of the optic lobe in relation to cell division patterns. Patterns of Der expression were determined by in situ hybridization while the patterns of cell division were determined by incorporation of bromodeoxyuridine. Der transcripts within the CNS are located almost exclusively within a small subset of cells in the optic lobe. These cells represent the precursors of the lamina, the outermost synaptic region of the optic lobe. Laminal cells are postmitotic, being derived from the terminal cell divisions of ganglion mother cells in adjacent proliferation centers. Induction of Der expression coincides with the birth of laminal cells and continues through the first day after puparium formation, a time when extensive interactions between photoreceptor cell axons and developing laminal cells occur. By forty-eight hours postpupariation, the lamina is well developed and Der transcripts are no longer present. Mutant Der alleles do not affect the structure of the lamina in the larval stage but minor optic lobe defects are seen in adults bearing the Elp1 allele of the Der gene. These results suggest a non-mitotic role for DER in the development of the lamina. In the second part of this thesis, a P-element mutagenesis was carried out to identify second site mutations that suppress the Elp phenotype. Elp represents a hypermorphic mutation in the Der gene. Analysis of Elp protein by protein blotting indicates that the increased activity is not due to an increased level of gene expression. Three dominant mutations that suppress the Elp phenotype were identified and designated as Su(Elp)2, Su(Elp)3-1, and Su(Elp)3-2. Su(Elp)2 is a recessive embryonic lethal mutation which also affects viability of heterozygotes. Embryos collected from Su(Elp)2 parents have both cuticle and CNS defects. The cuticle is frequently missing or aberrant. The CNS is often hypertrophied and aberrant. Su(Elp)2 does not enhance Der loss of function mutations or suppress torRL3. Su(Elp)3-1 is a post-embryonic recessive lethal mutation. The Su(Elp)3-1 mutation reduces the severity of the eye pattern defects in Elp homozygotes and increases the viability of Elp homozygotes. In addition, Su(Elp)3-1 rescues the viablility of Elp/flb trans-heterozygotes. Su(Elp)3-1 is complex and may consist of multiple P-element insertions that act as suppressors of Elp. Su(Elp)3-2 also appears to be a complex suppressor mutation