Regulation of Drosophila eye development by the dual-function transcription factor/protein tyrosine phosphatase eyes absent

The development from a fertilized embryo to an adult multicellular organism relies on a limited collection of highly conserved and reiteratively deployed signaling pathways. Instead of acting independently, these signaling pathways are integrated at multiple levels and must converge in a spatially and temporally regulated manner to make a diverse array of cellular decisions during the generation of different tissues and organs. The Drosophila compound eye has served as a superb experimental system for studying the integration of different signaling pathways during animal development. In this dissertation, I have focused on the roles of Eyes absent (Eya), a protein with two distinct functions that influence and are influenced by multiple signaling pathways, in Drosophila eye development.;Eya, initially identified and named for its role in Drosophila eye development but broadly conserved in metazoans, possesses dual functions as a transcriptional cofactor and protein tyrosine phosphatase. While Eya's transcriptional activity has been extensively studied in multiple developmental contexts across species, the physiological requirements for its phosphatase activity remain obscure. Impaired phosphatase activity has been associated with defects in both Drosophila and human development, indicating an essential contribution to Eya function. In this study, I demonstrated that Eya functions cooperatively with the cytoplasmic tyrosine kinase Abelson (Abl) in Drosophila eye development. Mechanistically, Eya is phosphorylated by Abl at multiple tyrosine residues and relocalized from the nucleus to the cytoplasm, where in vivo studies reveal a requirement for its phosphatase function. The requirement of eya as well as its genetic synergy with abl in the photoreceptor cell axon targeting process suggests that it is a likely physiological context for the cytoplasmic Eya phosphatase activity.;To investigate the signaling context for the cytoplasmic Eya phosphatase activity, I performed a sequence of RNAi-mediated genetic screens and identified a number of Src Homology 2 (SH2) and Phosphotyrosine binding (PTB) domain containing proteins that interact with the cytoplasmic Eya function in the photoreceptor axon targeting process. Subsequent biochemical and cell biological assays with one candidate protein, Socs44A, found that Socs44A co-immunoprecipitates with Eya and colocalizes with Eya at the plasma membrane in the cultured cells, consistent with the hypothesis that interactions with specific SH2/PTB proteins sequester Eya in the cytoplasm after its phosphorylation by Abl and organize downstream signaling complexes.;In addition, I have examined the functions of Abl in patterning the Drosophila retina by analyzing abl loss-of-function phenotypes in larval and pupal imaginal discs. Abl is widely expressed in Drosophila neural and epithelial tissues and influences their morphogenetic processes through regulation of the actin cytoskeleton. My results show that Abl is required for photoreceptor morphogenesis and cell fate maintenance, but is largely dispensable for initial cell fate specification. This result, together with our finding that Abl is required for the photoreceptor axon targeting process, suggests that Abl plays a key role in establishing and maintaining the terminally differentiated state of Drosophila photoreceptor cells.