The role of Ras in dorsoventral patterning and morphogenesis, and the developmental mechanism of eggshell evolution in Drosophila

The study of developmental mechanisms in Drosophila melanogaster continues to yield valuable insights into the processes of multicellular development as a whole and the development of disease in humans. During oogenesis in D. melanogaster, epidermal growth factor receptor (Egfr) signaling through the Ras pathway patterns the somatic follicular epithelium, establishing the dorsoventral asymmetry of eggshell and embryo. Unlike mutations in gurken and Egfr, which compromise dorsoventral patterning of both eggshell and embryo, hypomorphic mutations in Ras mainly disrupt eggshell patterning. Analysis of follicle cell clones homozygous for a null allele of Ras demonstrates that Ras is required cell autonomously to repress pipe transcription, the critical first step in embryonic dorsoventral patterning. The effects of aberrant pipe expression in Ras mosaic egg chambers can be ameliorated, however, by post-pipe patterning events, which salvage normal dorsoventral polarity in most embryos derived from egg chambers with dorsal Ras clones. The patterned follicular epithelium also determines the final shape of the eggshell, including the dorsal respiratory appendages, which are formed by the migration of two dorsolateral follicle cell populations. Confocal analyses of mosaic egg chambers demonstrate that Ras is required both cell and non-cell autonomously for morphogenetic behaviors characteristic of dorsal follicle cell migration, and reveal a novel, Ras-dependent pattern of basal E-Cadherin localization in dorsal midline follicle cells. A central question in biology is how developmental mechanisms are altered to bring about the evolution of morphological diversity. Drosophilids boast a remarkable diversity in eggshell appendage number. Appendage patterning in Drosophila melanogaster is well-characterized, inviting candidate-gene based approaches that identify developmental mechanisms underlying appendage number diversity. Previous studies show that both Egfr and transforming growth factor-β (Dpp) signaling pathways regulate appendage patterning. Broad-complex expression requires both Egfr and Dpp signaling and predicts future appendage position. Confocal analyses of Broad-complex immunofluorescence and appendage morphogenesis in Drosophila melanogaster (two appendages), and Drosophila virilis (four appendages), suggest a Dpp-mediated mechanism determines appendage number. Regulation of Dpp signaling or response may underlie the evolution of appendage-number diversity in Drosophilid eggshells.