The Effect of Development and Ecology on the Evolution of Ovary Size in Drosophila

Abstract

How the size of an organ is established and altered during evolution is poorly understood. The ovary of fruit flies of the genus Drosophila serves as an interesting model for understanding organ size evolution, as the number of egg-producing structures called ovarioles determines the ovary’s functional ‘size’. Species with more ovarioles can lay more eggs, and ovariole number can evolve rapidly between closely related species. However, the developmental and genetic mechanisms that determine and alter ovariole number were poorly characterized at the beginning of this thesis. I first analyzed the developmental basis of plasticity and species-specific ovariole number changes in D. melanogaster and closely related species. This analysis revealed distinct developmental mechanisms that alter ovariole number via changes in one cell type (terminal filament cells) in the developing ovary. To characterize the genetic mechanisms underlying proliferation patterns and potential cell-type interactions within the ovary, I then studied the role of the Hippo pathway in the somatic and germ cells of D. melanogaster. I uncovered a complex interaction between somatic cells and germ line cells, where proportional growth of these cell types is maintained by the Hippo pathway via interactions with the EGFR and JAK/STAT pathways. Finally, I expanded this work to investigate the physical, ecological, and developmental parameters that influence ovariole number evolution in Hawaiian Drosophila, where previous studies suggested that ovariole number correlated with larval food substrate. I describe my ongoing efforts to test correlations of ecology and ovariole number in a phylogenetic context in Hawaiian Drosophila. Primary differences in ovariole number between species occur through changes in cell number.