FGF mutants exhibit pleiotropic ovariole phenotypes relating to loss of epithelial sheath. Jihyun Irizarry^1,2, Angelike Stathopoulos¹. 1) California Institute of Technology, Division of Biology, Pasadena, CA; 2) CIRM Bridges to Stem Cell Research Program, California State Los Angeles, Los Angeles, CA.

   Fibroblast growth factor (FGF) signaling is crucial for many developmental processes including cell migration, survival, and differentiation in many species. Despite the broad importance of FGF signaling, no study has reported a function within the Drosophila ovary. The main objective of this study has been to elucidate the roles of FGF signaling during Drosophila ovarian morphogenesis. In particular, our study has focused on characterization of the thisbe gene, which encodes a Drosophila FGF ligand. We found that thisbe mutant females are sterile, and examination of the ovaries revealed significant phenotypes. The mutant ovaries lack two types of muscle tissue: the peritoneal sheath, an outer tissue which ensheaths ~20 ovarioles, and the epithelial sheath, a tissue that surrounds each individual ovariole thereby keeping them separate. Despite loss of these tissues, surprisingly, the sub-ovariolar compartments (i.e. germarium, egg chamber, stalk cell) were generated. However, these compartments exhibited intriguing phenotypes. In the ovarioles of thisbe mutant females, ectopic localization of polar cells and over-proliferation of stalk cells, follicle cells associated with developing egg chambers, were observed. Furthermore, the mutant exhibited developmental defects in oocyte development. We are currently investigating whether FGFs role in oocyte development is direct or indirect. For example, FGF signaling may be required to directly support proper GSC division/differentiation or, alternatively, the absence of the epithelial sheath may indirectly affect GSCs. As an initial step toward answering this question, we are examining ovary phenotypes at the pupal stage, when the peritoneal and epithelial sheaths form. Based on our preliminary results, we hypothesize that FGF signaling controls apical cell migration at the pupal stage.