The specification and patterning of the Drosophila egg chamber

Abstract

The generation of cell polarity through the localization of specific mRNAs and proteins to discrete subcellular sites is fundamental to asymmetric cell division, tissue morphogenesis, cell migration, and most other developmental processes. While many different localized mRNAs and proteins have been described, the mechanisms by which such molecules become localized are only poorly understood. In the first part of this dissertation, I describe my efforts to unravel the mechanism by which gurken (grk) mRNA becomes localized to the anterodorsal corner of the Drosophila oocyte during mid-oogenesis. Such localization is a key step in the polarization of the mature Drosophila egg and future embryo; defects in grk mRNA localization result in the production of depolarized eggs that give rise to embryos that fail to specify ectodermal, endodermal and mesodermal germ layers and die before hatching. I show, using a transgenic fly assay system, that a conserved sequence element within the grk mRNA, called the GLS (grk localization sequence) is essential for anterodorsal localization. My studies indicate that the GLS functions by mediating the association of grk transcripts with a minus end directed microtubule (MT) motor protein, most probably cytoplasmic dynein. Although MT minus ends are enriched around the nuclear membrane in the oocyte's anterodorsal corner, MT minus ends are also abundant along other regions of the anterior cortex. My data force reconsideration of previous models of grk mRNA localization which propose that grk mRNA transport complexes specifically associate with that subset of MTs whose minus ends are concentrated around the oocyte nucleus. Indeed my data suggest that grk mRNA transport particles associate with all MT populations equally and that anterodorsal localization is brought about through repeated rounds of MT association and anterior transport accompanied by specific trapping of the mRNA at the anterodorsal cortex. The mechanism by which grk becomes trapped is unclear, but probably requires at least one RNA element in addition to the GLS. The second part of the dissertation is focused on the mechanism by which Rab11, a small GTPase best known for its role in trafficking vesicles from recycling endosomes to the plasma membrane, polarizes Drosophila germline stem cells (GSCs). Specifically, I present my characterization of a new Rab11 effector, dRip11 (Drosophila Rab11-family interacting protein). First, I show that dRip11 binds to Rab11 in vitro. Second, I identify a region within the Rab11 protein that is required for binding to dRip11. Third, I show that dRip11 has overlapping expression pattern with Rab11 in GSCs and border cells within the Drosophila ovary. Finally, I describe my attempt to generate and analyze dRip11 mutations.