The Drosophila ovarian terminal filament imports molecules needed to produce lipid droplets, the fusome, and functional germ cells

Abstract

Oogenesis in Drosophila requires small molecules as biosynthetic precursors and regulators of follicle development. But how such molecules reach germline stem cells (GSCs), developing germline cysts and follicles in the germarium and early ovariole is poorly understood. The flat, stacked cells of the terminal filament (TF) form a specialized somatic structure positioned at the anterior end of ovarioles in virtually all insect ovaries, but physiological roles TFs play in adult ovaries remain little known. By briefly knocking down, specifically in the TF, exocyst components affecting vesicle trafficking, lipid importers such as LpR2, and organic anion importers such as Oatp30B, we found that the TF provides lipophilic molecules to GSCs and downstream germarium cells needed to maintain lipid droplets, and germ cell differentiation. When exocyst component Sec6 is knocked down, vesicles containing lipophilic cargos back up at the TF-germ cell junction, suggesting that endosomes move between the stacked TF cells and into cap cells by transcytosis. Some of these may be shuttled during the GSC cell cycle between the TF and newly forming fusome. Our studies suggest that TFs import lipophilic precursors and regulators to autonomously coordinate their ovarioles stem cell activity and cyst development.