Fig. 2. Diagram of germ cell sexual development. Embryonic stages are as described previously (Campos-Ortega and Hartenstein, 1985). L1, 1st instar larvae; L3, 3rd instar larvae. The adult stage depicts the apical end of a single ovariole in the female and the testis in the male. The germ cells and somatic gonadal precursors (SGPs) interact to form the embryonic gonad by stage 15. Both the germline and somatic gonad are already sexually dimorphic at this time. The female gonad undergoes ovary morphogenesis during late L3 [modeled after Godt and Laski (Godt and Laski, 1995)] to make individual ovarioles, and oogenesis begins in early pupae. In the male, the embryonic hub forms during stage 17 and spermatogenesis begins during L1. In adults, germline stem cells (GSCs) contact the somatic niche formed by cap cells in females and hub cells in males. Somatic stem cells (cyst progenitor cells in males and escort stem cells in females) also contact the niche. GSCs and somatic stem cells produce daughter cells that form differentiating oogenic or spermatogenic cysts of interconnected cells with branched fusomes. Later in female cyst development, the escort cells are replaced by follicle cells produced by the somatic (follicle) stem cells. Stage 12: SGPs, green; male-specific SGPs (msSGPs), brown; germ cells, yellow. Male: germ cells, dark blue; putative GSCs, light blue; msSGPs, brown; embryonic and adult hub cells, orange; fusomes, lime green; cyst progenitor cells, light green; cyst cells, green; testis sheath, yellow. Female: germ cells, dark pink; GSCs, light pink; terminal filament cells, orange; cap cells, red; stalk cell precursors, purple; basal cells, light blue; escort stem cells, light green; spectrosomes and fusomes, lime green; somatic (follicle) stem cells, brown; follicle cells, yellow.