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First published online July 11, 2006
doi: 10.1242/10.1242/dev.02415

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The control of sexual identity in the Drosophila germline

Department of Biology, 302 Mudd Hall, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA.

View larger version (29K): [in a new window]   Fig. 1. A simplified view of sex determination pathways in the somatic gonad and germline. In somatic cells, the ratio of X chromosomes to autosomes (X:A) influences the activity of Sex-lethal (Sxl), which, in turn, activates transformer (tra). tra, along with transformer 2 (tra2), controls the alternative RNA splicing of doublesex (dsx), which determines whether the somatic gonad will develop as male or as female. In the germ cells, the X:A ratio also influences sexual identity, and ovo, ovarian tumor (otu) and Sxl promote female germ cell development. Interactions between germ cells and somatic cells also influence germline sex determination and act through extracellular ligands, such as Unpaired (Upd), which promotes male development. Gap junctions (red) may also facilitate communication between the two cell types.

View larger version (36K): [in a new window]   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.

View larger version (25K): [in a new window]   Fig. 3. Enlarged view of the male and female germline stem cell (GSC) niches. In both the male and female GSC niches, GSCs are attached to the niche via DE-cadherin-rich cell-cell contacts. GSCs usually divide so that one daughter remains associated with the niche, and retains GSC identity, while the other daughter is displaced from the niche and enters gametogenesis. Signaling from the female niche (terminal filament and cap cells) uses the Tgfß signaling pathway to maintain GSCs and the Jak/Stat pathway to maintain the escort cell population. Signaling from the male niche (hub) uses both the Jak/Stat and Tgfß pathways to maintain the GSCs. Male: germ cells, dark blue; putative GSCs, light blue; hub cells, orange; cyst progenitor cells, light green; testis sheath, yellow. Female: germ cells, dark pink; GSCs, light pink; terminal filament cells, orange; cap cells, red; escort cells, light green; epithelial sheath, blue.

View larger version (85K): [in a new window]   Fig. 4. Sexual dimorphism in the embryonic gonad. (A,B) Stage 15 embryonic gonad labeled to reveal the germ cells (anti-Vasa, blue), male-specific somatic gonadal precursors (msSGPs) (anti-Sox100b, red) and esgG66B enhancer trap (anti-ß-gal, green). msSGPs are found only in male gonads at this stage, and anterior SGPs have a sex-specific identity as they express esgG66B in males but not females. The esgG66B and mgm1 enhancer traps are expressed differently (in male SGPs and germ cells, respectively), even though both are in the esg locus. Images courtesy of Stephanie Le Bras. (C,D) mgm1 expression (X-gal staining) in stage 16 gonads (outlined). mgm1 expression is specific to germ cells in male embryos (D) and is not expressed in females (C). Images reproduced, with permission, from Wawersik et al. (Wawersik et al., 2005). (E,F) Stage 17 embryonic gonad labeled to reveal the germ cells (anti-Vasa, red) and embryonic hub cells (anti-Fasciclin 3, green). The embryonic hub forms in males, but not in females (E), and anterior germ cells adopt a specific rosette distribution around the embryonic hub (F). Images courtesy of Stephanie Le Bras.

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