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. 2019 Jan 29;146(2):dev167643. doi: 10.1242/dev.167643

Fig. 2.

Fig. 2.

JAK/STAT signaling in homeostasis and regeneration in the Drosophila testis. (A) Schematic of the adult Drosophila testis. A group of quiescent somatic cells (green) forms the niche (also referred to as the ‘hub’) and secretes self-renewal cues for resident stem cells. GSCs (dark pink) and CySCs (dark blue) adhere to the niche. GSCs divide with oriented division to produce a gonialblast (light pink) that undergoes transit-amplifying divisions, resulting in a pre-meiotic cyst that gives rise to spermatids. The CySC divides to produce cyst cells (light blue) that become quiescent and ensheath the gonialblast. Two cyst cells continue to ensheath the associated spermatogonial cyst throughout spermatogenesis. (B) JAK/STAT signaling influences multiple processes in the testis; the insets detail events occurring in niche cells (green box), GSCs (pink box) and CySCs (blue box). Niche cells produce the cytokine Upd, which activates Stat92E (pStat92E) in GSCs and CySCs. Through its targets Zfh1 and Chinmo, pStat92E endows CySCs with the ability to support GSCs through BMP production. Upd can signal to Dome but not to Hop or Stat92E to promote spindle orientation during GSC division. In niche cells, upd mRNA is bound by Imp, which protects it from siRNAs, while Imp mRNA is a target of the let-7 microRNA. In GSCs, pStat92E regulates niche adhesion through E-Cad and the cytoskeleton via Chic. pStat92E also promotes male germline sexual identity through unknown targets and protects GSCs from irradiation-induced apoptosis through Diap1. In CySCs, pStat92E protects cells from irradiation-induced death through Diap1 and represses somatic differentiation through ImpL2. Chinmo and Zfh1 promote CySC self-renewal, and both are positively regulated by Cullin. pStat92E and Zfh1 are negatively regulated by E(Pc)/Tip60. Niche competition is controlled by the JAK/STAT target Socs36E, which represses Egfr/Ras/MAPK signaling. Niche competition is also regulated by another potential Stat92E target, Robo2, which represses the kinase Abl. pStat92E is positively regulated by Nurf301.