JAK/STAT pathway plays two opposite roles in Drosophila spermatogenesis. Lingfeng Tang, Douglas Harrison. Department of Biology, University of Kentucky, Lexington, KY.

   Germline cells in the testis are derived from germline stem cells (GSCs) at the tip and undergo a stereotyped pattern of divisions and differentiation to form mature sperm. The somatic hub cells at the tip express upd, a ligand for the JAK/STAT pathway that has roles in the maintenance of both Cyst stem cells (CySCs) and GSCs in the testis. We found that upd3 is also expressed in the hub, and mutants of upd3 have fewer CySCs and GSCs. Interestingly, JAK/STAT is also activated in elongated cyst cells which are away from the tip. The knockdown of JAK/STAT in the somatic cyst cells leads to impaired spermatid individualization, as shown by fewer cystic bulges, waste bags and individualization complexes and no sperm in the seminal vesicle. Activation of caspases in elongated spermatids is required for individualization. The knockdown of JAK/STAT in cyst cells almost completely eliminated the activation of two effector caspases: Drice and DCP1. Forced expression of hid, the initiator caspase, significantly rescued the impaired individualization phenotype. JAK/STAT is activated in elongated cyst cells, while caspases are activated in spermatids enclosed by cyst cells. Candidate downstream signals from cyst cells that might regulate caspase activation in spermatids were examined. Hedgehog is expressed in the cyst cells, and over-expression impaired the activation of caspases. Knockdown of hedgehog and STAT simultaneously in cyst cells is able to partly rescue the phenotype of STAT knockdown. We concluded that JAK/STAT activity in somatic cyst cells promotes individualization in spermatids by stimulating caspase activity, perhaps partly by inhibiting Hedgehog activity. JAK/STAT pathway is not only required for the maintenance of stem cells in the tip, but also required for individualization away from the tip during late differentiation, thus plays two opposite roles in Drosophila spermatogenesis.