sisterless Ais required for activation ofSex lethalin theDrosophilagermline

Both somatic cells and germ cells must establish their correct sexual identity for proper gametogenesis. InDrosophila,sex determination in somatic cells is controlled by the switch geneSex lethal(Sxl), which is activated in females by the presence of two X chromosomes. Though germline sex determination is much less well understood,Sxlis also essential for the female identity in germ cells. Loss ofSxlfunction in the germline results in ovarian germline tumors, a characteristic of male germ cells developing in a female soma. Further,Sxlexpression is sufficient for XY (male) germ cells to produce eggs when transplanted into XX (female) somatic gonads. As in the soma, the presence of two X chromosomes activatesSxlin the germline, but the mechanism for “counting” X chromosomes in the germline is thought to be different from the soma. Here we have explored this mechanism at bothcis- andtrans-levels. Our data support the model that theSxl“establishment” promoter (SxlPE) is activated in a female-specific manner in the germline, as in the soma, but that the timing ofSxlPEactivation, and the DNA elements that regulateSxlPE,are different in the germline. Nevertheless, we find that the X chromosome genesisterless A (sisA),which helps activateSxlin the soma, is also essential forSxlactivation in the germline. Loss ofsisAleads causes of Sxl expression in the germline, and to ovarian tumors and germline loss. These defects can be rescued by Sxl expression, demonstrating thatsisAlies upstream ofSxlin germline sex determination. We conclude thatsisAacts as an X chromosome counting element in both the soma and the germline, but that additional factors regulating female-specific expression ofSxlin the germline remain to be discovered.