Sex determination of germ cells in Drosophila.

Many lines of evidence indicate that in Drosophila the mechanism for establishing the sex of the female germline is different from that acting in somatic cells. In the soma Sxl has an embryonic determinative role and is required throughout the life of female flies; in germ cells its expression begins only in the larval ovary. Both the mechanism for activating Sxl and the genes controlled by Sxl are different in the germline. A number of genes have been identified that are essential either for survival (e.g. ovo, otu) or differentiation (snf, Sxl, fl(2)d, bgcn) of female germ cells. ovo is required during embryogenesis for survival of pole cells. Genetic interactions with dominant alleles of ovo and/or Sxl indicate that otu, Sxl, snf and fl(2)d act in the same pathway as does ovo. bgcn differs in that neither ovo nor SxlD mutations affect the bgcn phenotype even though XX bgcn germ cells enter the male pathway. bgcn causes sterility in both sexes. Although the germline defect is cell autonomous in mosaic gonads, bgcn is also required in the somatic tissue for maintaining oogenesis of wild-type germ cells. Several dominant suppressors of bgcn have been identified and some have properties similar to Suppressors of variegation, suggesting that chromatin structure is critical for proper germ cell sex determination.