virilizer regulates Sex-lethal in the germline of Drosophila melanogaster

In Drosophila, the gene Sex-lethal (Sxl) is required for female development. It controls sexual differentiation in the soma, dosage compensation and oogenesis. The continuous production of SXL proteins in XX animals is maintained by autoregulation and depends on virilizer (vir). This gene is required in somatic cells for the female-specific splicing of Sxl primary transcripts and for an unknown vital process in both sexes. In the soma, clones of XX cells lacking Sxl or vir are sexually transformed and form male structures; in the germline, XX cells mutant for Sxl extensively proliferate, but are unable to differentiate. We now studied the role of vir in the germline by generating germline chimeras. We found that XX germ cells mutant for vir, in contrast to cells mutant for Sxl, perform oogenesis. We show that the early production of SXL in undifferentiated germ cells is independent of vir while, later in oogenesis, expression of Sxl becomes dependent on vir. We conclude that the early SXL proteins are sufficient for the production of eggs whereas the later SXL proteins are dispensable for this process. However, vir must be active in the female germline to allow normal embryonic development because maternal products of vir are required for the early post-transcriptional regulation of Sxl in XX embryos and for a vital process in embryos of both sexes.