Expression of the Sex-lethal gene is controlled at multiple levels during Drosophila oogenesis

In addition to controlling somatic sexual development in Drosophila melanogaster, the Sex-lethal (Sxl) gene is required for proper differentiation of female germ cells. To investigate its role in germ-line development, we have examined the expression of Sxl in wild-type ovaries and ovaries that are defective in early steps of germ cell differentiation. As in the soma, the basic mechanism for on/off regulation of Sxl relies on sex-specific processing of its transcripts in germ cells. One class of female-sterile mutations, which includes fs(1)1621 and the tumorous-ovary-producing allele of the ovarian tumor gene, otu¹, is defective in the splicing process. These mutants have germ lines with high amounts of Sxl RNA spliced in the male mode and a severe reduction of protein levels in the germ cells. Another class of female-sterile mutations produces a phenotype similar to that seen in fs(1)1621 and otu¹ but appears to express normal levels of Sxl protein in the germ cells. However, this second class does not show the changes in protein distribution normally observed in wild-type germ cells. In the wild-type germarium, the non-differentiated germ cells show a strong cytoplasmic accumulation of Sxl protein followed, as the germ cells differentiate, by a dramatic reduction and redistribution of the protein into nuclear foci. Interestingly, two female-sterile alleles of Sxl, Sxl^f4 and Sxl^f5 belong to the second class, which shows persistent cytoplasmic accumulation of Sxl protein. These Sxl female-sterile mutants encode an altered protein indicating that Sxl regulates processes that eventually lead to the changes in Sxl protein distribution. Lastly, we demonstrate that during the final stages of oogenesis several mechanisms must operate to prevent the progeny from inheriting Sxl protein. Conceivably, this regulation safeguards the inadvertent activation of the Sxl autoregulatory feedback loop in the male zygote.