The Drosophila female-specific sex-determination gene, Sex-lethal, has stage-, tissue-, and sex-specific RNAs suggesting multiple modes of regulation.

For proper sexual development of females, the Sex-lethal (Sxl) gene must be activated early in development and remain on during the rest of the life cycle. Conversely, in males, Sxl must remain functionally off through development. Here, we show that the Sxl transcription unit spans a DNA segment of greater than 20 kb and encodes at least 10 distinct, but overlapping, RNA species. These RNAs range in size from 4.4 to 1.7 kb and exhibit sex, stage, and tissue specificity. Six RNAs, three female specific and three male specific, are first detected by midembryogenesis and persist through the adult stage: Their expression reflects the on/off regulation of Sxl's activity at the level of sex-specific alternate splicing. Four Sxl RNAs are found in adult females. Two of these RNAs are dependent on the presence of a functional germ line and may be relevant to Sxl's role in adult germ-line development. All four are present in unfertilized eggs. Finally, three Sxl RNAs are found only transiently during very early embryogenesis; we suggest that the expression of these RNAs may reflect an early regulation of Sxl at the level of transcription and that these transcripts are involved in the initial selection of the Sxl activity state in response to the primary sex-determination signal, the X/A ratio.