Abstract
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.
Highlights
The binary switch gene Sex-lethal (Sxl)plays a central role in the development of sexual dimorphism in the fruit fly Drosophila melanogaster
Among all the genes known to be involved in controlling sexual dimorphism in fruit flies, Sx/participates in three different levels of developmental regulation: the initial selection of the developmental pathway, the maintenance of a heritable commitment to the pathway {determination} and, the expression of the pathway
A variety of lines of indirect evidence suggest that the initial selection of the female or male Sxl activity state occurs early in embryogenesis in response to the number of X chromosomes relative to the number of sets of autosomes (X/A ratio)(Sanchez and N6thiger, 1983; Cline 1984; Gergen 1987)
Summary
The binary switch gene Sex-lethal (Sxl)plays a central role in the development of sexual dimorphism in the fruit fly Drosophila melanogaster (for review, see Baker and Belote 1983; Cline 1985, 1988b; N6thiger and Steinmann-Zwicky 1985). One of these appears to be required for the stable activation of the Sxl gene (Cline 1984, 1988a, b; Maine et al 1985b; Salz et al 1987) Another very early function appears to be involved in the down-regulation of the X chromosome dosage-compensation system (see Gergen 1987). A number of genes involved in regulating some aspects of dosage compensation have been characterized (for review, see Lucchesi and Manning 1987), the most immediate target(s)for Sxl's action is not known (Cline 1984; Gergen 1987) In addition to these somatic functions, Sxl is required for the normal development of the female germ line (Schtipbach 1985; Salz et al 1987). The analysis of RNAs in female and male animals reported here provided the first evidence for the sex-specific processing of Sxl transcripts and sug-
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