Figure 1.

Drosophila sex determination. In males (X/Y), Sxl gene is not expressed and tra gene is not expressed. The dsx and fru genes undergo default splicing to produce the male isoforms (“M”), which then regulate male cell differentiation. In females (X/X), Sxl is expressed and negatively regulates MSL-2 expression to regulate dosage compensation. Sxl protein also regulates alternative splicing of tra such that tra is expressed. The tra protein in turn regulates female-specific splicing of dsx and fru to produce the female isoforms (“F”) and female cell differentiation. In addition, tra protein acts through as-yet unclear mechanisms (“?”) to regulate female gene expression and cell differentiation. Please see text for details.