Box 1 Figure I.

Sex determination pathway in Drosophila. Active gene products and gene interactions are shown in purple and inactive in grey. Primary sex determination occurs very early in embryonic development when zygotically transcribed genes located on the X chromosome and the autosomes activate Sxl in females but fail to activate it in males. Sxl makes sex determination permanent by establishing a positive autoregulatory loop in females and controls the splicing of tra. In females, the presence of functional Sxl and Tra proteins leads to female-specific splicing of dsx and suppresses male-specific splicing of fru. In males, the absence of Sxl and Tra allows the default splicing of dsx and fru to produce male-specific isoforms.