The JAK/STAT signaling pathway is required for the initial choice of sexual identity in Drosophila melanogaster

The choice of sexual identity in Drosophila is determined by a system that measures the X chromosome to autosome ratio (X/A). This system depends upon unequal expression of X-linked numerator genes in 1X and 2X nuclei. The nu merators activate a special Sri promoter, Sxl-Pe, in 2X/2A nuclei, but not 1X/2A nuclei. By multimerizing a conserved Sxl-Pe sequence block, we genera ted a gain-of-function promoter, Sxl-Pe(GOF), that is inappropriately activ e in 1X/2A nuclei. GOF activity requires the X-linked unpaired (upd) gene, which encodes a ligand for the Drosophila JAK/STAT signaling pathway. opd a lso functions as a numerator element in regulating wild-type Sxl-Pe reporte rs. We demonstrate that the JAK kinase, Hopscotch, and the STAT DNA-binding protein, Marelle, are also required for Sxl-Pe activation.