Drosophila JAK/STAT pathway reveals distinct initiation and reinforcement steps in early transcription of Sxl

X-linked signal elements (XSEs) communicate the dose of X chromosomes to the regulatory-switch gene Sex-lethal (Sxl) during Drosophila sex determination. Unequal XSE expression in precellular XX and XY nuclei ensures that only XX embryos will activate the establishment promoter, SxlPe, to produce a pulse of the RNA-binding protein, SXL [1]. Once XSE protein concentrations have been assessed, SxlPe is inactivated and the maintenance promoter, SxlPm, is turned on in both sexes; however, only in females is SXL present to direct the SxlPm-derived transcripts to be spliced into functional mRNA [2, 3]. Thereafter, Sxl is maintained in the on state by positive autoregulatory RNA splicing [2]. Once set in the stable on (female) or off (male) state, Sxl controls somatic sexual development through control of downstream effectors of sexual differentiation and dosage compensation [1, 4]. Most XSEs encode transcription factors that bind SxlPe, but the XSE unpaired (upd) encodes a secreted ligand for the JAK/STAT pathway [5-7]. We show that although STAT directly regulates SxlPe, it is dispensable for promoter activation. Instead, JAK/STAT is needed to maintain high-level SxlPe expression in order to ensure Sxl autoregulation in XX embryos. Thus, upd is a unique XSE that augments, rather than defines, the initial sex-determination signal.