pSTAT3 activation of <i>Foxl2</i> initiates the female pathway underlying temperature-dependent sex determination

Ovarian development was traditionally recognized as a "default" sexual outcome and therefore received much less scientific attention than testis development. In turtles with temperature-dependent sex determination (TSD), how the female pathway is initiated to induce ovary development remains unknown. In this study, we have found that phosphorylation of the signal transducer and activator of transcription 3 (pSTAT3) and <i>Foxl2</i> exhibit temperature-dependent sexually dimorphic patterns and tempo-spatial coexpression in early embryos of the red-eared slider turtle (<i>Trachemys scripta elegans</i>). Inhibition of pSTAT3 at a female-producing temperature of 31 °C induces 64.7% female-to-male sex reversal, whereas activation of pSTAT3 at a male-producing temperature of 26 °C triggers 75.6% male-to-female sex reversal. In addition, pSTAT3 directly binds to the locus of the female sex-determining gene <i>Foxl2</i> and promotes <i>Foxl2</i> transcription. Overexpression or knockdown of <i>Foxl2</i> can rescue the sex reversal induced by inhibition or activation of pSTAT3. This study has established a direct genetic link between warm temperature-induced STAT3 phosphorylation and female pathway initiation in a TSD system, highlighting the critical role of pSTAT3 in the cross talk between female and male pathways.