Testis determination requires a specific FGFR2 isoform to repress FOXL2

Male sex determination inmammals relies on sex determining regionY-mediated upregulation of sex determining region-box 9 (SOX9) expression in XY gonads, whereas Wnt family member (WNT)/ R-spondin 1 signaling and forkhead box L2 (FOXL2) drive female sex determination in XX gonads. Fibroblast growth factor (FGF) 9 signaling ensures sustained SOX9 expression through repression of one of the ovarian pathways (WNT signaling), whereas the significance of FGF-mediated repression of the FOXL2 pathway has not been studied. Previously, we demonstrated that FGFR2 is the receptor for FGF9 in the XY gonad. Whether a specific isoform (FGFR2b or FGFR2c) is required was puzzling. Here, we show that FGFR2c is required formale sex determination. Initially, in developingmouse embryos at 12.5 to 13.5 days postcoitum (dpc), XY Fgfr2c^-/- gonads appear as ovotestes, with SOX9 and FOXL2 expression predominantly localized to the posterior and anterior gonadal poles, respectively. However, by 15.5 dpc, XY Fgfr2c^-/- gonads show completemale-to-female sex reversal, evident by the lack of SOX9 and ectopic expression of FOXL2 throughout the gonads. Furthermore, ablation of the Foxl2 gene leads to partialor complete rescue of gonadal sex reversal inXYFgfr2c^-/-mice. Togetherwithprevious findings, our data suggest that testis determination involves FGFR2c-mediated repression of both the WNT4- and FOXL2-driven ovarian-determining pathways.