Sex-Determining Cascades in Gonadal Development: Insights from Ovotestes.

We have performed detailed molecular and cellular analyses of ovotestis development in mice in order to gain further insight into the normal sequence of events in mammalian testis determination and the etiology of XY sex reversal. In ovotestes of B6 XYPOS fetuses, testicular cords formed only in the central region of the gonad where SOX9 was up-regulated, even though SRY was expressed throughout the length of these gonads. At the poles the ovary-specific protein FOXL2 was expressed. FOXL2-expressing cells also were found among the SOX9-positive pre-Sertoli cells in the central region of B6 XYPOS ovotestes. Our results demonstrate that 1. Both timing and levels of SRY expression are critical for SOX9 expression, and that SOX9, not SRY, is the pivotal determinant of sex differentiation in mice, 2. An active ovarian-determining pathway operates from an early stage in gonadal development, 3. The ovarian pathway is normally suppressed in XY gonads by the action of SOX9, and 4. Compromise of the testis-determining pathway can tip the balance in favour of ovary development.