Testicular differentiation occurs in absence of R-spondin1 and Sox9 in mouse sex reversals.

In mammals, male sex determination is governed by SRY-dependent activation of Sox9, whereas female development involves R-spondin1 (RSPO1), an activator of the WNT/beta-catenin signaling pathway. Genetic analyses in mice have demonstrated Sry and Sox9 to be both required and sufficient to induce testicular development. These genes are therefore considered as master regulators of the male pathway. Indeed, female-to-male sex reversal in XX Rspo1 mutant mice correlates with Sox9 expression, suggesting that this transcription factor induces testicular differentiation in pathological conditions. Unexpectedly, here we show that testicular differentiation can occur in XX mutants lacking both Rspo1 and Sox9 (referred to as XX Rspo1(KO)Sox9(cKO) ()), indicating that Sry and Sox9 are dispensable to induce female-to-male sex reversal. Molecular analyses show expression of both Sox8 and Sox10, suggesting that activation of Sox genes other than Sox9 can induce male differentiation in Rspo1(KO)Sox9(cKO) mice. Moreover, since testis development occurs in XY Rspo1(KO)Sox9(cKO) mice, our data show that Rspo1 is the main effector for male-to-female sex reversal in XY Sox9(cKO) mice. Thus, Rspo1 is an essential activator of ovarian development not only in normal situations, but also in sex reversal situations. Taken together these data demonstrate that both male and female sex differentiation is induced by distinct, active, genetic pathways. The dogma that considers female differentiation as a default pathway therefore needs to be definitively revised.

XY Sox9 embryonic loss-of-function mouse mutants show complete sex reversal and produce partially fertile XY oocytes.

Gonadal differentiation is the first step of mammalian sex determination. The expression of the Y chromosomal testis determining factor Sry leads to up-regulation of the transcription factor Sox9 which promotes testis differentiation. Previous studies showed that Sox9 deficiency induces expression of ovarian markers in XY mutant fetal gonads before they die. To better understand the genome-wide transcriptional profile underlying this process we compared samples from XY Sf1:Cre(Tg/+); Sox9(flox/flox) mutant gonads in which Sox9 is ablated in Sertoli-precursor cells during early stages of gonad development to XX Sox9(flox/flox) ovaries and XY Sox9(flox/flox) testes at E13.5. We found a complex mRNA signature that indicates wide-spread transcriptional de-regulation and revealed for XY mutants at E13.5 an intermediate transcript profile between male and female gonads. However, XY Sf1:Cre(Tg/+); Sox9(flox/flox) mutant gonads develop as ovaries containing XY developing follicles at P0 but less frequently so than in XX control ovaries. Furthermore, we studied the extent to which developing XY mutant ovaries are able to mediate adult fertility and observed that XY oocytes from XY mutant ovaries are competent for fertilization; however, two thirds of them fail to develop beyond two-cell stage embryos. Taken together, we found that XY Sf1:Cre(Tg/+); Sox9(flox/flox) females are capable of producing viable offspring albeit at a reduced level.