Anatomical and molecular analyses of XY ovaries from the African pygmy mouse Mus minutoides.

The African pygmy mouse Mus minutoides is characterized by the presence of a high proportion of fertile XY females in natural populations. This species displays 2 morphologically different X chromosomes: the ancestral X and a shorter one designated as X*, feminizing the XY individuals. This strongly suggests that in the presence of an X chromosome, the male differentiation program is not activated despite a functional Y chromosome. In this study, we compared the histology of the adult ovaries of the 3 female genotypes (XX, XX* and XY) and investigated the expression of some of the main genes involved in male and female differentiation. We found that XY gonads display a typical ovarian structure without any testicular organization. Moreover, the ovarian somatic marker FOXL2 is detected in XY follicle cells and exhibits the same pattern as in XX and XX ovaries, whereas SOX9 and DMRT1 are absent at all stages of follicular differentiation. However, surprisingly, XY ovaries display a higher level of Sry transcripts compared to testes. Our findings confirm the complete sex reversal in XY individuals with no apparent sign of masculinization, providing an attractive model to unravel new gene interactions involved in the mammalian sex determination system.