SOX9 is not required for the cellular events of testicular organogenesis in XX mole ovotestes

Mammalian sex determination is the genetic process that commits the undifferentiated bipotential gonads to develop as either testes or ovaries. The differentiation of SOX9-expressing Sertoli cells is assumed to be necessary to initiate testis development. Insectivorous moles of the genus Talpa represent a unique case of generalized true hermaphroditism, as XX female moles constitutively develop two ovotestes instead of normal ovaries. In this work, we have investigated the expression patterns of a number of genes known to play key roles in gonad organogenesis, throughout the entire process of ovotestis development in female moles. Molecular and morphological evidence are provided that these ovotestes contain primary medullary testis-like cords, Leydig cells, peritubular myoid cells, and a testis-specific vasculature, but no Sertoli cells. Our results show for the first time that SOX9 is not required for the formation of the primary testis cords, but it is necessary for the maintenance and subsequent development of these cords. In addition, the expression pattern of WNT4 in male and female moles indicates that this gene inhibits Leydig cell differentiation and, contrary to the proposed scenario in the mouse, it is not required for the colonization and survival of primordial germ cells. According to our data, mole ovotestes result from a process of PDGFRalpha-mediated mesonephric cell migration, which occurs simultaneously in both sexes. The fact that FST remains inactive during the critical stages of female gonad development, explains the lack of migration inhibition, and may be a consequence of improper WNT4 signalling in the mole