Sex chromosome quadrivalents in oocytes of the African pygmy mouse Mus minutoides that harbors non-conventional sex chromosomes.

Eutherian mammals have an extremely conserved sex-determining system controlled by highly differentiated sex chromosomes. Females are XX and males XY, and any deviation generally leads to infertility, mainly due to meiosis disruption. The African pygmy mouse (Mus minutoides) presents an atypical sex determination system with three sex chromosomes: the classical X and Y chromosomes and a feminizing X chromosome variant, called X*. Thus, three types of females coexist (XX, XX*, and XY) that all show normal fertility. Moreover, the three chromosomes (X and Y on one side and X on the other side) are fused to different autosomes, which results in the inclusion of the sex chromosomes in a quadrivalent in XX* and XY females at meiotic prophase. Here, we characterized the configurations adopted by these sex chromosome quadrivalents during meiotic prophase. The XX quadrivalent displayed a closed structure in which all homologous chromosome arms were fully synapsed and with sufficient crossovers to ensure the reductional segregation of all chromosomes at the first meiotic division. Conversely, the XY quadrivalents adopted either a closed configuration with non-homologous synapsis of the X and Y chromosomes or an open chain configuration in which X* and Y remained asynapsed and possibly transcriptionally silenced. Moreover, the number of crossovers was insufficient to ensure chromosome segregation in a significant fraction of nuclei. Together, these findings raise questions about the mechanisms allowing XY females to have a level of fertility as good as that of XX and XX females, if not higher.