Menage à trois in the African pygmy mouse, Mus minutoides: a third sex chromosome and a third sexual phenotype

The biological process that initiates the development of the sexes is known as sex determination.However, despite significant advances, many aspects of this process remain poorly understood.

In mammals, sex is typically determined by the sex chromosomes: XY embryos develop as males, while XX embryos develop as females. This highlights the pivotal role of the Y chromosome in initiating male development. However, across vertebrates, a wide diversity of sex-determination mechanisms exists, including species with unconventional systems.

One striking example is the African pygmy mouse (Mus minutoides), in which some phenotypic fertile females carry an XY chromosome set. This remarkable exception challenges the traditional model and offers a unique and powerful system for studying the molecular and evolutionary mechanisms of sex determination, sexual dimorphism, and sex-specific behaviors.