Except for a few species, mammals have an extremely conserved sex determining system. However, within the African pygmy mouse species (genus Mus), we recently uncovered an extraordinary diversity of sex chromosomes: fusions between autosomes and the X and/or Y chromosomes, modifications of sex determinism (XY or XO females), diversification of the Y chromosome, etc. This unique set of features and their phylogenetic proximity with the laboratory mouse make the African pygmy mouse an excellent model to investigate the evolution of mammalian sex chromosomes and sex determination. The SEXYMUS project thus proposes to use pygmy mice as proxies to identify the micro-evolutionary processes involved in X and Y differentiation. Three tasks will be undertaken dealing with different and complementary aspects of sex chromosome evolution. Task 1: Emergence of atypical sex determining systems. Identification of the genetic basis and the selective forces at play The mutation causing male-to-female sex reversal in M. minutoides will be investigated by cytogenomic and molecular approaches. Preliminary results have already identified the X chromosome as the target of the mutation. This study is expected to contribute to the identification of new genes involved in the sex determination pathway in mammals in general, and may highlight new gene candidates of pathological sex reversals in human in particular. Understanding the evolution of such aberrant sexual systems is one of the main goals of evolutionary biology. As these modifications are considered as highly deleterious, selective mechanisms are expected to have favored their diffusion. These will be explored by a multidisciplinary study integrating different approaches: the nature of the genes involved in the chromosomal changes will be established (cytogenomics), their rate and mode of evolution measured (sequencing, RT-PCR), phenotypic correlations identified (behaviour), and finally evolutionary predictions tested (computer modelling). Task 2: Y chromosome degeneration. Estimation of the mode and tempo of genetic erosion. It is universally accepted that the Y chromosome degenerates progressively. However, its rate of degeneration is vigorously debated, as well as its dynamics. The morphology of the Y chromosome of African pygmy mice is extremely diverse, varying from a normal-sized to a minute chromosome, and even to a complete loss of the Y chromosome described in one species. These results suggest fast genetic erosion. Hence, a comparative genomic approach of several Y-linked genes between different species/populations of pygmy mice will provide a micro-evolutionary insight into the dynamics of mammalian Y degeneration. Task 3: Origin and evolution of neo-sex chromosomes. “Sexualisation” of autosomes In sex-autosome fusions, parts of the autosomal genome, which were previously inherited from both parents, become linked to the sex chromosomes, and are thus only transmitted to one of the two sexes. These modifications lead to dramatic changes of the selective regime acting on these regions that are expected to influence the evolution of their gene content (sexualisation), gene expression (differentiation between sexes), and sequences (rapid evolution under positive selection, or degeneration after the suppression of recombination). We will test these theoretical predictions by cytogenomic and molecular analyses in one species carrying a neo-Y chromosome. The same approach will be performed on an exceptional case population within M. minutoides where almost (if not all) females are XY, leading to the quasi-complete suppression of recombination in a X chromosome.