Sex chromosome and sex determination evolution in mammals – SEXYMUS

We try to integrate a multidisciplinary approach by exploring the nature of the genes involved in the sex chromosome rearrangements (cytogenomics: fluorescence in situ hybridization), their rate and mode of evolution (sequence analyses / molecular evolution), their expression (cellular biology: qPCR, immuno-histochemistry), and the phenotypic correlations identified (behavioural study associated with hormonal dosages).

Recently, we identified a novel sex determination system in a close relative of the house mouse, M. minutoides. This species shows indeed a very large proportion (75%) of fertile X*Y females .
The aim of this project is to get a better understanding of how such a system could have evolved. Therein lies a Darwinian paradox as this system is associated with a high reproductive cost (loss of 1/4 of embryos in X*Y females). Therefore we search for evolutionary mechanisms involved in the evolution of this aberrant system. Thus, in controlled populations, we estimated the reproductive cost, and against all odds we have shown that X*Y females have better reproductive success than XX or XX* females. For example, X*Y females have significantly larger litter size, and they breed almost one month earlier than the other females. The analyses also revealed that transmission distorters are involved in the system: there is a preferential transmission of Y (80%) in males mated with XX or XX* females and very surprisingly, it’s the X chromosome that is favored in males mated with X*Y females (only 33% of Y transmitted), limiting the production of YY embryos. To our knowledge, this is the first time that such a genome-dependent distortion is documented.
In parallel, we search for the gene(s) responsible of the sex reversal by cytogenomics techniques, cell biology, and functional development. These combined approaches have allowed us to identify a very strong candidate gene. These very recent results open new perspectives. We have never been so close to identify a new gene involved in the cascade of sex determination in mammals, localized on the X chromosome and that its concerted action with the SRY gene is essential for the formation of a testis.

In this context, we plan to begin a study on human patients with Disorders of Sex Development (DSD) . The DSDs include a wide variety of diseases, from minor (such as lack of foreskin) to rare and severe (eg XY woman). Such aberrations can be caused by mutations on genes involved in the embryonic development of the testis, but not only. Indeed, the embryonic environment and the exposure to chemicals such as endocrine-disrupting compounds (eg pesticides), may also affect the development and reduce the ability to reproduce (decrease in the number and quality of sperm). The prevalence of DSDs is nearly one in 100 births, but epidemiological data showed an increase of these disorders in the last fifty years. It is therefore a real public health problem. Studies on DSDs led to the identification of several mutations and several genes involved in sex determination, but more than 50% of these pathological cases are still not determined. Thus, dissecting the atypical sex determinism of M. minutoides allowed to identify a strong candidate gene for sex reversal. The role of this gene in the cascade of sex determination was previously unknown. We will colaborate with laboratories and hospitals that offered us their cohort of human patients with DSD in order to detect possible mutations on this gene.

- Rahmoun & Veyrunes. Mouse sex-reversed rearrangements. In Encyclopedia of Genetics (2012)
- Veyrunes, et al (in press) Insights into the evolutionary history of the X-linked sex reversal mutation in Mus minutoides; clues from sequence analyses of the Y-linked Sry gene. Sexual Development
- Veyrunes, et al (submitted) A new cytotype of the African pygmy mouse Mus minutoides from Tanzania. Implications for the evolution of sex-autosome translocations. Chromosome Research

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.