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Evolution d'un déterminisme du sexe atypique chez un mammifère : causes et conséquences

Abstract : Therian mammals have an extremely conserved XX/XY sex determination system. Their highly differentiated and specialised sex chromosomes are thought to prevent any modification; however, a dozen species harbour unconventional systems. In the African pygmy mouse Mus minutoides, all males are XY, and there are three types of females: the usual XX but also XX* and X*Y ones (the asterisk designates a sex reversal mutation on the X chromosome, which evolved almost 1 million years ago). The evolution of such a system is a paradox, as X*Y females are expected to face high reproductive costs (loss of YY embryos, meiotic problems…), which should prevent the maintenance of the mutation. To better understand the evolution of this curious system, we first tried to identify the evolutionary mechanisms involved in the emergence and maintenance of the X*. The combination of empirical data and a theoretical approach based on population genetics models showed that two mechanisms participate in the maintenance of the system: the greater breeding success of X*Y females and the presence of sex chromosome transmission distorters (males transmit their Y more often in crosses with XX or XX* females and their X in crosses with X*Y females), the second mechanism likely being the trigger for the initial spread of the feminising chromosome. We then investigated the consequences of the evolution of this unusual system with three sex chromosomes. First on the phenotype, revealing that despite X*Y females have typical female anatomy and morphology, they resemble males on certain aspects of behaviour: they are more aggressive and less anxious than XX and XX* females. Then on the sequence and structural evolution of the X and X* (based on NGS data), showing that the two chromosomes have started diverging. Altogether, these results shed light on the constraints acting on sex determination systems with highly heteromorphic sex chromosomes and show that rare conditions can loosen these constraints. They also provide valuable insight into the impact of sex chromosome complement on phenotype, and inform on the evolutionary forces acting on sex chromosomes in that kind of polygenic sex determination system.
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Submitted on : Tuesday, September 22, 2020 - 1:00:09 PM
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Paul Saunders. Evolution d'un déterminisme du sexe atypique chez un mammifère : causes et conséquences. Evolution [q-bio.PE]. Université Montpellier, 2015. Français. ⟨NNT : 2015MONTS280⟩. ⟨tel-02945498⟩



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