Data from: XY females do better than the XX in the African pygmy mouse, Mus minutoides

All therian mammals have a similar XY/XX sex determination system except for a dozen species. The African pygmy mouse, Mus minutoides, harbors an unconventional system in which 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). The long-term evolution of such a system is a paradox, since X*Y females are expected to face high reproductive costs (e.g. meiotic disruption and loss of unviable YY embryos), which should prevent invasion and maintenance of a sex-reversal mutation. Hence, mechanisms for compensating for the costs could have evolved in M. minutoides. Data gathered from our laboratory colony revealed that X*Y females do compensate and even show enhanced reproductive performance in comparison to the XX and XX*; they produce significantly more offspring due to (i) a higher probability of breeding, (ii) an earlier first litter, and (iii) a larger litter size, linked to (iv) a greater ovulation rate. These findings confirm that rare conditions are needed for an atypical sex determination mechanism to evolve in mammals, and provide valuable insight into understanding modifications of systems with highly heteromorphic sex chromosomes.除十余种物种外，所有兽亚纲哺乳动物（Therian mammals）均拥有保守的XY/XX性别决定系统。非洲侏儒鼠（Mus minutoides）便是其一，其拥有非典型性别决定系统：所有雄性均为XY型，而雌性则分为三类——常规的XX型，以及XX*与X*Y型（星号代表X染色体上的性别反转突变）。这类系统的长期演化堪称悖论：X*Y雌性理论上会承受极高的繁殖成本（例如减数分裂紊乱、无存活能力的YY胚胎丢失），这原本应当阻碍该性别反转突变在种群中的扩散与维持。因此，非洲侏儒鼠（M. minutoides）或许已演化出补偿此类繁殖成本的相关机制。我们从实验室饲养种群中收集的数据显示，X*Y雌性不仅能够补偿此类成本，甚至相较于XX与XX*雌性展现出更优异的繁殖性能：它们的后代数量显著更多，这得益于（1）更高的交配成功率、（2）更早的首胎产仔时间，以及（3）更大的每胎产仔数，而这又与（4）更高的排卵率直接相关。这些研究结果证实，哺乳动物中非典型性别决定机制的演化需要极为严苛的稀有条件，同时也为理解高度异型性染色体系统的演化修饰提供了宝贵的学术视角。