XY FEMALES DO BETTER THAN THE XX IN THE AFRICAN PYGMY MOUSE,MUS MINUTOIDES

Abstract

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, because 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 reproductiveperformance 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. K E Y W O R D S :Breeding performance, life-history traits, sex chromosome evolution, sex-determination system, sex-reversed females, X * chromosome.

Sex reversal induces size and performance differences among females of the African pygmy mouse, Mus minutoides

“…Considering our results and those of Saunders et al (2014Saunders et al ( , 2016, it appears that the X*Y females obtain selective advantages through many aspects of their fitness: reproductive success, behaviour and whole-organism performance. The effect of the X* chromosome thus goes well beyond feminization, and is at the origin of a complex and multi-factorial X* syndrome that extends to life history traits, behaviour (Saunders et al, , 2016, size variation and performance ( present study).…”

“…The colony was established from wild-caught animals from South Africa (see details in Saunders et al, 2014). Overall, 54 mice were used in the experiments with 19 males and 35 females, all adults.…”

Anatomical and Molecular Analyses of XY Ovaries from the African Pygmy Mouse <b><i>Mus minutoides</i></b>

“…1 I and J, respectively). This observation may explain the lower breeding performances of XX females compared to the X * Y [Saunders et al, 2014]. However, the fact that the XX * females have large ovaries but comparable breeding performances to the XX rules out any causal link between ovary size and reproductive output.…”

Sex Reversal in Reptiles: Reproductive Oddity or Powerful Driver of Evolutionary Change?

“…Whether the high fecundity and increased boldness of sex-reversed dragons observed in laboratory studies translates to fitness increases in the field remains to be tested, but clearly inferior fitness of sex-reversed individuals cannot be assumed in reptiles or other vertebrates [Saunders et al, 2014[Saunders et al, , 2016. Indeed, the reverse might be true.…”