Résumé
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.