Résumé
Eukaryotes with separate males and females display a great diversity in
the way they determine sex, but it is still unclear what evolutionary
forces cause transitions between sex-determining systems. Rather that the
lack of hypotheses, the problem is the scarcity of adequate biological
systems to test them. Here, we take advantage of the recent evolution of a
feminizing X chromosome (called X*) in the African pygmy
mouse Mus minutoides, to investigate one of the evolutionary
forces hypothesized to cause such transitions, namely sex chromosome drive
(i.e., biased transmission of sex chromosomes to the next generation).
Through extensive molecular sexing of pups at weaning, we reveal the
existence of a remarkable male sex chromosome drive system in this
species, whereby direction and strength of drive is conditional upon the
genotype of males’ partners: males transmit their Y at a rate close to 80%
when mating with XX or XX* females, and only 36% when mating with X*Y
females. Using mathematical modelling, we explore the joint evolution of
these unusual sex-determining and drive systems, revealing that different
sequences of events could have led to the evolution of this bizarre
system, and that the “conditional” nature of sex chromosome drive plays a
crucial role in the short- and long-term maintenance of the three sex
chromosomes.