Sperm quality parameters are increased and asymmetric in house mouse hybrids

Spermatogenesis is a tuned cascade of processes producing sperm; impairment of any phase of this process can affect fitness of males. The level of impairment can be pronounced in hybrids between genetically divergent populations. To explore the effect of hybridization on sperm quality we produced F1 hybrids from 29 wild derived strains of two house mouse subspecies, M. m. musculus and M. m. domesticus, which diverged 0.5 MY ago. The measured sperm quality traits did not significantly differ between intrasubspecific crosses. Effects of intersubspecific hybridization were dependent on sperm trait and cross direction. The proportion of sperm head abnormalities was increased in F1 intersubspecific hybrids. The frequency of dissociated sperm heads was increased in the M. m. musculus x M. m. domesticus ([female]x[male]) F1 but decreased in M. m. domesticus x M. m. musculus ([female]x[male]) F1 hybrids, with the difference in medians being more than 180%. We deduce that the dissociated sperm heads trait is associated with the X chromosome and modulated by interaction with the Y chromosome; nevertheless, the high proportion of unexplained variance (55.46 %) suggests the presence of polymorphic autosomal interactions. The reported differences in sperm quality between cross types may be highly relevant to male fitness in zones of secondary contact between the two subspecies. The cross direction asymmetry in frequency of dissociated sperm heads should favour the M. m. musculus Y chromosome. This is consistent with the spread of the M. m. musculus Y chromosome in nature across the hybrid zone between these two subspecies.