• Haldane's rule;
  • hybrid sterility;
  • hybrid zone;
  • incompatibility;
  • reproductive isolation;
  • spermatocyte-to-spermatid ratio;
  • testis weight;
  • X chromosome

We assessed the fertility (reproductive success, litter size, testis weight, spermatocyte-to-spermatid ratio) of F1s and backcrosses between different wild-derived outbred and inbred strains of two mouse subspecies, Mus musculus domesticus and M. m. musculus. A significant proportion of the F1 females between the outbred crosses did not reproduce, suggesting that female infertility was present. As the spermatocyte-to-spermatid ratio was correlated with testis weight, the latter was used to attribute a sterile vs. fertile phenotype to all males. Segregation proportions in the backcrosses of F1 females yielded 11 (inbred) to 17% (outbred) sterile males, suggesting the contribution of two to three major genetic factors to hybrid male sterility. Only one direction of cross between the inbred strains produced sterile F1 males, indicating that one factor was borne by the musculus X-chromosome. No such differences were observed between reciprocal crosses in the outbred strains. The involvement of the X chromosome in male sterility thus could not be assessed, but its contribution appears likely given the limited introgression of X-linked markers through the hybrid zone between the subspecies. However, we observed no sterile phenotypes in wild males from the hybrid zone, although testis weight tended to decrease in the centre of the transect. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 379–393.