• gametogenic dysfunction;
  • genetic incompatibilities;
  • litter size;
  • oogenesis –Robertsonian fusion;
  • spermatogenesis

The divergence in reproductive features and hybrid fertility patterns between two chromosomal races (2n = 40, 40St, and 2n = 22, 22Rb) of the house mouse in Tunisia were re-assessed on a larger sample of wild and laboratory-bred individuals than studied hitherto. Results showed that litter sizes were significantly smaller in 40St than in 22Rb mice, contrary to previous analyses. This suggests that variation in litter size between the two chromosomal races is more likely related to selective and/or environmental factors acting locally than to interracial reproductive trait divergence. However, the significantly reduced litter size of F1 hybrids compared with parental individuals was confirmed, and further highlighted a sex difference in hybrid infertility, as F1 females produced fewer litters and of smaller size than males. Histological analyses of F1 and backcrosses showed a breakdown of spermatogenesis in males and a significantly reduced primordial follicle pool in females. The degree of gametogenic dysfunction was not related to the level of chromosomal heterozygosity per se, but a significant effect of two Rb fusions on follicle number was observed in hybrid females. These results suggest that genetic incompatibilities contribute to primary gametogenic dysfunction in hybrids between the chromosomal races in Tunisia. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 407–416.