Fertility estimates in the Tunisian all-acrocentric and Robertsonian populations of the house mouse and their chromosomal hybrids.

The reproductive features of wild all-acrocentric and 2n = 22 Robertsonian (Rb) house mice (M. m. domesticus) from Tunisia were studied. The aim was to examine the possibility of a reproductive selective advantage associated with chromosomal change as well as to measure the effect of heterozygosity for a large number of Rb fusions on the fertility of hybrids. Results showed that litter sizes were significantly smaller in Rb than in all-acrocentric mice. This difference, which may represent a favourable demographic strategy related to the habitat segregation observed in the Tunisian mice, needs to be studied further. The F1 hybrids between the two chromosomal races showed a significantly reduced reproductive success and litter size (respectively, 53 per cent and 60 per cent less than either parental race). Analysis of the testicular histology of F1 and backcross males showed in some cases a breakdown of spermatogenesis. The degree of this disturbance was not related to the level of chromosomal heterozygosity suggesting that genetic incompatibilities between the two genomes might be involved. The strong reduction in fertility measured in these hybrids represents a reproductive isolating mechanism effectively reducing gene flow between the all-acrocentric and 22Rb mice populations of Tunisia.