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 Robertso nian (Rb) house mice (M. m. domesticus) from Tunisia were studied. The aim was to examine the possibility of a reproductive selective advant age associated with chromosomal change as well as to measure the effec t of heterozygosity for a large number of Rb fusions on the fertility of hybrids. Results showed that litter sizes were significantly smalle r in Rb than in all-acrocentric mice. This difference, which may repre sent a favourable demographic strategy related to the habitat segregat ion observed in the Tunisian mice, needs to be studied further. The F1 hybrids between the two chromosomal races showed a significantly redu ced reproductive success and litter size (respectively, 53 per cent an d 60 per cent less than either parental race). Analysis of the testicu lar histology of F1 and backcross males showed in some cases a breakdo wn of spermatogenesis. The degree of this disturbance was not related to the level of chromosomal heterozygosity suggesting that genetic inc ompatibilities between the two genomes might be involved. The strong r eduction in fertility measured in these hybrids represents a reproduct ive isolating mechanism effectively reducing gene flow between the all -acrocentric and 22Rb mice populations of Tunisia.