Chromosomal introgression in house mice from the hybrid zone between M. m. domesticus and M. m. musculus in Denmark

The recent discovery of Robertsonian (Rb) translocations in Danish mice from the hybrid zone between Mus musculus musculus and M. m. domesticus stimulated the chromosomal analysis of populations along a north-south transect through this zone. G-Banding identified the Rb fusions as Rb(3.8), Rb(2.5) and Rb(6.9). The cytogenetic results show that there is a gradual decrease in the number of fusions as one proceeds north, the translocations abruptly ending in populations from the centre of the hybrid zone determined by seven diagnostic allozymic markers. These results indicate that Rb fusions are present only in domesticus or predominantly domesticus-genotype mice and that they do not introgress into M. m. musculus . To test if genie incompatibilities between the musculus genetic background and Rb fusions were involved in the systematic elimination of the latter, predominantly musculus mice from the hybrid zone were crossed with Rb domesticus mice carrying Rb(3.8). The karyotypic analysis of the progeny showed no distortion of the transmission ratio of this fusion.
The chromosomal and allozymic analysis of these mice further indicates that (i) recombination is not suppressed between metacentrics and their acrocentric homologues and (ii) specific domesticus chromosomal segments are tolerated in the musculus genomes whereas the Rb centromeres are not.