Empirical demonstration of hybrid chromosomal races in house mice

Western house mice (Mus musculus domesticus) and common shrews (Sorex araneus) are important models for study of chromosomal speciation. The ancestral karyotypes of both species consisted of telocentric chromosomes, and they are subdivided into numerous chromosomal races many of which resulted from fixation of mutations (Robertsonian fusions and whole-arm reciprocal translocations). However, some chromosomal races in these species may alternatively have originated through hybridization, by fixation of particular homozygous recombinant products. This explains findings in hybrid zones and reduces the number of evolutionary steps in chromosomal phylogenies. Here we demonstrate the process of generation of hybrid chromosomal races for the first time in either species using molecular markers. Analysis of centromeric microsatellite markers showed that the Mid Valtellina (IMVA) and Upper Valtellina (IUVA) chromosomal races of the house mouse are recombinant products of hybridization of the Lower Valtellina (ILVA) and Poschiavo (CHPO) chromosomal races, supporting earlier theoretical analysis. IMVA and IUVA occupy a small area of the Italian Alps where ILVA makes contact with CHPO. IUVA and CHPO have previously been shown to be reproductively isolated in one village, emphasizing that hybrid chromosomal races in small mammals, as in plants, have the potential to be part of the speciation process.