Chromosomal hybrid zones in the house mouse: sites of raciation and speciation
One of the most important evolutionary processes is the formation of new species, and the contacts between major genetic forms within species (‘races’) have been important model systems for studying speciation. Hybridisation occurs in these areas of contact, and so they are termed ‘hybrid zones’.
The hybrid zones between chromosomal races are of particular interest, because there are clear expectations that hybrids may be infertile due to meiotic abnormalities and/or gene flow in specific chromosomal regions may be reduced due to recombination suppression (viz. close to the breakpoints of the rearrangements that distinguish the chromosomal races). Either of these processes could promote speciation.
The house mouse is not only a biomedical model, it is also an exceptional study system with respect hybrid zones between chromosomal races. In particular, the western house mouse (Mus musculus domesticus) is characterised by numerous local chromosomal races, with chromosome numbers 2n = 22 – 38 as opposed to the standard 40-chromosome complement. These are particularly distributed in the vicinity of the Alps and on islands around the periphery of Europe. In the hybrid zones between the chromosomal races of the house mouse, the meiotic problems in the hybrids cause the expected reduced fertility. Under these circumstances it appears that selection may favour raciation as an alternative to speciation. If a new homozygous karyotype arises within a chromosomal hybrid zone with unfit hybrids, then selection may favour that new karyotype if its presence leads to a reduction in frequency of hybrids of the unfit type. The new homozygous karyotype may expand its range and in this way chromosomal hybrid zones may become sites of origin of new races, and there is evidence that this process has occurred. In other situations, selection against unfit chromosomal hybrids has apparently led to assortative mating and local speciation. Given the genomic tools in the house mouse, it is also possible to study gene flow across stable chromosomal hybrid zones in this species, with evidence that the chromosomal rearrangements promote genetic differentiation in this situation too. There are real opportunities to further develop chromosomal hybrid zones in the house mouse as model systems for the study of speciation.