Raciation and speciation in house mice from the Alps: the role of chromosomes

There are at least 24 different karyotypic races of house mouse in the central Alps, each characterized by a different complement of ancestral acrocentric and derived metacentric chromosomes; altogether 55 different metacentric chromosomes have been described from the region. We argue that this chromosome variation largely arose in situ. If these races were to make contact, in most cases they would produce F1 hybrids with substantial infertility (sometimes complete sterility), due to nondisjunction and germ cell death associated with the formation of long-chain and/or ring configurations at meiosis. We present fertility estimates to confirm this for two particular hybrid types, one of which demonstrates male-limited sterility (in accordance with Haldane's Rule). As well as a model for speciation in allopatry, the Alpine mouse populations are of interest with regards speciation in parapatry: we discuss a possible reinforcement event. Raciation of house mice appears to have happened on numerous occasions within the central Alps. To investigate one possible source of new karyotypic races, we use a two-dimensional stepping stone model to examine the generation of recombinant races within chromosomal hybrid zones. Using field-derived ecological data and laboratory-derived fertility estimates, we show that hybrid karyotypic races can be generated at a reasonable frequency in simulations. Our model complements others developed for flowering plants that also emphasize the potential of chromosomal hybrid zones in generating new stable karyotypic forms.