Population genetic structure in a Robertsonian race of house mice: evidence from microsatellite polymorphism

Genetic evidence was assessed for inbreeding and population subdivision in a Robertsonian fusion (Rb) race of the western European form of house mouse, Mus musculus domesticus, in central Belgium. Inbreeding, and the factors responsible for subdivision (genetic drift and extinction–recolonization) can theoretically influence the fixation of underdominant Rb variants. The data consisted of allele frequencies of eight microsatellite loci and of the Rb(4.12) and Rb(5.10) chromosomes. Six populations were sampled once, and a seventh was sampled successively over 3 years. No evidence for inbreeding within populations was found. Levels of between-population subdivision were high (θ=0.15−0.39), and showed no association with either karyotype or geographical distance over 8–60 km. In addition, low values of effective size were found in the successively sampled population (Ne=5–20). Cases of significant two-locus disequilibria were associated with the most closely linked pair of microsatellite loci (r=0.15): also consistent with small effective sizes. These results suggest that both the lack of inbreeding, and the combined effects of genetic drift and extinction–recolonization, may promote Rb polymorphism in M. m. domesticus.