Chromosomal Races Of House Mouse Research Articles

Genetic differentiation within and away from the chromosomal rearrangements characterising hybridising chromosomal races of the western house mouse (Mus musculus domesticus).

The importance of chromosomal rearrangements for speciation can be inferred from studies of genetic exchange between hybridising chromosomal races within species. Reduced fertility or recombination suppression in karyotypic hybrids has the potential to maintain or promote genetic differentiation in genomic regions near rearrangement breakpoints. We studied genetic exchange between two hybridising groups of chromosomal races of house mouse in Upper Valtellina (Lombardy, Italy), using microsatellites. These groups differ by Robertsonian fusions and/or whole-arm reciprocal translocations such that F1 hybrids have a chain-of-five meiotic configuration. Previous studies showed genetic differentiation in two chromosomes in the chain-of-five (10 and 12) close to their centromeres (i.e. the rearrangement breakpoints); we have shown here that the centromeric regions of the other two chromosomes in the chain (2 and 8) are similarly differentiated. The internal chromosomes of the chain (8 and 12) show the greatest differentiation, which may reflect pairing and recombination properties of internal and external elements in a meiotic chain. Importantly, we found that centromeric regions of some non-rearranged chromosomes also showed genetic differentiation between the hybridising groups, indicating a complex interplay between chromosomal rearrangements and other parts of the genome in maintaining or promoting differentiation and potentially driving speciation between chromosomal races.

Chromosomal speciation in mice: a cytogenetic analysis of recombination

Within species, populations differing by chromosomal rearrangements ("chromosomal races") may become reproductively isolated in association with reduced hybrid fertility due to meiotic aberrations. Speciation is also possible if hybridizing chromosomal races accumulate genetic differences because of reduced meiotic recombination in the heterozygous configuration in hybrids. Here, we examine recombination in pure races and hybrids within a model system for chromosomal speciation: the hybridization of the Poschiavo (CHPO) and Upper Valtellina (IUVA) chromosomal races of house mouse in Upper Valtellina, Italy. These races differ by Robertsonian fusions/whole-arm reciprocal translocations, such that hybrids produce a pentavalent meiotic configuration. We determined the number and position of the recombination points (using an antibody against the MutL homolog 1 [MLH1] protein) on synaptonemal complexes at pachytene in laboratory-reared CHPO, IUVA, and hybrid males, analyzing at least 112 spermatocytes per karyotypic group, up to a total of 534 spermatocytes. The mean ± standard deviation numbers of MLH1 foci per spermatocyte were 22.2 ± 3.2, 20.1 ± 2.9, 20.7 ± 2.3, and 21.9 ± 2.9 for CHPO, IUVA, CHPO × IUVA, and IUVA × CHPO, respectively. Altogether, 10,146 chromosome arms were examined, allowing multiple comparisons. Overall, recombination events were more frequently distal than proximal or interstitial. The average number of proximal MLH1 foci per chromosome arm decreased going from telocentric to metacentric bivalents to pentavalents (when present), which (together with other factors) influenced the average number of MLH1 foci per cell between CHPO, IUVA, and hybrid mice. The low frequency of proximal recombination in pentavalents of CHPO-IUVA hybrids may promote reproductive isolation between the CHPO and IUVA races, when coupled with reduced hybrid unfitness.