Résumé
In the present study, a phylogeographical approach was developed to analyse the influence of selection and history on a major histocompatibility complex (Mhc) class II gene polymorphism in European bank vole (Myodes glareolus) populations. We focused on exon 2 of the Dqa gene because it is highly variable in a large array of species and appears to evolve under pathogen-mediated selection in several rodent species. Using single-strand conformation polymorphism analysis and sequencing techniques, 17 Dqa-exon2 alleles, belonging to at least two different copies of Dqa gene, were detected over the distribution range of M. glareolus. Evidence of selection was found using molecular and population analyses. At the molecular level, we detected 13 codons evolving under positive selection pressures, most of them corresponding to regions coding for putative antigen binding sites of the protein. At the European level, we compared patterns of population structure for the Dqa-exon2 and cytochrome b (cyt b) gene. We did not detect any spatial genetic structure among M. glareolus populations for the Dqa-exon2. These results strongly differed from those obtained using the cyt b gene, which indicated a recent phylogeographical history closely linked to the last glacial events. Seven mitochondrial lineages have yet been described, which correspond to major glacial refugia. Altogether, our results revealed clear evidence of balancing selection acting on Dqa-exon2 and maintaining polymorphism over large geographical areas despite M. glareolus history. It is thus likely that Mhc phylogeographical variability could have been shaped by local adaptation to pathogens.