Unusual mitochondrial DNA polymorphism in two local populations of blue tit Parus caeruleus

Mitochondrial DNA (mtDNA) from 25 blue tits Parus caeruleus sampled from two populations of the Grenoble region (France) was assayed for polymorphism with 17 restriction endonucleases. Nine genotypes were found. Several mtDNA genotypes were also analysed by amplification via the polymerase chain reaction (PCR) and direct sequencing of 903 bp of the cytochrome b gene. The mtDNA polymorphism is greater in P. caeruleus than in other comparable bird species and results from the presence of two clearly differentiated mitochondrial lineages. Using the data of restriction polymorphism, the mean sequence divergence between individuals of the two lineages is 1.23%. Therefore, P. caeruleus should fall into the category II of phylogeographic pattern sensu Avise et al. (1987): discontinuous mtDNA genotypes which co-occur in the same region. P. caeruleus, like humans and other mobile species with high gene flow, seems to have lost its geographic structure in terms of mtDNA phylogeny. This unusual mitochondrial polymorphism can be explained by the recent admixture of two long-term isolated populations. This could be accounted for by two different scenarios. One assumes a simultaneous post-glacial colonization of the Grenoble region by two isolated European populations of P. caeruleus. Alternatively, hybridization between P. caeruleus and P. cyanus could have caused the observed pattern of mtDNA variation.