Lack of genetic structure among Eurasian populations of the tick Ixodesricinus contrasts with marked divergence from north-African populations

Host-parasite interactions may select for significant novel mutations with major evolutionary consequences for both partners. In poor active dispersers such as ticks, their population structures are shaped by their host movements. Here, we use population genetics and phylogeography to investigate the evolutionary history of the most common tick in Europe, Ixodes ricinus, a vector of pathogenic agents causing diseases in humans and animals. Two mitochondrial and four nuclear genes were sequenced for 60 individuals collected on four geographical scales (local, regional, Eurasian and western Palearctic scales). The overall level of nucleotide diversity was low and the variability did not differ at the local, regional or Eurasian scales but increased two fold for the western Palearctic scale. Moreover, the phylogenetic trees indicated an absence of genetic structure among Eurasian ticks, contrasting with a strong differentiation of the north-African ticks which formed a divergent clade. The homogeneity in Eurasian ticks may be explained by gene flows due to passive dispersal of ticks by hosts within a continuous population and recent range expansion of I. ricinus as shown by the fit of the observed frequency distribution of numbers of mismatches between pairwise sequences with the demographic expansion model (Harpending raggedness index, P = 0.74). The genetic divergence of the north-African populations could be explained by genetic drift in these small populations that are geographically isolated and/or selection pressures due to different ecological conditions (seasonal activity, pathogenic agents and hosts communities). The consequences of these results on the epidemiology of vector-borne diseases are discussed.