Phylogeography of the tick Ixodes ricinus: strong genetic divergence between North African and European populations.

Context / challenges

Ticks are haematophagous mites capable of transmitting many diseases, both to humans (Lyme disease, tick-borne encephalitis...) and to animals (bovine piroplasmosis and anaplasmosis...). Worldwide, they are considered the second most important vector group after mosquitoes. In addition to these human and animal health issues, ticks and the diseases they transmit can be associated with environmental issues because of the important role of wildlife in tick gorgement but also as a reservoir of pathogens. In order to effectively control these vectors, for example by using tick vaccines or acaricidal products, it is essential to know their genetic variability. Ixodes ricinus is the most common tick species in Europe: its range covers most of the western Palearctic zone, from North Africa to southern Scandinavia, and from Ireland to the Caspian Sea. The objective of this study was to characterize the genetic variability of a large panel of populations covering the entire range of this tick by analyzing the nucleotide polymorphism of several genes and to describe the partitioning of this inter- and intra-population variability.

Results

We sampled 40 populations covering the entire range of Ixodes ricinus and sequenced 2 mitochondrial and 4 nuclear genes from 60 individuals. The genetic diversity observed is not structured according to the geographical origin of the samples. The diversity observed throughout Europe is similar to that observed in a single French population. Despite the low active dispersal capacities of these non-winged vectors, these results suggest the existence of significant gene flows between these populations, linked to the movements of wild (birds, deer...) or domestic (trade) hosts on which ticks remain fixed for several days (the time of their blood meal). Patterns of nucleotide diversity suggest that these populations are recently expanding. In contrast to what has been observed in European populations, ticks in North African populations - where ticks are confined to more humid mountainous areas - have been found to be genetically very different from all other populations in the Western Palearctic, indicating ancient genetic isolation.

Perspectives

This study is the first to highlight the existence of a strong genetic divergence of certain populations of I. ricinus within its range. Further studies are in progress to determine whether North African and European populations of this tick are interfertile (existence of pre- or post-copulatory reproductive isolation?). Experiments on the vectorial competence of these different tick populations for different pathogens will have to be carried out in order to find out whether these genetic differences also correspond to phenotypic differences that would have major consequences for the epidemiology of tick-borne diseases.

Partners

Pasteur Institute of Tunis (Professor Ali Bouattour)

Publications

• Noureddine R., Chauvin A. et Plantard O. (2011). Lack of genetic structure among Eurasian populations of the tick Ixodes ricinus contrasts with marked divergence from north-African populations. International Journal for Parasitology 41. Pp 183-192.

Partners