Résumé
Despite the fact that parasite dispersal is likely to be one ofthe most important processes influencing the dynamics andcoevolution of host-parasite interactions, little information isavailable on the factors that affect it. In most cases,opportunities for parasite dispersal should be closely linked tohost biology. Here we use microsatellite genetic markers tocompare the population structure and dispersal of two host racesof the seabird tick Ixodes uriae at the scale of the NorthAtlantic. Interestingly, tick populations showed highwithin-population genetic variation and relatively low populationdifferentiation. However, gene flow at different spatial scalesseemed to depend on the host species exploited. The black-leggedkittiwake (Rissa tridactyla) had structured tick populationsshowing patterns of isolation by distance, whereas tickpopulations of the Atlantic puffin (Fratercula arctica) were onlyweakly structured at the largest scale considered. Host-dependentrates of tick dispersal between colonies will alter infestationprobabilities and local dynamics and may thus modify theadaptation potential of ticks to local hosts. Moreover, as I.uriae is a vector of the Lyme disease agent Borrelia burgdorferisensu lato in both hemispheres, the large-scale movements of birdsand the subsequent dispersal of ticks will have importantconsequences for the dynamics and coevolutionary interactions ofthis microparasite with its different vertebrate and invertebratehosts.