Inter-oceanic variation in patterns of host-associated divergence in a seabird ectoparasite

Aim Parasites with global distributions and wide host spectra provide excellentmodels for exploring the factors that drive parasite diversification. Here, we testedthe relative force of host and geography in shaping population structure of awidely distributed and common ectoparasite of colonial seabirds, the tick Ixodesuriae.Location Two natural geographic replicates of the system: numerous seabirdcolonies of the North Pacific and North Atlantic Ocean basins.Methods Using eight microsatellite markers and tick samples from a suite ofmulti-specific seabird colonies, we examined tick population structure in theNorth Pacific and compare patterns of diversity and structure to those in theAtlantic basin. Analyses included population genetic estimations of diversity andpopulation differentiation, exploratory multivariate analyses, and Bayesianclustering approaches. These different analyses explicitly took into accountboth the geographic distance among colonies and host use by the tick.Results Overall, little geographic structure was observed among Pacific tickpopulations. However, host-related genetic differentiation was evident, but wasvariable among host types and lower than in the North Atlantic.Main conclusions Tick population structure is concordant with the geneticstructure observed in seabird host species within each ocean basin, where seabirdpopulations tend to be less structured in the North Pacific than in the NorthAtlantic. Reduced tick genetic structure in the North Pacific suggests that hostmovement among colonies, and thus tick dispersal, is higher in this region. Inaddition to information on parasite diversity and gene flow, our findings raiseinteresting questions about the subtle ways that host behaviour, distribution andphylogeographic history shape the genetics of associated parasites acrossgeographic landscapes.