Population genetic structure of the tree-hole tick Ixodes arboricola (Acari: Ixodidae) at different spatial scales

The endophilic tick Ixodes arboricola infests cavity-nesting birds, and itsdispersal strongly depends on the movements of its host. Population genetic structureof I. arboricola was studied with seven polymorphic microsatellite markers.We collected 268 ticks from 76 nest boxes in four woodlots near Antwerp, Belgium.These nest boxes are mainly used by the principal hosts of I. arboricola,the great tit Parus major and the blue tit Cyanistes caeruleus. Asthese birds typically return to the same cavity for roosting or breeding, tickswithin nest boxes were expected to be highly related, and tick populations wereexpected to be spatially structured among woodlots and among nest boxes withinwoodlots. In line with the expectations, genetic population structure was found amongwoodlots and among nest boxes within woodlots. Surprisingly, there was considerablegenetic variation among ticks within nest boxes. This could be explained bycontinuous gene flow from ticks from nearby tree holes, yet this remains to betested. A pairwise relatedness analysis conducted for all pairs of ticks within nestboxes showed that relatedness among larvae was much higher than among later instars,which suggests that larvae are the most important instar for tick dispersal. Overall,tick populations at the studied spatial scale are not as differentiated as predicted,which may influence the scale at which host–parasite evolution occurs.