Host specificity of a generalist parasite: genetic evidence of sympatric host races in the seabird tick Ixodes uriae

Due to the close association between parasites and their hosts, many 'gener alist' parasites have a high potential to become specialized on different h ost species. We investigated this hypothesis for a common ectoparasite of s eabirds, the tick Ixodes uriae that is often found in mixed host sites. We examined patterns of neutral genetic variation between ticks collected from Black-legged kittiwakes (Rissa tridactyla) and Atlantic puffins (Fratercul a arctica) in sympatry. To control for a potential distance effect, values were compared to differences among ticks from the same host in nearby monos pecific sites. As predicted, there was higher genetic differentiation betwe en ticks from different sympatric host species than between ticks from near by allopatric populations of the same host species. Patterns suggesting iso lation by distance were found among tick populations of each host group, bu t no such patterns existed between tick populations of different hosts. Ove rall, results suggest that host-related selection pressures have led to the specialization of I. uriae and that host race formation may be an importan t diversifying mechanism in parasites.