The cost of travel: how dispersal ability limits local adaptation in host-parasite interactions

Classical theory suggests that parasites will exhibit higher fitness in sympatric relative to allopatric host populations (local adaptation). However, evidence for local adaptation in natural host-parasite systems is often equivocal, emphasizing the need for cross-infection experiments conducted over realistic geographic scales and comparisons among species with varied life history traits. Here, we conducted cross-infection experiments to test how two trematode (flatworm) species (Paralechriorchis syntomentera and Ribeiroia ondatrae) with differing dispersal abilities varied in the strength of local adaptation to their amphibian hosts. By pairing 26 host-by-parasite population cross-infections from across the western USA with analyses of host and parasite spatial genetic structure, we found that increasing geographic distance – and corresponding increases in host population genetic distance – reduced infection success for P. syntomentera, which is dispersed by snake definitive hosts. For the avian-dispersed R. ondatrae, in contrast, the geographic distance between the parasite and host populations had no influence on infection success. Differences in local adaptation corresponded to parasite genetic structure; while populations of P. syntomentera exhibited ~10% mtDNA sequence divergence, those of R. ondatrae were nearly identical (