Is the population genetic structure of complex life cycle parasites determined by the geographic range of the most motile host?

Due to their particular way of life, dispersal of parasites is often mediated by their host's biology. Dispersal distance is relevant for parasites because high degree of dispersal leads to high gene flow, which counters the rate of parasite local adaptation in the host populations. Parasites with complex life cycles need to exploit sequentially more than one host species to complete their life cycle. Most trematode parasites have such complex life cycles involving invertebrate and vertebrate hosts. The spatial scales of invertebrate and vertebrate host populations are often different, which may decrease the probability that the parasite cycles locally in the intermediate host population. We used neutral microsatellite markers to determine genetic structure in Diplostomum pseudospathaceum parasites collected from local populations of freshwater snails (Lymnaea stagnalis). D. pseudospathaceum is a trematode that has two intermediate hosts (snail and fish) and a highly motile definitive host (bird). We found that the parasite population infecting the local snail populations showed no genetic structure over a large geographic range (over 300km). We also did not detect evidence for isolation by distance in the parasite. We conclude that dispersal in the motile definitive host is likely to prevent emergence of local population genetic structure in the parasite. Our results suggest that parasite dispersal in the definitive host may limit local cycling of the parasites in the intermediate host populations.