Host sex and local adaptation by parasites in a snail-trematode interaction.

One of the leading theories for the evolutionary stability of sex in eukaryotes relies on parasite-mediated selection against locally common host genotypes (the Red Queen hypothesis). As such, parasites would be expected to be better at infecting sympatric host populations than allopatric host populations. Here we examined all published and unpublished infection experiments on a snail-trematode system (Potamopyrgus antipodarum and Microphallus sp., respectively). A meta-analysis demonstrated significant local adaptation by the parasite, and a variance components analysis showed that the variance due to the host-parasite interaction far exceeded the variance due to the main effects of host source and parasite source. The meta-analysis also indicated that asexual host populations were more resistant to allopatric sources of parasites than were (mostly) sexual host populations, but we found no significant differences among parasite populations in the strength of local adaptation. This result suggests that triploid asexual snails are more resistant to remote sources of parasites, but the parasite has, through coevolution, overcome the difference. Finally, we found that the degree of local adaptation did not depend on the genetic distance among host populations. Taken together, the results demonstrate that the parasites are adapted, on average, to infecting their local host populations and suggest that they may be a factor in selecting against common host genotypes in natural populations.