Co-occurrences of parasite clones and altered host phenotype in a snail-trematode system.

The frequent co-occurrence of two or more genotypes of the same parasite species in the same individual hosts has often been predicted to select for higher levels of virulence. Thus, if parasites can adjust their level of host exploitation in response to competition for resources, mixed-clone infections should have more profound impacts on the host. Trematode parasites are known to induce a wide range of modifications in the morphology (size, shell shape or ornamentation) of their snail intermediate host. Still, whether mixed-clone trematode infections have additive effects on the phenotypic alterations of the host remains to be tested. Here, we used the snail Potamopyrgus antipodarum-infected by the trematode Coitocaecum parvum to test for both the general effect of the parasite on host phenotype and possible increased host exploitation in multi-clone infections. Significant differences in size, shell shape and spinosity were found between infected and uninfected snails, and we determined that one quarter of naturally infected snails supported mixed-clone infections of C. parvum. From the parasite perspective, this meant that almost half of the clones identified in this study shared their snail host with at least one other clone. Intra-host competition may be intense, with each clone in a mixed-clone infection experiencing major reductions in volume and number of sporocysts (and consequently multiplication rate and cercarial production) compared with single-clone infections. However, there was no significant difference in the intensity of host phenotype modifications between single and multiple-clone infections. These results demonstrate that competition between parasite genotypes may be strong, and suggest that the frequency of mixed-clone infections in this system may have selected for an increased level of host exploitation in the parasite population, such that a single-clone is associated with a high degree of host phenotypic alteration.