Effects of host condition on susceptibility to infection, parasite developmental rate, and parasite transmission in a snail-trematode interaction.

Whether or not organisms become infected by parasites is likely to be a complex interplay between host and parasite genotypes, as well as the physiological condition of both species. Details of this interplay are very important because physiology-driven susceptibility has the potential to confound genetic coevolutionary responses. Here we concentrate on how physiological aspects of infection may interfere with genetic-based infectivity in a snail-trematode (Potamopyrgus antipodarum/Microphallus sp.) interaction by asking: (1) how does host condition affect susceptibility to infection? and (2) how does host condition affect the survival of infected individuals? We manipulated host condition by experimentally varying resources. Contrary to our expectation, host condition did not affect susceptibility to infection, suggesting that genetics are more important than physiology in this regard. However, hosts in poor condition had higher parasite-induced mortality than hosts in good condition. Taken together, these results suggest that coevolutionary interactions with parasites may depend on host condition, not by altering susceptibility, but rather by affecting the likelihood of parasite transmission.