Parasite dose, prevalence of infection and local adaptation in a host-parasite system.

Parasites have been found to be more infective to sympatric hosts (local adaptation) in some systems but not in others. The variable nature of results might arise due to differences in host and/or parasite migration rates, parasite virulence, specificity of infection, and to differences in the dose-response functions. We tested this latter possibility by manipulating the dose of trematode (Microphallus sp.) eggs on sympatric and allopatric host populations (Potamopyrgus antipodarum). We found that infection rapidly increased to a high asymptote (0.88 +/- 0.02, 1 S.E.) in the sympatric host population, but infections were low and surprisingly unrelated to dose in the allopatric host. We also found that host survival and growth rate were not negatively affected by increasing parasite dose in either population. These results suggest that defences in the allopatric host were not overwhelmed at high parasite doses, and that any life-history costs of defence are not plastic responses to parasite dose.