Dose effects and density-dependent regulation of two microparasites of Daphnia magna

Individual hosts constitute a limited resource for parasites, suggesting that density-dependent effects may play a role in within-host growth and parasite regulation. This hypothesis has been tested for several helminth parasites, but not for microparasites. We therefore examined dose-response patterns for the microparasitic bacterium Pasteuria ramosa and the fungus Metschnikowiella biscuspidata infecting the planktonic crustacean Daphnia magna. With increasing numbers of transmission stages administered to the host we found that host fecundity and survival and parasite transmission-stage production declined. Using a k-value analysis, a method that quantifies the strength of density dependence, we found for both parasites that density dependence acted at all doses, indicating the absence of a minimum density below which parasite fitness is density- independent. At low doses density was exactly compensated, but it was overcompensated at high doses. Overcompensation at high doses was weak for P. ramosa, but high for M. biscuspidata. At the two highest doses M. biscuspidata killed its hosts before any transmission stages were produced. Our data indicate that density dependence is expressed through retarded spore development in P. ramosa, but through both host mortality and reduced parasite fecundity in M. biscuspidata. A further experiment (P. ramosa only) revealed that in well-fed hosts more parasite transmission stages are produced than in poorly fed hosts, suggesting that competition for host resources retards P. ramosa development. Our data for P. ramosa, but not for M. biscuspidata, are largely consistent with assumptions made in models on microparasite epidemiology. We draw attention to the relevance of dose effects and within-host competition for the evolution of virulence.