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Abstract
BackgroundMultiple infections of the same host by different strains of the same microparasite species are believed to play a crucial role during the evolution of parasite virulence. We investigated the role of specificity, relative virulence and relative dose in determining the competitive outcome of multiple infections in the Daphnia magna-Pasteuria ramosa host-parasite system.ResultsWe found that infections by P. ramosa clones (single genotype) were less virulent and produced more spores than infections by P. ramosa isolates (possibly containing multiple genotypes). We also found that two similarly virulent isolates of P. ramosa differed considerably in their within-host competitiveness and their effects on host offspring production when faced with coinfecting P. ramosa isolates and clones. Although the relative virulence of a P. ramosa isolate/clone appears to be a good indicator of its competitiveness during multiple infections, the relative dose may alter the competitive outcome. Moreover, spore counts on day 20 post-infection indicate that the competitive outcome is largely decided early in the parasite’s growth phase, possibly mediated by direct interference or apparent competition.ConclusionsOur results emphasize the importance of epidemiology as well as of various parasite traits in determining the outcome of within-host competition. Incorporating realistic epidemiological and ecological conditions when testing theoretical models of multiple infections, as well as using a wider range of host and parasite genotypes, will enable us to better understand the course of virulence evolution.
Highlights
Multiple infections of the same host by different strains of the same microparasite species are believed to play a crucial role during the evolution of parasite virulence
The competitive outcome of multiple infections appears to be driven by several interrelated factors: the relative virulence of the coinfecting strains as measured in single infections [19,24], prior residency of one of the parasite strains or species [25,26,27,28], and the infectious dose used during simultaneous exposure [18,29,30]
The present study experimentally investigates multiple infections in D. magna using infectious doses containing isolates and clones of P. ramosa in equal (50:50) and unequal proportions (90:10 and 10:90)
Summary
Multiple infections of the same host by different strains of the same microparasite species are believed to play a crucial role during the evolution of parasite virulence. The competitive outcome of multiple infections appears to be driven by several interrelated factors: the relative virulence of the coinfecting strains as measured in single infections [19,24], prior residency of one of the parasite strains or species [25,26,27,28], and the infectious dose used during simultaneous exposure [18,29,30]. The latter factor – infectious dose – is strongly tied to the parasites’ epidemiology. Few studies examined how the relative parasite dose influences the expression of virulence and the production of transmission stages in the presence of multiple infections [18,30]
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