The expression and evolution of virulence in multiple infections: the role of specificity, relative virulence and relative dose.

BACKGROUND: 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. 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. RESULTS: We 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. CONCLUSIONS: Our 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.

Sex-specific effects of a parasite evolving in a female-biased host population.

BACKGROUND: Males and females differ in many ways and might present different opportunities and challenges to their parasites. In the same way that parasites adapt to the most common host type, they may adapt to the characteristics of the host sex they encounter most often. To explore this hypothesis, we characterized host sex-specific effects of the parasite Pasteuria ramosa, a bacterium evolving in naturally, strongly, female-biased populations of its host Daphnia magna. RESULTS: We show that the parasite proliferates more successfully in female hosts than in male hosts, even though males and females are genetically identical. In addition, when exposure occurred when hosts expressed a sexual dimorphism, females were more infected. In both host sexes, the parasite causes a similar reduction in longevity and leads to some level of castration. However, only in females does parasite-induced castration result in the gigantism that increases the carrying capacity for the proliferating parasite. CONCLUSIONS: We show that mature male and female Daphnia represent different environments and reveal one parasite-induced symptom (host castration), which leads to increased carrying capacity for parasite proliferation in female but not male hosts. We propose that parasite induced host castration is a property of parasites that evolved as an adaptation to specifically exploit female hosts.