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

Abstract

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.

Figure 1

Time-to-host-death (virulence) in single and mixed infections. Time-to-host-death (virulence) in single and mixed infections by the parasite isolates/clones (A) P1 and P4 (B) P1 and C1 (C) P1 and C14 (D) P2 and P4 (E) P2 and C1, and (F) P2 and C14. Error bars are standard errors.

Figure 2

Lifetime number of host offspring produced (host fitness) in single and mixed infections. Lifetime number of host offspring produced (host fitness) in single and mixed infections by the parasite isolates/clones (A) P1 and P4 (B) P1 and C1 (C) P1 and C14 (D) P2 and P4 (E) P2 and C1, and (F) P2 and C14. Error bars are standard errors.

Figure 3

Parasite spore production on day 20 post-infection in mixed infections. Parasite spore production on day 20 post-infection by parasite isolates/clones P1 and P4 (left panel), P1 and C1 (middle panel), and P1 and C14 (right panel). Error bars are standard errors.

Figure 4

Lifetime spore production of an infection (parasite fitness) in single and mixed infections. Lifetime spore production of an infection (parasite fitness) in single and mixed infections by the parasite isolates/clones (A) P1 and P4 (B) P1 and C1 (C) P1 and C14 (D) P2 and P4 (E) P2 and C1, and (F) P2 and C14. Error bars are standard errors.