Abstract
BackgroundEfficient host exploitation by parasites is frequently likely to depend on cooperative behaviour. Under these conditions, mixed-strain infections are predicted to show lower virulence (host mortality) than are single-clone infections, due to competition favouring non-contributing social 'cheats' whose presence will reduce within-host growth. We tested this hypothesis using the cooperative production of iron-scavenging siderophores by the pathogenic bacterium Pseudomonas aeruginosa in an insect host.ResultsWe found that infection by siderophore-producing bacteria (cooperators) results in more rapid host death than does infection by non-producers (cheats), and that mixtures of both result in intermediate levels of virulence. Within-host bacterial growth rates exhibited the same pattern. Crucially, cheats were more successful in mixed infections compared with single-clone infections, while the opposite was true of cooperators.ConclusionThese data demonstrate that mixed clone infections can favour the evolution of social cheats, and thus decrease virulence when parasite growth is dependent on cooperative behaviours.
Highlights
Efficient host exploitation by parasites is frequently likely to depend on cooperative behaviour
These data demonstrate that mixed clone infections can favour the evolution of social cheats, and decrease virulence when parasite growth is dependent on cooperative behaviours
Time to death in hours of waxmoth larvae inoculated with pure cooperator, pure cheat or mixed clone infections of P. aeruginosa
Summary
Efficient host exploitation by parasites is frequently likely to depend on cooperative behaviour Under these conditions, mixed-strain infections are predicted to show lower virulence (host mortality) than are single-clone infections, due to competition favouring non-contributing social 'cheats' whose presence will reduce within-host growth. We tested this hypothesis using the cooperative production of iron-scavenging siderophores by the pathogenic bacterium Pseudomonas aeruginosa in an insect host. Numerous models of parasite evolution predict that mixed infections have higher virulence (host mortality rate) than do single-genotype infections This is attributed to increased competition for host resources, favouring more rapid host exploitation [1,2,3,4,5].
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