Cheating, trade-offs and the evolution of virulence in a natural pathogen population
The evolutionary dynamics of pathogens are critically important for disease outcomes, prevalence and emergence. In this talk I will discuss some specific ecological conditions that promote the long-term maintenance of virulence polymorphisms in a pathogen population. Recent theory predicts that evolution towards increased virulence can be reversed if less virulent social ‘cheats’ exploit virulent ‘cooperator’ pathogens. However, there is little evidence that social exploitation operates within natural pathogen populations. I will demonstrate that for the bacterium Pseudomonas syringae, major virulence polymorphisms are maintained at unexpectedly high frequencies in the host Arabidopsis thaliana. Experiments reveal that the fitness costs of decreased virulence are eliminated in mixed infections, whereas less virulent strains have a fitness advantage in non-host environments. These results suggest that niche differentiation contributes to the maintenance of virulence polymorphisms, and that both within-host and between-host pathogen growth must be considered to understand the roles of cheating and cooperation in pathogen populations.