Maintenance of host variation in tolerance to pathogens and parasites

TLDR

Tolerance and resistance are distinct host defense strategies, with tolerance mitigating damage rather than fighting parasites, and recent work shows they can evolve differently, exhibiting variable costs and trade‑offs that influence the persistence of tolerance variation. The study reconciles empirical and theoretical discrepancies by employing dynamic game‑theoretical models. The models distinguish mortality tolerance, which mitigates disease‑induced mortality, from sterility tolerance, which addresses reductions in fecundity. The analysis shows that mortality tolerance benefits parasite fitness while sterility tolerance is neutral, and that measuring each mechanism’s impact on parasite epidemiology is essential to understand defense evolution.

Abstract

Tolerance and resistance provide hosts with two distinct defense strategies against parasitism. In resistance the hosts "fight" the parasite directly, whereas in tolerance the hosts fight the disease by ameliorating the damage that infection causes. There is increasing recognition that the two mechanisms may exhibit very different evolutionary behaviors. Although empirical work has often noted considerable variance in tolerance within hosts, theory has predicted the fixation of tolerance due to positive frequency dependence through a feedback with disease prevalence. Here we reconcile these findings through a series of dynamic game theoretical models. We emphasize that there is a crucial distinction between tolerance to the effects of disease-induced mortality and tolerance to the effect of the disease-induced reductions in fecundity. Only mortality tolerance has a positive effect on parasite fitness, whereas sterility tolerance is neutral and may therefore result in polymorphisms. The nature of the costs to defense and their relationship to trade-offs between resistance and tolerance are crucial in determining the likelihood of variation, whereas the co-evolution of the parasite will not affect diversity. Our findings stress that it is important to measure the effects of different mechanisms on characteristics that affect the epidemiology of the parasite to completely understand the evolutionary dynamics of defense.