Meeting Abstract
Mechanisms that reduce the costs of infection are expected to evolve when populations encounter novel pathogens. Individuals can reduce the costs of infection in two main ways: resistance and tolerance. Resistance decreases the costs of infection by decreasing pathogen load. In contrast, tolerance does not result in decreased pathogen load but rather the per-pathogen cost of infection. Most work in animals has focused on resistance, but we hypothesize that tolerance should evolve in host-parasite systems where resistance itself is very costly (e.g., due to strong inflammatory reactions that can damage host tissues). We tested this hypothesis in several populations of house finches (Haemorhous mexicanus) in which the bacterial pathogen Mycoplasma gallisepticum (MG) has been endemic for different amounts of time. Finches infected with MG display strong inflammatory immune responses and severe conjunctivitis, impairing predator avoidance and reducing survival probability. To determine if tolerance to MG has evolved in house finches, we measured pathogen load and the severity of conjunctivitis after experimentally infecting individuals from four populations with different histories of MG endemism. We predicted that, when infected with the same pathogen load, individuals from populations that have been coevolving with MG for a greater amount of time (>20 years) would have less severe conjunctivitis than individuals from populations that have been coevolving with MG for less time (5-10 years), or those from populations naïve to MG. This work allows us to determine if, and how quickly, tolerance to a pathogen can evolve and whether tolerance repeatedly evolves in different populations faced with the same novel pathogen.
