Invertebrate and other animal populations harbour genetic variation for immune capacity, which may seem paradoxical given the importance of immune performance to fitness. Why is functional variation in immune capacity not purged by natural selection? Why are susceptibility alleles not eliminated? Accumulating evidence suggests that environmental heterogeneity may retard the long term efficiency of natural selection and even maintain polymorphism, provided alternate host genotypes are favoured under different environmental conditions. “Environment” in this context may refer to abiotic factors such as temperature or food availability, the population density of hosts, or the genetic diversity of pathogens. These factors are controlled in many laboratory experiments measuring pathogen resistance or immune responses, and yet they may be overwhelmingly important in host-parasite coevolution in the wild.

Results/Conclusions

In a series of experiments, we exposed the crustacean Daphnia magna to its bacterial parasite Pasteuria ramosa under a variety of conditions to show how environment interacts with host and parasite genotypes to shape immune responses and resistance. The goal of this talk is to indicate how these data help us to understand the magnitude and importance of coevoltutionary dynamics in natural settings.