Effect of host-type heterogeneity and competition through cross-immunity on pathogen diversity
A major challenge in the ecology and evolutionary biology of infectious diseases is to understand how pathogen diversity is maintained over time. Previous studies of antigenic diversity have mostly focused on mechanisms related to host immunity, and more more specifically on frequency-dependent selection generated by competition for hosts through cross-immunity. This kind of competition is neutral with respect to other traits, as there are no fixed fitness differences between the individual pathogens. Emerging evidences in rotavirus and norovirus suggest however that viral diversity is also related to genetically determined variation in the host population, which introduces heterogeneity in the resource pathogens are competing for. Here, we address how the binding affinity of the virus, which depends on the host phenotype, influences pathogen diversity by using a model with two hosts and two pathogens. We ask about the evolution of specialization in the context of cross-immunity. An adaptive dynamics framework was implemented to analyze the role of a pathogen trade-off in the ability to infect different host types and the effect of specific and generalized immunity on the long-term stable coexistence of strains.
We show that on the one hand, under weak cross-immunity and weak host affinity trade-offs, the system evolves to a unique Evolutionarily Stable Strategy (ESS), in which the two strains have the same ability to infect both host types. In this case, the strains become neutral with respect to the binding affinity of the virus. On the other hand, when cross-immunity is stronger but the affinity trade-off is still weak, evolutionary branching occurs, where the population evolves to a dimorphic population. These strains stably coexist and are partial specialists for different hosts. Finally, under a strong trade-off, the most likely scenario is the one in which both strains become completely specialized for different hosts. These two strains will not overlap in the type of host that they infect, avoiding competition. However, it is also possible for both viruses to specialize for the same host, where the ESS onto which to the population settles will be determined by initial conditions. These results demonstrate the importance of considering both immunity and host genetic heterogeneity to understand the long-term coexistence of multiple pathogen strains, in particular, how niche differentiation is influenced by host affinity as well as immune response.