Dynasym

Protective symbionts can provide hosts with immunity against virulent parasites. Yet, symbionts themselves may also be costly for the host. The form of symbiosis is thuscontext dependent: mutualistic upon exposure (or infection) with the virulent parasite but parasitic in its absence. The probability of one or the other depends on the parasite prevalence in the host population and changes thus in the course of a parasite epidemic. The strength of protection as well as the costs of the symbiont may increase with its density within the host, and this density may vary across hosts, generating a heterogeneous host population. This heterogeneity is expected to influence the parasite dynamics. In turn, as the parasite epidemic progresses, the optimal per-host density of symbionts changes and the population composition is thus expected to shift over time, creating a feedback between the population heterogeneity and the parasite dynamics. Classical theory on host-parasite systems only considers two players – the host and the parasite. The possibility of protective symbiosis clearly demonstrates the need for theory that goes beyond two-species systems. In this project, we will develop theoretical models to study the joint changes in the densities of hosts, symbionts, and parasites and the associated changes in the form of symbiosis. We are particularly interested in the question of when diversity in symbiont densities across hosts can be maintained and how the properties of the symbiont and the host, such as the mechanism of protection and the mode of parasite transmission, matter for the results. In a last step, we will allow for coevolution between symbionts and parasites to identify conditions that lead to coevolutionary dynamics vs those that lead to break-down of the symbiosis. We will at least initially mostly develop general theory, but we will later also explore models that match the biology of Daphnia and its parasites.