Density dependence of symbiont-mediated immunity and parasite epidemics in symbiont-protected host populations

• Applicant: Dr. Hildegard Uecker
• Subject Area: Bioinformatics and Theoretical Biology ; Evolution, Anthropology ; Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 528028597

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 thus context 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. As the parasite epidemic progresses, the optimal per-host density changes and the population composition is thus expected to shift over time. 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. Current theory is based on compartmental between host models and furthermore neglects heterogeneity in the immunity of symbiont-carriers. Yet, we know from other systems that the precise within-host dynamics as well as host heterogeneity influence parasite dynamics and evolution and therefore changes in the form of symbiosis from mutualistic to parasitic and back. Given the limited knowledge of protective symbiosis in plankton systems, current model assumptions are not tailored to such systems. In this project, we aim to study the joint changes in the densities of hosts, symbionts, and parasites and the associated changes in the form of symbiosis. We will both develop general theory and explore models that match the biology of Daphnia and its parasites. In a first step, we will set up models for the dynamics of symbionts and parasites within hosts, accounting for different mechanisms of host protection. These models will then be linked to between-host models to describe parasite spread in host populations for a range of assumptions on symbiont inheritance and transmission. In this way, we can derive how the precise within-host dynamics and the mechanism of protection feed into the dynamics of all three players at the host population level. We will thereby allow for variability in the symbiont density across hosts and characterize the context-dependent form of symbiosis over time. In a last step, we will allow for co-evolution between symbionts and parasites to identify conditions that lead to co-evolutionary dynamics vs those that lead to break-down of the symbiosis.

• DFG Programme: Research Units
• Subproject of: FOR 5726: Density dependent symbiosis in planktonic systems – DynaSym