Research question :

How within-host competition regime shapes the coevolutionary dynamics of host investment in defence and parasite investment in exploitation ?

Background :

Most theoretical models of multiple infections rely on specific hypotheses about parasite competition and local coexistence within-host. Very few of these models assess the coevolution of host and parasite traits. Parasite evolution is expected to modulate selective pressures acting upon host, and alter its response to infection. In such a context, parasite competition seems be a key variable for the evolutionary dynamics and epidemic features.

Methods :

We develop a nested coevolutionary model of host resistance and parasite
virulence. From individual-based traits, we define a within-host model and derive from the within-host equilibrium the main epidemiological features of interest -transmission, virulence, recovery-. We use this first model to build an adaptive dynamics model in order to assess the joint evolution of the parasite virulence and host resistance. We compared the coevolutionarily stable states predicted by the model under different competition regimes, including single infections (preemption), superinfection (dominance) and coinfections (mixed). We tested the sensitivity of our results to various epidemiological scenarios, notably by varying the force of infection and the infectious period.

Results :

Our results suggest that parasite virulence under multiple infections evolved
towards higher values than under single infection, while the opposite trend was observed for host resistance. In addition, the local coexistence of parasites enables kin selection effects that reduce both virulence and the subsequent host response. We showed that the magnitude of multiple infections effects varies with the ecological context. Coevolutionary outcomes deviate from simple optimization of persistence depending on the degree of spatial coupling of hosts (parasite dispersal). I will discuss the findings of the model and adress particular considerations and perspectives relating to the importance of strains interaction and contact structure.