Trade-offs between Host Defense Mechanisms: Impacts on Evolutionary and Coevolutionary Dynamics of Host-Parasite Interactions

Evolution plays a key role in shaping the trajectory of infectious diseases and it is important to comprehend the mechanisms that allow parasites to mutate and adapt to new environments. Studying host-parasite interactions provides essential insights that are crucial for disease control and prevention. In this thesis, I utilise mathematical models and evolutionary invasion theory to explore how hosts allocate investments in defence mechanisms against parasites. I specifically examine how investments are influenced by the presence of negative correlations between these mechanisms.

Hosts' defence mechanisms against parasites are usually divided into two classes: resistance and tolerance. Resistance strategies act to reduce the fitness of parasites, while tolerance strategies mitigate the negative impact on host fitness without directly harming the parasites. First, I investigate the simultaneous evolution of resistance and tolerance in a host population where they are negatively correlated by a trade-off. Here I focus on examining the optimal investment patterns in resistance and tolerance and predicting the favoured strategy under diverse ecological and epidemiological conditions. Next, I study a case where the host can evolve both sterility tolerance and mortality tolerance as defences against parasitic infection, with a direct trade-off between the two strategies. The primary objective here is to predict the evolutionary outcomes based on the trade-off shape: polymorphism, stable investments, or maximization/minimization. I then extend this model into a coevolutionary framework, investigating the coevolution of hosts and parasites while considering explicit trade-offs between host sterility-mortality tolerance and parasite recovery-transmission.

My results reveal the underlying feedbacks created by the impact of trade-offs, disease prevalence, fluctuating ecological dynamics, and various epidemiological traits on the selection of defence strategies. This work contributes to a better understanding of host-parasite evolution and serves as a potential theoretical base for future experiments in this field.