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Abstract

Host-parasite coevolution is expected to present a strong force in nature, with interactions between hosts and parasites leading to rapid reciprocal adaptations. In recent years several coevolution experiments have been carried out, testing predictions on the dynamics and outcomes of coevolutionary processes. One aspect of host and parasite ecology that has received little attention in host-parasite coevolution experiments is the ability of both parties to shape their environment. A number of parasites are able to produce transmissive stages that persist in the environment across host generations and many host species are able to manipulate their environment through secretion of antimicrobial compounds or hygienic behaviours. The main objective of my PhD thesis is to extend our knowledge of how these traits may affect the outcome of coevolution. In chapter 1 I review previous evolution experiments and discuss how transmission mechanisms may have been responsible for confounding results that have been seen in the past. The subsequent chapters relate to a host-parasite coevolution experiment where I allowed the red flour beetle, Tribolium castaneum, to coevolve separately with two parasites, the general entomopathogenic fungus, Beauveria bassiana, and its natural microsporidian parasite, Paranosema whitei. T. castaneum is able to secrete antimicrobial compounds into its environment and both parasites are able to produce persistent environmental stages. Within my experimental design I allowed for both the benefits of releasing transmissive stages into the environment for the parasites and secretion of antimicrobial compounds for the beetles to be realised. Coevolution resulted in both parasites increasing in virulence chapters 2 and 3. Fascinatingly, I was also able to show that in B. bassiana increased virulence was obtained via evolved resistance to the external immune compounds of the beetles chapter 3. Subtle reciprocal changes were also seen in hosts coevolved with B. bassiana chapter 4. This work highlights the importance of external immunity and transmissive parasite stages in coevolutionary interactions and the need for more studies where such effects are taken into account.