| Summary: | <p>Hosts, parasites, and the environment are interlinked in the disease triangle. Thus, environmental changes can lead to shifts in the ecological and evolutionary dynamics of host-parasite
interactions. In this thesis, I combine meta-analytical and experimental approaches to explore the role of abiotic and biotic factors, specifically nutrition and within-host parasite-parasite
interactions, on infection outcomes. Firstly, I conducted a literature search to gather data from a wide range of host-parasite systems and carried out a meta-analysis to investigate
whether changes to host nutrition affect virulence. Among other findings, this study reveals that changes to host nutrition may have diverse effects across host and parasite species, depending upon a balance between the effect of host nutrition on the immune system and on parasite resources. Subsequently, using a similar meta-analytic approach, I investigate the
extent to which coinfections affect virulence relative to single infections. Across host-parasite systems, I find that coinfections are overall more virulent than single infections, with phylogenetic divergence of coinfecting parasites and their transmission strategies key determinants. Extending on this analytical work, I experimentally test the role of specific pathogen traits (a ‘producer’ and two ‘non-producers’ for siderophore production and quorum sensing) on within-and-between-host dynamics in single infection and coinfections, using a nematode host (Caenorhabditis elegans) and pathogenic bacterium (Pseudomonas aeruginosa). I find that producer strains are consistently better at both colonising and transmitting between nematode hosts than non-producers in single infections and
coinfections. On a different theme to the preceding research, in the final data chapter, I ask the broad evolutionary question: why don’t all animals avoid inbreeding? I demonstrate that
inbreeding avoidance only evolves when there is a risk of inbreeding depression and when related sexual partners frequently come into contact. In this thesis, I use the disease triangle as a tool to investigate abiotic and biotic drivers of host-parasite interaction outcomes and dynamics, extending on previous research highlighting the importance of studying host-parasite dynamics within an ecological context at both the level of within-host and between-host dynamics. In a changing world, this work is a timely contribution to our understanding
of the broad and varied effects of environmental change on infection outcomes.</p>
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