Evolutionary and ecological immunology of Toll-like receptors in mammals: patterns of selection within and across taxa

Abstract

Infection from pathogens is consequential in host survival and fitness. Genetic differences in immune components can provide benefits in defending a host from pathogens. The Toll-like receptors (TLRs) are a family on innate immune receptors which detect pathogen components and have been found to show signs of positive selection across taxa. Genetic variation within TLRs has also been shown to relate to susceptibility to infection. In the first chapter I examine signatures of positive selection across the order Carnivora. The chapter aims to look for trends within and between TLRs using tests for selection, location of positively selected codons (PSCs), and TLR cellular location. I also investigate strength of selection over each species’ evolutionary history to find relationships with ecological and life history characteristics. The majority of TLRs show robust evidence of positive selection, with PSCs located within areas of biological significance for pathogen interaction. Stronger signs of positive selection were found in extra-cellular receptors than intra-cellular. Significant relationships between strength of selection and ecological characteristics and life history traits were found in multiples TLRs. Most notably, these were with the reproductive traits of gestation length, interbirth interval, and litter size, which potentially illustrate theories of life history trade off in investment between reproduction and immune function. The second chapter of this thesis focuses on genetic and behavioural defences which are crucial in host-pathogen relationships but not often combined into one study. This study looks at population-level TLR polymorphism in the bank vole (Myodes glareolus) and boldness and exploration personality scores. The haplotypes of the voles formed three clusters (A, B and C) but no significant relationships were found between the clusters and personality. However, there was a trend between B cluster haplotypes and increased explorations scores. These prospective relationships have implications in the maintenance of personality and genetic variation within populations and their roles in the spread of disease. The study highlights the importance of considering multiple aspects of host immune defence and their interactions rather than individualising the components. Overall, interaction with pathogens plays a large role in patterns in selection seen across taxa and within species. This thesis shows how the inclusion and consideration of ecological and behavioural factors into immunogenetics can provide a more representative idea of the complex interactions that are at work in host immune defences.