The evolution of ecological immunity in the decorated cricket (Gryllodes sigillatus)

Abstract

Organisms are constantly under attack from pathogens and must invest in immune function to survive. Natural selection should therefore have favoured universally perfect immunity, yet variation in immunity persists, both at the scale of a single individual, as well as between individuals. Understanding how animals regulate their investment in immune function is an important goal of the field of ecological immunology. Central to this field is the idea that different environmental conditions pose different evolutionary constraints on immune function and that the sexes might adopt different strategies to solve these problems. In this thesis, I examine the role of environmental factors and host behaviour in shaping immune function. To achieve this, I perform a series of experiments using the decorated cricket (Gryllodes sigillatus) as a model to understand how an abiotic and biotic environmental factor (i.e. diet and the social environment, respectively) affect immunity in male and female crickets. I also evaluate how the sexes adjust their feeding behaviour to survive infection. While my work on the social environment and feeding behaviour focuses on the phenotypic effects of these factors on immunity within a single generation of crickets, I use experimental evolution to explore the consequences of dietary adaptation on immune function over multiple generations. Overall, my thesis demonstrates the importance of considering the multifaceted nature of immune function (from physiological to behavioural defences) and of carefully selecting a set of ecologically relevant conditions when assessing immunity. My work highlights the need to rigorously validate the use of immune proxies within a particular species prior to testing any hypothesis regarding immune function. Moreover, I provide evidence suggesting that immune function can evolve both with abiotic (e.g. diet) and biotic (e.g. social environment) environmental factors. Finally, I demonstrate that male and female crickets can actively improve their survival following infection by modifying their feeding behaviour, suggesting that animals are not simply passive actors subjected to the effects of their sex, genes or environment when it comes to disease resistance. Collectively, the results I present in this thesis provide an important foundation for understanding the role of environmental factors, social interactions and host behaviour in shaping immune function.

Date of Award	2021
Original language	English