Nutritional Ecoimmunology Under Low-dose Infections

Abstract

Investigating immune defence within an ecological context is crucial for comprehending the coevolution of hosts and pathogens. Traditional immunology has revealed a broad range of immune genetic molecular mechanisms and cascading responses of related effectors. However, immune activation post-infection is only part of the overall host defence against disease. The classical immunological approach of inducing high-dose systemic infections is insufficient to elucidate the fundamental principles of non-immunological defences. The cost of immunity is substantial, involving high-nutrient-demand physiological activities. How animals minimise these costs by altering life history strategies is a central question in ecoimmunology.

My PhD project is dedicated to addressing this issue. By establishing a fruit fly fungus infection system, we implement low-dose infections, including both needle tapped topical infections and sexually transmitted infections, to detect nuanced changes in life history traits and differential adaptive behaviours. Additionally, my research investigates the effects of nutritional availability and specific nutrients on life history trade-offs, behavioural thermoregulation, and self-medication. Specifically, in Chapter 2, we show that insects respond to low-dose infections by boosting fecundity via a dose-dependent terminal investment strategy. The potential mechanisms underlying this behavioural strategy are associated with the absence of Dorsal-related immunity factor (Dif), a canonical innate immune gene, where individuals with immune deficiencies exhibit a stronger reproductive response. We also find that the immune factor Turandot C (TotC) is a potential driving force in mediating terminal investment. In Chapter 3, we show that the trade-off between survival and fecundity during cold-seeking is reversed only under ad libitum yeast consumption. Further studies revealed that the ingestion of a single amino acid, tryptophan, is effective in reducing virulence and microbe loads in infected hosts. In chapter 4, we find that 2Phenylethanol (2-PE), a volatile metabolite produced by yeast to attract insects, mediates self-medication of insects, providing both therapeutic and preventative effects against infections. And, this self-medication behaviour is highly correlated with mating cues. The underlying physiological mechanism involves the host regulating 2-PE intake through TotC expression to enhance protein metabolism.

Overall, this thesis contributes to our understanding of ecological immunology by integrating physiological, genetic, and ecological perspectives. It provides insights into how organisms mitigate pathogen threats and fitness costs through non immunological defences. These findings underscore the importance of accurately reflecting host behavioural immune defences and resource acquisition under natural exposure levels for understanding the evolution of immunity. Finally, we recommend incorporating immunological research within the naturalistic contexts of ecological and evolutionary dynamics, which has implications for disease management and conservation efforts in insects.

Date of Award	2 Oct 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Nicholas Priest (Supervisor) & Christopher Pudney (Supervisor)