Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Brent J. Sinclair

Abstract

The influence of temperature on interactions with pathogenic or symbiotic microbes is a driving force behind the survival of insects under climate change. However, we know little of how insects physiologically respond to these pressures. In temperate climates, winter dominates the thermal landscape; thus, I am particularly interested in how cold interacts with insect responses to microbes. Here I explore the thermal biology of the insect immune system and the impacts of cold on host-microbe interactions. First, I demonstrate that acute exposure to cold activates selective components of immunity in Drosophila melanogaster, as a compensatory response to trade-offs or injury. Next, I show that cold acclimation decreases immune function at low temperatures in Gryllus veletis at the same time that cold tolerance increases. I conclude that this is a trade-off between immunity and the response to cold. Third, I demonstrate that immune activity varies seasonally in insects, but that each species responds differently. These shifts were likely driven by species-specific responses to multiple overwintering pressures. Fourth, I demonstrate that thermal plasticity in both Gryllus veletis and the fungal pathogen Metarhizium brunneum contribute to the outcome of infection. Further, fluctuating temperatures produce different outcomes of infection than constant temperatures, but we can predict these outcomes based on additive thermal performance under constant conditions. Lastly, I observe that the composition of the hindgut microbiome in Gryllus veletis, containing both beneficial and pathogenic microbes, shifts irreversibly across seasons. Further, microbial shifts coincide with changes in both cold tolerance and immune activity, which indicate that there is a functional relationship between the microbiome and host survival of low temperatures. Overall, changes in temperature are inextricably linked to changes in insect responses to both pathogenic and symbiotic microbes, which has likely selected for an adaptive physiological connection between insect immunity and the response to cold. I demonstrate that the connection between physiological responses to abiotic and biotic pressures modify our interpretation of phenotype. Therefore, we cannot rely on a univariate and species-isolated understanding of how insects respond to temperature if we are to predict the impact of climate change on their fitness.

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