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Abstract
Symbiotic associations between eukaryotes and microorganisms are common in nature. Aphids harbor an array of heritable bacterial symbionts known to mediate diverse ecological interactions. While roles for most aphid symbionts are known, less is known about how they are impacted by environmental conditions or interactions with other biotic agents; factors that influence symbiont persistence in nature. Since climate change theory and empirical evidence show that nights are warming faster than days, I first conducted laboratory experiments to evaluate how the timing of warming influences Hamiltonella defensa-mediated anti-parasitoid defenses in the pea aphid Acyrthosiphon pisum. I found that an increase of just 2.5°C disabled symbiont defenses regardless of the timing of warming, and that warming negatively impacted symbiont‐mediated interactions more than symbiont‐free ones. Results suggest that predicted near-term temperature increases may cause widespread disruption of ecologically important symbioses. Since nothing is known about how aphid viral pathogens interact with resident symbionts, I next investigated whether Acyrthosiphon pisum virus (APV) interacts with the H. defensa as well as Regiella insecticola, which protects against fungal pathogens. I found that APV did not influence symbiont protection against wasps or fungi. However, R. insecticola reduced APV titers and ameliorated APV infection costs, thus providing tolerance to this pathogenic virus. In contrast, H. defensa infection had no effect on virus titers, and APV infection costs were instead exacerbated. I also determined that APV is primarily horizontally transmitted through food plants and occasionally via parasitoid ovipositors. This work shows that heritable symbionts and pathogenic viruses likely influence each other’s maintenance in nature. Finally, I characterized roles of the ubiquitous arthropod symbiont Wolbachia (wPe) in banana aphids (Pentalonia). I found that wPe protects against the specialized fungal pathogen, Pandora neoaphidis, but not parasitoids. wPe also improved aphid fecundity, development time and longevity. These findings support a new protective function for Wolbachia and may partially explain the prevalence of wPe in Pentalonia. The Pentalonia-wPe system provides unique opportunities to merge two key models of symbiosis to better understand mechanisms and infection dynamics in nature. My work shows that a multitude of factors contribute to the maintenance of protective symbioses.