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Novel immunological interactions as an overlooked aspect of global change: insights from the host range expansion of Lycaeides melissa
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There is accumulating evidence that insect populations globally are experiencing alarming declines. The combination of factors cumulatively exacerbating global insect declines has been referred to as “death by a thousand cuts”. One of these “cuts” is introduced species, however, recent recommendation articles have failed to address the issue of non-native plant species and their role in declining insect populations. The purpose of my dissertation was to investigate another potential indirect effect of non-native plant species on native insect fauna: immunological consequences. I chose this focus because although there is vast evidence that nutritional, phytochemical, and microbial variation can impact the insect immune system, these questions have been understudied in the context of novel host plant use. Given that non-native host plants often contain novel traits, such as secondary metabolites, these introductions effectively represent natural experiments which are excellent opportunities to test ecological immunology theory.To address these questions, I conducted four experimental projects on wild populations of the Melissa blue butterfly (Lycaeides melissa) in the Great Basin Desert. This plant-feeding butterfly is of interest from an evolutionary ecology standpoint because it has recently undergone a host expansion; it has incorporated a novel host plant into its diet. I have used this host expansion as a comparative framework to understand how ecological components can change the immune response; the novel host plant represents a novel nutritional, chemical, and microbial resource, all factors that can potentially impact the insect immune response. In my first chapter, I tested whether host plant use directly affected the insect immune response and whether host plant associated traits such as phytochemistry, microbes, or foliar protein had direct or indirect effects on insect immunity. For my second chapter, I investigated the role of maternal microbes in mediating the larval immune response of L. melissa. For my third chapter, I investigated how novel host plant use affects the transcriptional regulation of L. melissa genes when infected with the lepidopteran virus, Junonia coenia densovirus (JcDV). Finally, for my fourth chapter I tested whether novel host use impacted the ability of L. melissa to resist a viral pathogen, Junonia coenia densovirus (JcDV).Taken together, the results from my dissertation work suggest that novel species interactions (between native insect fauna and non-native plants) have immunological consequences. While use of the native host plant A. canadensis did not always result in a immune response, my final chapter revealed strong evidence for this host plant increasing survivorship in the presence of a live viral pathogen. Further, there is accumulating evidence from the literature that nutritionally superior host plants frequently result in a stronger immune response in lepidopterans.