DISSERTATION RESEARCH: Inter-strain variation and evolution of resistance to phytochemicals in the bumblebee trypanosome parasite, Crithidia bombi

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Animal pollination maintains ecosystem biodiversity and is valued at $215B/year to world agriculture. Bumblebees and other wild pollinators enable reproduction of wild plant species and contribute $3B/year to US food production; bumblebees may increase in importance if honeybee populations continue to collapse. However, parasites have contributed to US bumblebee population decreases of up to 96%. Plants produce an arsenal of chemicals, termed 'phytochemicals', that confer protection against herbivores and pathogens. These chemicals can be potent antimicrobials; they are used in traditional and modern medicines and confer herbs and spices with gustatory and food-preserving properties. Antimicrobial nectar chemicals consumed by bees could reduce pollinator infection, but might also select for bee parasites that are resistant to common chemicals, which would then lose medicinal value. This project will enhance understanding of how floral landscapes affect and could be manipulated to reduce pollinator disease. This work will use a bumblebee parasite, Crithidia bombi, to determine how phytochemicals affect parasite growth and evolution, specifically: (1) Do strains vary in phytochemical resistance? Laboratory data suggest that effects of phytochemical ingestion depend on the parasite strain with which bees are infected. Such variation may reflect or allow differential strain fitness in response to common floral chemotypes. The proposed experiments will test the effects of 8 nectar phytochemicals on 4 C. bombi strains. (2) Can strains evolve resistance to individual phytochemicals? Chronic phytochemical exposure could select for tolerant genotypes, thereby reducing medicinal effects realized by pollinators. Experiments will evaluate whether growth-inhibiting effects of phytochemicals decrease over 4 weeks of phytochemical exposure. (3) Do phytochemical blends slow the evolution of resistance? In nature, pollinators encounter phytochemicals in blends rather than single compounds. In agricultural and clinical studies, evolution of resistance is delayed when parasites are simultaneously exposed to multiple chemicals, as would occur when bees forage in diverse landscapes. This project will test whether phytochemical resistance in C. bombi is retarded when the parasite is exposed to phytochemical blends rather than single compounds. Results will generate hypotheses about how plant interactions with animal pathogens influence pollinator health; and suggest strategies for using phytochemicals to mitigate pollinator decline.