Drosophila sechellia is a dietary specialist endemic to the Seychelles islands that has evolved to consume the fruit of Morinda citrifolia. When ripe, the fruit of M. citrifolia contains octanoic acid and hexanoic acid, two medium-chain fatty acid volatiles that deter and are toxic to generalist insects. Drosophila sechellia has evolved resistance to these volatiles allowing it to feed almost exclusively on this host plant. The genetic basis of octanoic acid resistance has been the focus of multiple recent studies, but the mechanisms that govern hexanoic acid resistance in D. sechellia remain unknown. To understand how D. sechellia has evolved to specialize on M. citrifolia fruit and avoid the toxic effects of hexanoic acid, we exposed adult D. sechellia, D. melanogaster and D. simulans to hexanoic acid and performed RNA sequencing comparing their transcriptional responses to identify D. sechellia specific responses. Our analysis identified many more genes responding transcriptionally to hexanoic acid in the susceptible generalist species than in the specialist D. sechellia. Interrogation of the sets of differentially expressed genes showed that generalists regulated the expression of many genes involved in metabolism and detoxification whereas the specialist primarily downregulated genes involved in the innate immunity. Using these data, we have identified interesting candidate genes that may be critically important in aspects of adaptation to their food source that contains high concentrations of HA. Understanding how gene expression evolves during dietary specialization is crucial for our understanding of how ecological communities are built and how evolution shapes trophic interactions.

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Thesis submitted to the Faculty of Wesleyan University in partial fulfillment for the degree of Master of Arts in Molecular Biology and Biochemistry Middletown, ConnecticutMay 2019I would like to thank my committee members Professor Amy MacQueen and Professor Scott Holmes for taking the time to reading and critiquing this thesis. I would also like to thank any, and all, Wesleyan faculty that has helped me throughout my time here at Wesleyan.To present and previous lab mates, thank you for helping me along this journey. I have had the privilege to learn from, and work with, Joyce Noble. I will always be grateful for Joyce's patience whenever I had thousands of questions but am most indebted to her for her compassion and empathy whenever lab work felt overwhelming. I would also like to thank Rutesh Vyas, Diane Meredith, and James Farber. They all took it upon themselves to teach me as much as they could.I also need to thank the friends that have become my "East Coast" family and have made my time at Wesleyan an unforgettable experience. Thank you to Rachel