DISSERTATION RESEARCH: Linking functional evolution of odorant receptors to behavioral adaptations and the emergence of herbivory in drosophilids

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This research investigates the role of sensory adaptation of smell in closely related species of flies in transitions from feeding on microbes to feeding on plants. Plant-feeding are the most diverse group of animals, including a quarter of all named species. Yet little is known about how insects evolve to feed on plants from other types of diets, or how insects evolve to target specific kinds of plants. This project will study how the smell receptors and underlying genetics of these flies have changed over evolutionary time as their diet switches from yeast to plants. It will determine the changes in smell receptors that allow the switch to new diets and how the underlying genetics shifts to target new host plants. This increased understanding of the evolution of the sense of smell will inform our general understanding of changes in ecological interactions. This research may furthermore provide insights that allow manipulation of insect food choice. It therefore has the potential to broadly impact society through applications to agriculture, where insects are major crop pests, and human health since insects frequently spread disease.This study will investigate the evolution of olfaction (sense of smell) after a niche change by comparing the olfactory systems of Scaptomyza flava and Scaptomyza pallida. These two fly species are closely related but differ in diet. S. flava is a true herbivore feeding on mustard plants, while S. pallida retains the more ancestral diet of microbes. Gas chromatography (GC) -electroantennographic detection (EAD) will be used to differentiate which volatile compounds (smells) produced by leaves of host species the flies can detect. This technique divides a stream of leaf volatiles into two identical streams. One stream is sent into a GC detector for identification by essentially artificially smelling the compound. The other stream is aimed at a fly where EAD will determine if the compound is detectable naturally by determining if the compound triggers a response in single nerve cell. Leaf volatiles that are detected by herbivorous flies will thereby be identified and compared to those detected by flies that feed on microbes. The volatiles identified by GC-EAD as differentiating the two species will then be used as baits in olfactory assays where four calibrated airstreams containing different volatiles are fed into the corners of a square arena. The attraction to each plant volatile will then measured to determine if the species behavior matches their detection profile. Finally, genes that appear to be important in differentiating these two species will be engineered into a model organism Drosophila melanogaster to test if the genes effect on food choice is fully understood.