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dc.contributor.advisorEdwards, Scott V.en_US
dc.contributor.authorBaldwin, Maude Wheeleren_US
dc.date.accessioned2015-07-17T16:53:03Z
dash.embargo.terms2017-05-01en_US
dc.date.created2015-05en_US
dc.date.issued2015-05-17en_US
dc.date.submitted2015en_US
dc.identifier.citationBaldwin, Maude Wheeler. 2015. Evolution of sweet taste perception in hummingbirds. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.en_US
dc.identifier.urihttp://nrs.harvard.edu/urn-3:HUL.InstRepos:17467228
dc.description.abstractMammals have three members of the small taste receptor gene family responsible for the perception of sweet and savory tastes: two genes (T1R2 and T1R3) comprise the canonical sweet receptor, and a third gene, T1R1, acts with T1R3 to make the savory receptor. Here, in a joint effort with a team of international collaborators, we show that even though birds are missing the taste receptor gene (T1R2) required by other vertebrates to perceive carbohydrates and sweeteners, hummingbirds still detect sugars—but in a novel way. This project spanned multiple fields and field sites, integrating taste tests on wild birds, behavioral analysis of captive animals, bioinformatics, receptor cloning, and cell-based functional assays. The first published avian genome, that of the chicken, revealed a surprising lack of T1R2. Chickens are sweet-insensitive: however, many nectar-feeding birds appear highly attuned to sugars like sucrose, fructose and glucose. Our initial field experiments with a panel of artificial sweeteners as well as high-speed filming and choice tests on captive birds indicated a rapid response to sugars rather than a post-ingestive metabolic sensing of caloric value. As the response appeared sensory, we pursued a candidate gene approach to search for possible taste receptors, and cloned T1R taste receptors from chickens, hummingbirds, and swifts. By analyzing genomes from an additional 10 birds and an alligator, we documented widespread absence of T1R2 and identified signatures of positive selection in the remaining hummingbird T1Rs. Together with Dr. Yasuka Toda at the University of Tokyo, we were able to test the function of these receptors in cell culture. We used a cell-based luminescence assay to measure functional responses. As expected, chicken and swift receptors responded to amino acids, but, surprisingly, the umami receptor in hummingbirds had acquired a new function and was now sensitive to carbohydrates as well. Chimeric studies of receptors containing hummingbird and chicken sequence identified 19 mutations involved in this functional change: since divergence from swifts, the umami receptor underwent extensive re-modeling. Further behavioral tests with wild hummingbirds revealed that most agonists from the cell-based assay were appetitive, while artificial sweeteners which did not activate the receptors were not preferred—a concordance between in vivo and in vitro studies that indicates that this re-purposed receptor guides hummingbird taste behavior. Diet shifts have profound physiological effects and evolutionary ramifications: the radiation of hummingbirds is likely due, at least in part, to their ability to colonize an empty niche. However, much remains to be learned about the roles of taste in changes in diet, and the causes and effects of shifts in diet and perception are often unclear. For instance, birds appear to have lost T1R2 early in their evolutionary history. As they are the descendants of carnivorous dinosaurs, birds may have experienced relaxed selection on the sweet receptor similar to that seen in mammalian carnivores; alternatively, the loss could be due to the overall genome-wide reduction seen in birds. In Chapter 3, we begin to investigate causes and consequences of the changes in hummingbird taste receptors, and present new behavioral studies regarding the ability to detect amino acids. Together, these findings raise questions about the evolution of sensory systems and of neural circuits underlying perception: studying taste receptors in a comparative context yields insight into basic aspects of the evolutionary process as well as molecular mechanisms underlying behavior.en_US
dc.description.sponsorshipBiology, Organismic and Evolutionaryen_US
dc.format.mimetypeapplication/pdfen_US
dc.language.isoenen_US
dash.licenseLAAen_US
dc.subjectBiology, Generalen_US
dc.subjectBiology, Molecularen_US
dc.subjectBiology, Animal Physiologyen_US
dc.titleEvolution of sweet taste perception in hummingbirdsen_US
dc.typeThesis or Dissertationen_US
dash.depositing.authorBaldwin, Maude Wheeleren_US
dc.date.available2018-01-01T08:30:51Z
thesis.degree.date2015en_US
thesis.degree.grantorGraduate School of Arts & Sciencesen_US
thesis.degree.levelDoctoralen_US
thesis.degree.nameDoctor of Philosophyen_US
dc.contributor.committeeMemberHoekstra, Hopi E.en_US
dc.contributor.committeeMemberPierce, Naomi E.en_US
dc.contributor.committeeMemberDulac, Catherineen_US
dc.contributor.committeeMemberLiberles, Stephenen_US
dc.type.materialtexten_US
thesis.degree.departmentBiology, Organismic and Evolutionaryen_US
dash.identifier.vireohttp://etds.lib.harvard.edu/gsas/admin/view/468en_US
dc.description.keywordshummingbird; taste; evolution; sweet; umamien_US
dash.author.emailmaudebaldwin@gmail.comen_US
dash.identifier.drsurn-3:HUL.DRS.OBJECT:25164391en_US
dash.contributor.affiliatedBaldwin, Maude Wheeler


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