Evolution of sweet taste perception inhummingbirds by transformation of the ancestralumami receptor
Citation
Baldwin, M. W., Y. Toda, T. Nakagita, M. J. O’Connell, K. C. Klasing, T. Misaka, S. V. Edwards, andS. D. Liberles. 2014. “Evolution of Sweet Taste Perception in Hummingbirds by Transformationof the Ancestral Umami Receptor.” Science 345 (6199) (August 21): 929–933. doi:10.1126/
science.1255097.

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doi:10.1126/science.1255097

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Published in final edited form as:
Science. 2014 August 22; 345(6199): 929–933. doi:10.1126/science.1255097.

Evolution of sweet taste perception in hummingbirds bytransformation of the ancestral umami receptor
Maude W. Baldwin1,†,*, Yasuka Toda2,*, Tomoya Nakagita2, Mary J. O'Connell3, Kirk C.
Klasing4, Takumi Misaka2, Scott V. Edwards1, and Stephen D. Liberles5,†
1Department of Organismic and Evolutionary Biology, Harvard University, and Museum of

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Comparative Zoology, Cambridge, MA 02138, USA 2Department of Applied Biological Chemistry,
Graduate School of Agricultural and Life Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku,
Tokyo, Japan 3Bioinformatics and Molecular Evolution Group, School of Biotechnology, Dublin
City University, Glasnevin, Dublin 9, Ireland 4Department of Animal Science, University ofCalifornia, Davis, Davis, CA 95616, USA 5Department of Cell Biology, Harvard Medical School,
Boston, MA 02115, USA

Abstract
Sensory systems define an animal's capacity for perception and can evolve to promote survival in
new environmental niches. We have uncovered a noncanonical mechanism for sweet taste
perception that evolved in hummingbirds since their divergence from insectivorous swifts, theirclosest relatives. We observed the widespread absence in birds of an essential subunit (T1R2) ofthe only known vertebrate sweet receptor, raising questions about how specialized nectar feeders
such as hummingbirds sense sugars. Receptor expression studies revealed that the ancestral umami
receptor (the T1R1-T1R3 heterodimer) was repurposed in hummingbirds to function as acarbohydrate receptor. Furthermore, the molecular recognition properties of T1R1-T1R3 guided
taste behavior in captive and wild hummingbirds. We propose that changing taste receptor
function enabled hummingbirds to perceive and use nectar, facilitating the massive radiation ofhummingbird species.

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Sensory systems display remarkable flexibility across vertebrates, with some animals losing
sensory modalities that are no longer key for survival (1, 2) and others evolving new
adaptive sensory capabilities (3). The repertoires of sensory receptors for odors,
pheromones, and tastes reflect species-specific ecology, with receptor families rapidly

Copyright 2014 by the American Association for the Advancement of Science; all rights reserved.
†

Corresponding author. maudebaldwin@gmail.com (M.W.B.); stephen_liberles@hms.harvard.edu (S.D.L.).
*These authors contributed equally to this work.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/345/6199/929/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S6Tables S1 to S4
References (26–44)
Movies S1 and S2

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expanding and contracting (4), and in some lineages, new receptor families evolving (5). In
the olfactory system, functional expansion of the receptor repertoire predominantly involves
a pattern of gene duplication and mutation, leading to novel receptors with altered ligand
recognition properties (6). This pattern of gene duplication and mutation is also observed invomeronasal receptors and bitter taste receptors, but not in sweet and savory taste receptors(4, 7). Receptors for these palatable tastes are unique among the chemosensory receptor
families in that they are highly conserved in number and amino acid identity. New vertebrate
sweet receptors, and the evolutionary mechanisms that underlie their acquisition, have notpreviously been identified.

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In vertebrates, sweet and savory (“umami”) tastes are sensed by G protein–coupled receptors(GPCRs) termed T1Rs (8). Most vertebrates have three T1Rs, with the T1R1-T1R3
heterodimer mediating umami taste and the T1R2-T1R3 heterodimer mediating sweet taste(8, 9). Human T1R2-T1R3 detects carbohydrates and artificial sweeteners (10), andknockout mice lacking T1R2 or T1R3 have defective sweet taste perception (9, 11). In
genomes analyzed so far, T1R expansions are observed only in some fish species (12),whereas losses are observed in other vertebrates, often in accordance with diet. Someobligate carnivores, such as cats, lost T1R2 and appetitive behaviors toward carbohydrates(1), whereas the giant panda, which feeds predominantly on bamboo, lost T1R1 (2).
Chickens, turkeys, and zebra finches also do not have T1R2 (13), but the relationshipbetween T1R repertoire and avian ecology is unclear. Birds display tremendous
heterogeneity in diet, with different lineages primarily consuming fruits, nectars, animals,
and seeds. Hummingbirds are specialized nectar feeders, and their ability to perceive and use
sugar-rich resources allowed them to colonize a nectarivorous niche, enabling theirextensive diversification (14). However, how hummingbirds detect sugars remains
unknown, so we characterized the repertoires and functions of bird taste receptors tounderstand the underlying mechanisms of sugar perception.

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We identified T1Rs in whole-genome sequences available for 10 birds with different dietsand compared them to T1Rs from other vertebrates (Fig. 1A). Also, we cloned T1Rs fromthe oral tissue of Anna's hummingbirds (Calypte anna); the domestic chicken (Gallus
gallus), which does not prefer sugars (7); and the insectivorous chimney swift (Chaetura
pelagica), because swifts are the closest living relatives of hummingbirds (Fig. 1B and fig.
S2). Expression in oral tissue was verified by reverse transcription polymerase chain
reaction (fig. S2). Two T1R genes—T1R1 and T1R3—were detected in each available bird
genome, and candidate signatures of positive selection were identified in the hummingbird
lineage (Fig. 1B and table S2), but not the chicken or swift lineages. We failed to detect
T1R2 in bird genomes, despite the presence of flanking loci. Non-avian reptiles retained
T1R2, including the Chinese alligator (Alligator sinensis), a member of the sister group to
birds (fig. S1), suggesting that the loss of T1R2 occurred within Dinosauria. These findings
suggest that an alternative T1R2-independent mechanism for sugar detection arose in avian
species that display high behavioral affinity for nectar or sweet fruit.
To identify avian sweet receptors, we analyzed responses of bird taste receptors to sugars
and amino acids (Fig. 2). Responses of bird T1Rs were measured in heterologous cells bymeans of calcium-sensitive photoprotein reporters (15). Hummingbird T1R1-T1R3
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responded to several carbohydrates, including sucrose, fructose, and glucose. Responses
were not observed when T1R1 or T1R3 alone was used, suggesting that hummingbird
T1R1-T1R3 functions as an obligate heterodimer. Hummingbird T1R1-T1R3 also detectedsucralose and various sugar alcohols, including sorbitol and erythritol, but not cyclamate,acesulfame K, and aspartame, which are sweet to humans (16). Low-affinity responses wereobserved to some amino acids, as with the human sweet receptor, which recognizes
carbohydrates as well as proteins, dipeptides, and amino acids (8). In contrast, cells
expressing chicken or swift T1R1-T1R3 failed to detect carbohydrates at any concentration
tested and instead recognized alanine and serine. Thus, T1R1-T1R3 heterodimers fromswifts, chickens, primates (humans, squirrel monkeys, baboons, and macaques), rodents(mouse and rat), and teleost fish (zebrafish and medaka) detect palatable amino acids (8, 17,18). In contrast, in the hummingbird lineage, this receptor complex acquired a new functionin the past 42 to 72 million years (14, 19), evolving the capacity for carbohydrate
recognition.

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Next, we sought to understand the critical changes in hummingbird T1R1-T1R3 that enabledsugar detection. We designed protein chimeras involving portions of hummingbird T1R1T1R3, which responds to sugars, and chicken T1R1-T1R3, which does not (Fig. 3 and fig.
S3). We focused on the venus flytrap domain, an extracellular region of family C GPCRs
that mediates ligand binding (20). Introducing the venus flytrap domain of chicken T1R3
into hummingbird T1R3 (chimera 1) rendered the heterodimeric receptor sensitive to aminoacids rather than sugars. Reintroducing 109 amino acids (residues 158 to 266) of
hummingbird T1R3 into the chicken T1R3 venus flytrap domain restored sucrose responses(chimera 2). Further analysis of this 109–amino acid region identified 19 nonconsecutive
amino acids (chimera 3; sites: fig. S3), which were collectively sufficient to impart sucroseand sucralose sensitivity (fig. S4). Subsets of these 19 residues did not similarly support
sugar binding (fig. S3). Two identified sites (I206 and S237) displayed evidence of positive
selection (Fig. 1B). Hummingbird T1R1 also contains sites that are under putative positive
selection and mutations that contribute to acquired sugar responsiveness, because a mixed
receptor pair of chicken T1R1 and hummingbird T1R3 prefers amino acids (fig. S5). Thus,
the evolution of carbohydrate detection by hummingbird T1R1-T1R3 involved widespreadmutation of both receptor subunits.

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We created a homology model of the T1R3 venus flytrap domain based on the x-ray crystal
structure of the same region in a related GPCR, metabotropic glutamate receptor 1(mGluR1) (20) (Fig. 3D). This model predicted that the 19 sugar response–conferring
substitutions in T1R3 were clustered in three distinct regions of the protein. Three residues(G165, I167, and N211) were in the putative orthosteric ligand-binding site, which in
mGluR1 is located at the interface between the two lobes of the venus flytrap domain on theextracellular surface (20). G165 and I167 align near S186 and T188 of mGluR1, which form
salt bridges to the glutamate ligand (20), and all three align near T1R1 residues that areimportant for ligand responses (18, 21). The remaining residues clustered in two otherlocations, whose functions in family C GPCRs are unknown but may be important forfolding topology, interdomain or intersubunit interactions, or G protein activation. The
dramatic redecoration of the T1R1-T1R3 protein surface that occurred in hummingbirds toScience. Author manuscript; available in PMC 2015 February 22.

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allow for sugar binding makes sense, given that carbohydrates and amino acids adopt
completely different structures.

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We next asked whether T1R1-T1R3 function would dictate hummingbird taste behavior. We
reasoned that nonnutritive agonists of T1R1-T1R3 without caloric value would be palatableto hummingbirds, like artificial sweeteners are to humans. Hummingbirds prefer sugars (22),but behavioral responses to many other human sweeteners are unknown. We developed abrief-access, two-choice gustatory preference paradigm in captive ruby-throatedhummingbirds (Archilochus colubris) to measure taste responses to T1R1-T1R3 ligands(Fig. 4A). As expected, hummingbirds displayed strong behavioral affinity for sucrose overwater, as measured by an increase in mean drinking bout length, number of long bouts (>1 sof uninterrupted drinking), and overall time spent drinking. High-speed video recordings(movie S1) indicated extremely rapid choice decisions; water trials terminated within threeor four tongue licks (~250 ms), suggesting that sugar preference involves rapid processingof taste information rather than post-ingestive effects. Ruby-throated hummingbirds equallyconsumed solutions of sucrose and erythritol, a nonnutritive agonist of hummingbird T1R1T1R3, but displayed a strong preference for sucrose over aspartame, a sweetener to humans
that failed to activate hummingbird T1R1-T1R3.

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We also developed a behavioral assay involving Anna's hummingbirds because we cloned
T1R1 and T1R3 from this species (Fig. 4B and movie S2). Experiments were performed in
the Santa Monica Mountains at a field site frequented by wild hummingbirds. We recorded
the behavior of birds presented for 15 min with test stimuli, and in control experiments,
hummingbirds displayed strong preference for sucrose over water and high behavioral
affinity for several sugars abundant in nectar, including sucrose, glucose, and fructose. Next,
we presented solutions of sucrose and test stimuli, and measured the mean drinking boutlength. Anna's hummingbirds displayed a strong behavioral attraction to the T1R1-T1R3
agonists erythritol and sorbitol, with responses similar to those to sucrose. In contrast,
Anna's hummingbirds displayed a strong preference for sucrose over other structurally
diverse human sweeteners that failed to activate hummingbird T1R1-T1R3, including
aspartame, cyclamate, and acesulfame K. Furthermore, these synthetic human sweeteners
were aversive at high concentrations, because birds rejected mixed solutions containing
these chemicals and sucrose (fig. S6 and movie S2) and often displayed a characteristic
behavioral pattern involving beak withdrawal, head shaking, and/or spitting that waspreviously observed in response to the ingestion of bitter plant metabolites (23). This
reaction was also observed toward sucralose solutions, and mixtures of sucrose andsucralose were not consumed (fig. S6), indicating that sucralose is also actively rejected.
Other species of hummingbirds (black-chinned and Allen's hummingbirds) visited anddisplayed similar taste preferences (fig. S6). Together, these behavioral experiments showthat several agonists of hummingbird T1R1-T1R3, including simple sugars and sugar
alcohols, evoke fast, appetitive gustatory responses in hummingbirds. Other synthetic humansweeteners that do not activate hummingbird T1R1-T1R3 are not similarly attractive and areoften actively rejected. We conclude that the molecular recognition properties ofhummingbird T1R1-T1R3, together with those of other gustatory receptors, instruct taste
behavior in both captive and wild hummingbirds. We studied the repertoire and function of

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taste receptors to provide a molecular basis for variations in animal ecology and theevolutionary events that cause them. We identified a transformation of taste receptor
function that occurred in hummingbirds after their divergence from an insectivorous
ancestor. We propose this to be a key evolutionary adaptation that contributed to theacquisition of nectar-feeding behavior and enabled the extensive radiation of hummingbird
species. The molecular basis for this change in taste behavior is an altered ligand-binding
preference of T1R1-T1R3 from amino acids to carbohydrates, a complex feat that involved
dramatic structural changes in the receptor surface. It has been proposed that the ancestral
T1R heterodimer, as well as the ancestral family C GPCR, were amino acid receptors (17,24). The mammalian sweet receptor probably derived from a similar transformation that
occurred earlier in vertebrates. Birds descended from carnivorous theropod dinosaurs (25),and like mammalian carnivores, it appears that an ancestor of birds lost T1R2, perhaps
another example of the close relationship between diet and taste receptor repertoire. Basedon evidence presented here, hummingbirds recently evolved a new sugar receptor andconsequently regained sweet taste perception.

Supplementary Material
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Refer to Web version on PubMed Central for supplementary material.

ACKNOWLEDGMENTS
We thank those who supported these experiments (see the supplementary materials for the full list). Grants to
M.W.B. (NSF grant DDIG 1110487, Society for Integrative and Comparative Biology, and Sigma Xi); M.J.O'C.
(Fulbright Commission and Science Foundation Ireland Research Frontiers Program, grant EOB2673); S.D.L.
(NIH, grant RO1DC013289); S.V.E. (NSF); and T.M. (Japan Society for the Promotion of Science, grant LS037)
are acknowledged. T1R sequences and alignments are available at GenBank and Dryad.

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Fig. 1. Analysis of T1R sequences in birds(A) A maximum-likelihood tree was constructed using T1R sequences from 13 birds and theChinese alligator (∇ = nodal bootstrap <80%; scale bar, 0.4 substitutions per site). (B)Amino acid sequences of T1R3 cloned from birds. Gray, transmembrane domains; red,putatively selected sites (table S3).

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Fig. 2. Evolution of a sugar receptor in hummingbirds(A) Functional expression of avian and rodent taste receptors to stimuli [100 mM, except
aspartame (15 mM); n = 6 independent experiments, mean ± SE, *P ≤ 0.05]. (B) Sugar
responses of hummingbird T1Rs alone or in combination (n = 6 independent experiments,
mean ± SE, *P ≤ 0.05). (C) Dose-dependent responses of T1R1-T1R3 from species
indicated to amino acids (blue) and sugars (red).

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Fig. 3. Molecular basis for the acquisition of sugar binding in hummingbird T1R1-T1R3

NIH-PA Author Manuscript(A) T1R3 chimeras containing chicken (black) and hummingbird (red) amino acids were
designed (CRD, cysteine-rich domain; TM, transmembrane domains). (B) Responses of
T1R3 chimeras and hummingbird T1R1 to L-alanine, sucralose, and sucrose (100 mM). (C)Dose-dependent responses of T1R3 chimeras and hummingbird T1R1 to sucrose. (D) A
homology model of the venus flytrap domain of T1R3 shows the putative ligand binding site(yellow), predicted by alignment with ligand-contacting sites of rat mGluR1 (20), andmutations that confer sugar binding, which cluster in three distinct locations (red, green, andblue).

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Fig. 4. T1R1-T1R3 agonists evoke taste responses in captive and wild hummingbirds(A) Captive ruby-throated hummingbirds (n = 3 or 4, mean ± SE) were presented withsolutions of test stimuli and sucrose (333 mM), and the drinking bout lengths, time spent
drinking, and number of long bouts (>1 s) were recorded (linear mixed-effect models fordifferences between stimuli and sucrose, ***P ≤ 0.001). Red bars indicate palatability
similar to that of carbohydrates. (B) The taste preferences of wild Anna's hummingbirds
were measured (mean bout lengths ± SE, sample sizes: table S4, Kolmogorov-Smirnov tests
for differences between stimuli and sucrose: *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001).

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Concentrations: white, 500 mM; gray, 1 M; black, indicated. Red bars indicate equal
preference. [Photo credits: (A) M.W.B. and F. Peaudecerf, (B) M.W.B.]

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