Sensory biology. Evolution of sweet taste perception in hummingbirds by transformation of the ancestral umami receptor.
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Hummingbirds lack TAS1R2 (as do birds generally); the ancestral TAS1R1-TAS1R3 umami heterodimer was instead repurposed in the hummingbird lineage to detect carbohydrates rather than amino acids.
"We observed the widespread absence in birds of an essential subunit (T1R2) of the 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 a carbohydrate receptor."
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Neither TAS1R1 nor TAS1R3 alone produced a ligand response in heterologous cells; only the co-expressed heterodimer responded, establishing TAS1R3 as an obligate dimerization partner rather than a stand-alone receptor.
"Responses were not observed when T1R1 or T1R3 alone was used, suggesting that hummingbird T1R1-T1R3 functions as an obligate heterodimer."
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Chimera and mutagenesis experiments localized most of the sugar-response-conferring substitutions to the TAS1R3 Venus-flytrap (ligand-binding) domain, with additional contributing substitutions in TAS1R1.
"Reintroducing 109 amino acids (residues 158 to 266) of hummingbird T1R3 into the chicken T1R3 venus flytrap domain restored sucrose responses (chimera 2)."
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Hummingbird T1R1-T1R3 receptor pharmacology (responsiveness to sucrose/fructose/glucose/sucralose, non-responsiveness to human sweeteners such as aspartame/cyclamate/acesulfame-K) corresponded with behavioral sugar preference in captive and wild hummingbirds.
"We conclude that the molecular recognition properties of hummingbird T1R1-T1R3, together with those of other gustatory receptors, instruct taste behavior in both captive and wild hummingbirds."
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This study characterized T1Rs cloned from Anna's hummingbird (Calypte anna) and behavior in ruby-throated hummingbird (Archilochus colubris), not Oreotrochilus melanogaster; no data specific to O. melanogaster are presented.
"we cloned T1Rs from the 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)"