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Sweet Receptors In Hummingbirds

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In the paper “Evolution of sweet receptors in hummingbirds by transformation of the ancestral umami receptor,” Baldwin et al. proposed the evolution of the sensory receptor for sweet taste perception in captive and wild hummingbirds has survived in new environments over time. They proposed that the vertebrate sweet receptor subunit, T1R2, was missing in birds making them wonder how these hummingbirds recognized sugars considering their specialized nectar-based diets. Their studies revealed that an ancestral sweet and savory “umami” receptor, called T1R1-T1R3 heterodimer, taste function changed in hummingbirds to operate as a carbohydrate receptor and thus recognize and use nectar. This transformation was the catalyst for the expansion of the…show more content…
The sweet and savory “umami” taste receptor (the T1RI-TIR3 heterodimer) and the sweet taste receptor (the T1R2-TIR3 heterodimer) are sensed by g protein-coupled receptors called T1Rs. In certain vertebrates losses in taste receptors and perception are observed. The researchers set out to understand how hummingbirds can detect sugars since their nectar diets contain lots of sugars and their mechanisms of sugar perception began largely unknown. They started by identifying T1Rs for 10 different birds with different diets, and also cloned T1Rs from Anna’s hummingbird, chicken, and swifts’ (they are the hummingbird’s closest relative) oral tissue. T1R1 and T1R3 were successfully detected birds’ genome, except for the T1R2. Only non-avian species kept that receptor, which means that a different T1R2-indepenednt mechanism for sugar perception developed in avian species. To find these sweet receptors, they analyzed responses of bird taste receptors to sugars (carbohydrates), and amino acids. Hummingbirds with both the T1R1-T1R3 detected carbohydrates, sucralose, and sugar alcohols, but only when both were used in taste recognition. Weaker responses were observed with amino acids. The chicken or swift T1R1-T1R3 heterodimer failed to detect carbohydrates, but did detect amino acids.…show more content…
Chimera 1 introduced the flytrap of chicken T1R3 into hummingbird T1R3. The receptor became sensitive to amino acids. Chimera 2 reintroduced 109 amino acids of the hummingbird T1R3 into chicken T1R3. The receptor was able to regain its sugar perception. The hummingbird sugar detection was found in T1R1 since a mutation of a combination of it with the chicken T1R1 prefers amino acids, and from an identified 19 amino acids that were sugar sensitive, two amino acid sites displayed evidence of positive recognition. This means that the mutation of both of the receptor subunits is implicated in the evolution of sugar (carbohydrate) detection by the hummingbird
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