Genomics and Sweet Taste in Hummingbirds

Genomic analyses provide critical information about the evolution of sensory systems. A fascinating example is the evolution of sweet taste in hummingbirds, the only vertebrate group know to use a nonstandard receptor for tasting sugars. Box Extension 14.2 describes how hummingbirds are thought to have evolved this uniquely repurposed sensory mechanism.

We noted in the text that vertebrates use dimeric combinations of two G protein–coupled receptors for sweet and umami taste categories (see Figure 14.16). Sweet taste depends on a dimer of T1R2 and T1R3 proteins, whereas umami taste is mediated by a T1R1–T1R3 dimer. In mice, knockouts of either the gene for T1R2 or T1R3 lead to defective taste perception.

Birds appear to have lost the gene for T1R2 during their evolution, although nonavian reptiles have retained it. Birds do not usually eat carbohydrates, and researchers speculate that the gene for sweet taste was lost in their evolution with dinosaurs from nonavian reptiles. (Cats and other meat-eating felids have also lost the gene for T1R2 and lack sweet taste perception.)

Hummingbirds, like other birds, lack the gene for T1R2, but they strongly prefer sugar solutions. What is the basis of this ability, which allows them to fill a nectar-feeding ecological niche? The T1R1–T1R3 dimer, which is the umami receptor in other vertebrates, has evolved in hummingbirds to respond to sugars rather than to the amino acids that normally stimulate umami taste. Broad changes in the extracellular stimulant-binding domains of both receptor proteins underlie this change in receptor activation by sugars rather than amino acids. Genomic analysis shows that this transformation of taste receptor function occurred after divergence of hummingbirds from insect-eating ancestors, and probably was a key adaptation in the evolution of nectar-feeding behavior.

References

Baldwin, M. W., Y. Toda, T. Nakagita, M. J. O’Connell, K. C. Klasing, T. Misaka, S. V. Edwards, and S. D. Liberles. 2014. Evolution of sweet taste perception in hummingbirds by transformation of the ancestral umami receptor. Science 345: 929–933.