How hummingbirds evolved a sweet tooth decoded

Hummingbirds have evolved a sweet tooth that sets them apart from other birds and has made them highly specialised nectar feeders, Harvard scientists say.

Scientists analysed the genetic codes of 10 bird species and found that only hummingbirds had a genetic adaptation that meant they were drawn to sweetness.

Hummingbirds' ability to detect sweetness evolved from an ancestral savoury taste receptor that is mostly tuned to flavors in amino acids.

"It's a really nice example of how a species evolved at a molecular level to adopt a very complex phenotype," said Stephen Liberles, Harvard Medical School associate professor of cell biology.

"A change in a single receptor can actually drive a change in behaviour and, we propose, can contribute to species diversification," Liberles said.

After cloning the genes for taste receptors from chickens, swifts and hummingbirds - a three-year process - Maude Baldwin, co-first author of the research paper, needed to test what the proteins expressed by these genes were responding to.

She joined forces with Yasuka Toda, a graduate student of the University of Tokyo and co-first author of the paper, who had devised a method for testing taste receptors in cell culture.

Together they showed that in chickens and swifts the receptor responds strongly to amino acids - the umami flavors - but in hummingbirds only weakly.

But the receptor in hummingbirds responds strongly to carbohydrates - the sweet flavors.
"This is the first time that this umami receptor has ever been shown to respond to carbohydrates," Baldwin said.

Toda mixed and matched different subunits of the chicken and hummingbird taste receptors into hybrid chimeras to understand which parts of the gene were involved in this change in function.
She found 19 mutations, but there are likely more contributing to this sweet switch, Baldwin and Liberles suspect.

"If you look at the structure of the receptor, it involved really dramatic changes over its entire surface to accomplish this complex feat," Liberles said.

"Amino acids and sugars look very different structurally so in order to recognise them and sense them in the environment, you need a completely different lock and key. The key looks very different, so you have to change the lock almost entirely," said Liberles.

The research was published in the journal Science.