<p>Hummingbirds have evolved a sweet tooth that sets them apart from other birds and has made them highly specialised nectar feeders, Harvard scientists say.<br /><br />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.<br /><br />Hummingbirds' ability to detect sweetness evolved from an ancestral savoury taste receptor that is mostly tuned to flavors in amino acids.</p>.<p><br />"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.</p>.<p><br />"A change in a single receptor can actually drive a change in behaviour and, we propose, can contribute to species diversification," Liberles said.</p>.<p><br />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.</p>.<p><br />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.</p>.<p><br />Together they showed that in chickens and swifts the receptor responds strongly to amino acids - the umami flavors - but in hummingbirds only weakly.</p>.<p><br />But the receptor in hummingbirds responds strongly to carbohydrates - the sweet flavors.<br />"This is the first time that this umami receptor has ever been shown to respond to carbohydrates," Baldwin said.</p>.<p><br />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.<br />She found 19 mutations, but there are likely more contributing to this sweet switch, Baldwin and Liberles suspect.</p>.<p><br />"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.</p>.<p><br />"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.<br /><br />The research was published in the journal Science.</p>
<p>Hummingbirds have evolved a sweet tooth that sets them apart from other birds and has made them highly specialised nectar feeders, Harvard scientists say.<br /><br />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.<br /><br />Hummingbirds' ability to detect sweetness evolved from an ancestral savoury taste receptor that is mostly tuned to flavors in amino acids.</p>.<p><br />"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.</p>.<p><br />"A change in a single receptor can actually drive a change in behaviour and, we propose, can contribute to species diversification," Liberles said.</p>.<p><br />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.</p>.<p><br />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.</p>.<p><br />Together they showed that in chickens and swifts the receptor responds strongly to amino acids - the umami flavors - but in hummingbirds only weakly.</p>.<p><br />But the receptor in hummingbirds responds strongly to carbohydrates - the sweet flavors.<br />"This is the first time that this umami receptor has ever been shown to respond to carbohydrates," Baldwin said.</p>.<p><br />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.<br />She found 19 mutations, but there are likely more contributing to this sweet switch, Baldwin and Liberles suspect.</p>.<p><br />"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.</p>.<p><br />"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.<br /><br />The research was published in the journal Science.</p>