IMAGIC A WORLD without bird song. Still, this could have happened if it hadn’t been for a genetic change that occurred around 30 million years ago, at the start of Passeri evolution, to give songbirds their name. clean.
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Birds evolved from carnivorous dinosaurs called theropods. Meat eaters don’t need to detect sugar like, say, fruit eaters do, and genetic analyzes of modern birds suggest that their theropod ancestor had lost the ability to taste sweetness. Today, however, many birds have a high-sugar diet of nectar or fruit, so perceiving things as sweet is a useful attribute. And the research has just appeared in Science by Toda Yasuka of the University of Tokyo and Maude Baldwin of the Max Planck Institute of Ornithology in Seewiesen, Germany, suggest that songbirds can indeed perceive sweetness. This renewed ability may have played a role in their success. Considering that almost half of the bird species currently alive are Passeri, this is no small feat.
Vertebrate taste receptor genes normally include three that code for proteins called T1R1, T1R2 and T1R3. The taste receptors themselves are made up of pairs of these proteins. Sweetness receptors are a combination of T1R2 and T1R3. Birds, however, lack the gene for T1R2. Presumably, it was lost by their theropod ancestors, who did not need it. The experiments of Dr Toda and Dr Baldwin showed how this loss was reversed.
The couple’s first study, published in 2014, looked at hummingbirds, which feed on flower nectar. He found that hummingbirds regained the ability to taste sugars via mutations in the genes of T1R1 and T1R3. The receptor formed by combining T1R1 and T1R3 normally detects umami, a tasty flavor typical of meat. In hummingbirds, these mutations also allow this receptor to detect sugars. So Dr Toda and Dr Baldwin wondered if this was also the case with songbirds.
To find out, they cloned T1R1–T1R3 receptors from a variety of songbirds and tested their responses to sugar. All of the receptors they tested, from birds that had a diet high or low in sugar, interacted strongly with the sugar molecules. This confirmed that, as with hummingbirds, songbirds regained the perception of sweetness via mutations in the T1R1 and T1R3. In contrast, the cloned umami receptors of the Tyranni, a sister group of the Passeri, did not interact with sugars, although they did so strongly with amino acids typical of meat. Mutations in the songbird line must therefore have occurred after the divergence of the Passeri and Tyranni lines, but before the Passeri themselves began to proliferate in their current variety.
Strangely enough, when Dr Toda and Dr Baldwin looked at the molecular changes that allowed the T1R1–T1R3 hummingbird and Passeri receptors to detect sweetness, they found them to be completely different. Both, however, involved many changes in the DNA, suggesting a strong evolutionary pressure to optimize them. This pressure was probably a consequence of the competition to fill new ecological niches opened up by an ability to recognize sweet things as both edible and nutritious. And this is what made the current diversity of Passeri. How it all relates to the melodious songs sung by many of the band members is unclear. It may just be a coincidence. But if so, for those who love birdsong, this is luck. ■
This article appeared in the Science & Technology section of the print edition under the title “The Sweet Taste of Success”