IMAGINE A WORLD without bird song. Yet this might have come about if it had not been for a genetic change that happened some 30m years ago, at the beginning of the evolution of the Passeri, to give songbirds their proper name.
Birds evolved from carnivorous dinosaurs called theropods. Meat eaters need not detect sugar in the way that, say, fruit eaters do, and genetic analyses of modern birds suggest their theropod ancestor had lost the ability to taste sweetness. Today, however, many birds have sugar-rich diets of nectar or fruit, so perceiving things as sweet is a useful attribute. And research just published in Science by Toda Yasuka of Tokyo University and Maude Baldwin of the Max Planck Institute for Ornithology in Seewiesen, Germany, suggests songbirds can indeed perceive sweetness. This re-evolved ability may have been instrumental in their success. Since almost half the bird species now alive are Passeri, that is no small matter.
Vertebrates’ taste-receptor genes normally include three that encode proteins called T1R1, T1R2 and T1R3. The taste receptors themselves are formed from pairs of these proteins. Receptors for sweetness are a combination of T1R2 and T1R3. Birds, however, lack the gene for T1R2. Presumably, it was lost by their theropod ancestors, which did not need it. Dr Toda’s and Dr Baldwin’s experiments have shown how this loss was reversed.
The pair’s first study, published in 2014, was on hummingbirds, which feed on nectar from flowers. It found that hummingbirds regained the ability to taste sugars via mutations in the genes for T1R1 and T1R3. The receptor formed by combining T1R1 and T1R3 normally detects umami, a savoury flavour typical of meat. In hummingbirds, these mutations allow this receptor to detect sugars, too. Dr Toda and Dr Baldwin therefore wondered whether that was also the case for songbirds.
To find out, they cloned T1R1–T1R3 receptors from a variety of songbirds and tested their responses to sugar. All the receptors they tested—from birds with sugar-rich and sugar-poor diets alike—interacted strongly with sugar molecules. This confirmed that, as with hummingbirds, songbirds regained perception of sweetness via mutations of the gene for T1R1 and T1R3. By contrast, umami receptors cloned from the Tyranni, a sister group to the Passeri, did not interact with sugars, though they did so strongly with amino acids typical of meat. The mutations in the songbird lineage must thus have happened after the Passeri and Tyranni lines diverged, but before the Passeri themselves began proliferating into their current variety.
Intriguingly, when Dr Toda and Dr Baldwin looked at the molecular modifications which allowed the T1R1–T1R3 receptors of hummingbirds and Passeri to detect sweetness, they found them to be completely different. Both, though, involved numerous changes to the underlying DNA, suggesting a strong evolutionary pressure to optimise them. This pressure was probably a consequence of competition to fill the new ecological niches opened up by an ability to recognise sweet things as both edible and nutritious. And it was that which resulted in the Passeri’s current diversity. How all this ties up with the mellifluous songs sung by many members of the group is unclear. It may just be a coincidence. But if so, for those who enjoy bird song, it is a fortunate one. ■
This article appeared in the Science & technology section of the print edition under the headline “The sweet taste of success”