Science & technology
科技板块
The origin of songbirds
鸣禽的起源
The sweet taste of success
成功的甜蜜滋味
Songbirds can detect sugar. That may explain their ubiquity
鸣禽可以探测到糖，这或许可以解释它们无处不在的原因
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.
想象一个没有鸟儿歌唱的世界。然而，如果不是大约3000万年前雀形目进化初期发生的基因变化给了鸣禽真正的名字，可能这个世界就真的没有鸟儿歌唱。
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 reevolved ability may have been instrumental in their success. Since almost half the bird species now alive are Passeri, that is no small matter.
鸟类是从食肉恐龙进化而来的。肉食者不需要像水果食者那样检测糖分，对现代鸟类的基因分析表明，它们的兽脚亚目祖先已经失去了品尝甜味的能力。然而，今天，许多鸟类以富含糖分的花蜜或水果为食，所以“感知事物是甜的”是一个有用的属性。东京大学的户田康嘉和德国许威森的Max Planck鸟类研究所的莫德·鲍尔温刚刚在《科学》杂志上发表了一项研究，表明鸣禽确实能够感知甜味。这种再进化的能力可能对它们的成功起到了重要作用。因为现在几乎一半的鸟类都是雀形目，这不是一件小事。
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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.
脊椎动物的味觉受体基因通常包括三个编码T1R1、T1R2和T1R3的蛋白质。味觉感受器本身就是由这些成对的蛋白质组成的。甜味受体是T1R2和T1R3的组合。然而，鸟类缺乏T1R2基因。据推测，它是兽脚亚目动物的祖先遗失的，它们并不需要它。户田博士和鲍德温博士的实验显示了这种损失是如何逆转的。
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.
他们的第一个研究发表于2014年，研究对象是以花蜜为食的蜂鸟。研究发现，蜂鸟通过T1R1和T1R3基因的突变重新获得了尝糖的能力。由T1R1和T1R3组合而成的受体通常能检测鲜味，即肉的一种典型的咸味。在蜂鸟身上，这些突变也让这种受体能够检测糖。户田博士和鲍德温博士因此想知道鸣禽的情况是否也是如此。
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.
为了找到答案，他们从各种鸣禽中克隆了T1R1-T1R3受体，并测试了它们对糖的反应。他们测试的所有受体——来自高糖和低糖饮食的鸟类——都与糖分子发生了强烈的相互作用。该研究证实，鸣禽和蜂鸟一样，通过T1R1和T1R3基因的突变重新获得了对甜味的感知。相比之下，从霸鹟亚目(雀形目的姐妹群)克隆的鲜味受体不与糖相互作用，虽然他们对肉类中典型的氨基酸有着强烈的反应。因此，鸣禽谱系的突变一定发生在雀形目和霸鹟亚目品种的分化之后，而又在雀形目本身开始繁殖成目前的品种之前。
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.
有趣的是，户田博士和鲍德温博士研究了使蜂鸟和雀形目的T1R1-T1R3受体感知甜味的分子修饰，结果发现它们是完全不同的。然而，这两种方法都涉及到基础DNA的大量变化，这表明有强大的进化压力来优化它们。这种压力很可能是人类为了填补新的生态位而展开竞争的结果，这种生态位是人类识别甜味食物既可食用又有营养的能力所开启的。这也导致了雀形目的多样性。这一切与该群体许多成员所唱的优美歌曲有何联系尚不清楚，可能只是一个巧合。但如果是这样，对于那些喜欢鸟鸣的人来说，这是幸运的。
来源：经济学人
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