雙語閱讀熊貓基因的答案（雙語閱讀熊貓基因）

2023-11-01 06:54:20

CONVERGENT evolution—the arrival, independently, by different species at the same answer to a question posed by nature—is a topic of great interest to biologists. One aspect of the phenomenon which has not yet been much looked at, however, is its underlying genetics. In particular, an issue not previously addressed is how often such changes arise from similar mutations in the two convergent lines, and how often they have different genetic causes that happen to have similar effects on the organisms』 forms and functions. That has now been rectified by an examination of two creatures which, though only distantly related, share an unusual feeding habit, an unusual anatomical feature and an unusual name: panda.

趨同進化——生物學家最感興趣的主題——不同物種對大自然拋出的問題分別給出了相同的答案。然而，關於趨同進化現象中還沒有的深入了解的一面是在基因方面。兩條趨同線中相似基因突變帶來的變化多久會出現一次，這些變化擁有的不同遺傳原因（這些原因對機體的形式和功能效果類似）多久出現一次，這些問題之前都沒有得到解決。如今對兩種生物的檢驗給出了答案。雖然這兩種生物是遠親，但他們都有獨特的飲食習慣、解剖特徵和名字：熊貓

The giant panda is a black and white bear. The red panda (pictured above) is related to weasels, raccoons and skunks. Their habitats—mountainous areas of southern China and its neighbours—overlap, but their last common ancestor lived 43m years ago. They do, though, share a limited kinship, for both are members (along with dogs, cats, hyenas, mongooses, seals and so on) of the mammalian order Carnivora. Which is curious, because both are vegetarian.

大熊貓是一種黑白相間的熊。小熊貓（見以上圖片）是黃鼠狼、浣熊、臭鼬的親戚。它們的棲息地在中國南方山區及鄰國山區，但這兩個物種最後一個共同祖先生活在4300萬年前。它們確實是親屬關係，兩者都是食肉目哺乳動物的一員，這一目還有犬科、貓科、鬣狗、貓鼬、海豹等。大熊貓和小熊貓都是食草動物，這令人感到好奇。

More intriguing still, both subsist almost entirely on bamboo (some etymologists think their mutual name is derived from the Nepali phrase nigalya ponya, meaning 「bamboo-eater」). And most curiously of all, both have a sixth digit on their forepaws—a kind of ancillary thumb derived from one of the bones of the wrist that helps them hold bamboo stalks for consumption. These common features led Hu Yibo of the Institute of Zoology, in Beijing, and his colleagues, to wonder if the two vegetarian carnivores also shared genetic modifications that might explain those features. And, as Dr Hu observes in a paper just published in the Proceedings of the National Academy of Sciences, they do.

更奇怪的是，兩者都完全依賴竹子，有些詞源學家認為它們共同的名字來源於尼泊爾語中的詞組 nigalya ponya，意思是「食竹者」。最讓人好奇的是，兩個物種的前爪都有從手腕處的骨頭進化而來的六個腳趾，有助於吃竹子時抓住竹竿。這些共性吸引著北京動物研究所的胡義博和他的同事。他們想要知道這兩個食草動物是否有著相同的基因突變，而這些突變可能是這些共性的原因。胡義博士在《國家科學院學報》上發表的論文稱，確實如此。

To search for such genetic commonalities, Dr Hu and his colleagues compared the DNA of the two pandas with that of four other members of the Carnivora: polar bears (close relatives of giant pandas), ferrets (close relatives of red pandas), and tigers and dogs (close relatives of neither). If pandas share features of their DNA with each other, but not with the four comparison species, he reasoned, then it is likely that those features encode recent adaptations common to the two of them.

為了研究這些基因共性，胡義博士和同事將這兩個物種的基因和其他四種食肉目動物的基因進行比較：北極熊（大熊貓的近親）、雪豹（小熊貓的近親）、老虎和狗（和大小熊貓關係較遠）。胡義博士解釋道，如果大小熊貓的DNA特點彼此相同，但與其他四個物種的不同，那麼，這些共同特性有可能會是後來在它們身上出現的。

Altogether, the team identified 70 genes (out of the 20,000 or so that mammals have) which sport bits of DNA that seemed to be shared. They also found ten genes which have been disabled by crippling mutations in both of the pandas, but not in the other four species. Not all of the shared genetic features obviously tied in with the shared peculiarities of pandas, but some did. These fell into three categories: genes affecting anatomy, genes affecting appetite and genes affecting the digestion and metabolism of nutrients.

研究小組識別了70種有相似之處的基因（哺乳動物的基因有20000多種）。他們還發現，兩種熊貓體內劇烈的基因突變導致了10種基因殘缺，但另外四個物種沒有這種情況。並不是兩種熊貓身上所有的基因特性都存在這種情況，只有部分如此。這些基因分為三類：骨骼基因、飲食基因和消化及新陳代謝基因。

The anatomy-related adaptations were those that seemed to control the development of the pandas』 second thumbs. Two genes in particular, DYNC2H1 and pericentrin, have mutations that cause identical changes in each type of panda in the proteins encoded by these genes. In mice, mutations in these genes are known to encourage extra digits to grow, so it is not unreasonable to suspect that they are also the cause of this in pandas.

骨骼基因似乎控制著熊貓第二拇指的發育。尤其是 DYNC2H1 和中心粒周圍蛋白，這兩種基因導致兩種熊貓在加密蛋白質上發生了明顯變化。老鼠的基因突變就促進了另外一個腳趾的生長，因此，對熊貓的這一假設是有理可循的。

The appetite-modifying change involved one of the disabled genes. When working, this gene encodes a protein which forms part of the tongue’s taste buds for umami, a savoury 「meatiness」 separate from the other four tastes of sweetness, sourness, bitterness and saltiness. Umami perception is stimulated by glutamic acid, one of the 20 amino acids that make up proteins—but one that is more characteristic of animal proteins than plant ones. Sensitivity to umami is valuable to a carnivore but unnecessary (and possibly harmful) to a herbivore.

飲食基因中包括一種殘缺基因。這種基因在發揮作用時會產生一種蛋白質，使之成為味蕾的一部分，產生「鮮味」，與甜、酸、苦和鹹等味道不同。鮮味味覺的刺激源於穀氨酸，即構成蛋白質的20中胺基酸中的一種，而穀氨酸更具有動物蛋白的特色，植物蛋白特色較少。鮮味感知對食肉動物很有價值，對食草動物則不僅沒有意義，還可能造成傷害。

The other genetic convergences Dr Hu recorded were related to the digestion and metabolism of substances scarce in, or absent from, bamboo. He and his colleagues noted parallel changes in the genes for three digestive enzymes whose job is to liberate two particular amino acids, lysine and arginine, from proteins. Both lysine and arginine are abundant in meat, but in short supply in bamboo. The team also noted parallel changes in four genes that encode proteins involved in the metabolism of two vitamins, A and B12, and of arachidonic acid, a lipid essential for bodily function. All of these are scarce or non-existent in bamboo as well. Exactly how the genetic changes seen alter the effectiveness of the proteins involved remains to be determined. But the prediction would be that they enhance the availability of the nutrients in question.

胡義博士記錄的其他基因趨同與竹子中稀缺的物質消化和代謝有關。他及其同事發現了三種消化酶基因的平行變化。這些酶的作用是從蛋白質中分離賴氨酸和精氨酸這兩種穀氨酸。肉類中的賴氨酸和精氨酸很豐富，在竹子裡卻很稀缺。研究人員還注意到四種基因發生的平行變化。這些基因對負責代謝維生素A和維生素B12的蛋白質，以及新陳代謝花生四希酸的蛋白質產生，其中花生四希酸是人體機能必須的脂質。所有這些基因在竹子中都很稀缺甚至不存在。這些基因變化是怎樣改變有關蛋白質效果的這一問題還沒有定論。不過，有人預測這些基因有助於獲得缺失的營養。

The upshot is that the two pandas do indeed seem to have similar genetics underlying their similar peculiarities. Such similarities are, admittedly, easier to find than different genetic routes to the same outcome would be. But Dr Hu has established that, at least in the case of pandas, natural selection has often taken the same paths to arrive at the same outcomes.

結論是兩種熊貓的相似基因似乎確實決定了類似性徵。不可否認，與不同基因路徑得到相同結果相比，這種相似性更加容易發現。然而，胡義博士認為，自然選擇通常採取相同模式，最後得到同樣的結果，至少在熊貓案例中如此。

編譯：劉恬

審校：周小靖

編輯：翻吧君

來源：經濟學人

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