Nature: 线粒体是复杂生命进化的关键

Energy Revolution Key to Complex Life: Depends on Mitochondria, Cells' Tiny Power Stations
Nature: 线粒体是复杂生命进化的关键

译者：Docofsoul

ScienceDaily (Oct. 21, 2010) — The evolution of complex life is strictly dependent on mitochondria, the tiny power stations found in all complex cells, according to a new study by Dr Nick Lane, from UCL (University College London), and Dr William Martin, from the University of Dusseldorf.

《每日科学》2010年10月21日报道 —— 复杂生命的进化密切依赖于线粒体这种出现于所有复杂细胞的微型能量工厂。来自UCL（伦敦大学学院）的Nick Lane博士与来自德国杜塞尔多夫大学的William Martin日前在《Nature》发表论文宣布了这一新的发现。



Artist's rendering of basic cell structure, including mitochondria. (Credit: iStockphoto/Sebastian Kaulitzki)
包括线粒体在内的基本细胞结构的艺术构想图（图片来源：iStockphoto/Sebastian Kaulitzki)

"The underlying principles are universal. Energy is vital, even in the realm of evolutionary inventions," said Dr Lane, UCL Department of Genetics, Evolution and Environment. "Even aliens will need mitochondria."

UCL遗传学、进化与环境系Lane博士说：“基本原理是普适的。能量是关键，甚至在进化创新领域也是如此。陌生的生命也需要线粒体。”

For 70 years scientists have reasoned that evolution of nucleus was the key to complex life. Now, in work published in Nature, Lane and Martin reveal that in fact mitochondria were fundamental to the development of complex innovations like the nucleus because of their function as power stations in the cell.

整整七十年来，科学家们推断细胞核的进化是复杂生命的关键所在。而目前Lane 与Martin等在《Nature》发表的一项论文则揭示：实际上线粒体才是象细胞核这样的复杂创新事物的生育与发展的根基，这是因为线粒体在细胞中承担着能量工厂的使命。

"This overturns the traditional view that the jump to complex 'eukaryotic' cells simply required the right kinds of mutations. It actually required a kind of industrial revolution in terms of energy production," explained Dr Lane.
At the level of our cells, humans have far more in common with mushrooms, magnolias and marigolds than we do with bacteria. The reason is that complex cells like those of plants, animals and fungi have specialized compartments including an information centre, the nucleus, and power stations -- mitochondria. These compartmentalised cells are called 'eukaryotic', and they all share a common ancestor that arose just once in four billion years of evolution.

　　　Lane解释说：“这样就推翻了传统的观念，即向复杂‘真核’细胞方向的跃迁必然需要适当类型的突变。但是，从能量产生的角度而言，实际上需要的是产业上的重大变革。”　在细胞水平上，人类与蘑菇、木兰与金盏菊的相似之处远较与细菌的相似之处多。其中的道理在于：象植物、动物与真菌那种复杂的细胞有着包括一个信息中心、细胞核与能量工厂即线粒体在内的特殊功能腔室。这些区室化后的细胞被称为“真核细胞”，这些细胞有着一个在四十亿年的进化中只出现一次的共同祖先。

Scientists now know that this common ancestor, 'the first eukaryote', was a lot more sophisticated than any known bacterium. It had thousands more genes and proteins than any bacterium, despite sharing other features, like the genetic code. But what enabled eukaryotes to accumulate all these extra genes and proteins? And why don't bacteria bother?

　　科学家现在知道了这个共同祖先——“第一个真核细胞”，它比任何已知的细菌都要复杂得多，比任何细菌都多出了成千上万的基因与蛋白，尽管细菌也有着诸如遗传密码等等共同特征。但是，是什么促使真核细胞积累了这样额外的基因与蛋白呢？为什么细菌会对这些“外快”无动于衷？

By focusing on the energy available per gene, Lane and Martin showed that an average eukaryotic cell can support an astonishing 200,000 times more genes than bacteria.

　　　通过集中精力研究每个基因所能获得的能量的情况上，Lane与Martin显示：一个普通的真核细胞能够支持比细菌多出二十万倍的基因，（其实力）确实惊人。

"This gives eukaryotes the genetic raw material that enables them to accumulate new genes, big gene families and regulatory systems on a scale that is totally unaffordable to bacteria," said Dr Lane. "It's the basis of complexity, even if it's not always used."

　Lane博士说：“这就给了真核细胞遗传学上的原料，使之能够积累新的基因、（组织）更大的基因家庭并（拥有）调节系统，其规模达到细菌完成无法承担的水平。而这正是复杂性的基础，即使并未被经常使用。”

"Bacteria are at the bottom of a deep chasm in the energy landscape, and they never found a way out," explained Dr Martin. "Mitochondria give eukaryotes four or five orders of magnitude more energy per gene, and that enabled them to tunnel straight through the walls of the chasm."

　Martin博士对此作了解释：“细菌处于能量景观地形上出现的某一深渊的底层，而且（深陷其中）永远找不到出路。线粒体给了真核细胞的每个基因多出四或五个数量级的能量，从而使真核细胞（战斗力大增，）得以穿透深渊之壁走向自由之地。”

The authors went on to address a second question: why can't bacteria just compartmentalise themselves to gain all the advantages of having mitochondria? They often made a start but never got very far.

论文作者接着解决第二个问题：为什么细菌不能使自身区室化来获得所有拥有线粒休的优势呢？细菌（其实也这样做了，只不过）经常做得虎头蛇尾草草收场。

The answer lies in the tiny mitochondrial genome. These genes are needed for cell respiration, and without them eukaryotic cells die. If cells get bigger and more energetic, they need more copies of these mitochondrial genes to stay alive.

答案就在微小的线粒体基因组上。这些基因为细胞呼吸所需，没有这些基因真核细胞也就一命归西了。如果细胞想变得更大、得到更多的能量，就需要更多的线粒体基因的拷贝才能维生。

Bacteria face exactly the same problem. They can deal with it by making thousands of copies of their entire genome -- as many as 600,000 copies in the case of giant bacterial cells like Epulopiscium, an extreme case that lives only in the unusual guts of surgeonfish. But all this DNA has a big energetic cost that cripples even giant bacteria -- stopping them from turning into more complex eukaryotes. "The only way out," said Dr Lane, "is if one cell somehow gets inside another one -- an endosymbiosis."

细菌确实面临着相同的难题。细菌能够通过制作其整个基因组的成千上万的拷贝来对付这一困境，比如说巨型细菌Epulopiscium（该细菌仅生存于刺尾鱼科鱼的与众不同的内脏中，这里作为极端的实例用以说明问题）就有多达六十万个拷贝。但所有这些DNA必须消耗巨大的能量，结果就使巨型菌疲于奔命，无力转变为更为复杂的真核细胞。 Lane博士指出：“唯一的出路在于其中一个细胞在某种原因下进入了另一个细胞，即进入内共生状态。”

Cells compete among themselves. When living inside other cells they tend to cut corners, relying on their host cell wherever possible. Over evolutionary time, they lose unnecessary genes and become streamlined, ultimately leaving them with a tiny fraction of the genes they started out with: only the ones they really need.

细胞之间也存在着彼此竞争。在其它细胞内生活的时候，它们倾向于找捷径抄近路，只要可能，就尽可能傍上宿主。随着进化时间的延伸，他们丢掉了不必要的基因而变得简化了，这样最后使自己只拥有开始时所有的那一小部分基因：也正是他们实际上所需要的基因。

The key to complexity is that these few remaining genes weigh almost nothing. Calculate the energy needed to support a normal bacterial genome in thousands of copies and the cost is prohibitive. Do it for the tiny mitochondrial genome and the cost is easily affordable, as shown in the Nature paper. The difference is the amount of DNA that could be supported in the nucleus, not as repetitive copies of the same old genes, but as the raw material for new evolution.

复杂性的关键在于这些留下来的少许基因几乎无足轻重。从计算上看，要支持几千个拷贝中一个正常的细菌基因组，所需要的能量高得离谱。但是，如果按Nature论文所显示的来分析：计算支持微小的线粒体基因组所需要的能量，结果就会顺理成章。差别在于在细胞核内可得到支持的DNA的数量，不是以相同的旧的基因的重复拷贝的面目出现，而是以为新的进化提供新原料的身份亮相。

"If evolution works like a tinkerer, evolution with mitochondria works like a corps of engineers," said Dr Martin.

“如果进化象一个工艺师那样工作的话，那么线粒体的进化就象一队工程师那样工作着。” Martin博士说。

The trouble is that, while cells within cells are common in eukaryotes, which often engulf other cells, they're vanishingly rare in more rigid bacteria. And that, Lane and Martin conclude, may well explain why complex life -- eukaryotes -- only evolved once in all of Earth's history.

问题在于，对真核细胞而言，尽管细胞内共生是普遍现象（导致一个细胞吞噬了其它细胞），而这种现象在刚性细菌中基本上不出现。由此Lane与Martin总结说：这可能很好地解释了为何象真核生物这样的复杂的生命在地球的全部历史上只进化了一次。