模拟人生4卡顿完美解决:污染与进化

污染与进化

Waters of change

水域发生变化

An accidental experiment in America shows how evolution happens

美国实验表明如何演变为意外进化

IT IS not often that biologists have a chance to watch natural selection in action. The best-known cases—the evolution of resistance to antibiotics in bacteria and to pesticides in insects—are responses to deliberate changes people have made in the environment of the creatures concerned. But mankind has caused lots of accidental changes as well, and these also offer opportunities to study evolution.

生物学家们不常有机会看到自然抉择是如何进行的。最典型的例子——抵抗抗生素的细菌与抵抗杀虫剂昆虫的进化，这便是人们故意改变生存环境后的结果。于是，因为人类引起的诸多意外变化，为研究进化提供了诸多机会。

Recently, two groups of researchers, one at New York University (NYU) and the other at the Woods Hole Oceanographic Institute in Massachusetts, have taken advantage of one of these changes to look at how fish evolve in response to environmental stress. The stress in question is pollution by polychlorinated biphenyls (PCBs). These chemicals—widely used in the middle decades of the 20th century to manufacture electrical insulation, coolants, sealants and plasticisers—often ended up dumped in lakes, rivers and coastal waters. Eventually, such dumping was banned (in America, this happened in 1977). But PCBs are persistent chemicals, and their effects are felt even today. In particular, they disrupt the immune systems of animals such as fish, cause hormonal imbalances and promote tumours.

最近两组研究人员，一组在纽约大学，另一组在马萨诸塞州的伍兹霍尔海洋研究所进行一项试验，利用这些变化发现鱼，在面对环境变化压力时所做出的各种进化反应。压力来源即—受多氯联笨（PCBs）污染的物质。这类化学品在20世纪中期被广泛用于制造电气绝缘、冷却剂 、密封剂及增塑剂等，最终被丢弃进湖泊、江河或是沿海水域。其实，诸如此类的丢弃是被禁止的（1977美国曾出现过类似的情况）可是PCBs是一种持久化学物质，由此带来的影响至今还有。特别是，它严重破坏了动物的免疫系统，造成了荷尔蒙失衡，肿瘤激增等，如鱼类。

As is the way of evolution, however, some fish species have developed resistance to PCB poisoning. Isaac Wirgin, at NYU, and Mark Hahn, at Woods Hole, have been studying PCB-resistant fish, to see how they do it. After that, the two researchers will be able to look at how these populations evolve yet again as the environment is cleaned up.

在此进化过程中，某些鱼类标本出现了抵抗PCB毒性的特征。纽约大学伍兹霍尔的Isaac Wirgin为了见证这种抵抗是如何起作用的，便一直致力于研究对抗PCB的鱼。之后，2名研究人员将研究环境变干净时他们是如何进化的？

Breaking the chain

断开锁链

The species of interest to Dr Wirgin is the Atlantic tomcod of the Hudson river in upstate New York. Part of the Hudson was polluted with PCBs by two General Electric plants. Dr Hahn is looking at a different animal, the killifish (pictured), in New Bedford harbour, Massachusetts, which was polluted by other producers. Both Hudson tomcod and New Bedford killifish are able to tolerate levels of PCB far higher than those that would kill such fish in cleaner waters. The question is, why?

Wirgin博士感兴趣的试验种群是位于纽约哈德逊河上游的大西洋小鳕。此段河流已被2座发电厂排出的PCBs污染。Hahn博士正在观察另外一群，鳉鱼（上图所示），马萨诸塞州的新贝德福德港亦是深受其他工厂的污染。问题是，为什么干净水域中足以杀死鱼类的PCB？在这两个地方的小鳕鱼和鳉鱼均能忍受远远高于正常值PCB的毒性而继续生存下去呢？

PCBs do their damage by binding to a protein called the aryl hydrocarbon receptor, or AHR, thus stopping it working properly. AHR is a transcription factor, meaning that it controls the process by which messenger molecules are copied from genes. These messenger molecules go on to act as the blueprints for protein production, so preventing a transcription factor from working can cause all sorts of problems. Both Hudson tomcod and New Bedford killifish, however, have unusual AHR molecules. And it is this that seems to explain their immunity.

PCBs所造成的危害却合成了一种叫“芳香烃受体”或“芳香烃受体抗体”（AHR）的物质，这样便会导致功能失常。AHR是转录因子，就是它控制着传播分子复制基因的过程。这些传播分子的作用就是扮演蛋白合成的蓝图，以便防止转录因子引起类似反应。哈德逊小鳕与新贝德福德港鳉鱼具有了异常的AHR分子，因此也就能解释为什么它们具备这样的免疫力。

A protein is a chain of chemical units called amino acids. In tomcods, AHR is composed of 1,104 such units. Except that in Hudson tomcod it frequently isn’t. These fish generally have 1,102 amino acids in their AHRs. The two missing links in the chain (a phenylalanine and a leucine, for aficionados) are encoded in the gene for ordinary tomcod AHR by six genetic “letters” that are missing from the DNA found in PCB-resistant Hudson tomcod. The shortened version of AHR does not bind nearly so easily to PCBs. It still, however, seems to work as a transcription factor. The result is fish that are more or less immune to PCB poisoning.ny

蛋白质为一连串的化学单位，称作胺基酸。小鳕鱼体内，AHR是由1104个这样的单位组成的。除了哈德逊小鳕鱼，其他鱼类体内的AHR含有此类胺基酸数为1102个。2个在基因链上遗失了的（分别是苯丙氨酸和亮氨酸）基因，其实是普通小鳕鱼的AHR的6个遗传基因编码，也就是自哈德逊小鳕鱼体内PCB抗体遗失的DNA。简写AHR并不意味能很容易与PCBs绑定。这仍然跟转录因子的作用有关，结果表明，是这些鱼或多或少具备了PCB免疫力。

In the case of the New Bedford killifish the situation is similar, but more complicated. There are no missing amino acids. Dr Hahn has, however, found nine places along the amino-acid chain of killifish AHR where the link in the chain varies between individuals. Altogether, he has identified 26 such variations. Two of them seem particularly resistant to the effects of PCBs. It is not that the pollutants do not bind to the protein—they do. But the protein does not seem to mind. It appears to work equally well, whether or not it has PCB passengers on board.

就新贝德福德港的鳉鱼来说情况颇为相似，便更为复杂。因为它的体内没有丢失的上面提到的胺基酸。Hahn博士发现，9处鳉鱼的AHR的胺基酸链中，不同个体间是相连的，共找出26处。其中2处具有抵抗PCBs的作用。并不是说，污染物不与蛋白质绑定，它们是可以的。且蛋白质似乎并不介意，因为作用是相同的，无论PCB有没有在其中。

These fishy cases are reminiscent of the peppered moth in Britain. This, too, evolved in response to industrial pollution. It developed black wings, so that it was invisible when it settled on soot-covered tree trunks. Now, with the clean air brought by anti-pollution legislation, British peppered moths are once again peppered.

这些鱼类让我们联想起英国的斑点蛾。它们也是因为工业污染而发生的基因突变转而进化的。衍变到后来，黑色翅膀渐渐变成浅灰色，当它们落在树干上时你根本分辨不出它们在哪。如今，因为反污染法律的建立带来了洁净的空气，现在的斑点蛾正在渐渐恢复它原来的模样。

Both the Hudson river and New Bedford harbour are being cleaned up, too. This year, for example, General Electric will dredge 1.8m cubic metres (2.4m cubic yards) of PCB-contaminated sediment out of 60km (35 miles) of the Hudson. Dredging will continue over the next few years, after which the river should be PCB-free. It is possible that the tomcod and the killifish will then evolve again, just as the peppered moth did, if their PCB-resistant proteins are not absolutely as good as the original versions—which they might not be, given that evolution did not find them before. If that happens, Dr Wirgin and Dr Hahn will be watching.

如今的哈德逊河与新贝德福德港变得洁净许多。近些年，发电厂将疏浚1.8万平方米（2.4万平方英尺）的PCB污染物沉积物，并将其远离哈德逊60公里（35英里）以外。疏浚将在接下来数年中持续，直至这条河中没有PCB物质。亦有可能这里的小鳕鱼、鳉鱼还会继续进化，就像斑点蛾的进化过程一样。它们的PCB抗体蛋白未转化到原来那么好，也不可能会达到原先的水平。研究还未发现，如果出现Wirgin与Hahn博士会继续关注下去。