生物谷：当一个物种面临巨大的选择压力，进化过程将大大加快。一提到自然选择，人们自然而然地就会想到漫漫历史长河。但一个国际科学家小组最近发现，一种热带蝴蝶抵御细菌的自然选择过程可以在不到一年的时间内完成。相关论文发表在7月13日的《科学》杂志上。
在南太平洋岛国西萨摩亚，生长着一种美丽的热带蝴蝶，学名幻紫斑峡蝶（Hypolimnas bolina），通常称为“蓝月亮”。近几年，由于受到一种名为沃尔巴克氏体的细菌的侵扰，在乌波卢岛和萨瓦伊岛上的雄性“蓝月亮”曾一度濒临灭绝。但是经过短短几年时间的进化，“蓝月亮”就获得了抵御沃尔巴克氏体攻击的“秘密武器”———抑制基因，其家族的男丁重新兴旺。这可能是科学家观测到的最快的生物进化过程，也是对达尔文生物进化论的最新证明。
都是细菌惹的祸
沃尔巴克氏体（Wolbachia）是一类分布于节肢动物（如昆虫）生殖组织内的共生细菌。这些共生细菌通过卵细胞的细胞质传播，并参与多种调控其寄生动物生殖活动的机制，如诱导孤雌生殖、雄性致死和调节繁殖力等。通俗地讲，对于“蓝月亮”蝴蝶来说，沃尔巴克氏体喜欢搭乘雌性蝴蝶的卵细胞这辆“便车”来进行传播。由于雄性蝴蝶对它的生存繁殖来说是“无用的东西”，因此它会有选择地攻击杀死雌蝴蝶体内的雄性蝴蝶卵，使得雌蝴蝶产下的后代，绝大多数是雌性。经过几代传播，整个种群的雌性蝴蝶都会带有这种细菌，而雄性蝴蝶将越来越少。
2001年，研究人员经过调查发现，乌波卢岛和萨瓦伊岛上的雄性“蓝月亮”在其蝴蝶家族中所占的比例已不足1％。2002年，英国伦敦大学学院的格雷戈里.赫斯特及其同事经过研究证明，导致“蓝月亮”蝴蝶性别比例严重扭曲的罪魁祸首就是沃尔巴克氏体。沃尔巴克氏体实乃雄性蝴蝶的“冷血杀手”。
美丽的“蓝月亮”难道要成为“女儿国”？不必担心，大自然适者生存的法则永远在发挥作用。面对巨大的自然选择压力，为了捍卫雄性的生存权利，“蓝月亮”蝴蝶与沃尔巴克氏体进行了一场艰苦的“卫子”之战。
蓝月亮”的“绝地反击”
2006年，科学家对西萨摩亚几个岛上的“蓝月亮”蝴蝶进行了新一轮调查研究。在乌波卢岛，他们惊喜地发现，该岛上“蓝月亮”蝴蝶的雌雄比例已接近1∶1。他们捕捉了14只雌蝴蝶进行人工饲养，发现它们都顺利地产下了健康的“男丁”，雌雄比例基本持平。这说明沃尔巴克氏体对它们的杀伤力已大打折扣。
在萨瓦伊岛上的发现才真正称得上是让人惊讶。2006年初，这个岛上的雄性“蓝月亮”所占比例还不足1％。然而，到了2006年底，差不多经过了10代的繁衍进化，雄性“蓝月亮”的数量已火箭般爆增，所占比例接近40％。美国加州大学伯克利分校的博士后研究员西尔万.查兰特说，“就我所知，这是迄今为止科学家观测到的最快的进化过程。”
查兰特和赫斯特发表在7月13日《科学》杂志上的论文指出，在这两个岛上都没有观测到“蓝月亮”蝴蝶和其它种类蝴蝶的异种交配。因此，“蓝月亮”蝴蝶不是依靠其它物种，而是通过自身种群的进化，获得了对付沃尔巴克氏体的“秘密武器”，赫斯特称其为“抑制基因”。
通过进一步的基因分析发现，沃尔巴克氏体存在于这两个岛上的所有“蓝月亮”蝴蝶中，并且没有什么基因上的变化。他们通过基因渗入方法将沃尔巴克氏体菌注入到不带有抑制基因的“蓝月亮”蝴蝶的细胞中，仅仅经过三代的渗入，沃尔巴克氏体专杀雄性蝴蝶的能力就达到了颠峰，说明沃尔巴克氏体的传染能力并没有减弱。可以说，并非是细菌本身的改变而导致雄性蝴蝶再度恢复活力，这两个岛上“蓝月亮”蝴蝶性别比例的戏剧性变化只能归功于抑制基因的广泛获得。
查兰特表示，在未来3年的时间里他们将了解更多，那时抑制基因的精确位置就能被确认。现在尚不清楚，这种抑制基因是出自该岛“蓝月亮”蝴蝶自身的基因突变，还是由东南亚迁移过来的“蓝月亮”蝴蝶引入，东南亚的蝴蝶已经具备了这种基因。但不论该基因突变来自哪一种途径，接下来的一步都是自然选择。抑制基因使得雌蝴蝶可以产下雄性后代，这些雄蝴蝶和很多很多的雌蝴蝶交配，使得越来越多的后代具备了这种抑制基因。
赫斯特表示，人们通常认为自然选择是一个缓慢的过程，要经历几百或几千年的时间。但在这项研究中，从进化的时间尺度来看，“蓝月亮”蝴蝶的进化只不过是一眨眼的功夫，这是一个值得观察的显著事件。
“军备竞赛”仍在继续
“蓝月亮”蝴蝶和沃尔巴克氏体的斗争体现了生物进化中的一个有趣的现象———“红后原则”(RedQueenPrinciple)。查兰特解释说，“红后原则”来源于刘易斯.卡罗尔的名著《爱丽丝镜中奇遇记》，爱丽丝和红后在山顶上愈跑愈快，到头来却发现她们依然处在同一位置。也就是说，要保住原来的位置，就得马不停蹄地往前跑。在大自然的有机物中，无论是掠夺者和猎物，还是宿主和寄生物，都必须不断进化，才能维持双方的平衡。
事实上，发生在“蓝月亮”蝴蝶身上的故事并非绝无仅有，不少节肢动物都感染过专杀雄性后代的病菌。先前就有研究揭示，一些昆虫在面对几乎要沦为单性的繁殖压力下，会通过异乎寻常的方式来重新恢复其家族的性别平衡。
沃尔巴克氏体会善罢甘休吗？它会不会也进化出对付抑制基因的本领？查兰特认为这完全有可能，沃尔巴克氏体的基因也会发生变异，从而恢复其杀伤力。看来这是一场节节高升的“军备竞赛”，双方的本领都会愈来愈厉害，从“兵来将挡，水来土掩”，直到“你有狼牙棒、我有天灵盖”为止。不知最终的赢家会是谁。
UC伯克利网站报道全文：
Researchers witness natural selection at work in dramatic comeback of male butterflies
By Sarah Yang, Media Relations | 12 July 2007
BERKELEY – An international team of researchers has documented a remarkable example of natural selection in a tropical butterfly species that fought back - genetically speaking - against a highly invasive, male-killing bacteria.
Within 10 generations that spanned less than a year, the proportion of males of the Hypolimnas bolina butterfly on the South Pacific island of Savaii jumped from a meager 1 percent of the population to about 39 percent. The researchers considered this a stunning comeback and credited it to the rise of a suppressor gene that holds in check the Wolbachia bacteria, which is passed down from the mother and selectively kills males before they have a chance to hatch.
Male (above) and female Hypolimnas bolina, also called the Blue Moon or Great Eggfly butterfly. The proportion of females in some populations of H. bolina in the South Pacific reached 99 percent as a result of infection by a bacteria that kills males before they hatch. However, researchers recently witnessed a remarkable comeback of male butterflies on some islands thanks to the rise of a suppressor gene. (Sylvain Charlat photos)
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Print-quality images available for download
"To my knowledge, this is the fastest evolutionary change that has ever been observed," said Sylvain Charlat, lead author of the study and a post-doctoral researcher with joint appointments at the University of California, Berkeley, and University College London. "This study shows that when a population experiences very intense selective pressures, such as an extremely skewed sex ratio, evolution can happen very fast."
The researchers' findings are described in the July 13 issue of the journal Science.
Charlat pointed out that, unlike mutations that govern such traits as wing color or antennae length, a genetic change that affects the sex ratio of a population has a very wide impact on the biology of the species.
It is not yet clear whether the suppressor gene emerged from a chance mutation from within the local population, or if it was introduced by migratory Southeast Asian butterflies in which the mutation had already been established.
"We'll likely know more in three years' time when the exact location of the suppressor gene is identified," said Charlat. "But regardless of which of the two sources of the suppressor gene is correct, natural selection is the next step. The suppressor gene allows infected females to produce males, these males will mate with many, many females, and the suppressor gene will therefore be in more and more individuals over generations."
Charlat worked with Gregory Hurst, a reader in evolutionary genetics at University College London and senior author of the paper. Descriptions of all-female broods of H. bolina date back to the 1920s, but it wasn't until 2002 that Hurst and colleagues first identified Wolbachia bacteria as the culprit behind the distorted sex ratio.
"We usually think of natural selection as acting slowly, over hundreds or thousands of years," said Hurst. "But the example in this study happened in a blink of the eye, in terms of evolutionary time, and is a remarkable thing to get to observe."
Sylvain Charlat prepares to collect butterfly samples on an island in the South Pacific. (Philippe Paccou photo)
The researchers noted that bacteria that selectively kill male offspring are found among a range of arthropods, so what was seen in this study may not be unusual, despite the fact that it has never before been described in the scientific literature. Previous research has revealed some of the extraordinary ways in which insects adapt to the pressures inherent when nearly all its members are of one gender.
Notably, Charlat and Hurst reported in an earlier study that, thanks to Wolbachia, when males of H. bolina, commonly known as the Blue Moon or Great Eggfly butterfly, become a rare commodity, the number of mating sessions for both males and females jumps, possibly as an attempt to sustain the population despite the odds.
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Charlat added that the relationship between Wolbachia and the Blue Moon butterfly illustrates the so-called Red Queen Principle, an evolutionary term named after a scene in Lewis Carroll's famous book, "Through the Looking-Glass," in which the characters Alice and the Red Queen run faster and faster at the top of a hill, only to find that they remain in the same place.
"In essence, organisms must evolve or change to stay in the same place, whether it's a predator-prey relationship, or a parasite-host interaction," said Charlat. "In the case of H. bolina, we're witnessing an evolutionary arms race between the parasite and the host. This strengthens the view that parasites
can be major drivers in evolution."
The researchers focused on the Samoan islands of Upolu and Savaii, where in 2001, males of the Blue Moon butterfly made up only 1 percent of the population. In 2006, the researchers embarked on a new survey of the butterfly after an increase in reports of male-sightings at Upolu.
They found that males that year made up about 41 percent of the Blue Moon butterfly population in Upolu. They hatched eggs from 14 females in the lab and confirmed that the male offspring from this group were surviving with sex ratios near parity. For Savaii, the population was initially 99 percent female at the beginning of 2006. By the end of the year, researchers found that males made up 39 percent of the 54 butterflies collected.
The researchers tested for the continued presence of Wolbachia in the butterflies. By mating infected females with males from a different island that did not have the suppressor gene, they also confirmed that the bacteria were still effective at killing male embryos. The male-killing ability of the bacteria emerged again after three generations. Thus, they could rule out a change in the bacteria as an explanation for the resurgence of the males in the butterfly populations studied.
The field work for this study was based out of the UC Berkeley Richard B. Gump South Pacific Research Station on the island of Moorea in French Polynesia. The Gump station is part of the Moorea Coral Reef Long Term Ecological Research Site, one of 26 sites funded by the National Science Foundation to study long-term ecological phenomena.
The Gump Research Station is managed through UC Berkeley's Office of the Vice Chancellor for Research. George Roderick, UC Berkeley professor of environmental science, policy and management and curator of the Essig Museum of Entomology, is a former director of the station, and Neil Davies is the station's executive director and research scientist. Both Roderick and Davies are co-authors of this study.
Other study co-authors are Emily Hornett of University College London, James Fullard of the University of Toronto at Mississauga, and Nina Wedell of the University of Exeter in Cornwall, England.
The U.S. National Science Foundation, the U.K. Natural Environment Research Council and the Natural Sciences and Engineering Research Council of Canada helped support this research.
原始出处：
Science 13 July 2007:
Vol. 317. no. 5835, p. 214
DOI: 10.1126/science.1143369
Extraordinary Flux in Sex Ratio
Sylvain Charlat,1,2* Emily A. Hornett,1 James H. Fullard,3 Neil Davies,2 George K. Roderick,4 Nina Wedell,5 Gregory D. D. Hurst1
The ratio of males to females in a species is often considered to be relatively constant, at least over ecological time. Hamilton noted that the spread of "selfish" sex ratio-distorting elements could be rapid and produce a switch to highly biased population sex ratios. Selection against a highly skewed sex ratio should promote the spread of mutations that suppress the sex ratio distortion. We show that in the butterfly Hypolimnas bolina the suppression of sex biases occurs extremely fast, with a switch from a 100:1 population sex ratio to 1:1 occurring in fewer than 10 generations.
1 Department of Biology, University College London, 4 Stephenson Way, London NW1 2HE, UK.
2 Gump South Pacific Research Station, University of California, Berkeley, BP 244 Maharepa, 98728 Moorea, French Polynesia.
3 Department of Biology, University of Toronto at Mississauga, 3359 Mississauga Road N, Mississauga, ON L5L 1C6, Canada.
4 Environmental Science (ESPM), University of California, Berkeley, CA 94720–3114, USA.
5 School of Biosciences, University of Exeter, Cornwall Campus, Tremough, Penryn TR10 9EZ, UK.
* To whom correspondence should be addressed. E-mail: s.charlat@ucl.ac.uk
