生物谷：當(dāng)一個(gè)物種面臨巨大的選擇壓力，進(jìn)化過程將大大加快。一提到自然選擇,，人們自然而然地就會(huì)想到漫漫歷史長(zhǎng)河。但一個(gè)國(guó)際科學(xué)家小組最近發(fā)現(xiàn),，一種熱帶蝴蝶抵御細(xì)菌的自然選擇過程可以在不到一年的時(shí)間內(nèi)完成,。相關(guān)論文發(fā)表在7月13日的《科學(xué)》雜志上。
在南太平洋島國(guó)西薩摩亞,，生長(zhǎng)著一種美麗的熱帶蝴蝶,，學(xué)名幻紫斑峽蝶（Hypolimnas bolina），通常稱為“藍(lán)月亮”,。近幾年,，由于受到一種名為沃爾巴克氏體的細(xì)菌的侵?jǐn)_，在烏波盧島和薩瓦伊島上的雄性“藍(lán)月亮”曾一度瀕臨滅絕,。但是經(jīng)過短短幾年時(shí)間的進(jìn)化,，“藍(lán)月亮”就獲得了抵御沃爾巴克氏體攻擊的“秘密武器”———抑制基因，其家族的男丁重新興旺,。這可能是科學(xué)家觀測(cè)到的最快的生物進(jìn)化過程,，也是對(duì)達(dá)爾文生物進(jìn)化論的最新證明。
都是細(xì)菌惹的禍
沃爾巴克氏體（Wolbachia）是一類分布于節(jié)肢動(dòng)物（如昆蟲）生殖組織內(nèi)的共生細(xì)菌,。這些共生細(xì)菌通過卵細(xì)胞的細(xì)胞質(zhì)傳播，并參與多種調(diào)控其寄生動(dòng)物生殖活動(dòng)的機(jī)制,，如誘導(dǎo)孤雌生殖、雄性致死和調(diào)節(jié)繁殖力等,。通俗地講,，對(duì)于“藍(lán)月亮”蝴蝶來說，沃爾巴克氏體喜歡搭乘雌性蝴蝶的卵細(xì)胞這輛“便車”來進(jìn)行傳播,。由于雄性蝴蝶對(duì)它的生存繁殖來說是“無用的東西”,，因此它會(huì)有選擇地攻擊殺死雌蝴蝶體內(nèi)的雄性蝴蝶卵，使得雌蝴蝶產(chǎn)下的后代,，絕大多數(shù)是雌性,。經(jīng)過幾代傳播，整個(gè)種群的雌性蝴蝶都會(huì)帶有這種細(xì)菌,，而雄性蝴蝶將越來越少,。
2001年，研究人員經(jīng)過調(diào)查發(fā)現(xiàn),，烏波盧島和薩瓦伊島上的雄性“藍(lán)月亮”在其蝴蝶家族中所占的比例已不足1％,。2002年，英國(guó)倫敦大學(xué)學(xué)院的格雷戈里.赫斯特及其同事經(jīng)過研究證明，導(dǎo)致“藍(lán)月亮”蝴蝶性別比例嚴(yán)重扭曲的罪魁禍?zhǔn)拙褪俏譅柊涂耸象w,。沃爾巴克氏體實(shí)乃雄性蝴蝶的“冷血?dú)⑹?rdquo;,。
美麗的“藍(lán)月亮”難道要成為“女兒國(guó)”？不必?fù)?dān)心,，大自然適者生存的法則永遠(yuǎn)在發(fā)揮作用,。面對(duì)巨大的自然選擇壓力，為了捍衛(wèi)雄性的生存權(quán)利,，“藍(lán)月亮”蝴蝶與沃爾巴克氏體進(jìn)行了一場(chǎng)艱苦的“衛(wèi)子”之戰(zhàn),。
藍(lán)月亮”的“絕地反擊”
2006年，科學(xué)家對(duì)西薩摩亞幾個(gè)島上的“藍(lán)月亮”蝴蝶進(jìn)行了新一輪調(diào)查研究,。在烏波盧島,，他們驚喜地發(fā)現(xiàn)，該島上“藍(lán)月亮”蝴蝶的雌雄比例已接近1∶1,。他們捕捉了14只雌蝴蝶進(jìn)行人工飼養(yǎng),，發(fā)現(xiàn)它們都順利地產(chǎn)下了健康的“男丁”，雌雄比例基本持平,。這說明沃爾巴克氏體對(duì)它們的殺傷力已大打折扣,。
在薩瓦伊島上的發(fā)現(xiàn)才真正稱得上是讓人驚訝。2006年初,，這個(gè)島上的雄性“藍(lán)月亮”所占比例還不足1％,。然而，到了2006年底,，差不多經(jīng)過了10代的繁衍進(jìn)化,，雄性“藍(lán)月亮”的數(shù)量已火箭般爆增,，所占比例接近40％。美國(guó)加州大學(xué)伯克利分校的博士后研究員西爾萬.查蘭特說,，“就我所知，這是迄今為止科學(xué)家觀測(cè)到的最快的進(jìn)化過程,。”
查蘭特和赫斯特發(fā)表在7月13日《科學(xué)》雜志上的論文指出，在這兩個(gè)島上都沒有觀測(cè)到“藍(lán)月亮”蝴蝶和其它種類蝴蝶的異種交配,。因此,，“藍(lán)月亮”蝴蝶不是依靠其它物種，而是通過自身種群的進(jìn)化,，獲得了對(duì)付沃爾巴克氏體的“秘密武器”,，赫斯特稱其為“抑制基因”,。
通過進(jìn)一步的基因分析發(fā)現(xiàn)，沃爾巴克氏體存在于這兩個(gè)島上的所有“藍(lán)月亮”蝴蝶中,，并且沒有什么基因上的變化,。他們通過基因滲入方法將沃爾巴克氏體菌注入到不帶有抑制基因的“藍(lán)月亮”蝴蝶的細(xì)胞中，僅僅經(jīng)過三代的滲入,，沃爾巴克氏體專殺雄性蝴蝶的能力就達(dá)到了顛峰,，說明沃爾巴克氏體的傳染能力并沒有減弱?？梢哉f,，并非是細(xì)菌本身的改變而導(dǎo)致雄性蝴蝶再度恢復(fù)活力，這兩個(gè)島上“藍(lán)月亮”蝴蝶性別比例的戲劇性變化只能歸功于抑制基因的廣泛獲得,。
查蘭特表示,，在未來3年的時(shí)間里他們將了解更多，那時(shí)抑制基因的精確位置就能被確認(rèn)?，F(xiàn)在尚不清楚,，這種抑制基因是出自該島“藍(lán)月亮”蝴蝶自身的基因突變，還是由東南亞遷移過來的“藍(lán)月亮”蝴蝶引入,，東南亞的蝴蝶已經(jīng)具備了這種基因,。但不論該基因突變來自哪一種途徑，接下來的一步都是自然選擇,。抑制基因使得雌蝴蝶可以產(chǎn)下雄性后代,，這些雄蝴蝶和很多很多的雌蝴蝶交配，使得越來越多的后代具備了這種抑制基因,。
赫斯特表示,，人們通常認(rèn)為自然選擇是一個(gè)緩慢的過程，要經(jīng)歷幾百或幾千年的時(shí)間,。但在這項(xiàng)研究中,，從進(jìn)化的時(shí)間尺度來看,，“藍(lán)月亮”蝴蝶的進(jìn)化只不過是一眨眼的功夫,，這是一個(gè)值得觀察的顯著事件,。
“軍備競(jìng)賽”仍在繼續(xù)
“藍(lán)月亮”蝴蝶和沃爾巴克氏體的斗爭(zhēng)體現(xiàn)了生物進(jìn)化中的一個(gè)有趣的現(xiàn)象———“紅后原則”(RedQueenPrinciple),。查蘭特解釋說，“紅后原則”來源于劉易斯.卡羅爾的名著《愛麗絲鏡中奇遇記》,，愛麗絲和紅后在山頂上愈跑愈快,，到頭來卻發(fā)現(xiàn)她們依然處在同一位置。也就是說,，要保住原來的位置,，就得馬不停蹄地往前跑。在大自然的有機(jī)物中,，無論是掠奪者和獵物,，還是宿主和寄生物，都必須不斷進(jìn)化,，才能維持雙方的平衡,。
事實(shí)上，發(fā)生在“藍(lán)月亮”蝴蝶身上的故事并非絕無僅有,，不少節(jié)肢動(dòng)物都感染過專殺雄性后代的病菌,。先前就有研究揭示，一些昆蟲在面對(duì)幾乎要淪為單性的繁殖壓力下,，會(huì)通過異乎尋常的方式來重新恢復(fù)其家族的性別平衡,。
沃爾巴克氏體會(huì)善罷甘休嗎？它會(huì)不會(huì)也進(jìn)化出對(duì)付抑制基因的本領(lǐng),？查蘭特認(rèn)為這完全有可能,，沃爾巴克氏體的基因也會(huì)發(fā)生變異，從而恢復(fù)其殺傷力,?？磥磉@是一場(chǎng)節(jié)節(jié)高升的“軍備競(jìng)賽”，雙方的本領(lǐng)都會(huì)愈來愈厲害,，從“兵來將擋,，水來土掩”，直到“你有狼牙棒,、我有天靈蓋”為止,。不知最終的贏家會(huì)是誰。
UC伯克利網(wǎng)站報(bào)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.
原始出處：
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: [email protected]
