古代長毛象的基因復(fù)活，以了解物種的最后日子

古代長毛象的基因復(fù)活，以了解物種的最后日子

Researchers have resurrected the genes of a tiny population of ancient woolly mammoths that mysteriously died out some 4,000 years ago on Wrangel Island, a remote 125-kilometer-wide (78-mile) Arctic refuge located off the coast of Siberia. It is likely that this group was among the last in the world.

研究人員重新發(fā)現(xiàn)了一小群遠(yuǎn)古長毛猛犸象的基因，這些長毛猛犸象大約在4000年前神秘地消失在弗朗格爾島上。這個(gè)群體很可能是世界上最后的群體之一。

Madison Dapcevich

During the Ice Age, Wrangel Island was connected to Beringia and modern-day Alaska and Canada via the Bering Land Bridge. Mammuthus primigenius were among the most abundant species adapted for cold weather, but as temperatures rose in the millennia following the Pleistocene, the bridge was eventually engulfed by rising seas, forever isolating the Wrangel Island mammoths.

在冰河時(shí)代，弗蘭格爾島通過白令海峽陸橋與白令海峽、現(xiàn)在的阿拉斯加和加拿大相連。Mammuthus primigenius是適應(yīng)寒冷天氣最豐富的物種之一，但隨著更新世之后的數(shù)千年溫度的上升，這座橋最終被上升的海洋吞沒，永遠(yuǎn)地隔離了弗蘭格爾島猛犸象。

Dramatic environmental changes associated with a warming planet saw the loss of dry steppe tundra, also known as Mammoth steppe, as the world entered the Holocene. A number of other cold-adapted species were also lost, including cave bears and hyenas as well as woolly rhinoceros. Meanwhile, mammoths went extinct in two waves, which saw mainland populations first die out, followed by those that inhabited St. Paul Island (5,600 years ago).

當(dāng)世界進(jìn)入全新世時(shí)，與全球變暖相關(guān)的劇烈環(huán)境變化導(dǎo)致了干燥草原苔原(也稱猛犸象草原)的消失。許多其他適應(yīng)寒冷的物種也消失了，包括洞穴熊、土狼以及長毛犀牛。與此同時(shí)，猛犸象的滅絕分為兩波，首先是大陸種群的滅絕，接著是居住在圣保羅島的種群的滅絕(5600年前)。

The new research builds on evidence that the last mammoths on Wrangel Island suffered from a variety of genetic defects. Rebecca Farnham/University at Buffalo

The Wrangel Island mammoths held on until a “fairly abrupt” extinction event wiped them out 4,600 years ago. To determine what that catastrophic event might have been, a collaborative team of researchers from several US-based universities compared the DNA of Wrangel Island mammoths to three Asian elephants and two other ancient mammoth populations. Specific genetic mutations unique to Wrangel Island mammoths were identified, each thought to play a role in important functions, like neurological development, male fertility, insulin signaling, and sense of smell. These altered genes were synthesized and placed into cells in Petri dishes for researchers to test whether the genes would function normally.

弗蘭格爾島的猛犸象堅(jiān)持了下來，直到4600年前一次“相當(dāng)突然”的滅絕事件才將它們滅絕。為了確定這一災(zāi)難性事件可能是什么，來自美國幾所大學(xué)的一個(gè)合作研究小組將弗蘭格爾島猛犸象的DNA與三頭亞洲象和另外兩種古代猛犸象進(jìn)行了比較。弗蘭格爾島猛犸象特有的特定基因突變被發(fā)現(xiàn)，每一種突變都被認(rèn)為在重要的功能中發(fā)揮作用，如神經(jīng)發(fā)育、男性生育能力、胰島素信號和嗅覺。這些被改變的基因被合成并放入培養(yǎng)皿中的細(xì)胞中，以供研究人員測試這些基因是否能正常工作。

In short, they did not.

簡而言之，它們并不能。

"Our results suggest that once populations get small they harbor deleterious mutations, once those mutations are there they will affect the ability of the population to thrive. Even if the population were to recover large numbers the mutations are still there, dragging the population down," study author Vincent Lynch, from the University of Buffalo, told IFLScience.

“我們的研究結(jié)果表明，一旦種群數(shù)量減少，它們就會(huì)產(chǎn)生有害的突變，一旦這些突變出現(xiàn)，它們就會(huì)影響種群興旺的能力。即使種群數(shù)量大量恢復(fù)，突變?nèi)匀淮嬖?，將種群拉低，”布法羅大學(xué)的研究作者文森特·林奇告訴IFLScience。

It is believed that the environmental conditions would have gradually killed off the Wrangel Island mammoths, dwindling their already isolated population. Individuals that survived would have bred with others over generations, eventually reducing the genetic pool and eliminating the ability for a species to “breed out” harmful genetic mutations, write the researchers in Genome Biology and Evolution.

據(jù)信，環(huán)境條件會(huì)逐漸殺死弗蘭格爾島的猛犸象，減少它們已經(jīng)孤立的種群。研究人員在《基因組生物學(xué)與進(jìn)化》雜志上撰文稱，幸存下來的個(gè)體會(huì)在幾代人的時(shí)間里與其他個(gè)體交配，最終減少基因庫，并消除一個(gè)物種“繁殖”出有害基因突變的能力。

"Evolution is only really effective in large populations," added Lynch, adding that as populations "get smaller, the ability of selection to remove bad mutations – and select for the good ones – is decreased because the strength of randomness is greater than selection."

林奇補(bǔ)充說:“進(jìn)化只有在大種群中才真正有效。”他還補(bǔ)充說，隨著種群“變小，淘汰不好的突變并選擇好的突變的能力就會(huì)減弱，因?yàn)殡S機(jī)的力量大于選擇。”

The final nail on the Wrangel Island mammoth coffin remains mysterious, but the authors note that it is clear that it is likely no coincidence that the population declined shortly after isolation. The findings build on an understanding of the final days of the mammoths, highlighting that the last of their kind likely suffered from a variety of genetic effects that hindered their development, reproduction, and ability to smell. Lynch notes that there are several limitations associated with the study.

弗蘭格爾島猛犸象棺木的最后一顆釘子仍然是個(gè)謎，但作者指出，很明顯，猛犸象的數(shù)量在隔離后不久就減少了，這可能不是巧合。這些發(fā)現(xiàn)建立在對猛犸象最后的日子的理解之上，強(qiáng)調(diào)它們最后的種類可能遭受了各種各樣的遺傳影響，這些影響阻礙了它們的發(fā)育、繁殖和嗅覺能力。林奇指出，這項(xiàng)研究有幾個(gè)局限性。

"Alas, we have only one Wrangel Island genome. It could be some of these mutations are unique to that individual rather than the population. More genomes would help us know this. Also, we don't have mammoth cells to test these genes in, so maybe the mammoth cells had ways to minimize the deleterious mutations we found," he said.

“唉，我們只有一個(gè)弗蘭格爾島基因組?？赡苓@些突變中有一些是個(gè)體特有的而不是群體特有的。更多的基因組將幫助我們了解這一點(diǎn)。另外，我們沒有猛犸象細(xì)胞來測試這些基因，所以也許猛犸象細(xì)胞有辦法將我們發(fā)現(xiàn)的有害突變最小化。”他說。

Even so, Lynch tells IFLScience that this is a “cautionary tale” for living species threatened with extinction who may face similar genetic challenges in the generations to come.

即便如此，林奇告訴IFLScience，這對那些面臨滅絕威脅的物種來說是一個(gè)“警示故事”，這些物種可能在未來幾代面臨類似的基因挑戰(zhàn)。