Living plants have been generated from the fruit of a little arctic flower, the narrow-leafed campion, that died 32,000 years ago, a team of Russian scientists reports. The fruit was stored by an arctic ground squirrel in its burrow on the tundra of northeastern Siberia and lay permanently frozen until excavated by scientists a few years ago.
This would be the oldest plant by far that has ever been grown from ancient tissue. The present record is held by a date palm grown from a seed some 2,000 years old that was recovered from the ancient fortress of Masada in Israel.
Seeds and certain cells can last a long term under the right conditions, but many claims of extreme longevity have failed on closer examination, and biologists are likely to greet this claim, too, with reserve until it can be independently confirmed. Tales of wheat grown from seeds in the tombs of the pharaohs have long been discredited. Lupines were germinated from seeds in a 10,000-year-old lemming burrow found by a gold miner in the Yukon. But the seeds, later dated by the radiocarbon method, turned out to be modern contaminants.
Despite this unpromising background, the new claim is supported by a firm radiocarbon date. A similar avenue of inquiry into the deep past, the field of ancient DNA, was at first discredited after claims of retrieving dinosaur DNA proved erroneous, but with improved methods has produced spectacular results like the reconstitution of the Neanderthal genome.
The new report is by a team led by Svetlana Yashina and David Gilichinsky of the Russian Academy of Sciences research center at Pushchino, near Moscow, and appears in Tuesday’s issue of The Proceedings of the National Academy of Sciences of the United States of America.
“This is an amazing breakthrough,” said Grant Zazula of the Yukon Paleontology Program at Whitehorse in Yukon Territory, Canada. “I have no doubt in my mind that this is a legitimate claim.” It was Dr. Zazula who showed that the apparently ancient lupine seeds found by the Yukon gold miner were in fact modern.
But the Russians’ extraordinary report is likely to provoke calls for more proof. “It’s beyond the bounds of what we’d expect,” said Alastair Murdoch, an expert on seed viability at the University of Reading in England. When poppy seeds are kept at minus 7 degrees Celsius, the temperature the Russians reported for the campions, after only 160 years just 2 percent of the seeds will be able to germinate, Dr. Murdoch noted.
The Russian researchers excavated ancient squirrel burrows exposed on the bank of the lower Kolyma River, an area thronged with mammoth and woolly rhinoceroses during the last ice age. Soon after being dug, the burrows were sealed with windblown earth, buried under 125 feet of sediment and permanently frozen at minus 7 degrees Celsius.
Some of the storage chambers in the burrows contain more than 600,000 seeds and fruits. Many are from a species that most closely resembles a plant found today, the narrow-leafed campion (Silene stenophylla).
Working with a burrow from the site called Duvanny Yar, the Russian researchers tried to germinate the campion seeds, but failed. They then took cells from the placenta, the organ in the fruit that produces the seeds. They thawed out the cells and grew them in culture dishes into whole plants.
Many plants can be propagated from a single adult cell, and this cloning procedure worked with three of the placentas, the Russian researchers report. They grew 36 ancient plants, which appeared identical to the present day narrow-leafed campion until they flowered, when they produced narrower and more splayed-out petals. Seeds from the ancient plants germinated with 100 percent success, compared with 90 percent for seeds from living campions.
The Russian team says it obtained a radiocarbon date of 31,800 years from seeds attached to the same placenta from which the living plants were propagated.
The researchers suggest that special circumstances may have contributed to the remarkable longevity of the campion plant cells. Squirrels construct their larders next to permafrost to keep seeds cool during the arctic summers, so the fruits would have been chilled from the start. The fruit’s placenta contains high levels of sucrose and phenols, which are good antifreeze agents.
The Russians measured the ground radioactivity at the site, which can damage DNA, and say the amount of gamma radiation the campion fruit accumulated over 30,000 years is not much higher than that reported for a 1,300-year-old sacred lotus seed, from which a plant was successfully germinated.
The Russian article was edited by Buford Price of the University of California, Berkeley. Dr. Price, a physicist, chose two reviewers to help him. But neither he nor they are plant biologists. “I know nothing about plants,” he said. Ann Griswold, a spokeswoman for PNAS, as the journal is known, said the paper had been seen by an editorial board member who is a plant biologist.
Tragedy has now struck the Russian team. Dr. Gilichinksy, its leader, was hospitalized with an asthma attack and unable to respond to questions, his daughter Yana said on Friday. On Saturday, Dr. Price reported that Dr. Gilichinsky had died of a heart attack.
Eske Willerslev, an expert on ancient DNA at the University of Copenhagen, said the finding was “plausible in principle,” given the conditions in permafrost. But the claim depends on the radiocarbon date being correct: “It’s all resting on that — if there’s something wrong there it can all fall part.”
If the ancient campions are the ancestors of the living plants, this family relationship should be evident in their DNA. Dr. Willerslev said that the Russian researchers should analyze the DNA of their specimens and prove that this is the case. However, this is not easy to do with plants whose genetics are not well studied, Dr. Willerslev said.
If the claim is true, then scientists should be able to study evolution in real time by comparing the ancient and living campions. Possibly other ancient species can be resurrected from the permafrost, including plants that have long been extinct.
西伯利亞凍原之下 三萬年種子被喚醒 開花結果
研究人員採果實內含糖細胞作「胎盤」 技術若持續發展 或可復育長毛象等冰原時期動物
三萬多年前埋入西伯利亞凍原下的果實種子被成功「喚醒」，培育成完整的顯花植物，並且開花結實。這項技術如果持續發展，可望用來復育冰原時期的哺乳類動物，例如長毛象。
美國的「國家科學院院刊」（Proceedings of the National Academy of Sciences）21日刊出研究結果。這是目前為止科學家復活的最古老植物「柳葉蠅子草」（Silene stenophylla）。洛杉磯加州大學植物學家沈米勒指出：「植物組織在三萬年後仍能發育，這是非常有趣的進展。」
沈米勒曾讓一顆來自中國大陸北方、一千三百年前的蓮花種子復育成功。另一組研究團隊在2005年時曾讓以色列死海附近，一顆儲藏兩千年的椰棗種子復育成功。
主導這項研究的俄羅斯科學院專家指出，這個最新發現可能是古代生物素材研究的重要里程碑，並凸顯永凍土在「尋找古代基因庫的重要性，這些先前存在的生命原本假定早就從地表消失」。
科學家在俄國東北部西伯利亞柯里馬河河岸下20到40公尺深處，發現三萬多年前冰河時期松鼠冬眠的洞穴，所在地層含有晚更新世的長毛象、披毛犀、野牛、馬、鹿以及其他動物等大型哺乳類的骨頭。
科學家之所以能使如此古老的植物種子復活，主要原因在於這些冬眠洞穴很快就被冰覆蓋，接著就維持「持續冰凍且從未解凍」，這實際上避免永凍土劣化。
研究人員試圖使採集到的植物種子發芽，但未成功。不過他們最後找到辦法，取用果實內富含糖的細胞作為「胎盤」，並讓植物在實驗室生長。
柳葉蠅子草這種植物目前仍在西伯利亞凍原繁衍，但花瓣形狀與花朵性別與它的遠祖略有不同。論文作者之一的古賓表示：「如果運氣好，我們還能找到某些結凍松鼠的組織，最後有可能讓我們找到長毛象，予以復育。」
日本研究團隊已在同一區域搜尋長毛象的遺骸。古賓說：「這是我們的地盤，我們會設法率先拿到。」
原文參照：
http://www.nytimes.com/2012/02/21/science/new-life-from-an-arctic-flower-that-died-32000-years-ago.html
2012-02-22/聯合報/A1版/要聞 編譯李京倫
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