生物谷：這的確有些不可思議,。德國科學(xué)家利用一顆遠(yuǎn)古牙齒化石,，成功確定了乳齒象完整的線粒體基因組，這也是迄今為止科學(xué)家得到的最古老的線粒體基因組,。該研究成果有望加深科學(xué)家對于象類分化的理解,。相關(guān)論文發(fā)表在的7月24日的《PLoS生物學(xué)》上。
2800萬年前出現(xiàn)的乳齒象是現(xiàn)代大象的近親,，它們大約有3米高,，有和猛犸一樣的長毛。但是,，乳齒象主要生活在森林里,，以樹木的枝葉為食。1萬年前，它們在北美洲北部滅絕了,。最新研究的這顆乳齒象牙齒化石發(fā)現(xiàn)于美國阿拉斯加州北部,，距今已經(jīng)有5—13萬年的歷史，科學(xué)家當(dāng)時(shí)并未發(fā)現(xiàn)乳齒象身體的其他骨骼化石,。
DNA測序表明,，乳齒象與現(xiàn)代大象的祖先在2500萬年前開始彼此分離。領(lǐng)導(dǎo)該項(xiàng)研究的德國馬普進(jìn)化人類學(xué)研究所（Max Planck Institute for Evolutionary Anthropology）的Michael Hofreiter表示,，“這一古老的化石和完整的測序本身就是一件有趣的事情,。”
長期以來，科學(xué)家無法確定亞洲象,、非洲象和多毛的猛犸之間的關(guān)聯(lián),，這是由于現(xiàn)代大象沒有活著的近親——與現(xiàn)代大象關(guān)系最近的是海洋中的儒艮（Dugong）和類嚙齒動(dòng)物蹄兔。而新的研究結(jié)果無疑將有助于解決關(guān)于現(xiàn)代大象祖先的爭論,。
研究人員在現(xiàn)代象,、猛犸和乳齒象DNA測序結(jié)果的基礎(chǔ)上，構(gòu)建了象類的家族圖譜,。該圖譜表明,，與一般認(rèn)識不同，亞洲象與非洲象的親密程度不及亞洲象與猛犸,。據(jù)研究人員估測,，非洲象與亞洲象和猛犸在大約760萬年前開始分道揚(yáng)鑣，隨后大約不到100萬年,，亞洲象和猛犸也發(fā)生了分離,。
Hofreiter表示，這些事件很像人類,、黑猩猩和大猩猩的分化,。“科學(xué)家通常認(rèn)為，當(dāng)時(shí)非洲的氣候逐漸干燥,，森林變得稀少,，而草地卻更加繁茂。”
在最新的研究之前,，進(jìn)行線粒體基因組測序最古老物種是3.3萬年前的猛犸,，而唯一一種被完整測序的動(dòng)物是公元1500年于新西蘭滅絕的恐鳥（Moa）,，最新的研究無疑打破了這些紀(jì)錄,。
丹麥哥本哈根大學(xué)的Eske Willerslev最近剛剛對格陵蘭島45—80萬年前的細(xì)菌線粒體DNA進(jìn)行了測序。他說,，“DNA絕對可以在極低的溫度下維持?jǐn)?shù)百萬年,。不過，沒有人知道它們最長能存活多少時(shí)間。”（科學(xué)網(wǎng) 任霄鵬/編譯）
原始出處:
Received: January 19, 2007; Accepted: May 24, 2007; Published: July 24, 2007
Proboscidean Mitogenomics: Chronology and Mode of Elephant Evolution Using Mastodon as Outgroup
Nadin Rohland1, Anna-Sapfo Malaspinas2,3, Joshua L. Pollack2, Montgomery Slatkin2, Paul Matheus4, Michael Hofreiter1*
1 Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany, 2 Department of Integrative Biology, University of California Berkeley, Berkeley, California, United States of America, 3 Department of Zoology and Animal Biology, University of Geneva, Geneva, Switzerland, 4 Alaska Quaternary Center, University of Alaska Fairbanks, Fairbanks, Alaska, United States of America
We have sequenced the complete mitochondrial genome of the extinct American mastodon (Mammut americanum) from an Alaskan fossil that is between 50,000 and 130,000 y old, extending the age range of genomic analyses by almost a complete glacial cycle. The sequence we obtained is substantially different from previously reported partial mastodon mitochondrial DNA sequences. By comparing those partial sequences to other proboscidean sequences, we conclude that we have obtained the first sequence of mastodon DNA ever reported. Using the sequence of the mastodon, which diverged 24–28 million years ago (mya) from the Elephantidae lineage, as an outgroup, we infer that the ancestors of African elephants diverged from the lineage leading to mammoths and Asian elephants approximately 7.6 mya and that mammoths and Asian elephants diverged approximately 6.7 mya. We also conclude that the nuclear genomes of the African savannah and forest elephants diverged approximately 4.0 mya, supporting the view that these two groups represent different species. Finally, we found the mitochondrial mutation rate of proboscideans to be roughly half of the rate in primates during at least the last 24 million years.
Figure 1. Appearance and Location of Origin of the Mastodon Tooth (IK-99–237) Used for Sequencing of the mtDNA Genome
The scale bar shows centimeters.
全文鏈接：
http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0050207
