生物谷：这的确有些不可思议。德国科学家利用一颗远古牙齿化石，成功确定了乳齿象完整的线粒体基因组，这也是迄今为止科学家得到的最古老的线粒体基因组。该研究成果有望加深科学家对于象类分化的理解。相关论文发表在的7月24日的《PLoS生物学》上。
2800万年前出现的乳齿象是现代大象的近亲，它们大约有3米高，有和猛犸一样的长毛。但是，乳齿象主要生活在森林里，以树木的枝叶为食。1万年前，它们在北美洲北部灭绝了。最新研究的这颗乳齿象牙齿化石发现于美国阿拉斯加州北部，距今已经有5—13万年的历史，科学家当时并未发现乳齿象身体的其他骨骼化石。
DNA测序表明，乳齿象与现代大象的祖先在2500万年前开始彼此分离。领导该项研究的德国马普进化人类学研究所（Max Planck Institute for Evolutionary Anthropology）的Michael Hofreiter表示，“这一古老的化石和完整的测序本身就是一件有趣的事情。”
长期以来，科学家无法确定亚洲象、非洲象和多毛的猛犸之间的关联，这是由于现代大象没有活着的近亲——与现代大象关系最近的是海洋中的儒艮（Dugong）和类啮齿动物蹄兔。而新的研究结果无疑将有助于解决关于现代大象祖先的争论。
研究人员在现代象、猛犸和乳齿象DNA测序结果的基础上，构建了象类的家族图谱。该图谱表明，与一般认识不同，亚洲象与非洲象的亲密程度不及亚洲象与猛犸。据研究人员估测，非洲象与亚洲象和猛犸在大约760万年前开始分道扬镳，随后大约不到100万年，亚洲象和猛犸也发生了分离。
Hofreiter表示，这些事件很像人类、黑猩猩和大猩猩的分化。“科学家通常认为，当时非洲的气候逐渐干燥，森林变得稀少，而草地却更加繁茂。”
在最新的研究之前，进行线粒体基因组测序最古老物种是3.3万年前的猛犸，而唯一一种被完整测序的动物是公元1500年于新西兰灭绝的恐鸟（Moa），最新的研究无疑打破了这些纪录。
丹麦哥本哈根大学的Eske Willerslev最近刚刚对格陵兰岛45—80万年前的细菌线粒体DNA进行了测序。他说，“DNA绝对可以在极低的温度下维持数百万年。不过，没有人知道它们最长能存活多少时间。”（科学网 任霄鹏/编译）
原始出处:
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.
全文链接：
http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0050207
