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Mammoth is the brother of the Asian elephant, and the African elephant is their grandfather

17.02.2006


Approximately 10,000 years after the last mammoths used to roam across the North American and Eurasian spaces, they still remain an exciting subject of inquiry for researchers. Mammoths and elephants belong to the most ancient group of mammals, therefore, when studying mammoths the researchers reveal secrets of evolutionary origin of contemporary species. Discussions continue about genetic kinship of mammoths and contemporary elephants.



Now, Russian researchers working at several institutes of the Russian Academy of Sciences, Russian Academy of Medical Sciences and Lomonosov Moscow State University, as well as at Universities of Massachusetts and California managed to obtain independent proof this kinship. The researchers managed to reconstruct the most ancient (as of today) complete DNA sequence of mitochondrial genome received from remains of a mammoth that died about 33,000 years ago.

Professor Rogayev, Doctor of Biology, and his colleagues (from UMASS MS, Scientific Center of Mental Health, Russian Academy of Medical Sciences, Institute of General Genetics, Russian Academy of Sciences, Moscow State University, UCSD) published the article entitled “Complete mitochondrial genome and phylogeny of Pleistocene mammoth Mammuthus primigenius” on February 7 in the Plos Biology magazine, Public Library of Science (Plos), where they described the DNA sequence of 16,842 signs (16,842 pairs of nucleotides). This is a complete mitochondrial genome of a mammoth, whose remains had been extracted from the permafrost. The remains date back to the Pleistocene era – the period of time from 10,000 through 1.6-1.9 million years B.C. The investigations have showed that mammoths and Asia’s elephants are related species that drifted apart in the course of evolution soon after separation of their common ancestor from the line of African elephants.


Professor Rogayev and his colleagues used the DNA educed from a section of the mammoth’s leg with intact skin and muscles. Palaeontologists found it in Kolyma, in the Maly Anuy river basin in 1986. The age of the discovered specimen (determined with the help of radiocarbon method) is approximately between 31,950 and 33,750 years. After it was extracted from the permafrost, it turned out that the mammoth’s leg tissues were in a very good condition. Thanks to colleagues from Magadan, the samples were preserved for several years in a frozen condition. The researchers discovered that the DNA, regardless of degradation traces, was of very high quality and could be educed in sufficient quantity.

With the help of the polymerase chain reaction (PCR) method, the researchers twice reconstructed a complete mitochondrial genome: in the Russian laboratories back in 2000 and, independently, in University of Massachusetts – several years later. Findings of the unique and laborious work are published now.

DNA is present in the nucleus of each cell of the body, but cellular organellas – mitochondria – have their own genome (its size making about 16,500 pairs of nucleotides), which exists outside the cellular nucleus. It is mitochondrial DNA that offers the researchers the most precious information about evolutionary development. While the DNA in the nucleus undergoes recombination, in the process of which its sections received from father and mother get mixed, thus impeding genetic history interpretation, the mitochondrial DNA, which is inherited only maternally, allows to trace a more direct kindred line.

Using the interpreted mitochondrial genome sequence, Professor Rogayev and his colleagues determined evolutionary kinship between extinct mammoths. It does not always work out to judge correctly about kinship via comparing parameters of the body structure. Thus, for example, based on teeth characteristics, the assumption was made about immediate relationship of the mammoth and the Asia’s elephant, while the proboscis structure investigation refers the mammoth to the same group with the African elephant.

To solve the evolutionary puzzle, Professor Rogayev and his colleagues also performed sequencing of complete mitochondrial genomes of African and Asia’s elephants. To exclude even the minimal possibility of DNA contamination, the elephant samples investigation was started after the researchers determined primary sequence of the mammoth’s mitochondrial genome. The following evolutionary history analysis proved that the mammoth had a common ancestor line with the Asia’s elephant, the line being more contemporary than that with the African elephant. The researchers also pointed out that the lines of the mammoth and the Asia’s elephant diverged soon after their common ancestor’s line separated from that of the African elephant.

Utilization of the “ancient” DNA is connected with great difficulties. The fossilized remains’ DNA not only gets destroyed (normally, the researchers succeed at best to educe only short fragments, less than 100 to 300 pairs of nucleotides), but it also accumulates a lot of artifact mutations, which cause mistakes in determining the structure. Uniqueness of Professor Rogayev’s group effort is not only in the length of the interpreted DNA, but also in accuracy of its interpretation. The results of their investigation demonstrate that obtained long genomic sequences do not contain artifacts.

On top of establishing kinship between the mammoth and the contemporary elephants, the researchers also found out to what the extent different Pleistocene mammoths were genetically close the each other. Contemporary elephants’ populations are rather heterogeneous genetically. And mammoths from different regions of Siberia turned out to be very similar at the DNA level. That means that at least maternal genetic lines of mammoths that populated Siberia in different epochs are very close. Thus, preliminary findings assume that a rather genetically homogeneous mammoth population used to inhabit Siberia during the entire late Pleistocene epoch.

“Analysis of the complete mitochondrial DNA sequence for reconstruction of animal evolution history may become the most efficient method for determining kinship between extinct and now existent species, as well as for forensic medicine or ‘historic’ samples analysis”, says Evgeni Rogayev. This is the way to get genes of the animals that vanished from the face of the earth many thousands years ago.

Sergey Komarov | alfa
Further information:
http://www.informnauka.ru

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