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Mummy Mitochondrial Genome Sequence Yields Clues About Human Evolution

NEW YORK (GenomeWeb News) – Researchers reported today that they have sequenced the complete human mitochondrial genome of the Tyrolean Iceman, whose mummified carcass was discovered in the Eastern Alps nearly two decades ago.
 
In a paper appearing online today in Current Biology, researchers from the UK and Italy sequenced the mitochondrial genome of an individual, nicknamed “Ötzi,” who lived more than five millennia ago during the Late Neolithic-Copper Age. Based on phylogenetic analysis of the mitochondrial sequence, the team found that Ötzi belonged to the so-called K1 haplogroup, still relatively common in Europe. But the Iceman does not appear to be closely related to any known European populations.
 
“The frequency of genetic lineages tends to change over time, due to random variations in the number of children people have — a process known as ‘genetic drift’ — and as a result, some variants die out,” co-author Martin Richards, an archeo-geneticist at the University of Leeds, said in a statement. “Our results suggest that Ötzi’s lineage may indeed have become extinct.”
 
Ötzi’s mummified corpse was discovered in the Alps near the Austrian-Italian border in 1991. Subsequent research suggests Ötzi died when he was about 46 years old, after being wounded by an arrow and receiving a blow to the head. The body, which has been displayed at the South Tyrol Museum of Archeology in Bolzano, Italy, since 1998, was almost completely preserved over 5,100 to 5,350 years by a process akin to freeze-drying.
 
Researchers have been dabbling in Ötzi’s genetics for nearly a decade. In the mid- to late-1990’s research teams from Munich and Oxford sequenced the first hypervariable segment of Ötzi’s mitochondrial DNA, revealing that the mummy belonged to a west Eurasian haplogroup K, which includes the sub-groups K1 and K2.
 
Then in 2000, researchers defrosted the mummy for the first time, looking at his intestinal contents. Along with genetic material from plants and animals that Ötzi consumed, researchers also found some human DNA, which appears to have come from cells in Ötzi’s intestines. Over the past six years, the team has been delving into mitochondrial DNA from the intestinal sample.
 
PCR amplification and sequencing of bits of Ötzi’s mitochondria suggested that the Iceman belonged to the K1 sub-group, but did not belong to any of the three known K1 branches.
 
For this paper, the researchers used a combination of Sanger sequencing and Roche 454 sequencing of 235 short, pooled PCR amplification products with a FLX Genome Sequencer to sequence the entire 16,569 base pair mitochondrial genome. They then compared Ötzi’s mitochondrial genome to those of 115 living individuals in the K haplogroup for whom complete mitochondrial sequences were available.
 
In agreement with previous research, the team found that Ötzi’s mitochondrial genome fell within a unique branch of the K1 haplogroup and did not match that of any other individuals sequenced so far. In particular, the mitochondrial genome contained transitions at positions 3513 and 8137, which the team used to define a new branch in K1 that they call Ötzi’s branch or K1ö.
 
Based on their analysis, the researchers determined that the mitochondrial lineage from which Ötzi is descended arose roughly 20,000 years ago. Over the past 5,000 or so years, the lineage has either disappeared or become exceedingly uncommon.
 
“[I]n the past, there was a group — a branch of the phylogenetic tree — of men and women sharing the same mitochondrial DNA,” senior author Franco Rollo, an anthropology researcher at Italy’s University of Camerino, said in a statement. “Apparently this group is no longer present. We don’t know whether it is extinct or it has become extremely rare.”
 
Ötzi is just the second person whose sequenced mitochondrial genome contains K1 sequence falling outside of the three characterized K1 branches. Earlier this year, researchers from the University of Copenhagen and elsewhere sequenced the mitochondrial genome of a Paleo-Eskimo who died between 3,400 and 4,500 years ago and found that the mitochondrial sequence was incompatible with those of present day Native American or Greenlander populations.
 
The results so far suggest that such haplotypes are present in less than one percent of K1 lineages as a whole and less than 0.05 percent of modern European mitochondrial lineages.
 
In the future, researchers say, similar techniques could aid research efforts on ancient samples, including studies of animal domestication. For their part, the team plans to try analyzing Ötzi's Y-chromosome. But, they say, this may prove difficult given that DNA from the Y-chromosome is less abundant than mitochondrial DNA. The researchers are also interested in determining whether they can find others who share Ötzi’s genetic lineage.
 
“It will be also very interesting to see whether the descendants of Ötzi are all extinct,” Rollo told GenomeWeb Daily News in an e-mail message. “At the present state of knowledge no one can claim to be the descendant of Ötzi but, who knows, perhaps in a lonely Alpine valley.”
 

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