NEW YORK (GenomeWeb News) – Mitochondrial DNA harvested from ancient mammoth hair indicates that animals from two very different woolly mammoth clades roamed Siberia.
In a paper appearing in the Proceedings of the National Academy of Sciences this week, an international team of researchers delved into the history and population structure of Siberian woolly mammoths using a combination of comparative mitochondrial genomics and carbon dating. The research indicates that the two woolly mammoth clades overlapped temporally, but had different geographical ranges and extinction patterns — with one clade disappearing before they had the opportunity to interact with humans.
“This discovery is particularly interesting because it rules out human hunting as a contributing factor, leaving climate change and disease as the most probable cause of extinction,” co-senior author Stephan Schuster, a biochemist and molecular biologist at Pennsylvania State University, said in a statement.
Woolly mammoths resembled furry elephants with long, curling tusks. Most died out about 11,000 years ago, with the last living mammoths disappearing a few thousand years later. Now, mammoth specimens are limited to preserved carcasses and museum exhibits.
But that hasn’t stopped researchers such as Schuster and his team from doing sequencing analyses on mammoths. In a paper published last September in Science, they shotgun sequenced ten woolly mammoth mitochondrial genomes from mtDNA in mammoth hair shafts. For their latest paper, the researchers sequenced the mitochondrial genomes of five more woolly mammoths that died between 13,000 and 60,000 years ago with up to 73-fold coverage using a 454 Life Sciences’ FLX sequencer as well as standard PCR.
Based on the 300,000 nucleotides of sequence obtained, the researchers constructed five new woolly mammoth mitochondrial genomes and compared them with 13 that had been deciphered previously. Their results suggest that Siberian woolly mammoths consisted of two very different clades.
Clade I includes animals that lived some 14,000 to 18,500 years ago and spanned sites more than 3,700 miles east to west. Animals belonging to clade II, on the other hand, lived long before this and so far appear to be limited to sites just 700 miles or so across northern Siberia.
For example, the researchers noted that the two clade II animals assessed in this study lived more than 58,000 years ago, making them “the oldest of the 18 mammoth mitochondrial genomes that have so far been reconstructed.”
Based on their Bayesian phylogenetic analysis of mammoth, elephant (African and Asian), and mastodon mtDNA, the researchers estimated that the woolly mammoth clades diverged about 1 or 2 million years ago. That’s roughly a quarter of the genetic distance between woolly mammoths and each of their living relatives, the African elephants (about 7.8 million years) and Asian elephants (about 6.5 million years).
While there appears to have been a time during which the two mammoth clades co-existed, the team’s work indicates that clade II disappeared from mainland Siberia as many as 30,000 years before clade I, raising questions about the evolutionary relationship and difference in survival between the clades. The authors speculated, for instance, that the clade II extinction may have been a consequence of that group’s limited geographical distribution compared to clade I.
In general, the researchers found that, although many mammoths lived over a much larger geographical region than elephants, their genetic diversity was comparatively very low.
“This low genetic divergence is surprising because the woolly mammoth had an extraordinarily wide range: from Western Europe, to the Bering Strait in Siberia, to Northern America,” co-senior author Webb Miller, a Penn State biologist and computer scientist, said in a statement. “The low genetic divergence of mammoth, which we discovered, may have degraded the biological fitness of these animals in a time of changing environments and other challenges.”
The study also highlights the value of hair samples. The team noted that hair samples —particularly hair shafts — contain copious amounts of mitochondrial DNA that is ideal for analysis and easily decontaminated. In a paper published in May, some members of the same research team used mitochondrial DNA from ancient human hair to better understand human population patterns in the far North.
“We plan to continue using our techniques to untangle the secrets of populations that lived long ago and to learn what it might have taken for them to survive,” Schuster said. “Many of us also have a personal interest in learning as much as we can about how any species of large mammal can go extinct.”
The mammoth mtDNA sequences have been added to GenBank.