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Study Finds Evidence of Rapid Genetic Drift in Icelandic Mitochondria

NEW YORK (GenomeWeb News) – Icelandic mitochondrial DNA appears to have undergone more rapid genetic drift than other European populations, according to a new study on ancient mtDNA.

A team of researchers from Iceland, Spain, and the Netherlands sequenced mtDNA from dozens of Icelandic skeletons believed to be at least a thousand years old. The research, appearing online today in PLoS Genetics, suggest that the mitochondrial genomes of ancient Icelandic populations were more similar to those of existing populations in Europe and the UK than to present day Icelandic populations. Consequently, the team concluded that Icelandic populations have experienced higher rates of genetic drift than the original populations that settled the country.

"The results demonstrate that the ancient Icelandic mtDNA sequences are more closely related to sequences from present inhabitants of Scotland, Ireland, and Scandinavia (and several other European populations), than to those from their descendants, the modern Icelandic population," senior author Kari Stefánsson, CEO of deCODE genetics, and his colleagues wrote. "We propose that this is due to a faster genetic drift in the Icelandic mtDNA pool during the last 1,100 years, than in the mtDNA pools of the larger European settlements."

Archaeological and historical records suggest individuals from the British Isles and Scandinavia settled Iceland about 1100 years ago. Based on studies of today's Icelanders, researchers have suggested that less than 40 percent of Icelandic mtDNA — representing matrilinear ancestry — originated in Scandinavia, while as much as 75 to 80 percent of their paternal ancestry, gauged by Y-chromosome DNA, is Scandinavian.

That is consistent with the notion that Scandinavian men, namely Vikings, frequently settled the area with women from other regions.

Although the population hasn't been isolated long enough to have accumulated mutations in the gene pool, Stefánsson and his co-authors explained, the relatively small size of Iceland's population make it prone to genetic drift.

In an effort to understand genetic changes in Iceland's population over the relatively short time since the country was settled, the researchers found a DNA source that could give them a window into history: ancient skeletons housed in the National Museum of Iceland.

The precise age of the skeletons isn't yet known since only a fraction of the museum's more than 780 skeletons have been carbon dated. Even so, the authors explained, because Icelanders converted to Christianity about a thousand years ago, it's possible to classify the skeleton as being more or less than a thousand years old based on burial practices. The researchers noted that this classification scheme seems to be consistent with carbon-dating, when available.

Stefánsson and his co-workers initially extracted DNA from 95 skeletons: 91 skeletons classified as "pre-Christian" and four skeletons classified as either "Christian" or "unknown." The skeletons came from 67 different sites and were discovered between 1880 and 1996 during archaeological digs, by accident during construction work, or as a result of soil erosion.

The team attempted to amplify and clone six fragments of the mtDNA control region for each of the samples and sequenced the mtDNA using an ABI3730. A dozen of these samples were also independently sequenced at a lab in Barcelona. After doing quality control analysis and screening, the researchers were left with mitochondrial sequences representing 73 individuals.

Before doing population analyses, the researchers excluded another five samples due to a lack of information about the age of the remains or because their mtDNA samples were identical to those found in nearby graves.

As expected, the researchers found that early Icelanders' maternal lineage was largely Scottish and Irish. They found that an estimated 58 percent of modern day Icelanders' maternal ancestry is Scottish and Irish. For ancient Icelanders, that estimate was even higher — in the skeletal samples tested, the maternal lineage was estimated to be 65 percent Scottish and Irish.

But less than half of the ancient haplotypes — just 40 percent — matched sequences found in mtDNA sequences from more than 800 modern day Icelanders.

In contrast, when they compared a slightly smaller stretch of ancient Icelandic mtDNA, the researchers found more shared sequences in other European groups, including populations from Scotland, Ireland, the North Atlantic islands, and Scandinavia.

The team concluded that the results were attributable to genetic drift following a population bottleneck. Based on data extrapolated from deCODE's genealogical database, they found evidence suggesting half of the matrilinear sequences carried by Icelanders born after 1900 are derived from just 541 common matrilinear ancestors.

"[T]he population size of Iceland and the other North Atlantic islands has been small during the last 1100 years, compared to the other groups included in these analyses," the authors wrote. "Accordingly, there is evidence that the gene pools of these small populations have been more heavily affected by genetic drift than those of larger European populations."

The researchers expressed enthusiasm that the study would pave the way for more large studies of ancient DNA in other populations. "It is the first such study to be large enough to permit meaningful statistical methods to be applied to ancient DNA," Stefánsson said in a statement. "We very much hope this will aid and encourage others to follow with large studies in other parts of the world."

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