June 27, 2013
Prehistoric Horse Remains Lead To Oldest Genome Ever Sequenced
redOrbit Staff & Wire Reports - Your Universe Online
A fossilized bone fragment belonging to a prehistoric relative of modern-day horses has led to the oldest genome-sequencing effort of all time, according to new research published online Wednesday in the journal Nature.
According to Jonathan Ball of BBC News, the approximately 700,000-year-old fossil predates all previous DNA mapping efforts by more than 500,000 years. The bone was recovered from the Thistle Creek site in the west-central Yukon Territory of Canada, and paleontologists estimate the horse lived in the region at least half a million years ago, he added.
"The ancient find indicates that all horses today, as well as donkeys and zebras, shared a common ancestor that lived some four million years ago, twice as early as thought," AFPs Laurent Banguet explained. The breakthrough also raises hopes many fossils deemed useless for DNA sampling may in fact be crammed with genetic treasure, researchers said.
In addition, lead author Ludovic Orlando of Denmark's University of Copenhagen and his colleagues report their findings suggest ancient DNA could be recovered from frozen remains up to one million years old. That means it could be possible to someday map additional genomes of ancient pre-humans, USA Today writer Dan Vergano noted.
Orlando and an international team of experts began their work by comparing the genome of the 700,000 year old horse with that of a 43,000 year old horse, six modern horses, and a donkey. By doing so, they were able to estimate how quickly mutations accumulated through time, and thus establish a genome-wide mutation rate.
The researchers initially pulverized a fragment of the bone to recover its DNA, and then used cutting-edge genetic sequencing techniques in an attempt to map its genome. This approach proved inadequate, Ball said, so they switched to technology that allowed the direct analysis of single DNA molecules.
"Using high-powered computers and an existing horse genome sequence as a reference, the scientists sifted through the 12 billion sequencing reads to distinguish between DNA motifs belonging to the ancient horse and those from contaminating organisms, such as bacteria accumulated from the environment," the BBC News reporter said. "From the resulting equine DNA fragments, they reconstructed a draft of its genome."
Orlando's team was able to sequence approximately 70 percent of the horse's genome, Ball added.
Their work led them to find that the last common ancestor of all modern members of the horse family lived between 4.0 and 4.5 million years ago. This means the evolutionary radiation underlying the origin of horses, donkeys and zebras reached back in time twice as long as previously believed, and it also revealed several new instances of severe demographic fluctuation in the history of the equine species.
"The team assembled the 700,000-year-old gene map using 'short' DNA sequences preserved in a leg bone, a capability that technology has only recently afforded to researchers," Vergano explained. Had the DNA not been frozen for the entirety of the time since the horse died, team members expressed doubt even that would have been possible. In theory, the ancient horse DNA could be reconstructed and used to clone a version of the now-extinct species, a kind of 'de-extinction' now widely debated by genetics researchers.
According to Orlando, the bone fragment used in the process was a fragment of metapodial bone from the leg. It was approximately six inches long by 3.2 inches wide, and the radio-dating of the organic material on the ground surrounding the bone indicates it was deposited there approximately 735,000 years ago, he told Banguet. In addition, they detected a Y-chromosome, revealing the bone fragment belonged to a male horse.