Genetics

Making genetic sense of horses

Print edition : July 26, 2013

Two pieces of the 700,000-year-old horse metapodial bone, just before being extracted for ancient DNA. Credit: Ludovic Orlando Photo: Ludovic Orlando

SCIENTISTS at the Centre for GeoGenetics at the Natural History Museum of Denmark (University of Copenhagen) have sequenced the genome from a horse that lived 700,000 years ago by analysing short pieces of DNA molecules preserved in its bone remnants that had remained frozen in the permafrost of Yukon, Canada.

This would be by far the oldest genome sequences. By tracking the genomic changes that transformed prehistoric wild horses into domestic breeds, the researchers have revealed the genetic make-up of modern horses with unprecedented details. The spectacular results are now published in the journal Nature. Ludovic Orland and Eske Willerseley from the Centre for GeoGenetics, in collaboration with Danish and international colleagues, carried out this work.

DNA molecules can survive in fossils well after an organism dies—not as whole chromosomes, but as short pieces that could be assembled back together, like a puzzle. Sometimes enough molecules survive so that the full genome sequence of an extinct species can be resurrected. Over the years, the full genome sequences of a few ancient humans and archaic hominins have been characterised. But so far none dated back to before 70,000 years.

By comparing the genome in the 700,000-year-old horse with the genome of a 43,000-year-old horse, six present-day horses and the donkey, the researchers could estimate how fast mutations accumulate through time and calibrate a genome-wide mutation rate. This revealed that the last common ancestor of all modern equids lived about 4-4.5 million years ago. Therefore, the evolutionary tree underlying the origin of horses, donkeys and zebras reaches back in time twice as long as previously thought. Additionally, this new clock revealed multiple episodes of severe demographic fluctuation in horse history, in phase with major climatic changes such as the Last Glacial Maximum, some 20,000 years ago.

R. Ramachandran

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