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First Posted: June 27, 2013
Jun 27, 2013

Horse Evolutionary Full Genome Sequence/Przewalski's Horse


Image ©HorseHints.org


©HorseHints.org

Przewalski's Horse (Equus ferus przewalskii or Equus caballus przewalskii, classification is debated), pronounced in English as (væl.ski), also known as the Asian Wild Horse or Mongolian Wild Horse, or Takhi, is the closest living wild relative of the domestic Horse.

Most "wild" horses today, such as the American Mustang, are actually feral animals, horses that were once domesticated but escaped and reverted to an apparently wild status. The Przewalski's Horse, on the other hand, has never been successfully domesticated and remains a truly wild animal today. There were once several types of Equid that had never been successfully domesticated, including the Tarpan, Onager, and others. However, most have become extinct, with the Przewalski's Horse the only remaining truly wild horse in the world.

Poliakov, who concluded that the animal was a wild horse species, gave it the official name Equus przewalskii (Poliakov 1881). However, authorities differ about the correct classification. Some hold it is a separate species, the last remnant of the wild horse Equus ferus, others hold it is a subspecies of Equus caballus. The question will only be answered with finality if or when the common ancestor from which domestic and Przewalski's horses diverged is determined. Although the Przewalski's horse has 66 chromosomes, compared to 64 in a domestic horse, the Przewalski's horse and the domestic horse are the only equids that cross-breed and produce fertile offspring, possessing 65 chromosomes. ..."

It has recently come to light that the Przewalski's horse, recently brought back from the brink of extinction in Mongolia, is truly the last remaining wild horse. A specimen of a Przewalski's horse was found in the Canadian Arctic permafrost. This specimen was found to be more than one half a million years old. That DNA has been buried for 7,000 centuries! An article was published in Nature News-"First horses arose 4 million years ago" and released June 2013. With this finding the prevailing wisdom is moot. That ancient genome is 10 times as old as any recovered so far! This finding pushes back the equine lineage by approximately two million years. Also, from this finding, a host of evolutionary insights have come to light. Twenty-nine regions in the genome of domestic horses have been identified. This DNA shows that there is statistical evidence meaning that varient genes in these regions were favored as horses became domesticated. The researchers will have to carefully determine which of these genes targeted selection. Also significent in this new discovery of horse genome is that the Przewalski's horse shows no sign of inter-breeding with domestic horses. To read this article in its entirety: "First horses arose 4 million years ago"

In an article review, "Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse", ({Nature (2013} doi:10.1038/nature12323 Received 30 October 2012; Accepted 30 May 2013; Published online 26 June 2013) the following appeared: "The rich fossil record of equids has made them a model for evolutionary processes1. Here we present a 1.12-times coverage draft genome from a horse bone recovered from permafrost dated to approximately 560-780 thousand years before present (kyr bp)2, 3. Our data represent the oldest full genome sequence determined so far by almost an order of magnitude. For comparison, we sequenced the genome of a Late Pleistocene horse (43?kyr bp), and modern genomes of five domestic horse breeds (Equus ferus caballus), a Przewalski's horse (E. f. przewalskii) and a donkey (E. asinus). Our analyses suggest that the Equus lineage giving rise to all contemporary horses, zebras and donkeys originated 4.0-4.5? million years before present (Myr bp), twice the conventionally accepted time to the most recent common ancestor of the genus Equus4, 5. We also find that horse population size fluctuated multiple times over the past 2?Myr, particularly during periods of severe climatic changes. We estimate that the Przewalski's and domestic horse populations diverged 38-72?kyr bp, and find no evidence of recent admixture between the domestic horse breeds and the Przewalski's horse investigated. This supports the contention that Przewalski's horses represent the last surviving wild horse population6. We find similar levels of genetic variation among Przewalski's and domestic populations, indicating that the former are genetically viable and worthy of conservation efforts. We also find evidence for continuous selection on the immune system and olfaction throughout horse evolution. Finally, we identify 29 genomic regions among horse breeds that deviate from neutrality and show low levels of genetic variation compared to the Przewalski's horse. Such regions could correspond to loci selected early during domestication. ..."

For More Information:

My friend, Frank Harrell, has an emotional poem set to music which is quite moving and relevant to this article. Follow this link then click on the play song icon in the upper left: Equus Caballus, by Joel Nelson, Music, Wylie Gustafson
Red List of Threatened Species/Equus ferus ssp. przewalskii
Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse
"DNA Buried 7,000Centuries Is Retrieved", New York Times International, Thursday, June 27, 2013

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