Light cast on selective horse breeding from 2700 years ago

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The excavation of the Kurgan Arzhan 2 site in Tuva, Siberia. This is tomb number 16, revealing 14 exhumed horse skeletons. The tomb dates back 2700 years. Photo: © Michael Hochmuth, German Archaeological Institute, Berlin
The excavation of the Kurgan Arzhan 2 site in Tuva, Siberia. This is tomb number 16, revealing 14 exhumed horse skeletons. The tomb dates back 2700 years. © Michael Hochmuth, German Archaeological Institute, Berlin

Nomads from Central Asia were selectively breeding their horses during the Iron Age, research has revealed, showing a preference for robust body shape and good udders, pointing to their use of mare’s milk.

The international team of scientists has shed light on animal domestication by studying the genes of Scythian horses.

Nomadic Scythian herders roamed vast areas of the Central Asian steppes during the Iron Age, between 2900 and 2100 years ago.

They were known for their exceptional equestrian skills, and their leaders were buried with sacrificed stallions at grand funeral ceremonies.

The researchers sequenced the genomes of thirteen 2300 to 2700-year-old stallions from the Scythian royal burial tombs of Arzhan, found in the Russian Republic of Tuva, in the outer reaches of Mongolia, and Berel, in the Kazakh Altai Mountains.

They also sequenced the genome of a mare from an older culture, found in Chelyabinsk, Russia, which is 4100 years old.

 An excavated tomb at the Alaas Ebé archaeological site in Churapchinsky District, Yakutia, Russia.
An excavated tomb at the Alaas Ebé archaeological site in Churapchinsky District, Yakutia, Russia.  © Eric Crubézy

By studying the variants carried by certain specific genes, they were able to deduce a large diversity in the coat coloration patterns of Scythian horses, ranging from bay through chestnut to black.

Their analysis also revealed a total of 121 genes selected by Scythian breeders, most of which were involved in the development of forelimbs — the Scythians seem to have had a preference for horses with a robust body shape — but also in the development of mammary glands, which corroborates the view that mare’s milk has been used for millennia.

In their work, the researchers identified the genomic regions in which adaptive mutations were concentrated during the first three millennia of horse domestication, which began some 5500 years ago.

These regions often carry genes linked to a population of cells in the embryo known as the neural crest, which gives rise to many of the organism’s tissues. This provides some of the first experimental evidence in favor of the “neural crest hypothesis”, which aims to explain why all domestic animals, despite independent lineages, have converged toward common physical and behavioral characteristics.

Kazakh horses in North Central Kazakhstan. Photo: Ludovic Orlando, Natural History Museum of Denmark, CNRS.
Kazakh horses in North Central Kazakhstan. © Ludovic Orlando, Natural History Museum of Denmark, CNRS.

As the neural crest gives rise to many tissues, the reprogramming of this group of cells over the course of development can lead to the co-appearance of a host of traits of interest.

The over-representation of genes linked to the neural crest among genes carrying adaptive mutations suggests the importance of this structure for domestication.

By comparing the genetic diversity of these ancient horses with that of modern-day horses, the authors discovered that a demographic collapse has occurred over the last 2300 years, leading to a fall in the genetic diversity of horses.

This is due to breeding practices, which use an increasingly limited number of stallions in reproduction, to such an extent that, today, almost all domestic horses share the same Y chromosome, unlike Scythian horses.

These phenomena have been accompanied by an accumulation of harmful mutations.

The relatively recent accumulation of these harmful mutations, found in all domestic animals, contradicts the so-called “cost-of-domestication” hypothesis, which holds that this accumulation occurs from the earliest stages of domestication.

The researchers are extending their research to other human cultures in order to understand how the domestication of horses has influenced the destiny of civilizations.

The study team, whose findings were published in the journal Science, was led by Ludovic Orlando, a senior researcher with the Laboratory of Molecular Anthropology and Image Synthesis at France’s National Center for Scientific Research and professor at the Natural History Museum of Denmark.

Ancient genomic changes associated with domestication of the horse, Pablo Librado, Cristina Gamba, Charleen Gaunitz, Clio Der Sarkissian, Mélanie Pruvost, Anders Albrechtsen, Antoine Fages, Naveed Khan, Mikkel Schubert, Vidhya Jagannathan, Aitor Serres-Armero, Lukas F. K. Kuderna, Inna S. Povolotskaya, Andaine Seguin-Orlando, Sébastien Lepetz, Markus Neuditschko, Catherine Thèves, Saleh Alquraishi, Ahmed H. Alfarhan, Khaled Al-Rasheid, Stefan Rieder, Zainolla Samashev, Henri-Paul Francfort, Norbert Benecke, Michael Hofreiter, Arne Ludwig, Christine Keyser, Tomas Marques-Bonet, Bertrand Ludes, Eric Crubézy, Tosso Leeb, Eske Willerslev, Ludovic Orlando. Science, April 28, 2017. 
DOI: 10.1126/science.aam5298

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