Since the horse was domesticated around 5,500 years ago, the invention of the saddle, carts and cavalry have significantly transformed human civilization. The genetics behind the domestication of horses has been difficult to reconstruct because wild horses are almost extinct.

A collectively authored international study published in the Proceedings of the National Academy of Sciences on Monday 15 December, led by Ludovic Orlando, researcher at the Center for GeoGenetics at the University of Copenhagen (Denmark), concludes that a positive selection of the genes involved in locomotion, physiology and cognition took place during the domestication of the horse, and that modern horses have an excess of deleterious mutations, which is probably the genetic cost of domestication. This hypothesis has also been formulated for cereals and vegetables, as well as for other pets.

Among those participating in this study were Tomàs Marquès-Bonet, ICREA researcher at the Department of Experimental and Health Sciences at UPF (CEXS) and head of the Comparative Genomics Laboratory of the Institute of Evolutionary Biology, a joint CSIC-UPF centre. Marquès-Bonet also works at the National Genome Analysis Centre in Barcelona.

The genomes of ancient, wild and modern ungulates were compared

Recent breakthroughs in research on ancient DNA have made it possible to reconstruct the genomes of ancient individuals. Scientists have thus been able to sequence the genomes of two horses from Taymyr (Russia), for which the archaeological evidence is thought to predate the domestication of the species.

They compared the genomes of these ungulates with the genome of domesticated horses and the genome of one of the few wild horses still existing today - Przewalski's horse.

According to the authors of the study, a significant part of the genetic variation of modern domesticated horses could be attributed to restocking from the extinct population of wild horses different from Przewalski's horse, which does not have this genetic variation.

There is a positive selection of characters associated with domestication

The study also identified a set of genes that have undergone positive selection during the evolutionary process. A group of these genes govern aspects that may be related to the domestic use made of horses by the human species, which affect muscle and limbic development, the joints and the cardiac system. A second series of adaptations is related to genes associated with cognitive functions, including social behaviour, learning ability, fear and pleasure, which the authors attribute to domestication.

Inbreeding and an excess of deleterious mutations

Another issue highlighted in this paper is that domestication was associated with inbreeding and an excess of deleterious mutations, which corroborates the "cost of domestication" hypothesis previously formulated for rice and tomatoes, and other domesticated species, such as the dog.

As a result, sequencing the genome of ancient and modern horses shows which genes were favoured by humans during the domestication process, but also reveals the cost of domestication. It is also possible to infer the contex t of the population in which domestication took place, and a significant genetic contribution to modern domestic breeds by an extinct population of wild horses.

For further reference, see:

Mikkel Schubert, Hákon Jónsson, Dan Chang, Clio Der Sarkissiana, Luca Erminia, Aurélien Ginolhac, Anders Albrechtsen, Isabelle Dupanloup, Adrien Foucal, Bent Petersen, Matteo Fumagalli, Maanasa Raghavan, Andaine Seguin-Orlando, Thorfinn S. Korneliussen, Amhed M. V. Velazquez, Jesper Stenderup, Cindi A. Hoover, Carl-Johan Rubin, Ahmed H. Alfarhan, Saleh A. Alquraishi, Khaled A. S. Al-Rasheid, David E. MacHugh, Ted Kalbfleisch, James N. MacLeod, Edward M. Rubin, Thomas Sicheritz-Ponten, Leif Andersson, Michael Hofreiter, Tomas Marques-Bonet, M. Thomas P. Gilbert, Rasmus Nielsen, Laurent Excoffier, Eske Willerslev, Beth Shapiro, Ludovic Orlando (2014), "Prehistoric genomes reveal the genetic foundation and cost of horse domestication", PNAS, 15 December.