Domestic horses derived from wild restocking.
Pure-bred Akhal-Teke horse image; Credit: © Shutterstock
Dr. Mikkel Schubert and many colleagues have added to the genetic foundations of our knowledge of horse with a paper today that reveals the
cost of domestication to the (extinct) species. We wrote on how
Wild Horses in America were ancestral to the Asian horses we now have as a domestic breed, but this information updates us on the effects of war, trade on the horses genetic hoofprint.
Five-and-a-half thousand years ago, the undomesticated wild animal was not encumbered with genes that made it more malleable, more useful and with locomotory, physiological and cognitive change in the phenotype. There was positive selection of domesticated horse genes that do not appear in the ancient Przewalskis horse. We have to remember that only 13 individuals contributed to the present Przewalski genome, as extinction almost caught them out.
The deleterious nature of the mutations concerned with domestication represent a cost to the animal. Mounting a horse, pulling a chariot and other transports and cavalry action made humans, with the horse, dominant over others. As wild horses are now extinct, the Arctic Taymyr Peninsula in Russia provided 2 ancient and frozen horse genomes that were compared with those of living horses. Without such preservation in frozen conditions, the latest techniques of genome preparation from the bone would still have been impossible. They were dated to the Late Pleistocene of Eurasia, 16,000 and 43,000 years old, when no domestication can have been carried out.
125 genes were identified as being associated with the domestic horses characteristics. They concern muscle and limb development, articular junctions and the heart; social behaviour, learning capability, fear response and something the researchers label
agreeableness! Taming is still a key feature of horse training, so we are assuming this word conveys much more than the simple lack of fear or ability to learn how to cooperate - much more!
Inbreeding is a necessary feature of domestication. People cannot always have access to a wide selection of animals, and the result can only be inbreeding, as we have found to our cost in the dog, our cattle and sheep and many others (including rice and tomatoes!) The necessarily-strong selection of certain traits that breeders wanted is also deleterious to the species. These genes are already known to be associated with the domestic horse. The validation by genome research is much more valuable in that we can now work on certainty, rather than suspicion that some gene creates a special phenotypic effect.
Large horses, then, and those that can pull carts or hold a man in armour on its back become obvious targets for selection. You can work out why the smaller animal could be useful in requiring less fodder or in negotiating a track. We are all aware of the cost of feeding a single horse, if the human love affair with the horse (or even gambling!) has affected us at all. The cost to the horse is also very important.
As domestication proceeded, deleterious genes became more common. Every time a population crashed or inbreeding occurred, the chance was there for these genes to accumulate. Luckily, ancient horse genomes contributed great variation to the modern horse, too. Restocking from wild populations undoubtedly would explain this variety as the limited genome of our Przewalski population has few variations, but plenty of deleterious genes. The sequencing of 5 domestic horse genomes revealed that at least 13% and probably much more was acquired by this restocking. Perhaps ancient breeders found their own stock inadequate and usefully managed to incorporate better characteristics into their herds.
This magnificent paper gives us ideas and cultural concepts about humans as well as horses, as they became involved in much of our history and economies. Take a look at photographs of Victorian London if you dont believe me. The authors are too numerous to mention, just as horse have been throughout their time with us. They come from universities such as Copenhagen, Santa Cruz and Berkeley(California), Berne, University College (London), Potsdam, Kentucky (of course), Louisville (also Kentucky), University College Dublin, King Saud (Riyadh) and Uppsala, with many other institutions also involved in several more nations. The paper is available as Prehistoric genomes reveal the genetic foundation and cost of horse domestication from PNAS.