Tracking the origins of Yakutian horses and the genetic basis for their fast adaptation to subarctic environments. PNAS December 15, 2015 112 (50) E6889-E6897; first published November 23, 2015;
Yakutia, Sakha Republic, in the Siberian Far East, represents one of the coldest places on Earth, with winter record temperatures dropping below −70 °C.
Nevertheless, Yakutian horses survive all year round in the open air due to striking phenotypic adaptations, including compact body conformations, extremely hairy winter coats, and acute seasonal differences in metabolic activities. The evolutionary origins of Yakutian horses and the genetic basis of their adaptations remain, however, contentious. Here, we present the complete genomes of nine present-day Yakutian horses and two ancient specimens dating from the early 19th century and ∼5,200 y ago. By comparing these genomes with the genomes of two Late Pleistocene, 27 domesticated, and three wild Przewalski's horses, we find that contemporary Yakutian horses do not descend from the native horses that populated the region until the mid-Holocene, but were most likely introduced following the migration of the Yakut people a few centuries ago. Thus, they represent one of the fastest cases of adaptation to the extreme temperatures of the Arctic.
We find cis-regulatory mutations to have contributed more than nonsynonymous changes to their adaptation, likely due to the comparatively limited standing variation within gene bodies at the time the population was founded.
Genes involved in hair development,body size, and metabolic and hormone signaling pathways represent an essential part of the Yakutian horse adaptivegenetic toolkit. Finally, we find evidence for convergent evolution with native human populations and woolly mammoths, suggesting that only a few evolutionary strategies are compatible with survival in extremely cold environments.
Yakutia (Sakha Republic, Russian Federation) is the coldest country in the whole Northern Hemisphere, showing annual thermal amplitudes over 100 °C and its entire range covered by permafrost . Despite such extreme conditions, a group of Turkic-speaking horse-riders, likely originating from the Altai-Sayan and/or Baïkal area, migrated into this region between the 13th and 15th centuries, pressed by the expansion of Mongolic tribes . The Yakut people successfully developed a unique economy based on horse and cattle breeding, with Yakutian horses mostly exploited as sources of meat and milk.
The Yakutian horse is the most northerly distributed horse on the planet and certainly the most resistant to cold. In contrast to cattle, which are kept in stables during winter, horses live in the open air all year round, grazing on vegetation that is buried under deep snow cover for 7–8 mo.
Yakutian horses exhibit unique morphoanatomical adaptations to the subarctic climate, being extraordinarily hairy and, as predicted by Allen's rule, compactly built, with short limbs.
Their metabolic needs are in phase with seasonal conditions, because they accumulate important fat reserves during the extremely brief period of vegetation growth and lower their activities during winter (6). Yakutian horses also show an increased carbohydrate metabolism in the spring, likely supporting higher energy expenditure and fetal growth.
The evolutionary origins of Yakutian horses, however, still remain contentious. The most commonly accepted hypothesis proposes that they descend from a founding population brought by the Yakut people . Some authors have suggested that at the time of their arrival, founding horses admixed with local populations descending from wild Late Pleistocene horses.
In contrast, others have proposed that modern Yakutian horses exclusively originate from native Late Pleistocene populations, and were secondarily domesticated by the Yakut people upon their arrival.
To date, the genomic diversity of Yakutian horses is mostly unknown, and genetic analyses are limited to few microsatellites and mtDNA sequences. In this study, we have sequenced and analyzed the complete genomes of 11 Yakutian horses, including nine present-day horses and two ancient specimens that lived in the region ∼5,200 y ago and in the early 19th century. The results revealed a genetic discontinuity between the Pleistocene and mid-Holocene genomic landscapes, showing that the contemporary population fully descends from a stock of domesticated horses. Taken together, these findings support the predominant hypothesis that the Yakut people introduced the breed in the 13th–15th centuries, and implies that the unique genetic adaptations of Yakutian horses, which we reveal here for the first time to our knowledge, were selected on an extremely short evolutionary time scale."
Tracking the origins of Yakutian horses and the genetic basis for their fast adaptation to subarctic environments (.pdf)