Domestication in the stream of "wild" genes

Each domestic animal has a wild ancestor, from which it differs primarily with increased sociality, a change in the work of endocrine and metabolic systems and reproduction time. According to classical ideas, home forms arose as a result of reproductive isolation from wild relatives and targeted selection. For example, the ancient cattle breeders caught a group of wild boars and kept in the corral, selecting the largest and most affectionate or most affectionate to the tribe, until they received domestic pigs.

However, the complete separation between wild and home populations occurred relatively late: archaeological and genetic studies of recent years show that at the beginning of the agricultural era, people controlled the reproduction of domestic animals to a much lesser extent than it was customary to think.

Data on the time and place of domestication of different species was collected by genetics and archaeologists. According to archaeological data, domestication rarely took place in very few places (one of the signs of domestication among archaeologists is a decrease in size). Genetics, analyzing mainly mitochondrial DNA (MTDNK; it is transmitted strictly along the maternal line), found several haplotypes in home forms and concluded that they had several ancestors, that is, domestication was repeatedly and independently. However, in recent years, sequencing has become more and more affordable, and genetics began to determine the sequences of the Y-chromosomes (transmitted along the paternal line) and full-size genomes of pets and their wild relatives, computer programs appeared for their analysis, so that the contradiction between archaeological and genetic data seems to be resolved. Most likely, the domestication of each species really occurred in one or two regions, but at the same time the exchange of genes between wild and domesticated forms did not stop. The intensity of these flows was different, depended on specific conditions, vitality and sizes of the herd. Sometimes people could not interfere with the crossing of their animals with wild relatives, in other cases they consciously resorted to him.

A great interest in the problem of domestication was shown by the British Zoo -Aoarchaeologist Keith Dobney, who has worked in recent years at Daremsk and Aberdeen Universities [1]. He and his co -authors analyzed archaeological, ethnographic and genetic data on domestication and the content of homemade ungulates. Researchers divided animals into several groups. One of them includes species that are used as transport: yaks, horses, donkeys, camels, llama and alpaki. They were domesticated in the harsh conditions of deserts, dry steppes or highlands (see table). In such animals, the strength, endurance and resistance to adverse environmental factors are primarily appreciated. These qualities are more developed in wild forms, so people did not interfere with their crossing with pets and even encouraged it.

Donks stand apart in this group. Many animals are still on free grazing, and therefore it is difficult to talk about reproductive isolation. Males are sometimes castrated, but they prefer to ride on non -nasty, and the presence of many breeding male individuals complicates the targeted selection. Moreover, appreciating strength and endurance, cattle breeders encourage the crossing of females with wild donkeys. They almost do not reject their animals, the main causes of the death of donkeys - illness and drought.

Camels - both dromaders and bactrians - were also not isolated from wild relatives until they disappeared as a result of hunting or crossing with home forms. The nomads, although they appreciated the size, color and milk yield, made it possible to reproduce almost all females, that is, artificial selection was very weak, but the genes indicate a strong selection under the influence of the external environment.

In a similar situation (weak directed selection and lack of reproductive insulation) were American pets - lamas and alpaks. In their genomes, many haplotypes of MTDNK of wild related species - Guanaco and Vikuni were found - which may indicate regular recruiting of wild females from different populations. Until now, lamas and alpaks graze unattended, the likelihood of hybridization with wild animals is great. Shepherds prefer to maintain high genetic diversity, in which animals have more chances to survive in a severe climate of the Andes.

Homemade yaks on the Tibetan plateau more often dies during winter storms and resist the wolves worse, so the shepherds do not mind when wild males descend from the mountains to mating to home herds.

The selection of Jacobs comes down to limited face and castration.

Weak artificial selection and active hybridization with wild animals determined the process of horses. The international group of researchers, led by the specialists of the University of Cambridge Vera Warmuth and Anders Eriksson, analyzed the sequences of MTDNK and Y-chromosomes of 322 local working horses from 12 different areas of the Eurasian steppe [2]. The variety of MTDNK was great, the variety of Y Cromosomes is much lower. Having examined the possible causes of the existing distribution of sequences and taking into account the data of archeology, scientists came to the conclusion: the wild ancestor of a domestic horse appeared in East Eurasia 160 thousand years ago and spread throughout the continent, moving west at a speed of about 100 km in 240 years. About 6 thousand years ago, in the western part of the Eurasian steppe, horses were domesticated, after which they continued to cross the wild relatives. Researchers suggest that nomads caught wild mares to maintain the size of the herd. The horses of Przhevalsky, the closest relatives of wild extinct horses, are poorly propagated in captivity. Perhaps, at the dawn of domestication, the cattle breeders faced the same problem, and the easiest way was to maintain the livestock at the expense of wild mares.

The second group of home -made ungulates, according to the classification of Kita, with the co -authors, are goats, sheep and cattle. Their domestication occurred in more favorable climatic conditions (see table), and these animals were bred for the sake of wool, meat and milk, so their productivity was more relevant than endurance. People were selected on these signs, mainly crushing males (there are 8-10% of the total), but the selection was interfered with crossing with wild relatives, which at first it was difficult to avoid.

Cattle breeds and grows slower, in addition, it is used as a drain force, so the culling pace is lower than that of goats and sheep. The analysis of MTDNK does not allow us to conclude that European livestock has been crossed with wild animals. The exception, apparently, was only Italy, where farmers attracted wild females in their herds. Another picture in Africa. Even modern African cattle breeders protect their herd, rarely score animals and practically do not touch females, and retain many bulls. Mortality in the herd is often higher than the damage from the face. The directed selection in this case is extremely weak, and the slow growth in the number of the herd and poorly controlled grazing lead to the flow of "wild" genes.

Pigs are a special category. Perhaps they first appeared next to a person of their own free will as a commenala (“cables”; commentaryism is the coexistence of two different types, useful for one of them (commensal) and indifferent to the other (host). - Note. Ed. ). The selection among them is easier than among other ungulates, since they multiply rapidly, grow rapidly, and most importantly - multiple. The deliberate crossing of pigs with wild relatives does not create any advantages, but the genome of European domestic pigs is a mosaic in which many wild populations participated. Exploring the history of the origin of European domestic pigs, the specialists of several scientific centers analyzed 103 full genome of wild boars and domestic pigs of different breeds from Europe and Asia [3]. Then, less complete data on 600 genomes of other wild and domestic pigs were added to these results. There were at least two district domestication areas: Central China and Eastern Anatoly. Judging by the analysis of the MTDNK of modern Chinese pigs, Chinese farmers were guided by the classical rules of breeders, and even in the early stages of domestication they kept their animals in the corrals and carried out strict selection. And the pigs of Anatolia did not know such restrictions and received powerful genetic injections from wild Asian wild boars. Then they, together with the farmers, moved to Europe, where they participated in the creation of European domestic pigs.

In Europe, wild boars also lived in Europe. Scientists came to the conclusion that Asian aliens were actively crossed with them, as well as the emerging local home forms. Such a mixture is understandable, because pigs grazed and they had enough opportunities to meet wild boars. However, despite the fact that the flow of genes from wild animals to the domestic was much stronger than the opposite, domestication took place. Why?

Scientists have found that both European and Asian farmers selected, and according to the same signs associated with the processes of the development of bones, teeth and nervous system, as well as in size, lack of aggressiveness and food behavior. Such a regular selection counteracted the influence of wild boars and created in the genome the “home -bearing islands” - areas containing genes responsible for economic signs. They determined the difference in appearance and behavior between wild and domestic pigs. Such islands are in the genomes of both European and Asian pigs, and according to researchers, they should have other domesticated species, because they provide the possibility of domestication in the absence of reproductive isolation. The existence of the flow of "wild" genes is well consistent with the plurality of mitochondrial haplotypes and archaeological data. Thanks to him, the process of domestication was complicated and prolonged, his specific features depended on the type, place, time and needs of man.

Natalia Reznik

1. Marshall FB, Dobney K., Denham T. & Capriles JM Evaluating The Roles of Directed Breeding and Gene Fow in Animal Domestication // Proc. Natl. ACAD. SCI. USA, 2014, 111, 6153–6158.

2. Warmuth v, et al. Reconstructor the Origin and Spread of Horse Domestication in the Eurasian STEPPE // Proc Natl Acad Sci USA. 2012. 109 (21): 8202–8206.

3. Frantz Laf et al., Evidence of Long-Term Gene Fow and Selection During Domestation From Analysses of Eurasian Wild and Domestic Pig Genomes // Nature Genetics. 2015. 47, 1141–1148. DOI: 10.1038/NG.3394