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Equine genetics through the ages

Estimated reading time: 5 minutes

  • Horses have had a profound influence on human history, whether as a means of transport or as work animals. Likewise, humans have had a direct and indirect influence on horses.
  • The domestication and selection of horses used to be shrouded in mystery, but modern technology such as the study of ancient DNA (aDNA) in the fossil record have done much to reveal how horses were bred through the ages.
  • Records show that few stallion lines were domesticated, probably because they were more aggressive and difficult to handle, a trend still visible in modern equine species with its large variation in mitochondrial DNA.
  • Throughout the ages, selection was used to alter domesticated horses to better suit human requirements.
  • It is important to genomically analyse as many horse populations as possible worldwide, as this will broaden and enhance the information and genetic base.

Horses have had a profound influence on human history, whether as a means of transport or as work animals. Likewise, humans have had a direct and indirect influence on horses. According to the Food and Agriculture Organization (FAO) of the United Nations, 87 horse breeds are extinct and of the remaining 905, almost a quarter are categorised as endangered.

Domestication and selection

The domestication and selection of horses used to be shrouded in mystery, but modern technology such as the study of ancient DNA (aDNA) in the fossil record have done much to reveal how horses were bred through the ages. By comparing the DNA of modern horses with the DNA in fossils, sediments and other sources such as graves, it is possible to follow the genetic history of horses over time along with the effect humans had on especially domestication and selection.

From these records it appears that wild horses were domesticated over a fairly long period in history and in different places (from East Asia to the Iberian Peninsula in the west). It is generally believed that mares were the animals mainly captured and domesticated, which is inferred from the large variation in mitochondrial DNA passed from mother to daughter in the female line.

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Stallions and diversity

Records show that few stallion lines were domesticated, probably because they were more aggressive and difficult to handle, a trend still visible in modern equine species with its large variation in mitochondrial DNA ā€“ much more than what is found in the Y chromosome.

Apart from the fact that fewer stallions were probably domesticated, human breeding practices where only a small number of stallions were used to produce hundreds of foals further limited variation in the Y chromosome. The grouping of animals into breeds enhanced this effect even more. These types of breeding practices led to a dramatic decrease in genetic diversity, especially in the last 200 years.

In addition, the DNA of many of the domesticated horse groups (as per fossil records) is missing in modern equine species and therefore may have disappeared over time, reducing variation even further.

Selection and adaptability

Throughout the ages, selection was used to alter domesticated horses to better suit human requirements ā€“ this can be determined by comparing specific genomic markers in the DNA of ancient and modern horses. For example, the heart and circulatory system as well as conformation traits were genetically altered, possibly because of the requirements the horse, as a means of transport, had to satisfy.

Genomic studies also showed a rise in the number of harmful genes and ā€˜genetic diseasesā€™, all directly attributed to selection by humans. Only a few stallions were used on a large number of mares, which meant that no natural selection took place. This led to a rise in the number of harmful mutations; genomics is utilised to identify the carriers of harmful genes.

Certain types and traits have been preferred by people of different cultures throughout the ages. It can be deduced from prehistoric cave drawings and ancient DNA data that earlier populations had selected for colour. Spots (leopard complex spotting) were also popular. Heterozygous (Lp/lp) animals have spots, while homozygous animals (LP/LP) are white and night blind.

Despite the disadvantage of a night-blind horse, the LP allele was relatively common in early domesticated horses. It can be deduced from the gene frequencies for LP in available samples, that people sometimes actively selected for and sometimes actively against the LP gene.

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Work capacity and speed

The GYS1 gene, which has a rather complex effect on the process of storing glycogen in the muscles, was a key feature in draft horses in the past. However, modern horses are relatively inactive and have plenty of feed, which have rendered the GYS1 gene quite harmful ā€“ chances are good that this gene will disappear.

Selection is also based on the markers in the MSTN gene that influence speed in Thoroughbred horses. Animals that are homozygous for one marker perform well in fast, short races (<1 300m); heterozygotes perform better in middle-distance races (1 300 to 1 900m), and animals homozygous for the other marker fare well in long-distance races.

Since the Middle Ages, selection in other breeds have been based on a marker in the DMT3 gene that determines whether the horse has five gaits or not. The combination of genes found in the modern horse determines its performance in jumping, classical riding and harness racing.

It is important to genomically analyse as many horse populations as possible worldwide, as this will broaden and enhance the information and genetic base. – Dr Helena Theron, senior geneticist, SA Stud Book

For more information, send an email to Dr Helena Theron at helena@studbook.co.za or visit www.studbook.co.za and www.logix.org.za.

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