Horse Genetics. Ernest Bailey
our evidence becomes stronger. However, we have never found a gene which is characteristic of a breed. While horses of all breeds can have albumin allele A and albumin allele B, the frequency of the alleles in a population of horses provides a small bit of evidence for the breed of origin.
A single locus does not give us a lot of power to determine the breed of a population of horses. We need a lot of genetic markers to effectively distinguish between breeds. With the advent of DNA testing, we have millions of DNA variants to use in comparing horse populations. Another application of these genetic markers is to identify the genetic relationship among populations. The point is that closely related populations and breeds share more genetic characteristics than distantly related populations.
Through the efforts of collaborating scientists around the world, DNA markers were developed for the horse and have been used widely to compare horses. These studies have benefited greatly from the increased use of parentage testing by horse breed registries around the world and adoption of a common set of microsatellite DNA markers for routine testing.
The development of genetic markers based on single nucleotide polymorphisms (SNPs) of DNA (Chapters 4 and 6) provided greater resolution for investigations of relationships among horse breeds. Fig. 1.1 illustrates the results of analyses of genetic distance in 38 horse populations using 6028 SNP markers (Petersen et al., 2013).
Fig. 1.1. Majority rule, neighbor join tree created from 6028 SNP markers using Nei’s genetic distance and allele frequencies within each population. The percentage bootstrap support indicated on all branches was calculated from 1000 replicates. The approach creates a “family tree” of horse breeds where the length of the branches is proportional to the genetic distance between two populations. (Reprinted with permission from Petersen et al. 2013.)
Not all breeds or populations of horses appear on this tree. However, an effort was made to select horses from different parts of the world and with widely divergent breeding histories. Not surprising to students of horse breeds, the Arabian horse appears in the middle of the tree with the Thoroughbred breed at the opposite end from the pony breeds and draft horse breeds.
Summary
• Population, landrace, and breed are all terms used to designate related groups of horses and reflect their relationship to each other and the level of selection applied by breeders.
• From the time of domestication, diverse breeds of horses have been developed based on selective breeding.
• Genetic markers can be used to measure differences among these different populations.
References
Petersen, J.L., Mickelson, J.R., Cothran, E.G. et al. (2013) Genetic diversity in the modern horse illustrated from genome-wide SNP data. PLOS One 8: e54997.
Website
Oklahoma State University, Animal Science Department webpage on Breeds of Livestock: http://afs.okstate.edu/breeds/horses (accessed February 13, 2020).
2 Evolution and Domestication of Horses
The special relationship between horses and people began approximately 5500 years ago. However, the horse encountered at the dawn of domestication was already the product of more than 50 million years of evolution occurring across several continents (reviewed in MacFadden, 1992). Changes in climate, geography, and interactions with the natural flora and fauna influenced the genes and gene combinations that were successful in each generation. By the time that people encountered the horse, it was large, strong, fleet, and social. These traits facilitated the special relationship between horses and people.
Evolution and Migration of Early Equids
Earliest ancestor of horses
The earliest recognized ancestor of the modern horse belonged to a species in the clade hyracothere (an animal or fossil of the genus Hyracotherium). Hyracotheres were also referred to as Eohippus, for “dawn horse”, and made famous by Thomas Huxley’s 1876 humorous cartoon showing Eohomo (dawn man) riding Eohippus (Fig. 2.1).
Fig. 2.1. Thomas Huxley’s 1876 cartoon of Eohomo and Eohippus (courtesy of the Division of Vertebrate of Paleontology, Peabody Museum of Natural History, Yale University.)
Such an event as a man riding an Eohippus never occurred since Eohippus (aka Hyracothere) existed over 50 million years ago and our genus, Homo, only appeared in the evolutionary record within the last 2 million years. Hyracotheres were small, perhaps the size of a small dog, and browsed leafy vegetation across North America and Europe. The hyracotheres present in North America became isolated from the rest of the world when rising waters submerged the land bridges that existed between the American and other continents. The modern horse is descended from just one of the species of hyracothere present in North America 58 million years ago. Other hyracothere species gave rise to rhinoceros and tapir species. The evolutionary trajectory of hyracothere in North America led to the appearance of hundreds of descendant species that evolved and went extinct in the intervening 58 million years. Fig. 2.2 illustrates the current view of the evolutionary processes leading to the horse.
Fig. 2.2. The phylogeny, geographic distribution, diet, and body sizes of the family Equidae over the past 55 million years. Vertical lines represent the actual time ranges of equid genera. The first ~35 million years are characterized by browsing species of relatively small body size. The last 20 million years are characterized by genera that are primarily browsing/grazing or are mixed feeders, exhibiting wide diversity in body size. Horses became extinct in North America about 12,000 years ago but the family survived by migration across land bridges to Asia and Europe. (Figure reprinted with permission from MacFadden 2005.) Plio. = Pliocene; Quat. = Quaternary.
Climate, environment and evolution
The climate in America is profoundly different today than 50 million years ago. Geological records indicate North America had a tropical environment, characterized by marshy ground and lush, woody vegetation. The five toes of hyracotheria allowed it to walk easily through marshlands, its prehensile lips were ideal to browse tender shoots, leaves, and branches. As the climate became drier and marshes gave way to grasslands, species adapted. Characteristics that favored browsers were replaced by characteristics that favored grazers. For example, teeth evolved in response to the challenges of eating grasses in sandy soil (Fig. 2.3). Inclusion of sand and dirt during grazing completely wears down the enamel crowns of teeth. Individuals with higher-crowned teeth, and especially those with teeth that grew throughout life, were better able to eat grasses. They were better fed and had more offspring than equids with their short, crowned teeth.