Horse Genetics. Ernest Bailey
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The horse belongs to the family Equidae in the order of perissodactyl. The evolutionary history of perissodactyls is shown in Fig. 3.1 (Steiner and Ryder, 2012). The perissodactyls are herbivorous, hoofed mammals with an odd number of toes. The other extant perissodactyls include four species of Rhinocerotidae (Fig. 3.2) and four species of Tapiridae (Fig. 3.3).
Fig. 3.1. Phylogenetic tree for the perissodactyl family based on combined molecular genetic data. Dog (Canis) and cattle (Bos) serve as outgroups; data was collected from four species of Rhinocerotidae, four species of Tapiridae and nine species of Equidae (figure reprinted with permission from the Zoological Journal of the Linnean Society).
Fig. 3.2. Indian rhinoceros.
Fig. 3.3. Central American tapir.
The common ancestor of extant perissodactyls existed approximately 54 million years ago (MYA). Divergence of the three families occurred shortly after that (51 MYA) with the emergence of the Equidae family occurring only about 4–4.5 MYA.
The only extant members of the Equidae family are members of the genus Equus. The Equidae are generally regarded as belonging to four major groups: the horses (E. caballus and przewalskii), the donkeys (E. asinus), the Asiatic wild asses (E. kiang, kulan and onager), and the zebras (E. quagga, E. zebra, E. grevyi) (Table 3.1). Each species has the same general morphology and size as the domestic horse, but only the horse and donkey were amenable to domestication. Each species evolved adaptations to the environment in their region of the world. Consequently, they exhibit unique behaviors, unique physical characteristics, and unique genetic attributes.
Table 3.1. Extant species belonging to the genus Equus.
Classification and Differences Among the Equidae
Species classification and population standings
The classification of the Equidae is based on the Linnaean system as recommended through agencies of the nongovernmental organization the International Union for Conservation of Nature (IUCN). IUCN is devoted to global environmental science and the maintenance of biological diversity. When defining a biological species, scientists take into account physical characteristics and geographical distribution, as well as biological abilities to reproduce, behavioral barriers to reproduction, and genetic measures of evolutionary distance between populations. In former times, the separation of two populations by a mountain range may have been sufficient for the populations to have accumulated genetic, physical, and behavioral differences that merited their consideration as a discrete and unique population. In modern times, with pressure from growing human populations and mechanized travel, many small populations of animals have been pushed together, forcing the hybridization of groups of animals that had previously diverged significantly. As a result, precisely defining species and their characteristics can be challenging. Nevertheless, geneticists have been resourceful in applying a wide variety of techniques, including DNA sequencing, to discover and characterize the diverse species.
Chromosomal differences among the Equidae species
At the cellular level, one of the most striking differences among equids is the diversity in chromosome number. They range from 32 chromsomes in the Hartmann Zebra (E. hartmannae) to 66 in the Przewalski Wild horse (E. przewaslkii). Chromosomes are large molecules in the cell that contain the DNA and its associated proteins. (Chromosomes are discussed in more detail in Chapter 17.) The amount of DNA among all mammals is very similar, but it is packaged differently in different species. Chromosomal morphology and chromosome numbers in the living members of the genus Equus are distinctive for each of the extant species. As an illustration of this diversity, the chromosome numbers found for the species of the four major groups noted above are shown in Table 3.1. In several species, variation in chromosome number has been described and these numbers are shown. Usually, these variants appear to be simple fusions or fissions of two chromosomes. Data are compiled from information in Ryder et al. (1978), Benirschke and Ryder (1985), Bowling and Millon (1988), Ryder and Chemnick (1990), and Houck et al. (1998).