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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).
The scientific name of the species is given in the far-left column, followed in subsequent columns by common names and the number of chromosomes reported for each species. The practice is to type the genus and species name in full the first time it is used, and then simply abbreviate the first letter(s) thereafter. So Equus caballus becomes E. caballus. In some literature, the horse is designated Equus ferus and includes both the domestic horse and the Przewalski horse. When using that convention, the domestic horse is given the subspecies designation “caballus” (E. f. caballus) and the Przewalski horse is likewise designated E. f. przewalskii. As described in Chapter 2, the two species diverged between 41,000 and 70,000 years ago and are distinguishable species at the DNA level. Therefore, we will use the convention of referring to them with the species designations, E. caballus and E. przewalskii.
The chromosomes of domestic horses and donkeys are well known from clinical karyotyping studies, as well as genome mapping studies, and excellent ideograms exist that depict the number and structure of the chromosomes in these species (Bowling et al., 1997; Raudsepp et al., 2000; Di Meo et al., 2009). However, relatively fewer karyotyping studies have been undertaken for the wild equids, and some variation has been noted in their karyotypes. We do not know whether the individuals studied had clinically abnormal karyotypes or whether this variation represents hybridization between chromosomally heterogeneous populations that were brought together in recent history.
Endangered species
The conservation status of the different species of Equus according to the IUCN is identified on the far right of Table 3.1. Those listed as critically endangered (Somali wild ass), endangered (Przewalski’s horse, kulan, onager, and Grevy’s zebra), and vulnerable (mountain zebra) are those considered at risk of extinction. Those identified as of least concern (kiang and common or plains zebra) are considered stable populations for the present.
Genomic differences among the species of Equidae
The relationships among the species of Equidae continue to be clarified as we obtain more DNA sequence information. Whole genome sequence is reported for most, if not all, of the Equidae species and subspecies in publicly available databases.
Horses
This group comprises two species, commonly known as Przewalski’s horse (E. przewaslkii) and the domestic horse (E. caballus). The domestic horse is well known and is the primary subject of this book. One likely ancestor of the horse, sometimes called a tarpan (E. ferus ferus and E. f. sylvestris), has become extinct in the wild but the natural distribution was throughout modern-day Europe, Asia, and the Middle East (Groves and Ryder, 2000). Przewalski’s horse (Fig. 3.4) is a closely related species that was driven to extinction in the wild during the 1900s, and then reintroduced to the Hustai National Park of Mongolia in 1992 using captive born Przewalski’s horses (Boyd and King, 2011). Before its initial extinction in the wild, the natural distribution of Przewalski’s horses appears to have been from eastern Germany through to the northern regions of Asia, especially Mongolia. The extant Przewalski’s horse population is descended from only 12 horses brought into zoo populations in the early 1900s. Today, the number of Przewalski’s horses exceeds 1800, including more than 300 living in the wild.
Fig. 3.4. Przewalski Horse (E. przewalskii) (picture provided by Zoological Society of San Diego).
The relationship between Przewalski’s horses and domestic horses continues to be a topic of research and discussion. Hybrids of the two species can be fertile, and indeed, are likely to have produced occasional hybrids that contributed to the genomic constitution of domestic horses (Gaunitz et al., 2018). Differences in overall genome organization are minimal. The most obvious difference is the number of chromosomes with E. caballus having 64 and E. przewalskii having 66. The difference appears to be the result of what is called a “Robertsonian translocation” when two smaller chromosomes fuse to form a single larger chromosome (Myka et al., 2003). Chromosome EPR23 of the E. przewalskii
