Hack and Lorenzen (2008), Moehlman et al. (2008a), and Novellie (2008).
Species Hybrids
Despite extensive chromosomal differences, the various equine species generally can be successfully crossed to produce viable progeny. In the late 1800s, a series of experiments was conducted to construct hybrids between zebras and horses and these were described in a book called The Penycuik Experiments (Ewart, 1899; now available as a digitized book). Such hybrids show that, despite extensive chromosomal and genomic changes, the overall genomic content is similar within the genus Equus and viable offspring can be produced. However, the hybrids are usually sterile, probably due to the differences in chromosome arrangements and failure to produce viable gametes following meiosis. During meiosis in hybrids, the chromosomes of the two parent species pair undergo recombination of DNA between chromosomes. However, because the genes are organized on different chromosomes, the resulting sperm and eggs have unbalanced numbers of genes and are non-viable. Even simple chromosome rearrangements that are sometimes observed in horses can result in unbalanced gametes and reduce fertility. With so many chromosome differences among Equus species, hybrids of distantly related equids are sterile. Only hybrids between closely related species, such as between the horses or among the hemiones, produce fertile offspring and, even then, hybrids are likely to have reduced fertility.
Rare fertility in mules and hinnies
Historically, the hybrid cross between the domestic horse and donkey species was agriculturally highly important. The offspring of a female horse and a male donkey is called a mule; the reciprocal cross between a female donkey and a male horse produces a hybrid called a hinny. Both hybrids have 63 chromosomes; 32 from the horse parent and 31 from the donkey parent. The ovaries of most female horse/donkey hybrids are usually non-functional (atrophic) and males do not produce sperm (they are azoospermic). The occurrence of fertile mules is very rare because of the failure to produce viable gametes. Some rare exceptions have been documented. Ryder et al. (1985) used karyotyping and blood typing to confirm the parentage of a jack (male) mule foal by a Welsh pony stallion out of a molly (female) mule. Karyotyping studies of fertile female mules and hinnies in China have also been reported (Rong et al., 1988). The explanation of documented fertile mules has been suggested to be a consequence of hemiclonal transmission, a phenomenon in which the mother passes along the chromosomes from the mother but not the father. In other words, these viable offspring are not true species hybrids. In these rare and well-documented cases, the offspring inherited only the domestic horse chromosomes and were genetically horses insofar as their chromosomes were concerned. Hemiclonal transmission has been observed in some amphibians but, other than these exceptional cases, is unknown in mammals. Because it is so rare, the mechanism for hemiclonal transmission in equids is a mystery.
Feral Species
Wild species of equids have been extinct in North America for thousands of years (except for zoo collections), but feral horses and donkeys are untamed, referred to as wild, and are free ranging in several locations around the world. Feral animals are descended from domestic horses and asses.
Summary
• Domestic horses belong to the family of odd-toed mammals called Perissodactyla.
• Within the family the horses are members of the genus Equus, which includes horses, asses, Asiatic asses and zebras.
• Among the differences in equid species are vastly divergent chromosome numbers. The evolution of equids entailed extensive chromosome rearrangement.
• The different species can produce viable offspring; however, they are almost always sterile due to the differences in chromosome number.
References
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