the long arm of E. caballus chromosome ECA5. Chromosome EPR24 is homolgous to the short arm of ECA5. It appears that chromosome ECA5 of E. caballus is the product of an ancient, evolutionary fusion between two chromosomes in a species that is an ancestor of E. caballus and E. przewalskii (Myka et al., 2003). However, the mitochondrial genome and the Y chromosome of Przewalski’s horses and domestic horses are distinctly different, demonstrating that they are clearly different species (Oakenfull and Ryder, 1998; Oakenfull et al., 2000; Wallner et al., 2003; Lindgren et al., 2004; Lau et al., 2009).
Asses
Asses evolved in northern Africa. Following domestication, the donkey is found throughout the world because of its usefulness in agriculture. The Somali wild ass continues to exist in the wild in regions of Ethiopia and Eritrea. At present, the number of Somali wild asses may be less than 200 and the species is considered critically endangered (Moehlman et al., 2008c).
The ass karyotype differs numerically from that of the horse by just a single chromosome pair, but the morphology and banding patterns of the individual chromosomes are quite different from those of the horse. Chromosome painting studies have demonstrated large numbers of rearrangements in genome organization when comparing asses with horses (Raudsepp and Chowdhary, 2001). The karyotypes of the Somali wild ass (Fig. 3.5) and the domestic ass cannot be distinguished. A simple chromosome polymorphism has been described several times that involves the same large metacentric chromosome (Benirschke and Ryder, 1985; Bowling and Millon, 1988). Studies of mitochondrial DNA from the extant asses and museum specimens of the extinct Nubian wild ass (E. africanus africanus) indicated that the domestic donkey appears to have descended from the Nubian wild ass and another unknown, ancestral species, but to be distinct from the Somali wild ass (Kimura et al., 2011).
Fig. 3.5. Somali wild ass (E. africanus somaliensis) (picture provided by Zoological Society of San Diego).
Asiatic Wild Asses
The taxonomy of the Asiatic wild asses has a considerable degree of uncertainty. They are difficult to study because of their distribution in remote locations throughout Asia, and their scarcity both in zoo collections and in the wild. While popularly referred to as kiangs, onagers, and kulans, the existence of multiple species and subspecies of Asiatic wild asses is a matter of continuing research and discussion (Groves and Ryder, 2000). The kiang is occasionally described as composed of three subspecies, E. kiang kiang, E. k. holdereri, and E. k. polyodont (Fig. 3.6). Table 3.1 shows the results from karyotyping the kulan (E. h. kulan), the onager (E. h. onager), and the kiang. The kulan and onager (i.e. hemione) karyotypes are similar to each other while kiangs possess between two and four fewer chromosomes and are genetically distinct, based on studies of mitochondrial DNA (Oakenfull and Ryder, 1998; Oakenfull et al., 2000). Variations within the kulan, onager, and kiang populations typically involve Robertsonian rearrangements of chromosomes (Ryder and Chemnick, 1990). Mitochondrial DNA sequence comparisons of E. h. kulan and E. h. onager showed minimal differences and led the authors to raise questions about designations of these animals as separate subspecies (Oakenfull and Ryder, 1998; Oakenfull et al., 2000). For more information about the status of Asiatic wild asses, see Shah et al. (2008) and Moehlman et al. (2008b).
Fig. 3.6. Kiang (E. kiang) (picture provided by Zoological Society of San Diego).
Zebras
The best-known feature of the African zebras is probably their extensive, distinctive striped coat patterns. Despite this common characteristic, the zebra group is genetically quite diverse. As can be seen from Table 3.1, they are typically regarded as belonging to three species groups (E. grevyi, E. quagga and E. zebra), and have chromosome numbers ranging from 32 to 46.
Grevy’s zebra (E. grevyi; Fig. 3.7) is characterized as a single species. The species is not very numerous and is classified as endangered. Several subspecies have been described for E. quagga (Fig. 3.8). E. q. burchellii has the common name of Burchell’s zebra, common zebra or plains zebra. Other subspecies of E. quagga have been recognized as E. q. boehmi (Grant’s zebra), E. q. zambiensis, E. q. borensis (maneless zebra), E. q. chapmani (Chapman’s zebra), E. q. crawshayi (Crawshay’s zebra), and E. q. selousi (Selousi’s zebra). This group is denoted as of “least concern,” meaning that the populations appear to be stable.
Fig. 3.7. Grevy’s zebra (E.grevyi) (picture provided by Zoological Society of San Diego).
Fig. 3.8. Buchell’s zebra (E. quagga) (picture provided by Zoological Society of San Diego).
E. zebra (Fig. 3.9) is considered to be vulnerable and occurs in two populations: the Hartman’s mountain zebra (E. z. hartmannae) and the Cape Mountain zebra (E. z. zebra).
Fig. 3.9. Hartmann’s zebra (E. zebra hartmannae) (picture provided by Zoological Society of San Diego).
Zebra species occupy distinctive ranges with limited overlap, especially in modern time. Chromosome painting has been used to identify the differences in genome organization among the different species (Trifonov et al., 2008).
