Przewalski’s and domestic horses are very closely related and have a phylogenetic relationship as sister taxa diverging between 150,000 and 250,000 years ago.
Although Przewalski’s horse (Equus ferus przewalskii) can hybridize with domestic horses (Equus ferus caballus) to produce fertile offspring, the existence of 2n = 66 chromosomes in Przewalski’s horse identifies it as being more different from its domestic relatives (2n = 64) than are any two breeds of domestic horse. Furthermore, mitochondrial DNA research has shown that Przewalski’s horse is not the ancestor of modern domestic horses.
Przewalski’s horse also shows a number of consistent differences in its appearance as compared to domestic horse breeds. The mane is short and erect when in good body condition and the forelocks are nearly nonexistent. The upper part of the tail has short guard hairs and a dark dorsal stripe runs from the mane down the spine to the tail. Several dark stripes can be present on the carpus and generally the tarsus, as well. Przewalski’s horses grow a thick mane in winter, which contrary to domestic horses, they shed each spring with the rest of their winter coat.
Other studies of the genetic differences between Przewalski’s and domestic horses have indicated very little genetic distinction between them. Only four alleles at four separate serological marker loci have been identified as specific to Przewalski’s horse. The vast majority of blood protein variants are present in both Przewalski’s and domestic horses and even the fastest evolving DNA region known in mammals, the mitochondrial DNA control region, does not show significant differences between the two types of horse. Thus, it is clear that Przewalski’s and domestic horses are very closely related and have in the past interbred, but the fixed chromosomal number difference between them indicates that they are distinct populations. A variety of molecular studies support their phylogenetic relationship as sister taxa diverging between 150,000 and 250,000 years ago.
• Image | © Nicholas Turland, Some Rights Reserved, (CC BY-NC-ND 2.0)
• Sources | Bowling & Ryder, 1987; Côté, Viel, & Bienzle, 2013; Goto, et al., 2011; Groves, 1994; Ishida, et al., 1995; King, Boyd, Zimmermann, & Kendall, 2015; Oakenfull, Lim, & Ryder, 2000; Oakenfull & Ryder, 1998; Ryder, 1994; Ryder, Epel, & Benirschke, 1978, Steiner, Mitelberg, Tursi, & Ryder, 2012; Steiner & Ryder, 2011; Trommerhausen-Smith, Ryder, & Suzuki, 1979; Vilà, et al., 2001