Hybridization of Asses & Zebras in Genus Equus

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The discussion centers on the hybridization of species within the genus Equus, specifically focusing on the tarpan and Przewalski horse, which have different chromosome counts (64 and 66, respectively) but can produce fertile hybrids. In contrast, hybrids between the tarpan and African ass are sterile. The conversation also touches on the existence of hybrids between the African ass and onager, questioning their fertility status. The classification of extant species within Equus includes two horses, three asses, and three zebras, with notable distinctions in their reproductive compatibility.

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  • Understanding of species classification in biology
  • Knowledge of chromosome numbers and their implications for hybrid fertility
  • Familiarity with the genus Equus and its species
  • Basic concepts of hybridization and reproductive compatibility
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  • Research the fertility rates of hybrids between African ass and onager
  • Explore the genetic implications of chromosome differences in Equus species
  • Investigate the historical context and extinction of the tarpan
  • Examine the classification and characteristics of extinct species within the genus Equus
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Biologists, geneticists, zoologists, and anyone interested in the evolutionary relationships and hybridization of equine species.

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In genus Equus there seem to be 8 extant species, of which one - tarpan - is extinct in the wild.
Out of these, 2 are classified as horses:
  1. tarpan
  2. Przewalski horse
Tarpan and Przewalski horse famously have a different number of chromosomes (tarpan 64, Przewalski horse 66) and as famously produce fertile offspring with unpaired chromosomes.
Now, 3 species of Equus are classified as asses:
  1. African ass
  2. Onager
  3. Kiang
and 3 are classified as zebras:
  1. Grevy zebra
  2. Plains zebra
  3. Mountain zebra
Now, while hybrids of tarpan and Przewalski horse are fertile, hybrids of tarpan and African ass are readily produced and even more famously sterile.

The hybrids of African ass and onager have been mentioned to exist. Any details? Are they sterile (like ass/horse hybrids) or fertile (like tarpan/Przewalski horse hybrids)?
 
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I've been having trouble working out what you are talking about ... vis:

Genus. Equus

Sub G. Equus (Horse)
E. ferus:

Notice there is only one species of horse, but there are two extant sub-species:
E. ferus caballus (modern horse)
E. ferus przewalskii (Przewalski's horse)
... iirc subspecies are usually interfertile with fertile offspring.

E. ferus ferus (Tarpan) is extinct. But see:
http://www.horsetalk.co.nz/2016/06/02/dont-call-me-tarpan/#axzz4HHHLl2E5

Other species in Genus Equus, by sub-genus:

SubG. Asinus (Donkeys)
E. africanus, E. hermionus, E. kiang
(+5 extinct)

SubG. Dolichohippus (Zebra)
E Grevy (+2 extinct)

SubG. Hippotigris (Zebra)
E. Quagga
E. Zebra
+ 1 extinct

SubG. Incertae sedis (unclassified)
E. major <---<<< I don't know anything about this!
+12 extinct

So I make that 3 species of donkey and 3 of zebra in keeping with post #1 - although one of the species of zebra is in a separate sub-genus.
Since the donkeys are all distinct species, it seems reasonable to expect hybrids, if any, to be infertile or to have low fertility. However, I cannot find recent references for any. You'd think that donkeys would be more closely related to each other than to horses but life is messy.

Note: my source here is wikipedia - they do seem to get this sort of stuff right though.