Hybridization of Asses & Zebras in Genus Equus

[snorkack]

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)?

 

[Simon Bridge]

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.