Confirmed: The wisent is an aurochs hybrid

Finally I have the time to present some interesting news here, that are not that quite new anymore as the paper appeared in October 2016. But I never found the time to cover a peer-reviewed paper properly.

In a 2015post, I mentioned the possibility of a hybrid origin for the wisent on my blog for the first time. I cited a 2004 paper that found the mitochondrial lineage of wisents to cluster with domestic cattle instead of American bison, although by Y-data and morphology the wisent is clearly closest to the A. bison. This provokes the idea that the wisent originated from hybridization of bulls of a bison-related species with cows of a cattle-related species [1]. Actually, I recognized some wisent individuals with a horn curvature very reminiscent of that of the aurochs already in 2013, but was reluctant to propose hybridization or introgression without having genetic data to back it up.

Wisent with aurochs-like horns at Hellabrunn Zoo, Munich

Now, with the recent paper by Soubrier et al. being published, the hypothesis of the wisent being a hybrid of aurochs and steppe bison, Bison priscus, seems to be confirmed [2].

The mystery of the wisent’s origin starts with the fact that there is no Pleistocene fossil record of this species, while there is solid evidence for the aurochs or the Holarctic steppe bison. Now, Soubrier et al. have analysed the genome of specimen of B. priscus, aurochs, historical and modern wisents. They found that wisent, aurochs and cattle cluster together on the mitochondrial genome, while yak, A. bison and Steppe bison form a clade. Furthermore, they found that a number of Pleistocene remains form a group preliminary called “Clade X”, which is sister to modern and historical wisents and they diverged about 120.000 years ago. On the nuclear genome, however, the wisent is a bison. This implicates that the wisent evolved from hybridization events of Steppe bison bulls with aurochs cows from more than at least 120.000 years ago (for comparison: the divergence between taurine and zebuine cattle happened about 250.000 years ago). The polygynical reproduction system of bovines probably endorsed this asymmetrical hybridization. The hybrids must have been ecologically different from both ancestor species, as they were reproductively isolated from then on [2]. This lineage lead to the modern wisent, Bison bonasus. From about 20.000 years on, wisent-like cave paintings began to show up. Prior to that, they showed a typically Steppe bison-like morphology.

It is exciting to have it confirmed, although it does not surprise me at all. Not only were the occasionally very aurochs-like horns of some wisents suspicious to me, but also its overall morphology compared to the other fossil and extant species of Bison. The wisent has a shorter trunk and longer legs, resulting in a square-like build like in the aurochs, it has a neck bulge and more horizontally oriented pelvis resembling taurine cattle. Now we can be quite confident that these traits are not a coincidence, but probably the vestiges of hybridization with aurochs. After all, both wisent and Clade X have a proportion of 10,9% of aurochs DNA in their nuclear genome.

Comparison of the morphology of A. bison (top), wisent (middle) and aurochs (bottom). Image source 1 & 2

Interestingly, the wisent as a hybrid of two large-horned species ended up being a short-horned species. To me, there are two explanations for that on genetic level: either the interplay aurochs and bison genes resulted in short horns, while respectively having only bison or aurochs alleles on all of these loci would result in large horns in both species, or new mutations showed up. In any case, short horns apparently were not disadvantageous to the hybrids, otherwise they would not have become fixated in the gene pool. Whether this fixation was the result of selective pressure or genetic drift cannot be said.

Hybridization is not an uncommon phenomenon. Textbooks tell us that hybrids between wild species tend to be unsuccessful due to pre- and postzygotic isolation factors and this might be true for most cases. But it is also evident that hybridization played a role in the speciation of a number of mammal species, including wild goats, whales, canines, mice, and our own species, Homo sapiens*. The wisent is yet another example, and there are probably a lot to find in other vertebrate groups too.

* It has become well-established by genetic studies of the past several years that modern Homo sapiens has experienced introgression by H. neanderthalensis, the Denisova Man and possible another yet undescribed human species.

The wisent being an aurochs hybrid of course provokes the question if it might be possible to obtain nuclear aurochs genes that have vanished from the modern domestic cattle gene pool (you could expand that even further to the basis of Bos; Banteng and Gaur might share basal genes that the aurochs also possessed but domestic cattle lost). However, I am sceptical to this idea. Simply crossing-in wisent in “breeding back” herds and breeding against obvious bison traits at the maxim “well, it should work somehow” is probably not a good idea. You would have to know these specific aurochs genes preserved in wisent and find a way to select on them. It would probably work only with gene targeting, and with that method you could instead try to recreate a complete aurochs with the complete genome that already has been resolved. The influx of wisent into breeding-back herds could also be problematic for rewilding those herds, as it might increase the tendency of these cattle to hybridize with wisent in the wild and lower the pre- and postzygotic isolation barriers; the (not really existent [3]) danger domestic cattle provide to the genetic integrity of wisent in the wild is sometimes used as an argument against rewilding wisent and cattle in the same area[3]. So I would not opt for Frankenstein crossbreeds, also regarding Banteng and Gaur (the Yak would be the only species for which I am open for experimental crosses, but that is another story), for practical reasons and public relations.

An interesting question is if hybridization with bison might have also left a track in the aurochs. There is one detail that makes me consider it not unlikely: the curly hair on the forehead and sometimes also neck and face that we see in many European taurine bulls (and cows, but to a lesser extent). Cattle outside of Europe, especially zebuine cattle, always lack this trait. Perhaps it is a trait that found its way into the gene pool of B. p. primigenius by occasional hybridization with bison and got fixed by sexual selection. But that is only an idea of mine.  

Literature

[1] Verkaar, Nijman, Beeke, Hanekamp, Lenstra: Maternal and Paternal Lineages in Cross-breeding bovine species. Has Wisent a Hybrid Origin?. 2004.

[2] Soubrier et al.: Early cave art and ancient DNA record the origin of the European bison. 2016.

[3] Vera: Do European Bison and domestic cattle cross spontaneously? 2002.