Would it be crazy to use hybridization in "breeding-back"?

As my readers will know, I suggested cautious hybridization with extant wild bovines that would add traits hardly achievable using domestic cattle only in “breeding-back” in a couple of my posts, after I initially opposed that idea. In this one for example, I suggest using wild yaks in “breeding-back”. But the Java banteng would add value as well, and using both in cautious doses would be add all of the traits that are difficult to achieve with domestic cattle only. The kouprey, which would add all the desired traits both the wild yak and banteng would add and has a slightly more aurochs-like morphology, unfortunately is not available for hybridizing as it is most likely extinct. To give a quick summary of the benefits the wild yak and the Java banteng would add to “breeding-back”:

Wild yak: 

- Very large, up to 205 cm withers height in bulls what equals the large size of European aurochs; difficult to achieve with domestic cattle only, where the upper height limit might be roughly 180 cm; well-marked size dimorphism between the sexes 

- Perfectly aurochs-like horns in both sexes. The horns of the wild yak are really identical to those of the European aurochs in terms of size, curvature, orientation to the skull and even colour. They have a very pronounced inwards-curve which is lacking in most aurochs-like cattle 

- They have a large shoulder hump and a long snout, two traits which are rarely as expressed as in the aurochs in domestic cattle 

- The physiological fitness is higher than in domestic yak and most likely domestic cattle 

- The food choice overlaps with that of cattle/aurochs, they are predominantly grazers 

Java banteng:

- A very well-marked colour difference between the sexes that is always present. This is extremely rare in domestic cattle as domestication reduces sexual dimorphism

- Aurochs-like body size (up to 190 cm which is not quite as large as wild yaks but still significantly larger than most domestic cattle) 

- Roughly aurochs-like morphology (hump, short trunk) 

- It ecologically overlaps with cattle/aurochs, although they browse more, including fruits 

 

Cautious hybridization and wise selection might thus speed up the process of achieving an aurochs-like animal significantly and might have a better end result. But there are, as one can imagine, objections against hybridizing with other species, which I shared until recently. Those objections are two main ones: 

 

Argument #1. A different species would add many undesired, non-aurochs traits, some of which are practically invisible and thus hard to select against, which might distort the authenticity and ecologic suitability of the results. 

There is valid truth to that argument. For example, wild yaks are adapted to higher altitudes than the aurochs, which was a lowland animal, and have larger lungs. They are also adapted to more severe cold, having fewer sweat glands and a different pelage. Also, there are skeletal differences, such as a different vertebrae count. Wild yaks also browse less than cattle/aurochs as they are more strict grazers. Banteng are adapted to a much hotter climate and browse more than cattle. There are also behavioural differences in both species. Yaks, for instance, have a different aggression display behaviour by presenting their large tail tuft, while banteng rely more on display in intraspecific competition than fighting head to head like cattle/aurochs and wild yaks. 

However, it is not impossible to get rid of these undesired behavioural, ecological and morphological traits. The solution might be absorptive breeding while running a strict selection regime. That means that each hybrid generation would be backcrossed with taurine cattle, but only those hybrids that still have retained the desired traits added by the species would be kept for further breeding. After eight generations, for example, the genealogical portion from the other species would be as low as 0,3%, therefore there would be a high chance that the undesired traits are washed out while the desired ones remain in the population when selected appropriately. 

 

Argument #2. “Breeding-back” cattle with a tiny but important bit of introgression from wild yak and Java banteng would not be pure domesticated aurochs anymore. Thus, the argument that “breeding-back” cattle are a native species in European nature would be gone, as they would more or less be hybrids. 

There are two problems with this objection. On a more general note, introgression between closely related species at some point of time has been found everywhere it was looked for in mammals, including our own species Homo sapiens, so it seems that hybridization (or more precisely, introgression) is a very widespread phenomenon in the process of speciation and evolution in general. In the case of wild bovines, Cambodian banteng have introgression from the kouprey, wisent have homeopathic introgression from aurochs and there may be more cases which we are currently not aware of. So introgression per se is not “unnatural”, quite the contrary. Furthermore, it is not technically true that domestic cattle are “pure” albeit domesticated aurochs. Domestication itself is an anthropogenic process, resulting in organisms that differ from the wildtypes that were shaped by nonhuman evolution. A wild aurochs’ genome is 100% wild aurochs. A domestic cattle genome is 100 - x % wild aurochs, x being the portion of the genome that is made up by alleles that mutated after domestication. I do not even dare to speculate how large or small that portion is, and it is certainly different from breed to breed to a certain degree. Thus, domestic cattle are not “pure” aurochs, but rather partly wild aurochs and partly a domestic mutant. So, the originality of the aurochs is gone as a result of domestication anyway. If another species is bred in, the non-aurochs portion would increase dramatically in the first couple of generations, but decrease in the advanced generations as a result of absorptive breeding. It is even possible that, when selected wisely, the aurochs portion could actually increase by introgression from wild yak and banteng. I say that because I consider it likely that, as long as the aurochs-like traits in wild yak and banteng are not the result of convergent evolution (which I do not consider particularly likely), these traits are caused by the same wildtype alleles as in the aurochs. For example, the identical horn curvature between aurochs and wild yak might be caused by alleles shared from a common ancestor. Introgression might reintroduce these alleles into the domestic cattle population. The same could be the case for the sexual dichromatism in the Java banteng. In this case, introgression would reintroduce wildtype alleles found in the aurochs which are absent or at least very rare in domestic cattle. So the “genetic purity” argument is not convincing to me. Apart from that, the introgression might also introduce physiological fitness-related alleles from banteng and wild yak which are superior to those of domestic cattle in terms of physiological fitness, thus providing an advantage for “breeding-back” cattle compared to the domestication-induced disadvantage that domestic cattle usually might have. 

 

Therefore, I think small doses of controlled hybridization/introgression from the two wild bovines most suitable, and subsequent wise selection, is not a crazy idea at all and could be very beneficial for the ecologic and phenotypic goal of “breeding-back”. What would be crazy, on the other hand, would be hybridization without a plan and without strict selection.