Why I think purity makes no sense in "breeding-back"

Readers of my blog will know that I am not afraid of proposing mixing “breeding-back” cattle with less-derived zebu breeds and even different species such as the wild yak and the banteng. Some might object that this would undermine the “purity” of those cattle as domesticated Bos primigenius primigenius. This is, however, not a concern to me, for a number of reasons that I want to outline with this post. 

1. Hybridization/introgression is common in evolution 

With our increase in genetic knowledge, we have more and more cases of closely related species that experienced more or less intense introgression from each other through hybridization. I have the impression that introgression was found everywhere it was looked for, including our own genus Homo. So we can assume hybridization is a quite common factor in the evolution of species. Bovines are no exception to this, including the aurochs, where hybridization between bison and aurochs has been found in both directions (albeit the gene flow was comparably small), and I would not be surprised at all if gene flow was also detected between aurochs and banteng and between wild yak and aurochs.

2. Domestic cattle are already highly mixed between the different lineages of Bos primigenius 

The initial domestication events, as far as current knowledge goes there were two of them, concerned B. p. primigenius and B. p. namadicus. However, the resulting domestic lineages did not remain pure domestic descendants of those respective two subspecies, as there not only was introgression between them, from taurine to indicine and vice versa, but also introgression from other aurochs subspecies. The mitochondrial haplotype R was likely inherited from the African aurochs, and recently Hereford has been found to have traces of introgression from B. p. suxianensisfrom Mongolia. This introgression was probably not limited to this one breed, it just was detected there. Therefore, taurine cattle are a mix of all four mainland aurochs subspecies, and zebus possibly too, as they experienced gene flow from taurine cattle, especially in recent decades. Therefore, the remaining aurochs alleles from the four mainland subspecies are not found in four distinct domesticated lines, but are most likely all over the place in taurine and indicine cattle. 

3. Zebus share alleles with the European aurochs that taurine cattle do not have 

When the full genome of a Neolithic aurochs bull from Britain was resolved by Orlando 2015, it was found that zebus actually share some alleles with the European aurochs that taurine cattle do not have. Less-derived zebus also are more aurochs-like on some aspects than most taurine cattle, such as the trunk length and leg length, udder size and overall slenderness. Whether there is a connection between those alleles shared with the European aurochs and these phenotypic traits has not yet been investigated, those alleles could be responsible for any biological aspect of the organism. Therefore, if one wants to come as close as possible to the European aurochs, including less-derived zebus into the breeding would be wise, even if it sounds counterintuitive at first because they descend from a different aurochs subspecies. 

4. Some wild bovines might share alleles with the aurochs that domestic cattle do not have 

Just as zebus share alleles with the European aurochs that taurine cattle do not have, it is possible that living wild bovines might have some wildtype alleles found in the aurochs that cattle have lost. This might particularly be true of alleles for morphological traits that are likely homologous, such as the primigenius spiral (found in the wild yak) or a well-marked sexual dichromatism (found in the Java banteng). Thus, including these wild bovines into the breeding might be very beneficial to “breeding-back”. Species purity in the “breeding-back” results is not really a concern to me as long as the introgression is limited to genes for desired key traits, because taurine cattle are not “pure” European aurochs anyway but rather a composite of an unknown portion of mutated alleles and remaining wildtype alleles. The same goes for zebus in respect to the Indian aurochs. 

 

Thus, I would not be afraid of mixing lineages in “breeding-back”. Taurine cattle are already highly mixed, and they lack some traits and alleles that can only be effectively reintroduced by mixing lineages and selecting wisely. Note that I am not opting for rampant crossbreeding and hybridization, but rather to use small doses of introgression accompanied by targeted selection in order to keep the desired traits from each lineage and eliminate distinctive traits of the other lineage. This goes for morphological as much as behavioural, ecological and – to the degree that it is detectable – genetic traits. This way, aurochs-like bovines that blow our minds could be achieved, while that is very difficult using taurine cattle only.