Monday, 25 December 2017

What if the aurochs was genetically reconstructed?

This post plays with the possibility of a genetically resurrected, genuine aurochs. This possibility emerged since a full genome has been resolved in 2015. This animal would be a true aurochs in its original form, not domestic cattle with a number of morphological/optic wildtype traits that might be considered dedomesticated at some remote point of time of natural selection. What to do with such a reconstructed aurochs, and would it make existing “breeding-back” projects unnecessary?

Each time it comes to genetically reconstructing an extinct animal, there are people questioning the purpose of doing so. Well, reconstructing a genuine aurochs would of course be for the same purpose of breeding aurochs-like cattle and using it for nature restoration. It serves an ecologic good and is about authenticity.
A number of recent posts covered the probable genetic differences between aurochs and domestic cattle, especially this one. I outlined why I believe that we definitely need more than mere matches on SNPs and haplotypes as the genetic differences between aurochs and cattle include a lot of aspects that have a dramatic influence on the organism as a whole (neurology, endocrinology, ontogeny, sexual dimorphism, immunology) and are responsible for the big differences in morphology, development and behaviour that we see between the wild animal called aurochs and its domestic counterparts. It is very likely that  these differences are caused by a high number of loci whose individual role in the whole have not been resolved yet – let alone the identification of wild vs. domestic alleles on these loci and their distribution among domestic cattle today. This means that true breeding-back of a genuine, pre-domestic aurochs is not possible by the current state of knowledge and that it would take a very long time, dozens of decades, because of the very high number of loci involved. Not to speak of the fact that many of the original defining wildtype alleles probably have been lost in the dramatic and intense process of domestication anyway.

Possibilities to genetically reconstruct an aurochs

So that true “breeding-back” is most likely not feasible as the necessary material is a) not identified and traced down in modern cattle and b) very likely not fully preserved anymore, how to get to a genuine, pre-domestic aurochs then? Luckily, and finally, the full nuclear genome of a British aurochs bull from 8.000 years ago was fully resolved in 2015 (see here), after the full mitochondrial genome of the aurochs had already been known since 2011.
This makes it feasible to truly revive a genuine, pre-domestic aurochs individual. The fact that it is a male is especially advantageous, as male-specific aurochs genes on the Y chromosome are known this way that otherwise would be unknown, which would have an impact on sexual dimorphic traits. But how to get from the genetic information to a life aurochs? Two methods are usually mentioned for that are:  
a) Cloning. Both bantengs and gaurs have already been cloned successfully using a domestic cow as a surrogate. For an aurochs, a domestic cow would be even more closely related. For cloning, having the genetic information only is not enough. For the process of somatic nucleus transfer, an actual aurochs tissue cell with an intact nucleus that contains not only the DNA molecule but also the associated proteins that are necessary for the regulation of the cellular processes are needed in order to produce a viable embryo. As far as I know, what we have is not enough to produce a clone, let alone the usual obstacles of cloning (somatic nucleus transfer has a rather low success rate, and cloned individuals often suffer from developmental problems).
b) Genome editing with CRISPR-Cas9. This new method provides a lot of new possibilities regarding genetic engineering and also the recreation of recently extinct species. It enables to do very precise modifications of the DNA molecule and to change or exchange genes point by point. By this technique it would be possible to take the DNA of a surrogate cattle and exchange the domestic alleles with aurochs alleles, producing a viable cell with aurochs DNA, and eventually leading to an aurochs calf being born by a suitable surrogate mother. The mitochondria however will still be of domestic origin, but they should have only a minor influence on the phenotype of the organism. As for epigenetics, perhaps the difference would be small as well as aurochs and domestic cattle are closely related, but we shall see. However, as the DNA is from a somatic cell of a grown aurochs and not from an embryo or calf, the individual might experience similar developmental issues as clones do.

If a genuine aurochs was born

But when finally a genuine aurochs individual is successfully recreated and born and growing healthy, what do with it? It will be a male, and of course more than just one individual with this genome can be created. At first, it will be most interesting to see it growing and become a mature individual. Now that we have something directly to compare with, we would see how far “breeding-back” results or any primitive breed match the goal. Perhaps also some minor surprises about its life appearance would revealed. Those reconstructed aurochs bulls could go to zoos for display just like any wild animal. They could even be used as new breeding bulls for any “breeding-back” herd, be it in grazing projects, zoos or private farms in order to successively increase the aurochs-likeness of those herds. One would have to be aware of the fact that it is a wild animal though, with the behaviour and dangerousness of a wild bovine.

Building up a population

Theoretically, even a female can be constructed by removing the Y chromosome and replacing it with a duplication of the X chromosome of the aurochs bull. With that, you can theoretically build up a herd, but the genetic diversity would of course be extremely narrow. Probably a lot of the first-generation crosses if not all would end up stillborn or as miscarriages because lethal alleles have a high chance of homozygosity in the offspring. Furthermore, these aurochs, which are basically from 8.000 years ago, might also be vulnerable to many modern cattle diseases. The only way to create a viable population of aurochs would be to acquire more genetic diversity. If one is optimistic, perhaps the full nuclear genome one or even more further aurochs individuals, in the ideal case one of another time and region, can be resolved. In this case it might be enough – if we get, say, the full genome of five individuals from different regions of Europe and different geological times, we would probably have a way higher genetic diversity than in the Wisent already. But if one is pessimistic, that one British aurochs bull might remain the only individual for which the full genome can be resolved. In this case, there would be only one way to achieve more genetic diversity in order to create a viable aurochs population: controlled, systematic hybridization with domestic cattle.

I would use a technique called absorptive breeding, where the hybrid offspring is constantly back-crossed with pure individuals of the parental generation until the level of “purity” desired is achieved. This way, a fifth-generation hybrid would be genealogically almost 97% pure. Most alleles would be aurochs alleles, but it would also include domestic cattle alleles (however, genealogy is not everything – theoretically it is also possible that such an animal still has 50% domestic alleles, although statistically unlikely). It would be best to not to use just any domestic cattle, but to use robust and healthy breeds to introduce necessary immunological alleles for the resistance against modern cattle diseases, f.e. Chillingham cattle and other robust northern breeds, and primitive landraces.
I would build up a herd composed of third, fourth and fifth generation hybrids created by absorptive breeding, keep them under semi-feral conditions (therefore, using them under the frame of a grazing project in a natural area would be the best option, although their behaviour would be challenge to conventional grazing projects) and then let them breed for themselves but would still select the offspring. This is necessary in order to achieve the goal: the goal is to add genetic diversity for resistance against diseases and overcome developmental problems, but at the same time not altering the biologic integrity of these wild cattle. That means artificial selection should focus on breeding out individuals with obvious signs of hybridization that would show in the phenotype (remember: phenotype not only refers to morphological/optical aspects, but also such as behaviour). The genuine, unhybridized aurochs individuals should always be the template for that. At the same time, individuals that cope well with the natural circumstances, are fully fertile, give birth to healthy offspring and apparently are resistant to diseases should remain in the population. Since people have to work with those cattle, tame, relaxed, less aggressive and less nervous individuals will of course be easiest to handle. However, one has to pay attention not to favour these individuals and therefore domesticate those aurochs hybrids once again (which would be rather ironic). One would have to regard them as wild bovines, keep and handle them like wild bovines. It works with wisents, bison and buffaloes, so it should work with aurochs as well.
After some generations, a viable population of aurochs would have been created. The likelihood of deleterious alleles becoming fixed homozygously has been drastically reduced by hybridization, and hybridization introduced a lot of new alleles for the resistance against diseases and cope with other obstacles, while artificial selection preserved the biological integrity of the wildtype. Furthermore, there would still be pure individuals to backcross with, even if you have to genetically produce them again. However, alleles for domestic mutations that would show in the morphology of the animals would remain in the population for some time unless they are effectively selected against. It would be comparable to the situation of many American bison, where you still occasionally see individuals with vestiges of hybridization with domestic cattle around 1900 (for example here), but invisible in the majority. This could include divergent horn shapes or colour variants, especially when the alleles are recessive.

The perception of resurrected aurochs

A lot of people, especially conservationists, will complain about those aurochs having domestic cattle introgression. They will argue that these aurochs are not “pure” and therefore not genuine. This exhausting mindset is the result of underestimating the role of hybridization in evolution and the importance of genetic diversity and might one day cost the wisent’s life as a species, a mindset that is part of the 19th and 20th century conception of nature as a static and “highly balanced” system to which we humans are the “destabilizing factor”, a conception that is not tenable for obvious reasons. Not only is hybridization a not as rare aspect of speciation as once thought (I plan to do a post on that), in this case we would have introgression into a wildtype from its domestic descendants. Hybridization between wild and domestic forms takes place everywhere they share the habitat, and is also evident for wolves, wild boars and also been proven for Holocene wild horses. Nobody would consider timber wolves or McKanzie wolves less “genuine” because their coat colour diversity is suggested to be influenced by ancient domestic dogs, or condemn wild boar populations because they share some domestic haplotypes – so why not accepting a population of genetically resurrected aurochs whose genetic diversity has been increased by controlled, systematic introgression of some of its domestic descendants?  

The idea of hybridizing those aurochs with domestic cattle in order to increase genetic diversity of course evokes the question which cattle would be best-suited to do this. As mentioned above, these cattle should a) be as healthy and genetically diverse as possible, b) as less-derived as possible in order to introduce as few derived or deleterious alleles as possible – two points that often go hand in hand. Probably it would be best to choose a handful of less-derived, robust and healthy landraces to hybridize with. Once again a balancing act has to be done. If one would hybridize only with a herd of a breed like Maronesa, the impact on the visible traits would be very low in the outcome, but the gain of genetic diversity would be narrow. So more than one breed has to be chosen, and from more than just one region. You can find suited cattle even in Asia in the form of the turano-mongolian breeds (see here). But importing sufficient numbers of cattle is very costly and effortful, which is also one of the reasons why many grazing projects and “breeding-back” projects use breeds that are already local.

And here is where already existing “breeding-back” cattle are important and which is why I think they would not at all become meaningless if genuine aurochs would one day be reconstructed. If such an aurochs project is to be set up on a large, semi-natural area somewhere in Europe outside the Mediterranean range, “breeding-back” cattle would be the most aurochs-like cattle available and since they are composed of a number of landraces, would introduce genetic material of a number of breeds. If such a project came about in Germany or the Netherlands, and one choose to use Taurus and Tauros individuals as a base, you would have the genetic material of at least eight breeds already (Heck cattle, Sayaguesa, Chianina, Maremmana, Limia, Maronesa, Pajuna, Highland). This also means that the range of undesired domestic traits is large, though, but it will be minimized by absorptive breeding and selection, just like in the case of bison with domestic cattle introgression.  

What to do with resurrected aurochs?

So if one finally has the desired viable population of genuine aurochs, what to do with them? At first, it might be wise to create a herd book for them, perhaps right from the beginning, like it has been done for a number of endangered species. This can be advantageous to maintain an overview over the genetic composition of the animals after they are spread to other breeding sites. Surely some of them can be sold to zoos for preservation and education. They can also be used for grazing projects and private breeding by aurochs fanciers, but I doubt that this will be a common use for genuine aurochs, especially the latter one. These animals would display wild behaviour and would be difficult and dangerous to handle just like wisents, bison and wild buffaloes. There is a reason why domestic cattle are used that commonly in grazing projects, but there are only a handful that work with wisents, although the wisent is a native wild animal here whose reintroduction should be preferred over that of a domestic proxy.  

The ultimate goal would be to not only create a population that can be considered “pure” or more or less pure in the eye of enthusiasts but also in those of the IUCN. A recognition of those reconstructed aurochs as aurochs and wild animals from an “official” organization would be a great help for the acceptance of these cattle as wild animals and legitimate their reintroduction into nature areas. Zoos would also help to spread and increase the population number, where they could also be used for education.