When and how should the "breeding-back" projects cooperate?

Today in the 2020s, there are several “breeding-back” projects focusing on the European aurochs. I think that there can be no question that it would be most beneficial for “breeding-back” as a whole in order to achieve the goal – that is, a population of cattle that is as aurochs-like and at the same time as genetically diverse as possible to be fit for a reintroduction into Europe’s nature – if the projects would one day cooperate in some sort. The question is: when and how should the projects cooperate? 

First of all, it has to be visualized why cooperating between the different “breeding-back” projects would be beneficial. It would be helpful to maintain a higher level of genetic diversity than if the projects would work separately. Maintaining a certain level of genetic diversity while at the same time creating a homogeneously aurochs-like population is one of the challenges of “breeding-back”. This is less of a challenge when there are several projects that cooperate. When the crossbred cattle used in the breeding projects are bred selectively for a more homogeneous phenotype, some alleles become fixed. In the process of homogenizing the population, the genetic diversity is reduced. However, in each different project, different alleles become fixed. And when the populations of the projects are eventually combined to one large gene pool, the genetic diversity is larger than it would be if there was only one project. As an example, Project 1 has a cattle population with the alleles A, B, C and D on loci responsible for any trait that is not affected by the breeding objectives of “breeding-back” but relevant for genetic health. Project 2 has a cattle population with the alleles E, F, G, and H. Both projects establish a homogeneous aurochs-like phenotypes, and reduced their genetic diversity in the process. In Project 1, only the alleles B and D remained in the populations, being present homozygous now. D is deleterious when homozygous. In Project 2, only the alleles F and H remained in the now very aurochs-like animals, and allele F is deleterious when homozygous. So both projects achieved a very aurochs-like phenotype at the expense of genetic diversity. When both projects combine their gene pools into one large gene pool, we have a population with the alleles B, D, F and H. Now the allelic diversity is larger again and the number of individuals being homozygous for deleterious alleles dropped significantly and since both projects have very aurochs-like animals, the degree of aurochs-likeness was not affected by combining the two lineages. This is of course a simplified example, but combining two or more lineages always results in a greater allelic diversity and if all the individuals of those lineages are very aurochs-like, genetic diversity would not go at the expense of resemblance to the aurochs, which would be the case if a not related non-“breeding-back” breed was crossed in to increase the genetic diversity. 

While combining the different “breeding-back” lineages would be beneficial in the long run, it does not make sense if the different projects are yet at different levels of “breeding-back” progress. For example, if one project has great animals and another project is just starting or has animals of modest resemblance to the aurochs, and they exchange animals, the result being one project benefiting in terms of aurochs-likeness and one project perhaps introducing undesired traits from the breeds of the other projects, while the genetic diversity would not necessarily increase because the process of achieving very aurochs-like animals is not completed yet. Rather it could lead to the contrary, because the project with the less good animals will use the great animal from the other project on a larger scale in order to improve the aurochs-likeness of their animals, thereby narrowing its genetic diversity. Thus, I think exchanging animals between the “breeding-back” projects really only makes sense if all of them are at the same level of aurochs-likeness. 

When one project has an animal with a trait that all the other projects lack, for example if one project has absolutely perfectly aurochs-like horns and all the animals of the other projects have bad horns, it certainly would be beneficial for the aurochs-likeness of the other projects to acquire animals from the project with the perfect horns, but it would also narrow the genetic diversity in all of them. Therefore, it would be smarter before exchanging animals to ask the following questions: Why are the animals of the other project better in this respect? Did they use a breed that contributed the traits that are lacking in the one project, and could this or another breed with that trait be used in the project? For example, if one project used only medium-sized breeds as founding breeds, and another project included a very large breed and thus has larger animals, it would be the best decision for the overall genetic diversity of the “breeding-back” pool to cross-in very large founding animals instead of depleting the diversity by using an individual from the other project. 

Therefore, I think that for now, the different “breeding-back” projects should focus on their own gene pool and how they can improve the aurochs-likeness of their animals within the gene pool. That is not to say that never ever should individuals, cows in particular, be exchanged between the projects. Only the large-scale use of individuals from other projects, f.e. as sires for many years, however aurochs-like it may be, should be avoided so that the overall genetic diversity is not reduced. But once all of the projects achieved the same level of aurochs-likeness, i.e. that all of the animals of all of the projects are large, have the right colour, the right horns, the right sexual dimorphism, the right morphology et cetera, I suggest to exchange animals at a regular basis in order to create genetically diverse and healthy individuals. In zoos and reserves, the animals are exchanged on a regular basis in order not to diminish the genetic diversity of the herds, and “breeding-back” should do so as well once all projects have reached the same level of quality. The result would be one large and genetically diverse population of very aurochs-like cattle that are fit to be established in European wildlife reserves.