I did a couple of posts where I suggested cautious outbreeding for the wisent using its closest living relative, the American bison, in order to overcome a supposed inbreeding depression resulting from the extreme genetic bottleneck of the modern population (which descends from only 12 founding individuals). See
- Over-purity as a danger for the wisent?
- What to do with the wisent in the Caucasus?
- Controlled hybridization for saving the wisent?
Consequently, I used to regard the Caucasus population, which has been living in the wild for several decades now and has an American introgression of roughly 5%, as very valuable for the conservation of the species. It has been several years since I wrote these posts, and which this one, I want to give my current take-on to the question if outbreeding the wisent is actually necessary for its conservation.
This question is sometimes intermingled with the controversy revolving its taxonomical status in relation to the American species. Some consider both forms subspecies of the same species because they are fully interfertile, i.e. they can produce hybrids that are fertile in both sexes, others prefer to retain the separate species status for both forms. I have no firm opinion on this, because I think having one is not useful as there is a) no clear definition of a species that works universally and that everyone agrees on and b) it is only intuitive that not all species are differentiated to the same extent because the time of separation between two given lineages may differ, the intensity of the selective pressure the lineages may experience may differ, the impact of mutations that appear in those lineages may differ (one mutation that has a drastic effect may prevent hybridization in otherwise very closely related lineages). Thus, I think having a firm opinion on if European and American bison are two distinct species or subspecies of one species and fighting over it is not very useful. Furthermore, moving back to the actual topic of this post, it is not really relevant for the question if American introgression should be utilized in the conservation of the wisent, as the taxonomical status of both forms does not make both lineages any more similar or dissimilar to each other. Taxonomy describes facts and does not create facts.
In my posts linked above I describe detrimental consequences of inbreeding that can be found in the wisent. The fact that these consequences exist, however, does not tell us much about the frequency at which they appear and if they are a problem for the long-term wellbeing of the species. The status survey and conservation action plan for the wisent from 2004 describes the inbreeding depression in the wisent as “very small” . Tokarska et al., who evaluate the genetic variability in the Bialowieza herd, even note the “absence of any signs of inbreeding depression”, although the low genetic diversity is seen as a potential threat as it implies low adaptability .
It is true that wisent populations may suffer from wisent-specific balanoposthitis and the species is particularly sensitive to foot and mouth disease, and introgression from American bison might (or might not) help to overcome this (although American bison are not immune to the latter disease either), but the average percentage of males in Bialowieza that have had balanoposthitis from 1980 to 2005 was only 6,5%, rarely reaching above 10% in some years .
What the wisent really needs in order to ensure the long-term prosperity of the species would be space for large populations (several thousands) to thrive and – very important – protection from poaching . Larger populations, even if inbred, have a much higher chance of surviving than small, dispersed herds of only a few dozen individuals. If the population is large enough (the wisent conservation action plan from 2004 suggests numbers of up to 3000 individuals), the individuals that display negative consequences from inbreeding because they have a higher number of deleterious alleles would have less reproductive success than those that have fewer deleterious alleles. Eventually, natural selection in a population that only is large enough will lead to a relative increase of non-deleterious alleles and a decrease or even disappearance of deleterious alleles.
A scenario in which outbreeding with their American relative would be beneficial is when there is a gene at which all or many wisents are homozygous for a deleterious allele that has a noticeably detrimental effect on the organism, and American bison have healthy alleles on this very gene, or if there were even several of such genes in which this is the case. In this scenario, it would certainly be beneficial to use American introgression in a separate studbook and using genetic tests in order to ensure that the hybrids only carry the healthy alleles on the gene(s) in question. But no such scenario has been reported yet. That means that further research on the genetic health of the wisent is necessary to determine if outbreeding would be useful and worth the effort at all, and it is perhaps questionable if that was the case if there was enough space for large populations of wisents to thrive. We should not forget that both bison types are the result of unique evolutionary processes on different continents, and full transparency in the form of a separate studbook would be a prerequisite for carrying out introgression.
What to do with the Caucasus population that has roughly 5% of American introgression, then? I still think it is a very valuable population, for the very fact that several decades of natural selection followed the hybridization event in the 20th century. The genetic and phenotypic health of this population should be studied. For example, it should be examined if balanoposthitis is less frequent or even occurs at all in the Caucasus population. I think this population should be studied, and not exterminated, as suggested in the wisent conservation action plan from 2004 – considering that there is no continuous range that includes herds without American introgression the danger of contaminating the purity of other herds is low. And even if the herd merges with a pure herd, only the alleles advantageous for the survival of the population in that specific environment get to spread – there cannot be anything bad about that, from an evolutionary perspective. The original wisent form that was native there, B. bonasus caucasicus, has been hunted to extinction already anyway. Arguments that the wisents with American introgression are detrimental to the ecosystem or maladapted are very dubious, as I explain in the post “What to do with the wisent in the Caucasus?” linked above.
All in all, I think that trying to establish large (several thousand individuals) populations of the wisent in the wild that are protected from poaching should have priority over the idea of outbreeding in order to conserve this species in the long run. If one wants to execute outbreeding, more research on the genetic health of the wisent should be done before that, in order to see if it will be effective in the first place. Studying the Caucasus population that already has American introgression would be relevant for this.
 Pucek, Krasinski, Krasinska, Olech, Belousova: European bison – status survey and conservation action plan. 2004.
 Tokarska et al.: Genetic variability in the European bison (Bison bonasus) population from Bialowieza forest over 50 years. 2009.
 Krasinski & Krasinska: Der Wisent – Bison bonasus. Neue Brehm-Bücherei. 2008.