Over-purity as a danger for the Wisent?

EDIT: For an update on my take on to this subject, go here. 

Ok, this is a little bit off-topic, but it is bovine-related and certainly a very important issue that I don’t want to miss here.

It is well-known that the Wisent went through a dramatic bottleneck in the 1920s; only 54 individuals remained, and these descend from only 12 ancestors [1]. The contribution of these ancestral individuals to the modern population is uneven and dominated by one pair in particular, so that the modern gene pool is even smaller [1].

The small genetic diversity and the high degree of inbreeding is regarded as one of the main dangers to the global wisent population. Inbreeding affects the skeletal growth, leads to skull asymmetry, deformation of the male gonads, increases the rate of stillbirths, decreases female fertility and the resistance against diseases and parasites [1]. Wisents are vulnerable to diseases such as posthitis and balanoposthitis, foot-and-mouth disease, cattle tuberculosis, bluetongue disease and others. Epidemics are an acute danger to wisent populations, even in captivity. In the Bialowieza Primeval Forest, about 20% of the mortalities are caused by diseases [1].

The modern Wisent population is separated into two genetic lines. One is of pure bonasus individuals, and the other one has introgression of one single caucasicus bull. Curiously, the inbreeding effects are more severe in the LC-line. In the Lowland line the inbreeding coefficient is equal to 44%, in the Lowland-Caucasian line 26% [1].

"Kaukasus", the last pure Caucasian wisent

Line breeding separates the very small gene pool into two even smaller sub-pools and is something that usually would be avoided in the conservation of a species that is highly inbreed and homozygoteous. The reason behind this is to preserve the pure members of the lowland line, although the introgression of only one bull of a different subspecies eighty years ago probably would have little effect on the global wisent population, except that the genetic diversity would increase (what is, in fact, necessary to preserve the species). While the L line is a closed pool, the LC line is “open” and therefore pure lowland bison are constantly mixed in. This leads to a further decrease of genetic diversity, because the Caucasian influence gets increasingly diminished [1]. Therefore, line breeding increases the problem of inbreeding and homozygosity.

Therefore, one way to prevent the further loss of genetic diversity that I propose is to stop the line breeding by “opening” the L line, so that the two very small gene pools could fuse into a larger one and to prevent that the Caucasian influence disappears altogether. The affect of this influence of only one bull on “pure” Lowland wisents would probably be very small, as both bonasus and caucasicus belong to the same species and show only slight phenotypic and ecologic differences. It confuses me that there are efforts to separate subspecies and subspecies hybrids (with hardly any recognizable difference except inbreeding-related issues) in an endangered species with a critically small gene pool like the Wisent, while there is no such action in a genetically diverse, healthy and not endangered species like the American Bison, of which the two subspecies also intermixed in some populations (f.e. Wood Buffalo National Park) or not even to purge out the influence of domestic cattle in the American bison.

Another but certainly controversial step to increase the genetic diversity of the wisent is to cross-in single American bison and mate the results with pure Wisents again until the American influence isn’t recognizable anymore. This way of breeding is called absorptive breeding. The mere existence of such hybrids, or individuals with “unproven pedigree” are regarded as a threat for the Wisent in the action plan for the conservation of this species by the Polish Academy of Science [1], although it certainly would compensate the larger, immediate danger of the small genetic pool. It is true that uncontrolled and nontransparent hybridization, f.e. in a feral population, is an indirect danger for the Wisent as a species, but controlled introgression of the closely related American species might be helpful in overcoming the severe inbreeding depression of the European bison.

In the early 20^th century, hybrids of wisent and Am. Bison, but also domestic cattle hybrids, were not rare at zoos and were in fact a threat for the conservation of the species, because there was the danger that hybrids and pure wisents become all mixed up and indistinguishable. That’s why a pedigree book for the Wisent was set up in 1931 – the first pedigree book for any endangered species – to ensure that the Wisent will survive in its original form, un-altered by uncontrolled hybridization [2]. Therefore, it is understandable why hybridization per se is still regarded as a threat to the Wisent, but today it is also apparent that the high degree of inbreeding and the resulting low resistance to diseases and parasites and other negative consequences are a drastic and immediate danger for the global population. Controlled introgression by few hybridization events would be one way to add genetic diversity to the wisent gene pool, but what other effects would controlled hybridization and back-crossing have on the Wisent?

The European bison and the American bison are closely related and can interbreed without any fertility problems. Because of this, several authors treated them as one species in the past. Phenotypic differences are apparent; the American bison has a more longish and massive body, more opulent fur on the head forelegs and also the horns are not as long but thicker as in the Wisent, among other differences. However, both species are ecologically similar, except that grasses make up a higher portion in the diet of the American bison. The Wood bison, B. b. athabascae, inhabits a similar habitat as the Wisent did in ancient Europe. In 1940, hybrids of American and European bison were released in the Caucasus. After controlled culling and subsequent replacement with pure wisents, the portion of American blood within the Caucasus population is now estimated being only about 5%, although the American influence is still slightly visible in the phenotype of the animals. Allegedly this population has a destructive influence on the local flora because of the hybrid origin. I am not familiar with the facts behind this claim, but I am sceptic about it because according to the food choice of the A. bison, the only difference to be expected is that they eat a higher amount of grasses than pure wisents would do – and originally there were two other species of grazers in the Caucasus that relied on grasses to a large extent (aurochs and wild horse). But of course does the presence of herbivores change the frequency of certain plant species, because herbivores decimate some species more than others. This is to be expected in every ecosystem, and this observed change in the plant community might be interpreted as a “damage” if someone expects the animal to cause damage. It is further claimed that the population released in the Caucasus is less tolerant to cold and less adapted to mountainous habitat because of the influence of the American bison. But the lacking adaption to the mountainous habitat probably isn’t due to the American influence in particular, but the Caucasian Wisent very likely had special adaptions to its habitat that are not present in the Lowland Wisents as well. For example, it was smaller, had shorter higher hooves and a less shaggy coat [3]. The claim that the American influence made them less resistant to cold is very likely incorrect, since American Bison resist temperatures down to - 40° Celsius.

Wisents with American bison introgression in the Caucasus (Photo by Sergej Trepet) 

In order to conserve the Wisent as a species but to increase the genetic diversity by controlled hybridization, not a patchwork of American and European features should be the objective, but a homogenous Wisent population with slight American introgression that is not visible. This is possible, as we know from the fact that many modern American bison populations experienced introgression from domestic cattle in the past, but the cattle apparently left hardly a trace in the phenotype, behaviour or ecology of the bison (however, this isn’t the case for all herds). Controlled hybridization followed by “expulsion breeding” by constant back-crossing with pure wisents can result in a wisent population that looks like the wisent, behaves like the wisent and has the ecology of the wisent, but is more genetically diverse than the highly inbreed “pure” populations. Individuals that show clear traces of the American bison can be selected out. When American bulls are used and only their female offspring is chosen for further breeding, the entrance of “foreign” Y chromosomes is effectively prevented (vice versa for the mitochondrial DNA in cows).

What would be the effect on the genetic structure of the Wisent? As a population geneticist would say, its allele frequency would slightly change. But as we know through modern genetics, populations always change (genetic drift). The modern wisent is necessarily genetically different from the Wisent of the 19^th century before the bottleneck event. And the Wisent of the 19^th century was genetically different from that of the Bronze Age et cetera.

Surely, uncontrolled and nontransparent hybridization of wisent with American bison or even cattle, be it in the wild or captivity, is a threat for the conservation of the species. But a controlled and transparent breeding plan like described above might be seen as a chance for the Wisent to gain more genetic diversity to overcome the menacing effects of inbreeding in the species. Perhaps the European bison Pedigree book one time will allow a third genetic line with American introgression besides the Lowland line and the Lowland-Caucasian line, where every interspecies mating is transparent and recognizable. In this case it would always be possible to distinguish between wisents with introgression and those without, and the amount of introgression is always transparent as well. If the Pedigree book is against such a step, there is still the possibility to set up a new breeding book for that lineage. Surely this would be controversial and also cause disaffirmation, but if it turns out that Wisents with slight American introgression are healthier and more resistant to diseases than their “pure” counterparts and show no apparent signs of hybridization (neither in their phenotype, behaviour or ecology), maybe there will be shift in looking at this strategy to safe the wisent, for which, as the conservation plan states, “the danger of extinction still remains”. Maybe it will also cause a different look at the 700 wisents that are not registered in the Pedigree book and also the population in the Caucasus. The latter population has been living in the wild for more than 70 years now and grew to magnificently large numbers (though poaching is a serious problem), and therefore is certainly precious. Culling of bulls and replacing them with pure Wisents could further reduce the phenotypic effect of the American bison introgression.

In my opinion, studies comparing the food choice of American bison, European bison and hybrids should be set up. Also it is necessary to investigate the resistance of the hybrids against diseases and other negative effects of inbreeding (the existing population in the Caucasus could serve as a model) and to investigate which of the two subspecies of the American bison is better-suited for such an attempt. This could be a scientific basis for a breeding experiment executing controlled and transparent introgression into the Wisent pool as described above without affecting it as a species in order to overcome the most serious and immediate threat for the conservation of the species.

Here's a video of the wisents in the Caucasus with bison introgression: 

Website of the European Bison Conservation Center

Literature

• [1] Mammal Research Institute, Polish Academy of Science: European Bison Bison bonasus: Current state of the species and an action plan for its conservation. 2002
• [2] Jan Raczynski, Malgorzata Bolbot (2009): The European Bison Pedigree Book.
• [3] Ninell Melkadze, Nargiza Ninua, Izabella Skhirtladze (2009): Catalogue of the type specimens of Caucasian large mammalian fauna in the collection of the National Museum of Georgia