Is outbreeding the wisent necessary?

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” [1]. 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 [2]. 

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 [3]. 

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 [1]. 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 20^th 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. 

 

Literature

 

[1] Pucek, Krasinski, Krasinska, Olech, Belousova: European bison – status survey and conservation action plan. 2004. 

[2] Tokarska et al.: Genetic variability in the European bison (Bison bonasus) population from Bialowieza forest over 50 years. 2009. 

[3] Krasinski & Krasinska: Der Wisent – Bison bonasus. Neue Brehm-Bücherei. 2008. 

 

 

Extinct megafauna that could be revived using genome editing

Humans wiped out countless species of animals. Some of those species, particularly those that died out comparably recently, might actually be retrievable. Usually, cloning comes to mind when talking about reviving extinct animals. However, somatic nucleus transfer for reproductive cloning requires an intact cell and not just the DNA of the animal, which is why it is not available for most species wiped out by man. However, there are a few megafaunal species that have been wiped off from which we have at least one and in some cases several complete genomes. If there is a still extant species that is closely related and would make a suitable surrogate, it would be possible to exchange the alleles specific for species A with those for species B and thus creating a viable cell with the genome of the extinct species in question with genome editing. This limits the number of extinct animals that could be revived. For example, I would imagine it to be pretty difficult if not impossible for a species like the thylacine, which has been evolutionary separated for 30 million years from its closest living relatives. In some cases, it could be easy and much less effortful because of the lower number of genes that differ and the suitability of the surrogate because the animal has a very close living relative. In this post, I present a number of megafaunal species from which full genomes either have been acquired or at least could potentially be acquired and which have a more or less closely related relative that can be used for genome editing and as a surrogate. The mitochondrial DNA of the resulting animal would be that of the donor cell, which would be from the related species. However, as mitochondrial genes are highly conserved among mammals this would not have much of an influence. Using closely related species also has the advantage that the behaviour will be rather similar, especially among the large herbivore species that I am going to list. This makes the rearing by a surrogate mother and socialization of the result less problematic or maybe not even an issue at all. 

I see reviving an extinct animal that has been wiped out by man as a contribution to species conservation just as breeding an endangered species or subspecies. One might ask, particularly in the case of wiped-out subspecies, why doing that at all if there are suitable ecological proxies. Occasionally some even question the need to conserve endangered subspecies such as the Northern White rhino because their conspecifics from other subspecies would function ecologically the same or very similar. Personally, I cannot relate to that mindset. Conservation is about preserving biodiversity and the preservation of evolutionary more or less distinct subspecies is a vital part of that. The same goes for reviving extinct species or subspecies – it would greatly increase the biodiversity again, after it has been depleted by man when the species or subspecies was wiped out. 

 

Bubal hartebeest, Alcelaphus buselaphus  

This animal is sometimes also considered a subspecies, but that question is merely taxonomical and not relevant for “de-extincting” the animal. In any case, other members of Alcelaphus could be used for genome editing and as a surrogate. Many individuals must have been preserved as trophies, and it is potentially possible to acquire full nuclear genomes from them. 

 

Bluebuck, Hippotragus leucophaeus  

Other members of Hippotragus could be used as a surrogate and for genome editing. It can be tried to acquire a fully nuclear genome from taxidermies.  

 

European aurochs, Bos primigenius primigenius

One full nuclear genome has been resolved in 2015 from a well-preserved Neolithic bone. Considering the richness of recent aurochs material, it could be possible to obtain quite a few more complete genomes. And there is a very close living relative, modern cattle. The number of genes that would have to be exchanged would probably be lower than in most of the other cases I am listing here, and also the surrogate and the procedure of implanting an embryo would be completely unproblematic. And considering the similarities in behaviour between cattle and aurochs, socialization will not be problematic either. For these reasons, I consider the aurochs one of the most realistic candidates for a revival through genome editing. Even if the resolved genome remains the only one to be fully resolved, one revived aurochs individual still can be outbred using aurochs-like cattle. I wrote a post on that a few years ago. 

 

Several types of wild horses 

There are well-preserved mummies of Siberian wild horses, Equus caballus lenensis, and one of the Yukon wild horse, Equus caballus lambei, so it could be possible to obtain fully resolved nuclear genomes from that. A domestic horse could be used for genome editing and as a surrogate. Even if only one genome can be obtained, the revived horses can be outbred with Przewalski’s horses and/or robust landraces in the same manner as I suggested for revived aurochs. 

 

Kouprey, Bos sauveli  

Numerous kouprey specimen have been preserved as trophies and one skin. It could be possible to obtain full nuclear genomes from that very recent material. The closest living relative is the Cambodian banteng which hybridized with the kouprey in the past. It can be used for genome editing, as a surrogate and even for outbreeding if necessary. 

 

Quagga, Equus quagga quagga  

The quagga is not and cannot be bred-back from extinction with living Plains zebras, which is why it would be desirable to try to acquire full genomes from the numerous preserved skins and the few skeletal material that is preserved of this zebra. The zebras of the Quagga Project can be used for genome editing, as a surrogate and outbreeding if necessary. 

 

Pyrenean ibex, Capra pyrenaica pyrenaica  

This is the only extinct animal that has been cloned so far, unfortunately the clone died shortly after birth. Genome editing with individuals from other subspecies of Capra pyrenaica could be more successful, they can be used as surrogates and for outbreeding. 

 

Steppe bison, Bos (Bison) priscus  

There are plentiful of remains from steppe bison, including soft tissue. Perhaps it would be possible to obtain full nuclear genomes from that. Needless to say, that still existing bison, be it European or American, can be used for genome editing, as a surrogate and outbreeding. 

 

? Woolly mammoth, Mammuthus primigenius  

Currently, there is no de-extinction project in the strict sense focusing on the woolly mammoth. There is the attempt to create a “mammophant” by introducing mammoth alleles for certain traits into the genome of an Asian elephant, what is not what I would consider de-extinction in the strict sense. If doing that is possible, it might also be possible even if more effortful, to exchange all alleles of genes where Asian elephant and woolly mammoth differ. However, since implanting an embryo into an elephant is very complicated, and those who want to create plan to use an artificial womb, a technique which does not exist hitherto, I wonder if it is feasible to recreate the woolly mammoth for practical reasons. 

 

Caucasian Wisent, Bos (Bison) bonasus caucasicus  

Several skins and trophies of this wiped-out subspecies (or species or variety, there is no consensus on its taxonomic status) exist, so it could be possible to obtain full nuclear genomes from it. European bison of the Lowland-Caucasus line, which partly descend from the last Caucasian wisent bull, could be used for genome editing, as a surrogate and for outbreeding. Obtaining genomes from remains of wisent prior to the bottleneck in the 20^th century could also help to greatly increase the very limited genetic diversity of this endangered bovine. 

 

Cave lion, Panthera spelaea  

Several very well-preserved pubs of this feline have been found. It might be possible to acquire full nuclear genomes from that, and the closely related actual lion would be suitable for genome editing and as a surrogate. 

 

Schomburgk’s deer, Rucervus schomburgki  

Some remains of this deer species exist, a sister species from the Rucervus clade could be used for genome editing and as a surrogate. 

 

Japanese sea lion, Zalophus japonicus

There are taxidermied specimens of this sea lion, related species of the Zalophus clade can be used for genome editing, as a surrogate and possibly outbreeding if necessary. 

 

Caribbean monk seal, Neomonachus tropicalis

There should be some remains of this recently extinct species, the related Hawaiian monk seal can be used for genome editing, as a surrogate and possibly outbreeding if necessary. 

 

One common objection against the revival of extinct animals is “one individual is not enough to build a population”. Apart from the fact that even one individual could tell us a lot about the extinct animal species/subspecies, it could be possible to get several genomes of those recently extinct species. Getting the full genome of five or ten individuals from different regions and times would probably enable to get a genetic diversity comparable to that of the modern wisent population, which descends from only twelve founding individuals from the same population. Some wisent individuals show inbreeding-related problems, but not to the extent that it threatens the survival of the species. An even more extreme example would be the Mauritius kestrel. Apart from that, related species/subspecies can always be used for outbreeding to add genetic diversity. Hybridization among related species with neighboring or overlapping distributions is very common in the animal kingdom. 

 

Why is the aurochs extinct while the wisent survived?

As everybody reading my blog will know, Holocene Europe was originally home to two native bovines, the wisent and the European aurochs. But only the wisent survived, while the aurochs eventually died out in the 17^thcentury, as a result of anthropogenic influence. But why did the aurochs die out while the wisent survived human activity until today? 

 

It was two anthropogenic factors that drove the aurochs to extinction: one was hunting, the other one habitat limitation due to the expanding civilization. Both factors also apply in the case of the wisent. The wisent was also hunted, and its habitat was also limited increasingly as the human population in Europe grew continuously. So why did the wisent survive and did not die out at the same time the aurochs did? 

One of the reasons might be that aurochs were hunted more intensively than wisent. Aurochs had larger, more impressively shaped horns and a colour that was more aesthetically appealing than that of the wisent (Sigismund von Herberstein wrote: “…the wisent is not as beautifully black as the aurochs…” in the 16^th century), so that it is possible that trophy hunting focused more on the aurochs than on the wisent. Julius Caesar wrote in De bello gallico that the Germanic people liked to hunt the aurochs for its horns, while he made no mention of the wisent, although this species must undoubtedly have lived in the Hercynian forest as well. 

Another possible reason why the wisent survived human activities while the aurochs died out lies in the ecology of both species. I outlined the ecologic differences between the two bovines in my post The ecologic niche of the aurochs. The aurochs was likely in direct competition with domestic cattle (and likely also horses) for feeding grounds. The aurochs was predominantly a grass eater, as are cattle and horses, and it is historically documented that aurochs grazed on the same places as the livestock of farmers, who chased the aurochs away if they encountered one on the pastures (see Anton Schneeberger’s report in Gesner 1602). During the last millennia and centuries of its existence, the aurochs retreated to wet habitats, such as swamps and marshes as isotope analyses show [1]. But also these areas were cultivated increasingly. The last historically documented population of aurochs, which lived in the forest of Jaktorow in Poland, disappeared because the space available to them became ever smaller and smaller as farmers continuously let their cattle and horses graze in the forests, so that the aurochs had to retreat even further and could not thrive [2]. 

The same problems also applied to the wisent. It, on the other hand, had the advantage that it could retreat to more mountainous regions (while the aurochs lived in even lowlands), which was less invaded by farmers or their domestic cattle and horses. Also, the wisent is more adapted to a forested habitat than cattle (and consequently likely also the aurochs), as they consume more wooden vegetation and need less grass in their diet [3]. The ecology of the wisent is probably the main reason why the extinction of the species in the wild occurred three centuries after the extinction of the aurochs, in 1919 [4]. We should not forget that the wisent survived the extinction in the wild only due to the fact that it was bred in captivity. Without the captive population, there would be no wisents today. The Caucasus wisent, B. b. caucasicus, did not have that luck and was fully exterminated in 1927. 

 

Literature 

 

[1] Lynch et al.: Where the wild things are: aurochs and cattle in England. 2008. 

[2] van Vuure: Retracing the aurochs - history, morphology and ecology of an extinct wild ox. 2005. 

[3] Bunzel-Drüke et al.: Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung – “Wilde Weiden”. 2008. 

[4] Krasinska & Krasinski: Der Wisent: Bison bonasus. 2008. 

 

The ecologic niche of the aurochs

The ecologic niche of the aurochs and where it differed from that of the wisent is hard to ascertain as the aurochs is extinct. Right? Not exactly. There actually a lot of things on the ecology of the aurochs and its interplay with other European herbivores that we can infer from various sources. 

The aurochs and other European herbivores of the Holocene

1. The aurochs was a grazer and probably much like cattle ecologically

 

The aurochs had the hypsodont denture of a grazer, like cattle today [1]. Anton Schneeberger writes that aurochs fed on acorns, mown grain and hay they were provided, and on branches of shrubs and trees during winter [2]. It has been reported that aurochs grazed in the same places as cattle and horses [2]. Since cattle descend from aurochs, and domestication probably did influence the basic diet in cattle [2], it can be assumed that aurochs and cattle were very much alike in food choice. There are differences within cattle, but that is mainly between highly derived breeds and landraces, since the former do not consume plants that are less digestible while the latter do [3]. 

Isotope analyses suggest that aurochs were found in more forested and wetter habitats than cattle, which mainly grazed on pastures [1]. It is of course possible that this is a difference between the wildtype and the domestic form, but it is more likely that the aurochs was pushed into these less accessible regions because open habitats were reserved as pastures for domestic cattle. Also, free-roaming cattle have been found to prefer wet habitats as well [5]. The continuous habitat loss was one of the driving factors for the aurochs’ extinction [2].  

 

2. The aurochs preferred lowlands and wetter habitat than the wisent 

 

The distribution of fossil and subfossil remains of aurochs and wisent have shown that aurochs preferred plain habitat and lowlands while wisent were found much more often in mountainous habitat [4]. This is also in line with what is known of the historic distribution of both species [2]. While it is true that both the wisent and the aurochs have been limited to hideaway regions due to habitat loss because of the human civilization and hunting, it probably has a reason why the hideaway regions of aurochs were marshes and lowlands and that of the wisent was mountainous habitat, and this region were the ecologic adaptions of the respective species. The wisent avoids areas which are too wet [4], which is why a replacement of the cattle at Oostvaardersplassen with wisent, as it has been discussed on occasion, is not feasible (Bunzel-Drüke, pers. comm.)

Nevertheless, both bovines must have met each other [2]. 

 

3. There was niche partitioning among Europe’s large herbivores 

 

According to the competitive exclusion principle in ecology, two or more species cannot exist in one and the same ecologic niche. Therefore, there must have been niche partitioning among Europe’s large herbivores and apparently this was the case. Cattle consume more wooden material than horses, which are strict grazers. However, cattle do not include as much wooden material in their diet as the wisent, which is a so-called semi-intermediate feeder. So there is a graduation on how much the respective herbivore relies on either grasses or wooden material, with horses relying the most on grasses and wisents consuming the most wooden material, and cattle being intermediate [4]. Since this niche partitioning is the result of evolutionary adaptions of these species, it is likely that this would also be the niche partitioning between wild horse, aurochs and wisent.  

Therefore, considering the differing habitat and food preferences, cattle and wisent would concur only to a limited extent in Europe’s nature systems. So the fear that wisent and cattle cannot be kept in the same reserve is baseless as long as the reserve provides habitats for both species. Another reason why some people have objections against (re)introducing cattle into European nature systems is that they are concerned that wisent and cattle would hybridize. However, a work by Frans Vera has concluded that cattle and wisent do not hybridize spontaneously and only under artificial conditions, which also often lead to the assault of the domestic individual by the wisent [6]. Furthermore, parturition does not occur in those hybrid pregnancies [6], what makes human assistance necessary. Thus, cattle and wisent would not produce hybrid populations in the wild even if mating between both species would occur. 

 

 

[1] Lynch et al.: Where the wild things are: aurochs and cattle in England. 2008. 

[2] van Vuure: Retracing the aurochs – history, morphology and ecology of an extinct wild ox. 2005

[3] Poettinger, J.: Vergleichende Studie zur Haltung und zum Verhalten des Wisents und des Heckrinds. 2011. 

[4] Bunzel-Drüke, Finck, Kämmer, Luick, Reisinger, Riecken, Riedl, Scharf & Zimball: „Wilde Weiden: Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung“. 2011

[5] Gander et al.: Habitat use of Scottish Highland cattle in a lakeshore wetland. 2003. 

[6] Vera: Do European bison and domestic cattle hybridize spontaneously? 2002. 

 

 

 

Przewalski's horse cloned for genetic diversity

In 2015 I proposed cloning as a chance for the wisent's survival. Cloning pre-bottleneck wisents would greatly increase the genetic diversity of the species, since the modern population descends from a population of only about 50 individuals which itself descended from only 12 individuals. Adding the genetic diversity of wisents that lived before the dramatic bottleneck event in the 20th century would help the species to overcome its inbreeding depression. And if cloning is not possible, genome editing with CRISPR-Cas9 is a viable alternative. 

It seems that there are people who had the same idea for the Przewalski's horse, which descends from only 12 individuals as well. A stallion has been cloned from an individual that has been cryopreserved since 1980. For details, you can have a look at the article from Revive & Restore. 

I hope that cloning for conservation will not be restricted to this one individual. I hope this idea will be put into practice for other species as well, including the wisent. 

Did aurochs and wisent hybridize?

Hybridization is very common in the animal kingdom and takes place everywhere closely related species meet each other. There are plenty of examples where hybridization played a role in speciation, not only in amphibians and fish but also mammals, including us humans. For a post on hybridization, go here. 

In recent years, it has been suggested that the wisent is a species hybrid of aurochs and steppe bison because the wisent clusters with domestic cattle on mitochondrial level. For details, go here. This theory has recently been questioned by a 2016 paper that suggests the affiliation of wisent and cattle mitogenomes are more likely a result of incomplete lineage sorting. It also suggests an early split from the steppe bison, from which it differs in head orientation associated with food choice. While the American bison and Steppe bison have a lower head orientation than the wisent as they are primarly grazers, the wisent has a higher head orientation and is a mixed feeder, which is probably the result of living in a more forested habitat [1]. A 2017 study indeed suggests that Bison schoetensacki was the immediate ancestor of the wisent [2].  

However, the wisent does show signs of admixture with Bos in its nuclear genome [3]. The study compared both modern and pre-bottleneck wisents to cattle and the 8.000 year old British aurochs with the fully resolved genome and found signs of interbreeding with domestic cattle. The authors also emphasize that it is possible that these genes are not from domestic cattle but from aurochs closer to domestic cattle than the British aurochs. Either domestic cattle or aurochs or both left their track in the genome of the wisent. However, the very small portion of Bos DNA suggests that this introgression did not happen in recent times. Hybridization may indeed explain the diverging horn shapes sometimes found in wisent, like in this individual at Hellabrunn Zoo I photographed in 2011: 

It is important to note though that wisent and cattle do not interbreed spontaneously. Not a single case of hybridization between both species in the wild has been reported even if they share the same habitat [4]. All wisent-cattle hybrids were created in human custody. However, it might have happened that domestic cattle genes found their way into the wisent genome over those hybrids. If they escaped and joined wild wisent herds, they might have been more likely to interbreed than pure cattle. 

Another possibility would be that both species interbred more easily than today when aurochs arrived in Europe during the middle Pleistocene and later further diverged due to the so-called Wallace effect or reinforcement. In this case, hybrids between aurochs and wisent would have a lower evolutive fitness than pure individuals, thus decreasing the likelihood that both species interbreed. This is just a thought-experiment of mine. 

Another possibility if aurochs and wisent indeed interbred in the past could be that the influence from wisents helped the newly arriving aurochs, which migrated from subtropical areas, to cope with the European climate. I would not be surprised if the curly hair on the forehead that can also extend to the entire neck, dewlap and shoulders which are found in taurine cattle and European aurochs, were in fact vestiges of hybridization with wisents, as those curly hair is very opulent in bison but completely absent in zebus and other Bos cattle. 

Because of the fact that bison and Bos cattle can interbreed, some authors tend to list them all as one genus Bosin recent years. I tend not to. There are no objective measures to determine what is one genus or more than one, it entirely subjective just as all systematic ranks. Hybrids between genera are not that uncommon, for example in chicken, whales (see “Wholphin”) and there is even one case in elephants (see “Motty”). Paleonotological evidence suggests that bison descend from Leptobos, and genetic evidence also suggests that yaks are in fact part of the bison branch and might descend from Leptobos as well. Bos, on the other hand, might descend from Pelorovis. In this case it could well be that all Leptobos and Pelorovis species could interbreed with cattle and bison if they were alive today. Thus they would have to be included into Bos as well, making it an extremely variable super-genus based solely on the fact that they can interbred. And, to be consequent, other genera would have to be lumped as well. Pseudorca and Tursiops and all related genera would have to be listed as one genus, and one might even go that far to synonymize Loxodonta with Elephas. The ability to interbreed alone might not be the best criterion for synonymizing genera, especially as it is gradual from fully infertile offspring to only one sex being fertile to fully fertile offspring. 

Literature 

[1] Massilani et al.: Past climate changes, population dynamics and the origin of Bison in Europe. 2016.

[2] Palacio et al.: Genome data on the extinct Bison schoetensacki establish it as a sister species of the extant European bison. 2017.

[3] Wecek et al.: Complex admixture preceded and followed the extinction of wisent in the wild

[4] Vera: Do European bison and domestic cattle cross spontaneously? 2002. Vakblad Natuurbeheer 

Controlled hybridization for saving the wisent?

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

Long-term readers of this blog will know that the second large wild bovine of Europe, the wisent, is an animal that is not only very spectacular to look at but is also immediately threatened by its extremely low genetic diversity. I covered this topic in several posts already (Overpurity as a danger for the wisent, Donate for the Wisents in the Caucasus and What to do with the wisents in the Caucasus). 

I also repeatedly emphasized why I see the hybrid wisents in the Caucasus as a chance instead of a threat, and I also already introduced the idea of controlled hybridization with American bison in order to increase the genetic of the wisent. This is of course a provocative idea that sounds risky, but I have my reasons and I consider a careful way how to put it into practise worth a try. This is what I want to explain thoroughly with this post. 

The problem 

Inbreeding basically leads to a reduction of genetic diversity, which in turn leads to an increase of homozygosity in the genome. As long as the genome is purged off deleterious (harmful) alleles in the process a high level of homozygosity does not necessarily affect the healthiness of the population (such as in Chillingham cattle), but when the fixation of the alleles is coincidental, such as in the case of a population crash, the consequence is that a lot of deleterious alleles suddenly have a high chance of becoming fixated homozygously in the population, causing them to express their effects in the phenotype and thus leading to a so-called inbreeding depression. The consequences are increased frequency of diseases or distortions that can pose a long-term threat for the survival of the population. This phenomenon is well-known, not only to geneticists, and is a problem in the conservation of animal breeds and species.

The wisent is such a case. Due to habitat destruction and hunting, the population crashed at the beginning of the 20^th century, and all living individuals descend from only 12 individuals. Nowadays, the inbreeding coefficient in the population is very high in both lines (the Lowland line and the Lowland-Caucasian line; the latter one partly descends from the last bull of the B. b. caucasicus subspecies). The inbreeding depression affects skeletal growth leading to skull asymmetry, deformation of the male gonads, increased rate of stillbirths, decreased female fertility and a reduced resistance against diseases and parasites. Wisents are particularly vulnerable to posthitis and balanoposthitis, foot-and-mouth disease, cattle tuberculosis, bluetongue disease and others [1]. Epidemics are an acute danger to wisent populations, both in the wild (in Bialowieza, 20% of the mortalities are caused by diseases) and captivity (few years ago, the stocks at German zoos crashed because of foot-and-mouth disease). Often, reintroduction efforts fails or experience painful set-backs because the animals are too sensitive to diseases (see the herd in the Romanian Carpathians). This is why the action plan for the conservation of the wisent still sees it in the danger of extinction [1]. The genetic basis is simply too narrow to ensure a long-term survival for the species. 

Possible solutions 

The current conservation strategy for the wisent is to continue line breeding without loosing diversity any further. This will not overcome the inbreeding depression but is an attempt not to make it worse. The only way to overcome the inbreeding depression is to increase genetic diversity by introducing more alleles to the population. Except from “constructing” artificial genetic diversity by altering the nucleotide sequence of extant wisents with genetic engineering (which I do not know of genetically feasible), I see only two possible ways to achieve that: acquiring genetic material from wisents that lived prior to the population crash (pre-bottleneck) and reintroducing it into the modern gene pool, or controlled hybridization with the closest living relative, the American bison. 

            Pre-bottleneck wisents 

The genetic material of any wisent that lived prior to the bottleneck event would probably increase the genetic diversity within the modern population greatly, even if it is only one individual, be it from the 19^th century or anywhere else during the Holocene. It should not be impossible to acquire the full genome of such pre-bottleneck individuals. There are probably plenty of hides, trophies or skeletons gained from wisents during the most recent centuries that should all still contain enough genetic material. Also any skeletal remains from the latest millennia might be well-preserved enough for this purpose. Since it was possible to resolve the full genome from 8000 year old aurochs bones, the same might be possible with wisent remains. Once the full genome of one or more pre-bottleneck wisents is resolved, it would be possible to introduce the differing alleles into the genome of a recent wisent using CRISPR-Cas9 (the same method has been suggested to revive the woolly mammoth based on an elephants’ DNA). One or few individuals from prior to the end of the 19^th century would probably (re)introduce enough “new” (or actually old) alleles into the populations to greatly reduce the effects of the inbreeding depression. The downside of this idea is: a lot of research work has to be done that is expensive and the technique is effortful as well. At the moment all the resources of these “de-extinction” techniques are concentrated on more prestigious animals such as the woolly mammoth, and it is pretty unlikely that anyone would spend a lot of money and effort on reconstructing old individuals of a species that still exists, even tough it would be a crucial step to saving the species. 

            Controlled hybridization with bison  

The second method, crossbreeding with its closest living relative, the American bison, would definitely be less costly and could be started immediately. It has to be said that both species are very closely related, they hybridize readily without any barriers or limitations and there are many authors that list them as one species. I am not suggesting to just rampantly cross all wisents on this world with bison and to swamp the whole population with bison genes. Not at all, this would be a huge mistake that would lead to the extermination of genuine wisents on a genetic level. What I am suggesting is wise and moderate hybridization followed by selective absorptive breeding on a separate, new breeding line with a separate herd book. The ancestry of each of these individuals would be documented just as that of the pure individuals. By this way, the population of the pure but inbred wisents on this world would not be affected or threatened in their genetic integrity in the least, only a few starting individuals would have to be “sacrificed” (if you would start with say, only 100 individuals, I bet this number would be still less than all the losses in the global population caused by inbreeding effects each year). 

The ultimate goal of the hybridization and subsequent breeding would be a population where the frequency of the deleterious alleles has been considerably reduced without affecting the biological integrity of the species too much by bison alleles. The deleterious wisent alleles affect development and other health factors and are probably not crucial for defining wisent characters. Thus, replacing them with alleles from bison would have a beneficial effect on the development and health of the individuals without affecting their nature as Bison bonasus. Of course hybridization will introduce any alleles of the American bison, which is why some sort of controlled breeding is necessary.  In the end, the animals would have to be indistinguishable from other wisents in morphology and looks, behaviour and ecology but at the same time not suffer from the same effects of the inbreeding depression as their conspecifics do. 

The American bison is subdivided into two subspecies, the plains bison Bison bison bison, and the wood bison Bison bison athabascae. Both subspecies are different ecotypes and have different phenotypes. The wood bison is more adapted to woody habitat, is better adapted to cold and is phenotypically not a far removed from the Wisent as the plains bison. So it would probably be wiser to take wood bison for the hybrid project. 

See here for a comparison between wood bison (left) and plains bison (right). As you see, the wood bison also morphologically resembles the wisent better than the plains bison. 

How to execute such a project 

The larger the initial gene pool the better of course. It would be easiest to start with a herd of bison cows and put a wisent bull on it, but the purpose of the project would be to gather as much wisent diversity as possible, so it would be wiser to start with a herd of wisent cows (20 would be minimum I would say, 100 would be ideal) and put a bison bull on it for the first round. After that and when all cows produced 50% bison and 50% wisent individuals, the American bull would have to be replaced with a wisent bull. The half-wisent individuals would be backcrossed with a pure wisent, which is called absorptive breeding. In order to gain maximum diversity it would be best to replace the wisent bull in each round, although this would be incredibly effortful and costly. This would have to be continued until 1/8 or 1/16 bison individuals are born. Depending on how strongly the bison influence shows, one of these advanced hybrid individuals could be chosen for breeding. I would only take bulls where the hybrid influence is phenotypically as much as undetectable. This is now where the real deal is going to happen: breeding between individuals that both have hybrid ancestry. Only the mating of two wisent that both have hybrid ancestry can enable the true purge of deleterious alleles from the population and the fixation of healthy alleles. The hybrid wisent bull would mate with the pure wisent, half-wisent, quarter wisent et cetera that are present in the herd. Personally I would phase out the individuals with a high bison percentage in this stage. When there is a good mix in the population with all individuals having a high wisent percentage in their genome, phenotypic selection can begin. It would be necessary to remove well-mixed individuals that have a clearly visible hybrid mark in their morphology and external appearance, or show bison behaviour such as head-butting in combat fights (wisent fight horn to horn like cattle). Factors such as food choice could also be evaluated, as bison are more heavy grass eaters than wisent, while wisent also like to browse and peel trees. Another round of pure wisent backcrossing might be necessary in order to get the wisent percentage in the individuals as high as 95% and higher. 

 

A schematic illustration of absorptive breeding

After couple of generations, the wisent would hopefully be virtually indistinguishable or even completely indistinguishable from pure conspecifics (there would be some individuals with bison traits popping out on occasion as usual in breeding, selection would have to wheedle that out in the long term run). What would be crucial now is actually the main purpose of the project after all: studying whether these wisent with bison introgression are truly more healthy and evolutionary fit than their inbred conspecifics. If my expectation is right, symptoms of the inbreeding depression such as stillbirths, infertility, developmental disorders and asymmetries must have decreased in frequency or maybe completely disappeared. Also, they would hopefully be less prone to the typical inbred wisent diseases. If kept under semi-natural or natural conditions, it would be very interesting to see if the survival rate in winter is higher than in Bialowieza, where most of the wisent die of the usual diseases that are plaguing the species. 

If the wisent with the controlled and selected bison introgression are truly healthier and more evolutionary fit than the pure but inbred ones, and indistinguishable from pure ones at the same time, the project could be called a success and the wisent would finally have a perspective for an ensured healthy long-term survival. 

A herd book 

Setting up a herd book would be essential in order to carry out the project in a professional and transparent manner. It would be separate from the international herd book for pure wisents, and it should mark not only the genealogy of the individuals themselves but also their genetic composition, f.e. 25% Bison 75% Wisent. Perhaps also if both parental sides have hybrid ancestry or if it is merely a back-cross. In this way, the introgression project would be absolutely transparent, controlled and documented, and there has to be no fear of a rampant, irreversible swamping of the wisent pool with bison genes. Especially since all evidently pure wisents are listed in the official breeding book for the species anyway. 

A breeding book for wisents with introgression from bison also opens possibilities for the Caucasus population, which descend from hybrids with plains bison. While there are voices that want to cull them all (f.e. such as the international wisent conservation action plan), the population could be regarded as being of considerable worth as it is the only large wisent population with a long history of living in the wild under natural selection on this world at the moment, and the bison hybridization increased their genetic diversity and probably also evolutive fitness.  A few herds of these individuals could be taken and bred in European conservation projects, but registered in a breeding book. That is where the hybrid project’s breeding book could be an option for the recognizing of this population as something of worth for conservation. A second section in the hybrid breeding book could be set up for this particular genetic line (the plains bison hybrids from the Caucasus), and the largest concern of the opponents of introgression in order to safe the wisent, the danger of confusion with pure wisents, would be refuted as everything is transparent thanks to the herd books. As the Caucasus wisent still show phenotypic vestiges of hybridization (if you take a close look, you see that they are somewhere intermediate between bison and wisent), they could be back-crossed with wisent again. In the long term run, it could even be considered to fuse both the wood bison hybrids and the plains bison hybrids in order to gain maximum genetic diversity. 

A hybrid herd book would also be a prospect for those wisent not registered in the official purebred herd book. There are about 700 individuals not registered because their pedigree is not proven or documented. A number of them might have hybrids with bison or cattle in their ancestry, a number might be pure but not documented. Anyway, conservation acts as if those 700 wisent would not exist, despite it is a considerable number and some of them might be more healthy than those in the official herd book. Ignoring them completely is maybe not wise. They could be included into the hybrid herd book as well, in the form of a third section. 

References 

[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

What to do with the wisent in the Caucasus?

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

The Caucasus mountains were home to another subspecies of wisent, Bison bonasus caucasicus, until was exterminated in the first half of the 20^thcentury (it is also considered a separate species by some authors). Nowadays, wisents have been reintroduced in the western Caucasus reserve, and this population is controversial for its special history. In this post I am going to explain why, have a look at the arguments against this population, why I consider it beneficial instead and what to do with it for the future. 

First and most importantly it is to note that the modern wisent went through a severe genetic bottleneck event. All modern wisents descend from only 12 founder individuals and thus the genetic basis of the species today is extremely narrow. The consequences of this a considerable inbreeding depression affecting skeletal growth leading to skull asymmetry, deformation of the male gonads, increased rate of stillbirths, decreased female fertility and reduced resistance against diseases and parasites. They are particularly vulnerable to posthitis, balanoposthitis, foot-and-mouth disease, cattle tuberculosis, bluetongue disease and others. 20% of the mortalities in the Bialowieza Forest are caused by diseases. The inbreeding depression is an immediate danger for the long-term survival of the species and an obstacle for the establishment of populations in the wild [1]. It is not rare that reintroduction attempts fail due to these effects or that whole stocks in zoos diminish because of diseases. Therefore, the species is still in danger of extinction [1]. 

That is why during the population crash in the 1920s and 1930s wisents were crossbred with American bison systematically in order to prevent an inbreeding depression. When an international herd book for pure wisent was set up, most of the hybrids were exterminated (and the expected inbreeding depression came). However, a herd of hybrids was released in the Caucasus in 1940. Subsequently the wisent percentage in the herd was increased by culling of hybrid bulls and release of pure wisents (absorptive breeding), so that the bison percentage in the modern herd is estimated to be as low as 5%. However, the hybrid influence is definitely recognizable (see here, here or here for individuals with a clear bison influence; it is not that much visible in these for example)* 

* It is interesting that the genealogic percentage is estimated that low while their phenotype is looks exactly intermediate between both species. One explanation might be that individuals with a high portion of bison genes might simply have a higher evolutionary fitness and thus the phenotypic influence of bison is still present despite absorptive breeding. 

Some authors suggest a culling of the population. There are a few arguments on why those hybrids are supposed to be maladapted and have been a bad choice for the Caucasus ecosystem: 

- allegedly the hybrids lack mountainous adaptions due to the influence of the plains bison

- allegedly, the plains bison influence made them less resistant to cold 

- allegedly, the plains bison influence has a destructive influence on the local flora 

First of all, Lowland wisent (B. b. bonasus, the only remnants of this population) do not show the mountainous adaptions of the Caucasus wisent either (which was smaller, had shorter and more rounded hooves), so the bison influence is probably not to blame on that. Furthermore, the claim that they are less cold tolerant due to bison influence is most likely incorrect as American bison resist temperatures down to -40° Celsius. Whether or not they have a destructive influence on the flora is probably subjective. The only dietary difference between American and European bison is that the American species consumes more grass than the European one. However, in the ecosystem there once were two other grazer species, aurochs and horses, so that the impact from the hybrid bison cannot be destructive for this ecosystem as it still lies within the spectrum covered by wisent, aurochs and horse. As we know, large herbivores do have an impact on the vegetation – they tend to open the habitat and stop or reduce forest growth. This is considered beneficial for European landscapes and biodiversity (intermediate disturbance hypothesis) which is why large herbivores, wisents among them, are (re)introduced in many nature reserves. Ironically, the same kind of influence that is considered beneficial elsewhere is considered a damage in the Caucasus reserve, serving as an argument to cull the population. Pure wisent would probably do exactly the same. 

Actually, I see no objective ecologic justification for culling this population and no alleged maladaptation as a result of hybridization. The sole remaining reason why this population is considered a threat is their hybrid ancestry. 

Back in the 1920s and 1930s, when there was no international herd book, hybridization was indeed a threat for the genetic integrity of the whole species. The Barbary lion, for example, is a subspecies which died out on genetic level as there are probably no remaining pure individuals due to hybridization. However, nowadays the situation is different. The pedigree of each wisent considered pure is documented in the pedigree book, and wisents that are not listed in the pedigree book are ignored to death anyway (this concerns about 700 wisents in Europe). Therefore, the existence of a wild hybrid population in the Caucasus is not a danger to captive pure herds (however, there is a neighbouring pure herd in the region, but I come back to that later). It is probably the mere existence of the hybrids that irritates some authors so that they suggest the culling of the whole population. 

Nevertheless, I want to look into the positive aspects of this population.  The inbreeding depression we see in the wisent is the result of the high degree of homozygosity in the population which leads to an expression of the effects of deleterious alleles. This can only be overcome by the introduction of new alleles in order to reduce the frequency of the deleterious alleles. Hybridization introduces new alleles and the closest living relative of the species is the American bison, so this species would offer as a possibility to increase genetic diversity. 

Hybridization of course introduces alleles specific (=diagnostic) for the foreign species, but could also introduce alleles on loci that are not relevant for the differences between wisent and bison as such but where wisent are homozygous for deleterious alleles. Thus, hybridization, when the right alleles are introduced, can help to overcome the inbreeding depression of the wisent affecting development, fertility and resistance against diseases without affecting its genetic integrity and thus be beneficial for its chances of survival as a species. Of course rampant hybridization without any kind of selection just produces a phenotypic and genotypic mosaic or mess, which is why some sort of directive selection would result in the desired goal: increasing the genetic fitness of the wisent without grossly affecting its genetic integrity as a species. Therefore just keeping those hybrids in zoos without any selective breeding would lead to technically nothing. However, in the population in the Caucasus, we have something special or even ideal acting upon their gene pool: natural selection. The start-up population was a bunch of hybrids from zoos, but since 1940 they experienced more than 70 years of natural selection. It is likely that the frequency of deleterious wisent alleles on the respective loci has decreased as a result of the higher fitness of newly introduced alleles from American bison, therefore decreasing the effects of the inbreeding depression. To put it simply, those hybrids with a healthier genetic make-up have a selective advantage over those with a high degree of deleterious alleles. I therefore expect the hybrid wisents in the Caucasus to be healthier, more resistant against diseases and have less problems with fertility and development than pure but highly inbred wisents. I strongly opt for studying the health and fitness of this population. As far as I know, this has not been studied before as there is little scientific interest in hybrids, especially for those in the wild. If the hybrid population in the Caucasus is indeed healthier and more evolutionary fit than inbred pure wisents, conservationist have one way to interpret it that can be expressed in two ways:

- Non-hybrid wisents are pure but dangerously inbred. Hybrid-influenced wisents are not pure but healthy and evolutionary fit. 

- Non-hybrid wisents are dangerously inbred but pure. Hybrid-influenced wisents are healthy and fit but not pure. 

In any case, the hybrid population in the Caucasus is probably the largest (in their best times up to 2300 animals) and healthiest wisent population since the population crash in the 1920s due to hybridization and several decades of natural selection. I am definitely not suggesting to crossbreed wisent and bison rampantly and simply release them in the wild. One argument against the hybrid wisents would be the phenotypic mosaic they are displaying which is probably the result of lacking selection on diagnostic wisent traits. The goal should be a population of wisents with increased fitness (i.e. reduced frequency of deleterious alleles) due to hybridization without any recognizable phenotypic influence from American bison in ecology, morphology or behaviour*. 

* Bison and wisent have different modes of combat behaviour. Bison butt their heads against each other while wisents fight more in a cattle-like manner by pushing and pulling with their horns. I know of now records on how the Caucasus population behaves in this respect. 

This could be achieved in a large-scale breeding project, but for the Caucasus population the solution is to increase the wisent portion in the gene pool while retaining their wild state, so that the fitness-increasing alleles donated by bison keep on replacing the deleterious alleles. One way would be to continue the absorptive breeding by culling wild hybrid bulls and releasing pure wisent bulls in the population. The problem is that this way is comparably un-directed. It would increase the wisent alleles in the gene pool, but the diagnostic wisent alleles as much as the deleterious alleles that are to be purged out. Therefore another possibility would be to release a sufficient number of pure wisents that merge into the hybrid population, so that the wisent genes find their way into the population by natural selection alone without human help. This way the survival chance for beneficial alleles is higher while the wisent percentage is still increased. This could be done several times in subsequent decades, until no American bison influence is visible in the population. 

As I wrote above, there is a neighbouring population of pure wisents in the Caucasus. The authors of the action plan for the conservation of the wisent 2002 fear an intermix between the pure and the hybrid population because they consider the hybrids, as written above, worthless and ecologically maladapted (for which, as I explained, there is no basis). I, however, hopethat there will be an intermix. It would be beneficial for both populations: beneficial alleles might enter the pure population while the wisent percentage in the hybrid population will increase. 

Although hybridization has been proven to be part of evolution in nearly all groups of related vertebrate species where it has been exanimated (see here), it is still deemed the absolute doom in conservation. This is, at one hand, probably a relict of the 19^thcentury picture of nature as a stable household system as much as the lesson from cases where uncontrolled hybridization indeed threatened or threaten the genetic integrity of endangered species. However, in the wisent it is the extremely narrow genetic base that threatens the long-term survival of the species. A rigorous study of the fitness and health of the Caucasus population could indeed provide a good argument for cautiously increasing the genetic diversity in the wisent via bison introgression in single controlled populations. Due to the pedigree book, the influence would be transparent anyway (a separate herd book for these new lines could be set up). 

In the eyes of others, it is more sensible to shoot them all and replace them with a population pure wisents that have the same fitness problems as anywhere else and might crash at any time.  

If anyone was to ask me personally which kind of animal I would prefer, a wisent that is 1/8 or 1/16 bison on both its parental sides and therefore healthier and more evolutionary fit or a pure wisent that is highly inbred with all the according problems, I would, honestly, go for the first option. 

I already covered the wisent population in the Caucasus in these two posts: 

- Over-purity as a danger for the wisent? 

- Donate for the wisent in the Caucasus! 

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

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