What is Introgression?
Simply put, introgression is the movement of genes from one species into the gene pool of another species by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introgression is not the same as simple crosses that create hybrids, such as the mule or hinny or wolfdog. Rather, introgression is a long-term process which often involves many hybrid generations before the backcrossing occurs. An example of introgression would be the Beefalo which is 3/8 (37.5%) bison. Depending on the amount of generational backcrossing, animals may be almost indistinguishable from the initial species. Once introgression occurs, regardless of the amount of backcrossing and generations, traces of that introgression will always be present. For instance, over 70,000 years ago, modern-day humans apparently interbred with Homo neanderthalensis (Neanderthals) and to this date there are small traces of Neanderthal DNA in the modern human genome.
Bison-cattle introgression occurred in the past and within the so-called "foundation herds
It is well documented that most (if not all) ranchers that saved the bison were cattle ranchers and experimented with what they called cattalo, i.e., bison-cattle hybrids and crosses. The most well-known ranchers, Charles Goodnight, Michel Pablo and Charles Allard, Charles "Buffalo" Jones and others, all experimented with cattalo and often traded these animals among themselves and others. Would it be surprising to learn that some of these bison-cattle backcrosses made their way back into a bison herd?
It is well known that all modern day bison can be traced back to about 5 so-called "foundation herds" although considering the trading and inbreeding that went on, it may not have even been that many. All of these "foundation herds" were exposed to cattle introgression by historical fact and genetic testing has shown that remnants of past bison cattle introgression exist within the current bison population.
There are 2 types of tests: mitochondrial and nuclear DNA tests. Without going into detailed genetics, there are some basic understandings necessary. Every living animal has DNA, RNA, and proteins. In general, closely related animals have a high degree of agreement in their DNA (i.e., their DNA is very similar), while the DNA of animals distantly related show patterns of dissimilarity and divergence. There are basically 2 types of DNA in mammals, the nuclear DNA found within the center of a cell (nucleus) and mitochondrial found within small cytoplasmic organelles known as mitochondria.
Mitochondrial DNA (often abbreviated mtDNA) is a small piece of circular DNA found within small organelles known as mitochondria inside every cell. Mitochondrial DNA is maternally inherited (passed from the mother to her offspring) providing the ability to trace maternal lineage and making mtDNA the mainstay of phylogenetics and evolutionary biology. However, due to the slow mutation rates in mtDNA, it is often hard to distinguish between closely related species to any large degree and mtDNA does not provide any information on paternal (father) lineage. For instance, if a cattle bull mated with a female bison, the hybrid would be pure bison based on mtDNA analyses. Because of these deficiencies, other methods of analysis must also be used.
Microsatellites, also known as short tandem repeats or simple sequence repeats, are pieces of nuclear DNA in which small sequences (pieces) of DNA are repeated, typically 5–50 times. These microsatellites may occur at thousands of locations within an animal's genome. They have a higher mutation rate than most other areas of DNA, leading to high genetic variability and specificity. This variability (diversity) and specificity makes microsatellites ideal for a variety of investigations including human and wildlife forensics, parental analysis, degrees of relatedness, evidence of population bottlenecks, gene flow between populations, genetic fingerprinting, in addition to their application in cancer and other disease diagnoses. Because of the variability and specificity of microsatellites, selecting and examining appropriate microsatellites can provide the ability to differentiate closely related animals and even identify individual animals, e.g., "fingerprinting". As such, all animals can be separated and identified with microsatellite nuclear DNA examination.
In looking for cattle introgression (and other genetic analysis like parentage and diversity), differences in a particular genes and/or sequence of DNA are sought. These differences are called alleles, meaning an alternative form of the gene or section of DNA. Since microsatellites have been identified that are specific and distinct in modern day cattle and bison, a specific modern-day cattle-associated microsatellite allele in bison could only exist as a result of some past introgression.
There are currently 14 microsatellites in bison that can identify cattle introgression depending on the allele. This does not mean that there are only 14 cattle genes. These microsatellites are simply markers and do not reflect the amount of cattle-associated genes that may be present; although the number of cattle alleles detected could be an indication of amount, but not necessarily. Conversely, the lack of any cattle alleles does not necessarily mean that there was no introgression, only that it was not detected by the methods currently used. Thus, the term "pure bison" is used in the most liberal sense.
Cattle genes in bison not really that common
Cattle alleles in bison are not really as common as most make it sound. While it may be true that most (or many) private and federal herds have cattle-associated genes, the number of actual animals with cattle associated genes is generally low, somewhere around 7%, maybe even less. Some herds, however, have a much higher prevalence of cattle alleles within their bison herds. Unfortunately this can easily happen with the introduction of a single animal with cattle alleles into the breeding herd. If that animal happens to be your herd sire and his offspring becomes part of your breeding herd, cattle alleles can spread through a herd rather quickly. You can look at these cattle genes as a venereal disease, spreading through the herd with each reproductive cycle.
Modern-day cattle alleles versus normal ancient DNA.
There is no existing scientific evidence to support the allegations that the cattle alleles represent ancient and normal DNA, only unfounded speculations. Unfortunately this is misinformation and half-truths that have been propagated by individuals without a good understanding of genetics and for one reason or another do not wish to acknowledge the cattle-bison introgression problem.
Bison and cattle have a common ancestry and bison belong to the same taxonomic family as cattle (Family Bovidae, Subfamily Bovinae), just as we are in the same family as the great apes (Family Hominidae). But we don't have modern-day ape genes even through 95+% of our DNA is identical to that of the great apes (gorillas, chimpanzees, and orangutans); these are simply common genes, not ape genes. Similarly, bison and cattle share many common genes; North American bison share 93-95% identical DNA with cattle and yaks. The similarity of bison and cattle (Bos and Bison) is no different than the similarity between human and other great apes. Just as humans can be separated from the other Great Apes with mtDNA and nuclear microsatellites, so can bison be separated from modern-day cattle.
The data from the "Higgs" bison, allegedly the result of cross breeding (hybridization) between the ancestor of cattle (the extinct Auroch) and the Steppe Bison some 120,000 years ago is an example of ancient introgression between pre-modern day cattle and pre-modern day bison. This is similar to the introgression (cross-breeding) of modern day humans (Homo sapiens) with Neanderthals and Denisovans some 70,000 years ago. To claim that ancient or common DNA is the cause of the modern-day cattle alleles in bison is unjustified, scientifically unfounded, and absurd.
The DNA being used to determine cattle-bison introgression are not common or ancestral genes, but represent recent events. Because of the small number of animals from which all modern-day bison are derived (the bison bottleneck), if the cattle alleles found in bison were ancient DNA, they would be detected in most, if not all, modern-day bison just not a small percentage of animals. It is ill-informed to assume or suggest otherwise.
Historical records and facts support the genetic studies on introgression
Each of the ranchers involved in establishing the plains foundation herds in the late 1800s either experimented with domestic cattle-bison introgression or purchased bison from others who were involved in such experiments. Tracing the origin and movement of these herds unequivocally shows that these cattle alleles are recent acquisitions. Take the Yellowstone and Goodnight herds for example.
The Charles Goodnight herd was based on the breeding of 5 wild-caught bison from the Texas panhandle in 1887. Over the last 120 years the Goodnight bison herd has remained reproductively isolated; in other words, all Goodnight bison are direct descendants and relatives of the original 5. The original 23 Wood bison of Yellowstone (according to the National Park Service and historical record, the original Yellowstone bison were Wood bison and not Plains) were crossed with 18 Plains bison cows from the Pablo-Allard herd and 3 bulls from the Goodnight Plains bison herd in 1902. Thus, the Yellowstone bison are hybrids of the Pablo-Allard and Goodnight Plains bison, and the original wild population of Wood bison. Yet, the Yellowstone bison do not have any "cattle genes". If the Goodnight bulls had cattle alleles, so would the Yellowstone herd. This is undisputed fact that the 3 bulls from the Goodnight herd did not have any cattle alleles and hence, neither did the original 5 wild-caught bison from Texas. But the Texas State Bison Herd which is the original Goodnight herd has both mitochondrial and nuclear cattle alleles, similar to the ones being detected in other herds. If these cattle alleles were "ancient", they would have to be in the 3 bulls sent to Yellowstone and the Yellowstone bison would not be free of cattle DNA. There is no other possible explanation except that, sometime after 1902, Goodnight's bison and cattalo were mixed. If these cattle-alleles were "ancient" DNA, they would be in all Goodnight and Yellowstone animals.
A similar historical trail can be made with the Pablo-Allard herd which originated from the 4 bison calves from Walking Coyote as well as other foundation herds.
In 1887, William T. Hornady seeing what ranchers were doing and all the cross-breeding of bison and cattle wrote "…it is to be feared that it will soon become a difficult matter to find a buffalo of absolutely pure breed." Can we really now deny that the problem exists when we did little to curtail it from the beginning? Do we continue to ignore this problem and see Hornady's prediction come true?
Do cattle alleles really matter?
That really all depends on your business plans. The commercial dairy or beef farmer could care less if his animals are pure bred or if they have a little bit of something else in its DNA as long as it a good milk or meat producer. Most people know that when they buy Angus beef at the supermarket that these are not all registered pure Angus but black cattle that look like Angus. By the same token, commercial Bison meat rancher shouldn't care if there are a few remnant cattle genes as long as his animals produce quality meat. Nobody is going to say these are not bison just because they have a few cattle genes floating around. As long as you are raising meat animals, cattle introgression genes should be of no concern.
But to deny the existence of these cattle alleles, to claim they are normal ancient DNA, or to claim that they are somehow beneficial to bison is irresponsible at the very least. There is no evidence whatsoever to support claims that bison with "detectable cattle ancestry at low levels have important genetic value and contain unique genetic variation that is absent from Yellowstone or other conservation herds with no molecular evidence of cattle ancestry" or that "genetic purity is less important than genetic variability". There is no current or "emerging research" that will support any of these claims. One can only suspect that there must be ulterior motives to those professing that cattle alleles are somehow normal or beneficial to the species.
By considering these genes potentially normal (ancient or otherwise) or somehow beneficial to the species and allowing them to propagate through the bison breed, we are not stewards of the bison or contributing to bison conservation but rather contributing to their demise as a species.
Genetic drift and erosion
As previously noted, modern day bison can be traced back to a few small foundation herds comprising a total of about 30 bison, maybe 50 at best. The Pablo-Allard herd in Montana had its foundation in 4 buffalo calves started by Walking Coyote; the Charles Goodnight herd in Texas was started with 5 captured animals; James "Scotty" Phillips herd in South Dakota had its origin in 5 animals of the Dupree herd; the Charles Alloway and James McKay herd in Canada also started with 5 animals; and the Charles "Buffalo" Jones herd in Kansas may have stared with as many as 18 animals (although their source is unclear). Hence, regardless of the diversity within the species, the modern day North American bison, in private and public herds combined, has a very small gene pool.
Because of this small genetic pool, bison are very susceptible to becoming genetically extinct through genetic drift (small isolated populations with a small gene pool that small events can change the genetic makeup of a species) and genetic erosion (when the gene pool of a species diminishes). Through the spread of cattle genes, bison are at risk of genetic drift and erosion, if not absolute genetic extinction.
Genetically we have essentially wiped out the Wood bison (Bison bison athabascae) as there no longer appears to exist enough genetic differences between the Wood and Plains bison to warrant their subspecies status. This was accomplished by the crossing of the Wood bison in Canada with the Plains bison of the Pablo-Allard herd and the crossing of the Yellowstone Wood bison with the Pablo-Allard and Goodnight Plains bison. Although this cross-breeding was done to save the bison, it nevertheless eliminated much of the wood bison population. Are we going to do the same to what is left of the North American bison through the propagation of cattle genes?
Stewards and Guardians of the Bison
Although the ranchers were the ones that saved the North American bison in the early 1900's, it may be current day ranches that lead the bison to genetic extinction. Although most bison ranchers today are meat producers with a primary interest in the meat market (and there is nothing wrong with that), most also love the bison and have a bona fide interest and desire to preserve the species. However, those that claim to be stewards of the bison while promoting misinformation about the normality or non-existence of modern-day cattle alleles in bison are threatening and jeopardizing the future of the North American bison as a species.
The stewards and guardians of the bison are the National Park Service, the Nature Conservatory, and private bison ranchers who are concerned about cattle introgression alleles and are making a concerted effort to eliminate these genes from their herds and prevent their further and continued dissemination.
All of our bison have been tested by mtDNA and nuclear DNA and found to be completely free of all cattle introgression alleles by currently available technologies. All bison sold will be accompanied by their actual laboratory test results which can be used at any time to register the animals with the North American Bison Registry if so desired.
References & Recommended Reading
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