A new study published in Scientific Reports has revealed the strongest evidence to date that all bison in North America carry multiple small, but clearly identifiable, regions of DNA that originated from domestic cattle.
In the study, Texas A&M College of Veterinary Medicine & Biomedical Sciences researchers, led by Drs. James Derr and Brian Davis, compared genome sequences among the major historical lineages of bison to 1,842 domestic cattle, establishing that all analyzed bison genomes contained evidence of cattle introgression.
This shared genetic ancestry is the result of multiple hybridization events between North American bison and cattle over the last 200 years, which followed the well-documented bison population crash of the 1800s.
"This comparative study clearly documents that the people responsible for saving the bison from extinction in the late 1800s are also responsible for introducing cattle genetics into this species," Derr says.
This study updates findings from a series of studies published 20 years ago, in which Derr's team revealed that only a few bison herds existed that appeared to be free of domestic cattle introgression. Now, with better genetic technology, these researchers have shown that even those herds are not free from hybridization.
"Today, it appears that all major public, private, tribal and non-governmental organization bison herds have low levels of cattle genomic introgression," says Sam Stroupe, a Ph.D. student in Derr's lab and first author of the study. "This includes Yellowstone National Park, as well as Elk Island National Park in Canada, which were thought to be free of cattle introgression based on previous genetic studies."
Derr says that these new findings will once again have ramifications for bison conservation efforts and long-term conservation planning since certain herds will no longer need to be managed in isolation. "For example, these findings really open up a number of opportunities for the establishment of new bison conservation herds and the augmentation of existing herds."
The history of bison includes one of the best-documented population crashes of any wildlife species and one of the most spectacular population recoveries in all of conservation biology due almost exclusively to the efforts of private herd ownership.
Prior to 1800, biologists estimated that more than 30 million bison roamed the central plains of Northern Mexico, the United States and Canada. A combination of indiscriminate killing, loss of access to suitable habitats and imported livestock diseases plunged bison numbers to a low of just a few hundred by the late 1870s.
This kind of population crash is known as a bottleneck, and every modern bison — of which there are now more than 430,000 according to the National Park Service — is descended from that small number of survivors.
Most surviving bison were under the control of only five different cattle ranchers who appear to have been motivated by the writings of fellow bison ranchers Charles Goodnight of Texas, and Charles Jones of Kansas. Both professional cattle breeders extensively documented and promoted crossbreeding between bison and domestic cattle in an attempt to develop a better beef-producing animal.
"The American bison were proven successful on this continent, surviving glaciation and multiple other climatic events over thousands of years," Derr says. "By crossing bison and cattle, those intrepid cattlemen hypothesized they could get a beef animal that is as resilient as bison but that produces as much quality meat as beef cattle. In the end, they failed due to inadvertent creation of a lackluster beef animal."
In the early 1900s, at about the same time that these ranchers were crossbreeding, biologist and early wildlife conservationist William Hornaday, the first director of the New York Zoological Park, began sounding the alarm that North American bison were being driven to extinction. Through Hornaday's efforts with the American Bison Society in 1905, a national movement was directed at establishing new bison conservation populations and preserving existing bison populations found in Yellowstone and Wood Buffalo National Parks.
However, the only bison available to establish these new conservation herds were almost exclusively animals from the cattlemen responsible for the species salvation.
"As a result, these well-intentioned hybridization efforts leave a complicated genetic legacy," Davis says. "Without these private herds, it is possible bison would have become extinct. At the same time, this intentional introduction of interspecies DNA resulted in remnant cattle footprints in the genomes of the entire contemporary species."
"We now have the computational and molecular tools to compare bison genomic sequences to thousands of cattle and conclusively determine the level and distribution of domestic cattle genetics in bison that represent each of these historical bison lineages," he says. "It is clear that some of the animals that were available to help save this species from extinction in the late 1800s were the products of these earlier hybridization experiments, and we are now left with the legacy of low levels of cattle genomic introgression across all modern bison populations."
According to Derr, it is important to recognize that although hybridization between closely related wildlife species has occurred naturally over time — well-known examples include coyotes and eastern wolves, grizzlies and polar bears, as well as bobcats and Canadian lynx — the bison-cattle hybridization is almost entirely a purposeful, human-made event that happened to coincide with the tremendous population bottleneck of the late 1800s.
"Two primary events, an extremely small bison population size and widespread interest in developing hybrid animals, changed and shaped the genomes of this species in ways we are just now starting to understand," Derr says. "Nevertheless, this species did survive and now they are thriving across the plains of North America."
As one of the world's most iconic animals, bison play a number of important, and sometimes conflicting, roles in society.
While some consider them a wildlife species that shouldn't be domesticated, others consider them an important economic livestock animal; although bison are raised as wildlife in state and federal parks and wildlife refuges, most bison alive today are owned by private ranchers and are raised for meat and fiber production.
To others, they hold religious and spiritual roles, as well as being icons of continental pride. In 2016, bison were even named the U.S. national mammal.
"Though viewed in different ways, bison conservation is a priority to many different groups, and it is imperative that we agree to use the best available scientific information to make decisions moving forward," Stroupe says. "These findings clearly show that, using modern genomic biotechnology, we can uncover many historical details regarding the past histories of a species and use this information to provide informed stewardship in establishing conservation policies into the future."
While many of these livestock herds are rather small with 100 animals or less, there are some exceptions. Turner Enterprises in Bozeman, Montana, is the largest private producer of bison, with over 45,000 animals spread across multiple states.
According to Mark Kossler, vice president of ranch operations, Turner Enterprises will use this new information to further improve its conservation efforts.
"Turner Enterprises has used the bison genetic work of Texas A&M University for the last 20 years to structure genetic management of our maternal herds concerning cattle mitochondrial DNA introgression," he says. "We were fully aware that advancing technology and mapping of the entire bison genome could reveal that all bison nuclear DNA could have cattle introgression as well.
"Knowing that the North American Bison herd has widespread introgression of cattle DNA, though in small amounts, will allow our operations to structure future genetic management between our herds to maintain broad genetic diversity without the worry of cross contaminating herds that were perceived to be 'clean' of introgression. This is helpful information for the bison community. We are appreciative of Texas A&M University's research and diligence in providing the bison community the final answer on this question of bison genetic purity."
Les Kroeger, president of the Canadian Bison Association, which helped fund Derr's research, agreed that better information is always a positive step for their sustainability and conservation efforts.
"As research tools improve, we gain a better understanding of the complex history of bison," he says. "With this information we can continue to lead the way to grow healthy populations of this iconic animal for future generations to enjoy. The commercial bison industry continues to support research and conservation initiatives while continuing to sustainably produce a high-quality protein for consumers to enjoy."
Chad Kremer, president of the National Bison Association, adds that the information will give bison producers across the country better information to manage their herds.
"The National Bison Association welcomes these research findings as we continue to unravel the long and complicated history of the American bison’s genetic lineage," he says. "Research projects such as this continue to assist producers in their ability to maintain healthy and diverse genetics in bison herds today. Genetics are a cornerstone of modern-day herd management, and something the National Bison Association promotes heavily in its outreach and education to producers across the continent through our North American Bison Registry. It is our hope that bison producers will utilize this data and technology to continue to improve their own herds' genetics while continuing to restore bison to their native landscape."
Source: Texas A&M University Veterinary Medicine and Biomedical Sciences, which is solely responsible for the information provided, and wholly owns the information. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.