Not Out of the Woods, Yet—Genetic Extinction (Part 2)

The mechanisms by which genetic extinction of the wild bison genome may occur were described in the September post.  If the prevention of loss of genetic information and the promotion of genetic diversity are to be achieved, how should we proceed?  What avenues are available or can be created? Broad objectives were laid out in the Vermejo Statement (see the Feb. 27, 2019 post From the Brink to the Foothills-Part 2).  More recently Paul Hedrick has laid out more specific objectives.  These include:

  • Keep cattle ancestry at a very low level,
  • Avoid inbreeding and artificial selection for livestock-related traits, and
  • Retain sufficient genetic variation for future adaptation.

Achieving these objectives requires a variety of strategies.

Cattle-Gene Introgression:

The greatest focus of conservation genetics has been identifying herds with cattle ancestry, since the efforts to restore the wild bison have been threatened by domestic cattle introgression.  Reduction of cattle-gene introgression involves several approaches because of various circumstances [1].

The popular tenet from the medical profession—Do No Harm—applies here as well.  The first and most logical strategy is to not introduce bison with known cattle ancestry into herds free of cattle  introgression.  Though this seems to be the easiest approach, there are only a few herds known to be free of cattle ancestry—e.g., the Yellowstone herd, the Henry Mountains herd, and more recently, the American Prairie Reserve herd.  This approach only protects these herds until other herds free of cattle ancestry can be established.  It should be noted the notion of cattle ancestry free is relative.  There may always be the presence of cattle genes.  Additionally, the complete eradication of cattle genes may not be desirable since the genetic testing has not matured enough to differentiate between genes unique to domesticated cattle and genes having common ancestry to bison and cattle (This issue will be explored in more depth in part 3).

A corollary to the above strategy is to introduce bison without cattle ancestry into herds with cattle-gene introgression.  The benefits could possibly include: a decrease in inbreeding depression, an increase in genetic variation, and genetic swamping of cattle ancestry. This would dilute the presence of cattle genes to the point at which natural selection would eventually take over and reduce the effects of cattle ancestry. A variation of the introduction of cattle-gene free bison strategy involves starting new herds.  

Another approach regarding cattle ancestry involves translocation of bison between herds with similar levels of cattle-gene introgression.  This, at least, would not raise the overall level of cattle ancestry, but would have the benefit of avoiding inbreeding depression.  But this requires more accurate tests to estimate the level of introgression and further examination of potential phenotypic effects [2].

Finally, culling may be used to reduce mitochondria DNA (mtDNA) and specific nuclear alleles (one of two or more alternative forms of a gene found at the same place on a chromosome) of cattle ancestry.  Culling involves separating out the undesirable animal with the objective of reducing or eliminating the traits, qualities or disease of that specific animal from the herd.  Undertaking this strategy to reduce the mtDNA, however, incorrectly assumes this also reduces nuclear DNA.  Care needs to be taken to retain variation at the nuclear level, requiring more extensive and accurate testing.  And culling to reduce specific nuclear alleles is also problematic. Unfortunately, this action will most likely have other alleles associated with the cattle ancestry remaining at other unidentified genetic regions [3].

Inbreeding and Genetic Drift:

Inbreeding and Genetic Drift are significant issues.  Most of the conservation herds are relatively small (i.e., less than 1000). Under these circumstances maintaining the genetic information and diversity required to promote the wild genome is difficult if not impossible.  To avoid these processes of genetic extinction, herd sizes of at least 2000 to 3000 are needed [4].  Out of the 44 conservation herds, only 10 herds have more than 400 animals, and out of these, only 4 have more than 1000 bison—Yellowstone National Park, Medano Ranch, Co., Tallgrass Preserve, OK, and Custer State Park, SD).  The herds smaller than 400, are most definitely, losing genetic diversity, and in danger of inbreeding.  Six of these herds are being managed as a meta-population with exchanges of animals.  This practice may alleviate some inbreeding but will not prevent loss of genetic diversity.  Only the Yellowstone herd is large enough (3000 to 4000) to limit that loss [5].  In addition to the four conservation herds mentioned above, the American Prairie Reserve [see link to the American Prairie Reserve’s website in the Favorite Links section of this blog] in Montana has a herd which is currently slightly less than 1000. 

The regular exchange of bison between herds is another method to avoid inbreeding and genetic drift. In moving bison to other herds, though, consideration must be given to disease control, handling practices, and state laws.  Animals would need to be tested prior to transfer to ensure diseases such as brucellosis and tuberculosis would not be transferred.  Handling of bison is difficult.  Care would be required to ensure the safety of the animal, not to mention the personnel involved.  Finally, laws defining the status of bison differ from state to state and would have to be taken into account.

Achieving genetic diversity requires ongoing assessment of genetic variation from which strategy decisions can be made.  In this regard, Hedrick offers several recommendations which are beyond the scope of this post [6].

Artificial Selection/Domestication:

A certain amount of human intervention in conservation herds cannot be avoided.  Even in Yellowstone the herd suffers from human management—the herd size is limited, the herd has been vaccinated, the average age of the herd has been artificially reduced, and  access to seasonal ranges has been restricted [7].  And this is the most “wild” bison we have!

Herd sizes are managed through random culling with the first animals coming through the chute being selected.  It has been observed, though, the largest animals are usually the first.  Bison traits, then, associated with large body size are being artificially selected out.  Thus, even random culling can have a negative effect on natural selection.  Culling along with vaccination is also used for disease control.  However, disease control treats low resistance bison equally with high-resistance bison, preventing natural selection from promoting bison with high-resistant immune systems. Intervention to control disease, then, tends to retain susceptible animals [8].  If the wild genome is to be encouraged, culling to limit herd size and efforts at disease control must be either eliminated or be rare and minimal.  Still, culling to reduce herd size may be necessary due to land and carrying capacity [9].

Keeping human intervention at a minimum is not enough.  As has been found with the re-introduction of wolves in Yellowstone bison have had to relearn their defensive traits. Avoidance of loss of defensive traits will require the introduction of the bison’s natural predators—the wolf and the grizzly. Predation is also a natural selective force. 

Summary:

The various genetic extinction mechanisms and the circumstances in which bison find themselves—large herds, small herds, land issues, etc.—require several strategies to prevent genetic extinction and promote genetic diversity. The American Prairie Reserve’s bison management approach is a good example of the implementation of some of those strategies discussed above:

  • Their overall goal is to achieve a herd size of 2000 to 3000 within the next 5 to 7 years
  • Their approach is “hands-off” as much as possible. 
  • Manipulation of bison population is minimized to allow for the development of natural sex ratio and age structure
  • Mortality from bull competition, predation, and other natural events is permitted (However, no wolves or grizzlies are currently present on the Reserve)
  • Continue to secure more land and habitat to support the herd and allow for continuous grazing
  • Ensure new bison introduced into the herd are free of cattle-gene introgression [10]

Implementing these strategies involves answering many questions. For instance, land is perhaps the most significant issue.  The common strategy to address the extinction mechanisms is to create large, free-ranging herds, requiring large amounts of land.  But not just any terrain will do.  The habitat must support large swaths of grazing land. How much land is needed for a large herd of free-ranging bison?   What needs to be done to prepare the habitat? Are there state and/or federal regulations involved?

Another concern involves genetic testing.  Ridding herds of cattle genes may cause the loss of common ancestry genes.  How do we differentiate?

If predation is to be re-introduced, what is required to make that happen?

These issues need to be worked out, and will be pursued in part 3 of this discussion.

End Notes:

[1] Hedrick, Paul W. “Conservation of Genetics and North American Bison (Bison bison).” Journal of Heredity 2009:100(4): 411-420.

[2] Phenotypic Effects—Effects on an organism’s observable characteristics or traits and covers the organism’s physical form and structure, developmental processes, biochemical and physiological properties, behavior and products of behavior (Wikipedia).

[3] Hedrick.

[4] Bailey, James A. 2013. American Plains Bison: Rewilding an Icon. Sweetgrass Books. Helena, MT. 179. and Hedrick.

[5] Bailey, 179.

[6] Hedrick.

[7] Bailey, 140.

[8] Bailey, 142-145.

[9] Carrying Capacity—the ability of a habitat to sustain a population (Bailey, 87).

[10] Retrieved 02-Oct-2019 from https://www.americanprairie.org/sites/default/files/APR_Bison%20Report_16_17.pdf). Also, email to author from Scott Heidebrink, Bison Restoration Manager, American Prairie Reserve. 03-Oct-2019.

Not Out of the Woods, Yet–Bison Genetic Extinction

The topic of genetic extinction of the wild bison genotype is rather extensive.  So, the plan is to discuss this issue in several parts, beginning with the mechanisms contributing to the loss of genetic information. Then follow with an exploration of how to avoid those mechanisms and a review of particular issues involved.

Part I: Mechanisms

Imagine a river running endlessly in both directions (nevermind the logical inconsistency) with only one bridge crossing the river, the only means to the other side. Furthermore, coming up to that bridge is a 12-lane highway, but the bridge only permits one lane with no merging lanes allowing access to that one lane.  Instead the other 11 lanes all end with a wall at the river’s shore.  But this is unknown until the ends of those lanes are reached.  Only those traveling on the lane accessing the bridge are able to cross the river.  This situation presents a bottleneck.  By the 1900s the North American bison entered a genetic bottleneck.   From a population of many millions only less than a 1000 were able to cross over our imagined bridge.  A genetic bottleneck occurs when a population is reduced to a small subset of the original population.  The last remaining individuals, then, represent the remaining genetic heritage of the entire initial population. But they do not represent the overall genetic diversity of the population before the bottleneck.  Such circumstances are detrimental to the viability of a species because of the loss of genetic diversity [1].

                Other than out-right extermination of a species, Bailey has identified five processes contributing to the genetic extinction of the wild bison genotype [2]:

  • Founder Effects (Initiating herds with few individuals having limited genetic diversity)
  • Genetic Introgression (Crossbreeding with cattle genes–hybridization)
  • Inbreeding depression in small herds
  • Genetic drift in small populations
  • Artificial selection by human intervention (domestication)

Before going further the concept of wildness needs definition.  According to Bailey, wildness refers to the impact humans have on an animal population or an ecosystem.  A species or an ecosystem is considered wild if it exists and functions with no human intervention [3].  Of course, in the case of bison, Native Americans have interacted with the bison.  But perhaps it could be argued their intervention had no more impact than that of natural, random events; certainly not the impact our culture has had. 

Founder Effects:

Founder effects involve initiating herds with few individuals. Today’s herds have been initiated with the few bison left from the slaughter that took place in the latter half of the 19th century.  Because of this, and as illustrated in the thought experiment above, with the loss of genetic diversity the potential for effective natural selection has been reduced.  In addition, with few founders the stage is set for other resulting mechanisms contributing to genetic extinction—for instance, inbreeding depression and genetic drift experienced in small herds.

Genetic Introgression and Hybridization:

Bison were saved from extinction primarily by 5 ranchers and the small remnant in Yellowstone National Park.  The ranchers during the end of the 19th century and early 1900s experimented with crossbreeding bison with cattle in an effort to raise a hybrid for meat production.  This endeavor quickly ended since bison-cattle hybrids almost always resulted in female offspring and no viable male offspring.  Hybridization that results in one sex being absent, rare or sterile indicates evolutionary incompatibility between the two species [4].  Even though hybridization was a dead-end, the attempt introduced cattle genes into bison herds, known as genetic introgression. Many of the bison from these early hybridization efforts were used to initiate or grow other herds, injecting traits related to domestication into the bison herds and effecting physiology.  One study by Derr found bison with cattle-gene introgression tend to be smaller at an early age and never grow as large as more pure, wild bison [5].

Inbreeding:

Inbreeding involves the breeding of closely-related individuals and occurs in small herds or in herds maintained with few bulls [6], limiting the genetic diversity. Bison bulls will mate with as many cows as is possible, and dominant bulls will father more calves while less dominant bulls may not father any calves [7].  In small herds the genetic material of the dominant bulls will tend to be concentrated and passed on with genetic material of others bulls lost. The negative effects of inbreeding replace natural selection in determining the future genetic make-up.

Genetic Drift:

A change in the relative frequencies of alleles (one of two or more alternative forms of a gene found at the same place on a chromosome) is known as genetic drift. This process occurs in a population due to random events during survival and reproduction [8]. Random chance determines which genes or animals survive and reproduce, causing genetic change in a population.  For instance, a bison could break through the ice when crossing a river and drown. But the major source of changes in allele frequency lies in reproduction during the production of ova and sperm.  When ova and sperm are formed in cell division, chromosomes split leaving the reproductive cells with only half the chromosome set. Thus, during reproduction some alleles are discarded both from the bull and the cow. In large populations random events effects are relatively unimportant because opportunities are present for natural selection to work, mitigating any loss of genetic information. However, in small populations genetic drift may cause some genes to disappear, reducing the genetic diversity and evolutionary potential [9].

Artificial Selection—Domestication:

Domestication results from the replacement or weakening of natural selection by artificial, human-managed selection.  Thus domestication is eradication of wild bison by modification. For example, aurochs were continually domesticated eventually leading to modern domesticated cattle.  These efforts were so extensive aurochs no longer exist.  The essence of selective breeding involves humans deciding which individuals will produce the next generation which will better serve human goals [10].

One of the goals involves handling. Wild bison are difficult to handle, causing harm to the animal, causing potential damage to shoots and pens, and involving more time and effort on the part of the ranchers.  To mitigate handling issues, bison ranchers and farmers have found that by increasing the level of serotonin and lowering the levels of dopamine bison become more docile.  Over time selecting those animals with increased serotonin and lower dopamine for breeding will artificially select the more manageable bison, moving from wildness to domestication. 

Additionally, since ranching and farming are bottom-line businesses, each bison is seen as a productive unit.  Under this perspective management of bison will increase the number of cows altering the natural sex ratio. Bulls can breed many cows, whereas a cow will only have one, possibly two calves per year. Maximizing the commercial herd requires few bulls but many cows.  Besides, cows are also easier to handle than bulls. A biased sex ratio shifts the breeding behavior.  Cows that do not incite competition between bulls will more likely be bred.  Thus the traits associated with competition between bulls become artificially selected out.  Unfortunately, this management perspective not only occurs with private herds.  Public herds are seen as a revenue source, and consequently, subjected to the same practice [11].

               Given the above mechanisms pushing us toward the genetic extinction of wild bison, the question becomes:  how do we mitigate or prevent these processes?  Complete avoidance may not be possible. The means and objectives involved may be in conflict with each other and may require trade-offs.

Prevention and mitigation of the genetic extinction mechanisms contributing to the loss of wildness in bison will be explored in the next post.

Endnotes:

[1] Bison Bellows. Retrieved 09-Sep-2019 from https://www.nps.gov/subjects/bison/bison-bellows-12-13-15.htm.

[2] Bailey, James A. 47.  2013. American Plains Bison: Rewilding an Icon. Helena, MT. Sweetgrass Books.

[3] Bailey, 73.

[4] Hedrick, Paul W. “Conservation of Genetics and North American Bison (Bison bison)”. Journal of Heredity 2009: 100(4): 411-420.

[5] Bailey, 48.

[6] Bailey, 76.

[7] Lott, Dale F. 194. 2002. American Bison: A Natural History.  Berkeley. University of California Press.

[8] Bailey, 78.

[9] Bailey, 49-50.  Also for a full discussion of genetic drift see Bailey, 78-80.

[10] Lott, 196-198.

[11] Lott, 198-200.