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livestock are actively embracing cloned animals by including them in their breed registry. They view cloning technology from a scientific based perspective and made the sound decision to include this powerful tool, and the animals produced using this technology, in their herd books. Of course, emotions and influence are
a part of any registry decision, but fortunately good, sound science eventually won the day.
Thank you for listening,
Dr. Shawn Walker, Ph.D.
Viagen
Vice President of Science and Technology
Ed. Note: While we appreciate that Mr. Thornton’s article expresses a different viewpoint
on cloning than your own, referring to it as an “opinion article” is misleading. Mr. Thornton was asked to address the most common concerns about cloning based on the results of AQHA’s member surveys from a genetic and scientific standpoint. The concerns noted in the article are not Mr. Thornton’s own personal opinion, but are the concerns of the American Quarter Horse racing industry most
often expressed to the AQHA and to Speedhorse
in numerous conversations about cloning. Mr. Thornton was given the opportunity to respond to your letter, and his response is included below.
Dear Editor,
I would like to thank Dr. Shawn Walker of Viagen for his response to my article “Cloning and the Mitochondrial DNA” that appeared in Speedhorse July 6, 2012. It is always good that when we are dealing with any controversial subject that we get both points of view. So with that in mind I would like to respond to his comments on the article.
One of the key objectives of the article was to point out some of the concerns that the industry has about cloning and how that might impact our breeding industry. One of those topics was the effect clones could have on the genetic diversity of the breed.
Genetic diversity is the key to breed improvement. I have been professionally serving as a pedigree analyst through my consulting
and writing for more than 20 years. One of the more common questions is—Where will our
next outcross come from? The reason we see this question has to do with how some phases of our industry are showing a lot of inbreeding through linebreeding or multiple crosses to key ancestors and the effect this is having on the genetic diversity of the breed. Dr. Walker is right in that we have embarked on some reproductive practices that can and have had an effect on the genetic diversity of the breed. This alone should give us cause to be concerned about the effect the clones
could have on the genetic diversity of our breed. We have to remember that the thoroughbred breeding industry has not adopted these breeding practices because they don’t want these practices to impact the genetic diversity of their breed.
As to the thought that a stallion’s mtDNA haplotype is irrelevant concerns me. It is common knowledge that the stallion does not pass on his mtDNA. This makes the mare being bred to a stallion important, as her mtDNA will pass to the foal. So why isn’t that important when we clone a stallion? Shouldn’t the clone get the same mtDNA making them “identical” to the animal being cloned? That is why I am still concerned that a stallion clone is not 100% identical to the stallion or gelding being cloned.
The next concern that Dr. Walker approached dealt with the mtDNA haplotypes. Haplotypes as I understand them are a series of genes that are linked and pass down together generation after generation. This is why we
can trace the migration patterns, as these “haplotypes” remain relatively constant in genetic makeup through certain markers or variants on a given location on the chromosome.
I have never thought much about the migration patterns in my studies of the female families but I have viewed the mtDNA as the key that we can use to identify the individual families established by a taproot mare. I found it very interesting that Dr. Walker confided to us that they sometimes by “random chance” have a match of the mtDNA haplotype in the course of their work on developing a clone.
I found Dr. Walker’s downplaying of the
role of the mtDNA haplotype interesting in the issues he outlined in sections three in his letter. I would have to ask Dr. Walker to clarify what he is talking about when he uses the term mtDNA haplotypes, is he referring to the variants that we use to trace those migration patterns or the whole mtDNA and the utilization of oxygen for energy.
Dr. Walker tells us that the “bulk of the scientific community has concluded that the mtDNA haplotype .... does not have a material impact on the phenotype of an animal.” His source of information for this statement came from 4 studies that were published with two in 2003; one in 2005 and one being published in 2011. I find this interesting because basic biology tells us that the mitochondria is the cells source of energy and energy is what makes our phenotype work. So how can the mtDNA not have an influence on the phenotype of the individual?
On the other hand the purpose of the study that was started in 2010 by Dr. Allan Davie and his colleagues is to find the genetic link between the genotype and the different phenotypes types or athletic ability of the individual. As Dr. Davie stated it, “Mitochondrial density
and function regulate aerobic power. Therefore,
the genotype of the mitochondria—genetic history—is one of the key factors that determine muscle utilization of oxygen and aerobic performance.” This is why I feel we need to know how the mtDNA functions to affect performance and despite the findings of some maybe we don’t have all the answers.
I find item number four in Dr. Walker’s letter very interesting. I have been frustrated
for years about the unknowns in the pedigrees of the quarter horse. The frustration of the unknowns is based on the deep desire I have always had to find out what makes a great horse. If a high percentage of the unknowns have mirror images in their mtDNA then they may all come from the same family. So I was very interested in the fact that Viagen has done some research in this area. I would very much like to see the European mtDNA expert publish his findings in a peer scientific journal.
Oh those mistakes in the pedigree and the integrity of the breed. This is another very frustrating area of pedigree research for me. You wouldn’t believe some of the stories I have been told about who begat who. This leads me to ask—How will cloning that is a new science that is not completely tested improve our integrity when the clone is not 100% identical to the animal being cloned?
Dr. Walker lists the factors that go into the making of a meat animal and that leads us to realize just how important different factors become to what we are trying to produce.
But there is no mention of how the mtDNA
can influence these factors in the animal’s development. The racehorse on the other
hand has its own factors that include areas of conformation and muscle type that are key
to their ability to run. We find those factors
in the nuclear DNA that contribute to the animal’s success as a sprinting or stamina type racehorse. The prime example of this is the individual being influenced mostly by “fast twitch” vs. “slow twitch” muscle fibers. Fast twitch being the sprinter type and the slow twitch being the stamina type runner. The
key to the efficient use of these muscle fibers is the oxygen utilization that is influenced by the mitochondria and its DNA in the cell located outside the nucleus. This is especially true in the stamina type runner who needs better utilization of oxygen to run the distance.
I may be a little idealistic but I don’t think we have all the answers about cloning and its impact on the breed. I believe it is a science in its infancy and needs time to grow and develop before we enter these individuals into our studbook.
Respectfully submitted, Larry Thornton
SPEEDHORSE, August 3, 2012 33