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tissue cell of the individual to be cloned. The donor cells are cultured, allowing them to develop before they are transferred into the enucleated oocyte.
3) Fusion of the donated cells with the enucleated oocyte takes place. The transferred DNA becomes part of the donor oocyte, form- ing what they call the cloned embryo.
4) The cloned embryo is then transferred into a recipient mare after it is cultured (allowed to develop) for a short time. The cloned embryo is allowed to develop into the cloned animal in the recipient mare, and is born naturally.
Proponents of cloning say that the resulting clone is genetically identical to the parent animal or the cloned individual. But there is one major exception to this statement and that has to do with the mitochondrial DNA. The clone inher- its the MtDNA of the donor oocyte, not the animal that was cloned. Although the MtDNA contains only a small group of genes, thanks to the Horse Genome Project, it is proving to have a large role in the performance of the individual. Yet the clone doesn’t carry the same MtDNA as the animal that was cloned carried.
An added note: If an oocyte from a mare with the same female family as our cloned indi- vidual is used, then the same MtDNA would be passed on to the clone. But a review of research and material on cloning doesn’t seem to promote that idea.
The part that makes this so interesting is that we all inherit all of our mitochondrial DNA from our mother. (For more informa- tion on the heredity of MtDNA through
the maternal line of a pedigree, read The Broodmare’s Influence in the June 8, 2012 issue of Speedhorse.) The discovery of this phenomenon and the consistency with which it is passed down the tail female line has lent
The clone is not 100% genetically
identical to the donor, because it does not have the same MtDNA.
an interesting insight into some of the early breeding practices and theories following the pedigree path. These theories include the Bruce Lowe Figure System.
Bruce Lowe was an Australian who became obsessed with the role of the mare in the produc- tion of a racehorse. He set out to determine the best mare lines by counting the number of times a mare was the tail female line descendant of a winner of the English Derby, English Oaks and St Leger. The tail female line is the bottom line of the pedigree that goes from the dam to the granddam, great granddam and so on down the line. Many of these tail female lines take us to a foundation mare for that line. She is called the taproot mare, and it is her MtDNA that is being passed down generation after generation.
The mare with the most descendants in Lowe’s system was given the number one. The taproot mare of the family number one was Tragonwell’s Natural Barb Mare. This lead to the formation of 50 families, some of which became noted for certain characteristics. Lowe’s system allowed the breeder to look at the pedi- gree to see which female family his horse came from. By doing so, they were inadvertently selecting for a specific MtDNA.
Some recent discoveries through research into the MtDNA of these families brought
out some discrepancies. One of the things
this testing brought out was that some of the founding mares were shown to be daughters
of other mares that were a part of the system, so the genetic base of the tail female line was not as widely spread as they originally thought. DNA research has shown that the MtDNA car- ries a set of genes known as haplotypes. These haplotypes don’t change from one generation to another but are passed down intact. This fact is so consistent that they trace migration patterns based on the MtDNA. But the point remains
that even early breeders like Bruce Lowe found something significant in the tail female line, they just didn’t understand the genetics of it.
As for cloning—if we don’t use the same MtDNA in a clone what have we lost? We have lost a part of the individual’s genetic identity.
The clone is not 100% genetically identical to
the donor, because it does not have the same MtDNA. We don’t know the complete role of how MtDNA influences an individual, but we do know that the MtDNA plays a role in what makes an individual not only great, but unique as well.
Of all the concerns raised about cloning, perhaps the most important take-home point is that the industry accepted definition of cloning is currently incorrect. A cloned animal is not genetically identical to the donor animal because of the loss of the genetic material contained out- side of the nucleus of the cell and in the mito- chondria—the mitochondrial DNA. Thanks to continuing genetic research, it is easy to see that the cloning process changes the dynamics of a horse’s pedigree (his genetics) by using a differ- ent MtDNA from a donor mare’s ooycte.
So, does changing the dynamics of the horse’s pedigree affect the integrity of the breed? Just as the AQHA fought to preserve the integrity of the breed by keeping horses prop- erly identified in the early days of the breed, they are currently facing a similar fight against cloning today.
One definition of integrity is “the state of whole, entire or undiminished.” Researchers have already proven that cloning does not reproduce the entirety of an animal’s DNA, and AQHA stands by its mission statement which starts with the words: “To record and preserve the pedigrees of the American Quarter Horse while maintaining the integrity of the breed.” In the end, it may be up to the courts to decide just how much integrity is enough.
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SPEEDHORSE, July 6, 2012