Mitochondria and Elite Performance
A paper on humans was released last week that outlined some variations in the control region of mitochondrial DNA associated with elite Japanese athletic status. A quick primer - Mitochondria are the cell's power producers. They convert energy into forms that are usable by the cell. Located in the cytoplasm, they are the sites of cellular respiration which ultimately generates fuel for the cell's activities so good mitochondria, in terms of its ability to respirate muscle and make it use lactic acid properly, would ultimately mean better athletic performance. Mitochondria are also involved in other cell processes such as cell division and growth, as well as cell death. Mitochondrial DNA is the DNA that resides inside the mitochondria and is different from nuclear DNA and is maternally inherited (you get it from your mother, but if you are a male, you can't pass it on to your kids...another reason to blame your mother for your own lack of athleticism!).
Now that we have that over with, within the mtDNA there is a control region as well as some protein coding genes. The control region of mitochondrial DNA (mtDNA) contains the main regulatory elements for mtDNA replication and transcription and certain polymorphisms (difference in the genetic code between two individuals) in this region could contribute to elite athletic performance, because mitochondrial function is one of determinants of physical performance. The study undertaken examined the effect of polymorphisms in the control region on elite athlete status by sequencing the entire mtDNA control region. The study gathered 185 elite Japanese athletes who had represented Japan at international competitions and broke them into endurance and sprint types, and 672 Japanese controls who had not participated in elite athletic competitions. Frequency differences of polymorphisms in the mtDNA control region between the three groups were examined. Endurance types EMA had three distinct changes in mtDNA code in the control region when compared to the control group, while the sprint group had a different change in code when compared to the control group. Additionally, while some of the control and endurance types had a certain genetic code, none of the sprinters did. The findings imply that several polymorphisms (changes in code) detected in the control region of mtDNA may influence physical performance probably in a functional manner.
So what does this mean for horses. Is it just a matter of sequencing the control region of mtDNA and working out what changes in genetic code mean a horse is going to be an elite horse?....not so fast. Unlike humans, the thoroughbred is a closed genetic book. As we posted earlier, if you trace all pedigrees back down the female line, as per the Bruce Lowe/Toru Shirai numbers, you get to a finite number of mares that actually started the thoroughbred. It is about 15 to 19, depending on if you include some colonial and polish families that have very small numbers in the total thoroughbred pool. As mitochondrial DNA doesn't mutate that frequently, if the polymorphisms in the mtDNA relating to elite performance in thoroughbreds existed, over the last 300 years we would have selected for it by now and all horses would come from the one, or two mitochondrial haplotypes. Humans haven't been selected the same way as horses and there are a lot of different mitochondrial groups, even if we all go back to a mitochondrial eve at some point.
Secondly, it is just as important to know that mitochondrial DNA is small in terms of size but does an awful lot in terms of function. How it does this is by using nuclear based genes to do the work. The limited coding capacity of mtDNA necessitates that nuclear genes make a major contribution to mitochondrial metabolic systems and molecular architecture. Consequently the vast majority of the mitochondrial proteins are nuclear-encoded and mitochondrial function depends on the coordinated expression of both nuclear and mitochondrial genomes through a process known as mitochondrial biogenesis. What is most likely, in terms of thoroughbreds, is that certain nuclear based genes will have variants within them to optimize mitochondrial function for the mitochondrial haplogroup that they are from. So, in terms of say, all the horses from family #1 (Tregonwell's Natural Barb), it is most likely that in order for that mitochondria to work efficiently, there are variants required within nuclear based genes that make it work optimally. The problem is, because they are nuclear based, they are subject to the laws of Mendel. In theory, you could have a mare that is from family line #1 and breed her to a stallion that has the variants in the genes that you are looking for your foal to get, but the foal inherits the mothers genes in this case and you don't get what you need to optimize the mtDNA function. Equally, the very next foal of the repeat mating could get the sire's genes and you have a potential stakes winner at foot.