Wednesday, June 26, 2013

Genetic Basis of Gait in Icelandic Horse--Part 1

Blessi was sold to me as a 4-gaited horse. Every Icelandic instructor (except one) who ever worked with Blessi confirmed that he was 4-gaited. Although when I first got Blessi, I think that Halldor, an instructor who worked with Lynn Alfonsi on the East Coast, said that he got Blessi into a flying pace. However, I was so ignorant of Icelandic gaits that I may have misunderstood what he was
saying.

For some odd reason (killing time I guess), I was looking at Blessi's lineage in World Fengur. I had always assumed that his dam was unevaluated but when I checked again, she is evaluated and she is 5-gaited with a 7 for pace. His sire is five-gaited with a 7.7 for pace.

Andersonn et al published a study "Mutations in DMRT3 affect locomotion in horses and spinal circuit function in mice" confirming that the ability to tolt is directly related to a SNP mutation on gene DMRT3.

You can read the study at:

http://www.nature.com/nature/journal/v488/n7413/full/nature11399.html

However Nancy Marie Brown has a much more understandable essay on the implications of this study at:

http://nancymariebrown.blogspot.com/search?updated-max=2012-10-03T10:53:00-04:00\
&max-results=8&start=5&by-date=false


The original study states "Thus, homozygosity for the DMRT3 nonsense mutation is required for the ability to pace in this breed." Therefore if Blessi's dam and sire were 5-gaited, genetically Blessi must be 5-gaited also. Unfortunately, he falls into what the study calls "...a considerable number of homozygous mutant horses are considered four-gaited may reflect phenotype misclassifications, but
more likely incomplete penetrance due to other genetic factors, maturity and environmental effects, in particular training." So if Blessi does have flying pace, he doesn't not exhibit it and it is not worth training it.

However, hidden in the tables of this study are some interesting questions.

Question 1:The first question comes "Table 1: Allele frequency of the DMRT3 nonsense mutation among horse populations"
The Missouri Fox Trotters, Paso Finos, Peruvian Pasos, and Rocky Mountain horses indicated 100 % of the tested population possessed the DMRT3 nonsense mutation. Tennessee Walkers and Kenturcky Mountain horses showed 98 and 95 % respectively. Of the Icelandic horses tested, the results indicated 65% of the 4-gaited horses, 99% of the 5-gaited horses, and 89% of a random populations showed the DMRT nonsense mutation. So my understanding is that if two homozygous (AA) for this mutant gene are bred, the result must be another 5-gaited horse. If two heterozygous (CA) horses are bred, some of the offspring will not be gaited. Does the table indicating that 89% of the random population tested indicate that approximately 1 in 10 of Icelandic horsed don't tolt?

Question 2:
"Supplementary Table 5 Differences in mean in scores from breeding field tests between homozygous mutant (AA)and homozygous wild-type (CC) or heterozygous (CA) horses all shown as four gaited" compares the gaits shown during breeding evaluations between homozygous and heterozygous horses. The scores for trot (82.28 vs 79.15), gallop (82.25 vs 79.71), slow tolt (80.10 vs. 76.40), walk (77.98 vs 74.14), slow gallop (82.33 vs 77.86) are significantly higher for the 4-gaited horses. The scores for tolt are basically the same for 4-gaited versus 5-gaited (82.87 versu 82.49 respectively). Does this mean that all gaits except the tolt (and pace of course) are going to better on average for a 4-gaited horse?

Question 3:
Table 1 shows that 0% of the Shetland ponies, Arabians, Gotland ponies, Thoroughbreds, Swedish warmbloods, and Przewalski horses possess the DMRT3 nonsense mutation. What does this imply about the source of ability to tolt among the Icelandic horse? It is known that the Vikings brought horses both from Shetlands and Northern British Islands and horses directly from Norway during the original settlement of Iceland. MtDNA studies confirm that the Icelandic horse is genetically similar to the northern European ponies such as the Shetland and Exmor plus additonal breeds such as Mongolian horse and Fjord. Icelandics are also very similar to the Nordland, a gaited Norwegian breed. So did the ability to tolt come from the common ancestors of the Nordland and the current Mongolian horse or was the ability to tolt bred out of the Shetland pony?

As breeders in Iceland have known, if you always breed a 4-gaited horse to a 4-gaited horse, eventually you lose the tolt. And it seems like if 4-gaited horses are not bred to 5-gaited, the quality of the other gaits may suffer. So if the Icelandic breed want to keep quality gaits other than the tolt, breeding will always result in an occassional 3-gaited horse. This has interesting implications for the buyer of a young, untrained Icelandic horse. Ah, I love genetics--that science always seems to raise more questions than it answers.

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