Scientists in Sweden have announced an important discovery that helps explain how some horses are limited to “natural” gaits, while others are capable of additional gaits.
Nearly all horses use “natural” gaits (walk, trot, canter and gallop) without special training. Additional “ambling” gaits may occur naturally in some individuals, but usually only in certain breeds.
Researchers have identified a single gene mutation that enables horses to perform gaits such as running walk and pacing. The pace is a lateral two-beat gait; the two legs on the same side of the horse move forward together, unlike the trot, in which the two legs diagonally opposite each other move forward together.
Not only does the mutation play a crucial role in the horse's ability to perform “ambling” gaits, it also affects performance in harness racing.
Leif Andersson and his research team in the Department of Animal Breeding and Genetics, at the Swedish University of Agricultural Science were looking for a genetic basis for gaits in horses. Only some horses can pace, and they wanted to find out why.
They studied the genomes of 70 Icelandic horses that could perform extra gaits — 40 could pace, and 30 could perform other alternate gaits. They found that a single mutation in a gene called DMRT3 was strongly associated with the ability to pace. The mutation resulted in the production of a shortened form of the DMRT3 protein. Both copies of the gene were mutated in the pacing horses.
Andersson explained that horses without this mutation cannot move their right hindleg and right foreleg forward at the same time. But with the mutation the movement is not regulated so strictly and becomes more flexible.
“We suspected a strong genetic component, but were almost shocked when we discovered that a single gene, DMRT3, largely explained the genetic difference between pacers and non-pacers’” said Lisa Andersson, one of the PhD students involved in the project.
The researchers developed a diagnostic test and found that the mutation is widespread among horses that show alternate gaits like Tennessee Walking Horse from the USA and Paso Fino from South America.
The mutation must have originated over a thousand years ago, Andersson said, judging by its widespread distribution among breeds. The horses with this gene would have given a smoother ride and thus been kept and bred.
While Andersson's group were investigating the presence of the gene mutation in pacing horses, a second research team was investigating the expression of the DMRT3 gene in the spinal cord of mice.
Klas Kullander and colleagues in the Department of Neuroscience, at Uppsala University, found that the gene was expressed in a previously unknown type of nerve cell within the spinal cord.
Further investigation showed that the DMRT3-neurons cross the midline of the spinal cord, connecting the left with the right side. They also connect with motor neurons that control flexor and extensor muscles. It seems that, as well as coordinating the activity of flexor and extensor muscles, this network of nerve cells also controls the alternate movement of left and right limbs.
‘The discovery of the DMRT3 mutation is an outstanding example of how genetic studies of evolution in domestic animals can lead to basic new knowledge concerning gene function and important biological mechanisms’, said Leif Andersson.
‘It is truly great when this type of interdisciplinary collaboration results in such ground breaking discoveries. There was no information in the scientific literature on the function of the DMRT3 prior to the publication of our article. This protein is present in all vertebrates for which data are available, and it is likely that DMRT3 nerve cells have a central role for coordinating movements in humans as well’, added Klas Kullander.
More details at equinescienceupdate.com