by Ben Sturgeon, Bsc, BVM&S, Cert EP, MRCVS
In the summer of 2012 the British Equestrian team, Olympians and Paralympians, were showered with medals of all colour. Perhaps one of the most surprising was that of Charlotte Dujardin who changed her final dressage routine, mid test.
Instead of performing the steps practised and memorised for months she and her horse did something entirely different. She won gold! It is a well known adage that “practise makes perfect” and we can even qualitatively identify practise as achieving muscle memory or motor learning. Indeed training has been shown to enhance motor neuron excitability and induce synaptogenesis, both of which aid communication between the nervous system and locomotory muscles themselves. But perhaps the greatest dream is to have a horse who can, perhaps like Charlottes and in the words of Nike, Just Do It.
The ability of a horse to perform in certain disciplines or indeed at certain specific gaits is not one enforced by a riders demands but one that is enhanced by a riders understanding and skill promoting the horses’ innate or evolutionary design to perform. It is unsurprising then that horse breeds “naturally” fall into disciplines aligned to their inherent abilities. It is as rare to see a Shire performing Dressage as a Warmblood pulling a plough.
The interest in enhancing and cajoling these natural traits has long been in “the eye of the beholder” selecting traits deemed desirable for progeny to possess, akin to Darwinian theory of selective advantage, although it is perhaps now akin to selection for financial gain.
Whilst the locomotory pattern or gait of a horse may be perceptively altered or effected by for example; limb conformation, muscle balance and arguably sex, and further influenced by riding aids, repetitive training and rider skill, it is perhaps more interesting to note that for a horse to be able to achieve a certain gait or type of gaits, irrespective of phylogenic (end result animal or indeed human) physical make up and environmental influences, its genetic status needs to be complicit.
It is recognised that the motor circuits in the spinal cord controlling locomotion, referred to as the Central Pattern Generator (CPG), upon appropriate stimulation, generate an organised motor rhythm i.e. movement or gait. A publication in 2012 reported, on chromosome 23, that a recessive single gene variant nominally called DMRT3, found within a 186Kb haplotype loci, largely accounted for the ability of some horses to demonstrate alternate gaits additional to the standard walk, trot, canter and gallop. Instead of the two-beat contralateral gait of the trot, some horses performed the pace, a two-beat ipsilateral gait and others four-beat ambling and intermediate gaits (such as the Icelandic Tolt, the running walk, lateral and diagonal gaits and the Trotters racing pace) and for others not to do so; in other words the gene regulated the pattern of locomotion. From genome analysis the mutation was identified worldwide with a predominance in the gaited breeds and in horses bred for harness racing. The gene mutation caused a premature “stop codon” resulting in the production of a modified protein by the DMRT3 gene (when evaluated in mice spinal cord neurons) which altered functional effects on the spinal cord neuronal circuitry or CPG coordinating locomotory movement. Specifically it regulated whether or not a horse had the theoretical ability to perform, to be trained or indeed even to demonstrate gaiting ability and was, with likely reaching consequences, demonstrated to have a favourable effect on harness racing performance. Perhaps most interestingly, the presence of the gene variant as a homozygous form (having the mutant gene present on both chromosomes) or as a heterozygous form (having only one copy on a chromosome) also influenced the level of gaitedness demonstrated, with a homozygous carrier performing a greater variety of alternate gaits. Furthermore, it was suggested that the homozygous form not only resulted in the ability to perform different gaits but also inhibited the transition to gallop.
As intimated, whilst the evolutionary adaptations of horses has led naturally to their use in niche disciplines, the discovery of this population of nerve cells, dependent on DMRT3 protein transcription critical for the control of gaits in horses, has interesting financial implications. A Standardbred galloping in a race would lead to disqualification whilst the innate, or perhaps we can now say genetic, predisposition to pace would favour higher speeds and hence racing performance; an American Saddlebred with a homozygous inheritance may show better ability to train and exhibit gaitedness in 5-gaited and roadster competition classes, whilst conversely a horse destined for disciplines where gaitedness is deemed less favourable the early identification of even a heterozygous inheritance may be a deciding factor in its promotion to a training programme. Furthermore, for the enterprising, the genetic potential in breeding stallions and broodmares can assessed and tested (SynchroGait Testing®) before aiming for selective offspring.
Genetics is a wonderfully foolhardy science where it seems that by tweaking here and there we can remove some of the doubts and mystery of life, whilst gaining many of the advantages. However, one final sum for you to consider; any phenotype is an amalgamation of its genotype (whole genetic profile) as well as its environment (feeding, training, disease etc). Whilst it is exciting to know that an identifiable and testable gene can inform us as to the potential of an animal or to certain aspects of the animal, that true qualitative potential is a quantitative summation of many genes working together and only influenced in part by you. Many biologists neatly liken this to baking a cake where “you can mix two sets of exactly the same ingredients together, but at the end of baking they will taste different.” The only caveat we can add here though is this, if you need gaitedness, you need at least one vital agreement.