The ACOX1 gene
This horse’s result
Ideal result for endurance
Sub-optimal use of fatty acids as slow-burning fuel.
This can hinder the horse’s ability to generate a sufficient, sustained energy supply to muscles over prolonged distances.
Why this is important for Anglo-Arabians
Throughout history, various breeds have been used to travel long distances, often over difficult terrain with a scarcity of food and water. Similarly, some horses needed to contend with harsh climates, predators, and a lack of sufficient shelter . Over time, the horses that survived and flourished were hardy and resilient, able to draw fuel from the fat supplies of their own body, rather than from external sources, leading to selection for a variant in the ACOX1 gene, which enabled them to use fat as a fuel, giving them better endurance and more hardiness.
This variant in the ACOX1 gene gives instructions for making an enzyme called peroxisomal acyl-coenzyme A oxidase 1 responsible for fat metabolism. It facilitates the generation of energy from fats over a long duration. Horses that are ACOX1 GG, which is the most common genotype in old, native breeds, can draw on energy from their body fat when food is in short supply, rather than using fast-burning glucose to power their movement. This use of fat as fuel enables the horse to survive harsh winters and travel over prolonged distances.
In contrast, the ACOX1 TT genotype is found much more frequently in thoroughbreds. Around 85% of thoroughbreds carry the ’T’ variant in the ACOX1 gene. This emerged as the most common thoroughbred result through selection for speed over short distances during the last 250 years. Horses with the ’T’ variant are much better at flat racing and are better able to gallop, drawing on glucose to power their explosive movement. While the ’T’ variant is extremely useful for galloping and flat racing, it is less advantageous for endurance. Over the last few centuries, especially in Polish Arabians, the ACOX1 ’T’ variant has appeared more frequently in Arabians selected and bred for flat racing. As with thoroughbreds, this ACOX1 TT or GT genotype gives Arabians and Anglo-Arabians better gallop performance and helps them to better at short-distance racing.
Thus, Anglo-Arabians can have either the ‘G’ variant or the ’T’ variant from their Arabian and thoroughbred inheritance, with those having the ‘G’ variant from their ancient ancestors enjoying the ability to burn fat for fuel, having greater hardiness, and being better able to travel over long distances.
Future breeding implications
Horses like this have inherited one ‘T’ variant on the ACOX1 gene from the sire and another ‘T’ variant from their dam. This gives them the result ACOX1 TT — one ‘T’ from each parent. Therefore, this horse will always pass one of these ACOX1 ‘T’ variants to their foal.
However, that foal could inherit a ‘G’ variant (good for endurance) or an ‘T’ variant (not so good for endurance) from the other parent. Therefore, it is important to know the genetic results for both parents to understand all possible breeding implications.
TT (this horse) bred with another ACOX1 TT horse (mate) =
- 100% chance of the foal being an ACOX1 TT horse
TT (this horse) bred with an ACOX1 GT horse (mate) =
- 50% chance of a ACOX1 TT foal.
- 50% chance of a ACOX1 GT foal.
How this gene influences endurance
The ACOX1 gene encodes the enzyme called acyl-coenzyme A oxidase 1, which is the first enzyme in the fatty acid β-oxidation pathway. This pathway oxidizes fat from the diet or the horse’s body to produce slow-burning energy, either during periods of starvation, or for long distance endurance, as glycogen burns out and is utilized too quickly. Those horses that have the ‘G’ variant in the ACOX1 gene are better able to make the acyl-coenzyme A oxidase 1 enzyme, enabling them to efficiently use fat for fuel compared to horses that have the ’T’ variant. This gives ACOX1 GG horses an advantage for endurance and for surviving periods when food is not readily available in sufficient amounts.