The Genetics of Trotting Racing Ability in Horses

As genomics improves and enables the design of more targeted studies relating genotypes to phenotypes, the opportunity for non-model organisms continues to expand - facilitating greater opportunities to gain novel insight into the mechanisms regulating biological homeostasis and health. Genomic studies of natural model species, domestic species in particular, give a complimentary view of genotype-phenotype relationships compared with the knowledge gained from the study of humans and experimental organisms.

Since the onset of domestication, horses have been strongly selected for, among other things, speed, strength, and endurance-exercise traits. This diverse and, at times, divergent selection has ultimately led to the development of highly specialized horse breeds. Within the last 400 years, breed specialization has focused primarily on preserving and improving traits related to aesthetics and performance. As a result, most horse breeds today are closed populations with high phenotypic and genetic uniformity within breed. However, a great deal of variation continues to exist among breeds. This variation, combined with breed specialization, has made the horse particularly well suited for genetic studies of locomotion patterns and provides a unique opportunity for genetic studies of athleticism. Generally speaking, athleticism describes the physical qualities that are characteristic of athletes and typically refers to traits such as strength, fitness, and agility.

Many modern day horse breeds exemplify some if not all of these traits, with shared selective pressures within breeds (e.g. health, fertility traits, conformation) and divergent selection between breeds (e.g. speed vs endurance) yielding a wide range of athletic phenotypes. Here we apply a genomic approach to investigate athletic phenotypes using a unique Nordic horse model consisting of the Norwegian-Swedish Coldblooded trotter (NSCT), the North Swedish Draught horse (NSD), and the Standardbred trotter (SB). Although both the NSCT and the NSD are horse breeds derived from the original North-Swedish horse, a small, heavy horse traditionally used in agriculture and forestry work, selection for traits beneficial to agricultural work only continues in the NSD. Since the 1960s, the NSCT has been intensively selected for harness racing performance and is now considered a true racing breed. As a result, a remarkable improvement in the racing performance of NSCTs has occurred during the last half-century. However, it is also well established that some degree of cross-breeding occurred between NSCT and SBs, a significantly faster breed of horse, before obligatory paternity testing was introduced in Sweden in 1969.

Consequently, the improvement in NSCT racing performance may be partially explained by a marked increase of favorable genetic variants originating from SBs. It is this specific attempt at gaining a competitive racing advantage that makes the NSCT ideal for genomic studies investigating athletic phenotypes.

Despite a dispersed history of crossbreeding with SBs, the relationship between the NSCT and NSD remains closer than either of the breeds with the SB. While both the NSCT and the SB are selected for racing performance, the Norwegian and Swedish breed organizations have remained highly committed to preserving the historical aesthetic of the NSCT breed. As a result, both NSCTs and NSDs can be classified as heavy horse breeds, with NSCTs sometimes referred to as “draft trotters”. Any lingering genetic similarities between the NSCT and the SB are therefore highly likely to be associated with favorable athletic traits for harness racing.