Morphometric Traits in Gaited Breeds of Horse: Potential Future Targets for Mapping

Abstract

s / Journal of Equine Veterinary Science 31 (2011) 230-356 242 [7] Poncet PA, Pfister W, Muntwyler J, Glowatzki-Mullis ML, Gaillard C. Analysis of pedigree and conformation data to explain genetic variability of the horse breed Franches-Montagnes. J Anim Breed Genet 2006;123:114-21. Morphometric Traits in Gaited Breeds of Horse: Potential Future Targets for Mapping E.A. Staiger , N.B. Sutter , R.R. Bellone , and S.A. Brooks 1 Department of Animal Science, Cornell University, Ithaca, NY, USA, 2 Department of Clinical Sciences, Cornell University, Ithaca, NY, USA, 3 Department of Biology, University of Tampa, Tampa, FL, USA Introduction: After initial domestication, the horse has been selected by man primarily for transportation purposes. Early uses were for war and labor; however, thanks to the industrial revolution, present day uses are for sport and recreation, resulting in divergent skeletal size, conformation, and locomotion in thehorse.Sinceancient times, the horse has shown polymorphism in gait; at intermediate speeds, the horse can perform a range of lateral and diagonal, 2-beat or 4-beat gaits [1]. These are known as the trot and pace, and the rack andrunningwalk, respectively.While therehavebeenstudies intothe mechanics of gait [2-4], there have been few studies that have investigated how morphology influences gait in horses. The aim of this study is to present preliminary findings of morphological traits that may influence gait in horses. Quantification of these morphometric traits will be valuable in future studies to identify genes associated with gait by whole genome association. Materials and Methods: The skeletal morphological assay was previously published in Brooks et al. [5]. The data gathered for that study included, in part, 34 body measurements that span the head, neck, body, and limbs collected from 1550 horses from 82 breeds, vital statistics (breed, age, sex etc) and owner reported gait information. Horses were categorized as if they were reported by their owner, or belong to a breed that exclusively performs a gait other than trot at intermediate speeds. The 34 measurements capture bone length and circumference as quantitative variables. Statistical analyses were conducted using JMP V8.0 software (SAS Institute Inc.). Results and Discussion: In our previously published study two principle components of skeletal variation in the horse were identified, PC1 (explains 62.6% of the variance) and PC2 thickness (explains 5.4% of the variance) [5]. PC3 from this study, although it accounted for a small 2.8% of the variance percentage of the total variance across all breeds, was statistically different in gaited horse (P < .0001). The top contributors to PC3 include the fore and hind pastern lengths, croup to dock length, ear length, and jaw width measures. To confirm differences in these measures specific to gait type the 34 measures were normalized by withers height and compared between the gaited (286 horses) and non-gaited categories (1264 horses) using ANOVA. Nineteen measurements were found to be statistically significant in gaited breeds. These included smaller jaw width (P < .0001), longer ear length (P < .0001), shorter dock height (P < .0001), longer croup to dock length (P < .0001), smaller heart girth (P < .0001), shorter gaskin length (P 1⁄4 .0333), smaller fore and hind cannon circumference (P < .0001), and smaller fore and hind pastern circumference (P < .0001). Conclusion: Body conformation is a critically important trait in nearly all horse breeds. It has been well documented that conformation influences movement; hip and shoulder angles influence the range of motion of the limbs, while limb segment lengths influence shock absorption and strength of the limbs. Principal components analysis of measurements from a sample of horses of different breeds has shown consistent trends in several quantified traits, including skeletal size (PC1) and shape (PC2). Studies are currently underway to map these PC-traits as QTLs in the horse using the EquineSNP50 genotyping chip (Illumina INC). Although it seems likely that gait preference is highly heritable, little previous work has documented the genetic and biomechanical components to gait. This preliminary study, therefore, has several implications regarding the relative significance of individual traits in determining a horse's quality of locomotion and predisposition towards ambling or trotting. Whether by chance, or design, gaited horses have been selected for a unique set of conformational traits. These may influence gait performance. For example, longer croup to dock lengths might indicate a horse is more predisposed to running walk than rack or pace. Additional data, particularly analysis of horses in motion, may help to shed light on how these individual components influence gait type and quality. Identification of genes responsible for gait preference and gaitspecific conformation in the horse, could inform studies of orthopedic disease susceptibility, and athletic performance, as well as provide valuable tools to breeders selecting for gait.