Vinsamlegast notið þetta auðkenni þegar þið vitnið til verksins eða tengið í það: http://hdl.handle.net/1946/20620
The breeding goal for the Icelandic horse promotes a five-gaited riding horse with a functional and aesthetically appealing conformation. The main objective of the thesis was to estimate the association between conformation and riding ability in Icelandic breeding horses. The first step was to apply and evaluate in terms of repeatability a three-dimensional (3-D) video morphometric method for quantification of the conformation. The second step was to assess the effect of the DMRT3 ‘Gait keeper’ mutation on gaiting ability in the Icelandic horse and estimate the development in frequency of the mutation within the population. The third and final step was to assess the association between conformation and riding ability, where both standard (direct) and 3-D conformational measurements were related to breeding field test scores for riding ability.
The conformation of 72 breeding horses was quantified with the 3-D morphometric method, which provided objective, detailed and repeatable data on the conformation and was found to be suitable for description of the breed and further studies. Comparison with earlier studies confirmed that the Icelandic horse has grown taller in recent years, changed from a rectangular to square body format and acquired a more uphill conformation. Measurements of the joint angles of the limbs revealed carpal and tarsal valgus and fetlock valgus to be
frequent findings in the breed. The second part of the thesis involved genotyping of 667 breeding horses with respect to the DMRT3 ‘Gait keeper’ mutation. The majority (76.3%) was homozygous mutant (AA) and 22.5% were heterozygous (CA) while only 1.2% was found homozygous for the wild type allele (CC). Homozygosity for the ‘Gait keeper’ mutation was confirmed to be permissive for the ability to pace and had a favourable effect on scores for the lateral gait tölt, demonstrated by better beat quality, speed capacity and suppleness. Horses that were heterozygote for the mutation had, on the other hand, significantly higher scores for the basic gaits and performed better beat and suspension in trot and gallop. These results indicate that the AA genotype reinforces the coordination of ipsilateral limbs, with the subsequent negative effect on the synchronized movement of diagonal limbs compared with the CA genotype.
Change in the frequency of the mutation was estimated on the basis of genotype probabilities of 146,763 horses registered in WorldFengur. The frequency of the A-allele has increased in the population in recent decades with a corresponding decrease in the frequency of the Callele, most likely promoted by the emphasis on lateral gaits in the breeding goal. The estimated frequency of the A-allele in the Icelandic horse population in 2012 was 0.94 with a predicted loss of the C-allele in relatively few years.
The final step was to estimate the phenotypic and genetic relationship between standard conformational measurements and scores for the riding ability and determine if 3-D morphometric measurements could discriminate between high-class and low-class horses based on scores for the different gaits. The data comprised records from all assessed breeding horses in Iceland in 2000-2013 (10,091 horses) and a subpopulation of 98 haphazardly selected breeding horses with a detailed quantification of the conformation in 3-D. Most of the standard measurements had a significant curvilinear relationship with the studied riding ability traits. Different 3-D measurements could discriminate between high-class and lowclass horses within each gait with high accuracy by multivariate analyses, as well as between AA horses that were presented as four-gaited horses (without pace) and five gaited horses with good pacing ability. Proportions in the top line of the horse describing the height of the horse at front compared to hind were found to be most important for the riding ability, revealing the advantage of an uphill conformation. Their estimated heritability and genetic correlation with total score for riding ability designate them as important indicators for performance.
The results have practical implications for breeding of Icelandic horses and can improve the assessment of the conformation at the breeding field tests and consequently the riding ability of the Icelandic horse.