Does heterozygosity at the DMRT3 gene make French trotters better racers?

Anne Ricard

doi:10.1186/s12711-015-0095-7

Abstract

BackgroundRecently, a mutation was discovered in the DMRT3 gene that controls pacing in horses. The mutant allele A is fixed in the American Standardbred trotter breed, while in the French trotter breed, the frequency of the wild-type allele C is still 24%. This study aimed at measuring the effect of DMRT3 genotypes on the performance of French trotters and explaining why the polymorphism still occurs in this breed. Using a mixed animal model, genetic parameters and environmental effects on performance traits were estimated from data on 173 176 French trotter races. The effect of the DMRT3 gene was then estimated by the effect of genotype at the highly linked SNP BIEC2-620109 (C-C, A-T) for 630 horses. A selection scheme that included qualification and racing performances was modeled to (1) verify if the observed superiority of heterozygous CT horses at this SNP could be explained only by selection and (2) understand why allele C has not disappeared in French trotters.ResultsHeritability of racing performance traits was high for qualification test (0.56), moderate for annual earnings per finished race (0.26 to 0.31) and low for proportion of disqualified races (0.06 to 0.09). Genotype CC was always unfavorable compared to genotype TT for qualification: the probability to be qualified was 20% for CC vs. 48% for TT and earnings were -0.96 σy lower for CC than for TT. Genotype CT was also unfavorable for qualification (40%) and earnings at 3 years (-0.21 σy), but favorable for earnings at ages greater than 5 years: +0.41 σy (P = 7.10−4). Selection on qualification could not explain more than 19% of the difference between genotypes CC and CT in earnings at ages greater than 5 years. Only a scenario for which genotype CT has a favorable effect on the performance of horses older than 5 years could explain that the polymorphism at the DMRT3 gene still exists in the French trotter breed.ConclusionsThe use of mature horses in the French racing circuit can explain that the CA genotype is still present in the French trotter horses.

Highlights

A mutation was discovered in the DMRT3 gene that controls pacing in horses
Allele A is fixed in most breeds that have been selected for trot racing or for pace racing, such as the American Standardbred [2], whereas allele C is fixed in breeds that have been selected for gallop racing, such as the Thoroughbred, the Arab and Swedish warmblood horses [2]
The aims of this study were to (1) measure the effect of DMRT3 genotypes on various traits associated with the performance of trotter horses in France, (2) explain why the polymorphism for this gene is still present in the French trotter breed, and (3) determine whether it is possible to predict when allele C might disappear from the population of trotter horses that are currently under selection in France

Summary

Introduction

A mutation was discovered in the DMRT3 gene that controls pacing in horses. In 2012, Andersson et al [1] reported a mutation in the DMRT3 (doublesex and mab-3 related transcription factor 3) gene that affects locomotion in horses. The role of this gene on a specific subset of spinal cord neurons was demonstrated in mice. In the original publication [1], the Swedish team genotyped 47 French trotters and found a frequency of only 77% for allele A in this breed This result was confirmed by Promerová et al [2]. The negative effect of allele C on the performance of trotter horses was clearly demonstrated by comparing the estimated breeding values for racing performances and earnings of Swedish Standardbred horses including crossbred imported French trotters

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