The genetic relationships of seven Japanese and four mainland-Asian horse populations, as well as two European horse populations, were estimated using data for 20 microsatellite loci. Mongolian horses showed the highest average heterozygosities (0.75-0.77) in all populations. Phylogenetic analysis showed the existence of three distinct clusters supported by high bootstrap values: the European cluster (Anglo-Arab and thoroughbreds), the Hokkaido-Kiso cluster, and the Mongolian cluster. The relationships of these clusters were consistent with their geographical distributions. Basing our assumptions on the phylogenetic tree and the genetic variation of horse populations, we suggest that Japanese horses originated from Mongolian horses migrating through the Korean Peninsula. The genetic relationship of Japanese horses corresponded to their geographical distribution. Microsatellite polymorphism data were shown to be useful for estimating the genetic relationships between Japanese horses and Asian horses.

A genetic linkage map of the horse consisting of 742 markers, which comprises a single linkage group for each of the autosomes and the X chromosome, is presented. The map has been generated from two three-generation full-sibling reference families, sired by the same stallion, in which there are 61 individuals in the F2 generation. Each linkage group has been assigned to a chromosome and oriented with reference to markers mapped by fluorescence in situ hybridization. The average interval between markers is 3.7 cM and the linkage groups collectively span 2772 cM. The 742 markers comprise 734 microsatellite and 8 gene-based markers. The utility of the microsatellite markers for comparative mapping has been significantly enhanced by comparing their flanking sequences with the human genome sequence; this enabled conserved segments between human and horse to be identified. The new map provides a valuable resource for genetically mapping traits of interest in the horse.

Summary The responses of plasma adrenocorticotropin (ACTH), cortisol, noradrenaline and adrenaline in 5 Thoroughbred horses to an incremental exercise and 2 relative workload exercises, at 105 and 80% maximal oxygen consumption (V̇O2max), on a treadmill were examined. These hormone concentrations increased (P<.05) with each exercise and the maximal plasma concentrations of ACTH, cortisol were observed between 5 and 30 min after the end of the exercise, while maximal catecholamine concentrations occurred just at exhaustion time. The plasma ACTH, noradrenaline and adrenaline responses during exercise were more sensitive to the intensity of exercise than that of cortisol and showed a significant correlation with blood lactate concentrations (r=0.605, P<.001 for ACTH; r=0.718, P<.001 for noradrenaline; r=0.738, P<.001 for adrenaline). The plasma cortisol response appeared to be connected with the duration of exercise (r=0.71, P<.05). The recovery of these hormones was related to the exercise styles. These results suggest that the autonomic nervous system and the pituitary‐adrenal axis of the horse are efficiently stimulated by various treadmill exercises, and these hormones may be used in the evaluation of exercise‐induced stress.

ABSTRACT. Myostatin is a member of the transforming growth factor- family with a key role in inhibition of muscle growth by negative regulation of both myoblast proliferation and differentiation. Recently, a genomic region on ECA18, which includes the MSTN gene, was identified as a candidate region influencing racing performance in Thoroughbreds. In this study, four SNPs on ECA18, g.65809482T>C, g.65868604G>T, g.66493737C>T, and g.66539967A>G, were genotyped in 91 Thoroughbred horses-in-training to evaluate the association between genotype and body composition traits, including body weight, withers height, chest circumference, cannon circumference, and body weight/withers height. Of these, statistically differences in body weight and body weight/withers height were associated with specific genotypes in males. Specifically, body weight/withers height showed statistically significant differences depending on genotype at g.658604G>T, g.66493737C>T, and g.66539967A>G (P<0.01) in males during the training period. Animals with a genotype associated with suitability for short-distance racing, C/C at g.66493737C>T, had the highest value (3.17  0.05 kg·cm -1 ) for body weight/withers height in March, while those with a genotype associated with suitability for long-distance racing, T/T, had the lowest (2.99  0.03 kg·cm -1 ). In females, the trends in the association of body weight/withers height with genotypes were similar to those observed in males. As the SNPs are not believed to be linked to coding variants in MSTN, these results suggest that regulation of MSTN gene expression influences skeletal muscle mass and hence racing performance, particularly optimum race distance, in Thoroughbred horses.KEY WORDS: body composition, myostatin, racing performance, Thoroughbred.J. Vet. Med. Sci. 73(12): 1617-1624 Thoroughbred horses originated from a small number of Arab, Barb, and Turk stallions and native British mares approximately 300 years ago [3,5,12]. Since then, they have been selectively bred to improve speed and stamina, and are consequently superior competitive racehorses. As a result, Thoroughbred horses have a very high skeletal muscle mass comprising over 55% of total body mass [10] ) is also superior to that of other species of similar size [16,17,26]. Such traits have been enhanced by artificial selection for the DNA sequence variants contributing to exceptional racing performance [8].Many significant advances have been made in the horse genome project (http://www.uky.edu/Ag/Horsemap/welcome.html), such as the completion of a high-quality draft horse genome sequence with over 1.1 million identified SNPs [25]. The advances in the genetic infrastructure for the horse has enabled the identification of a genomic region on ECA18 associated with racing performance phenotypes. Four case-control studies, including a candidate gene study [13], a microsatellite-based genome-wide association study [23], and two genome-wide SNP association studies [2,14] have identified the same genomic region on ECA18 as associated with r...

A microsatellite DNA typing with 17 microsatellites (mSats) including nine international minimum standard mSats was developed for parentage verification of racehorse, and 1, 308 racehorses, Thoroughbred and Anglo-Arab, were analyzed using the 17 mSats. Reasonable reliability and reproducibility of genotyping were observed based on data of sizing precision of PCR fragments.On 10-18 and over 0.99999, respectively. Therefore, it is theoretically suggested that the present DNA typing has high ability for individual identification and parentage verification.Actually, true paternities for 367 double mating cases could be verified using the 17 mSats. Furthermore, to confirm potentiality for resolution of multiple mating, paternity testing was simulated by selection from 502 provisional stallions in 367 mare-offspring pair cases. Decision rate for one candidate sire with the 17 loci was 97.3% indicating high ability for resolution of multiple mating. These data suggest that the present DNA typing is quite suitable to parentage verification for racehorses in Japan.