Abstract
The Thoroughbred horse breed was developed primarily for racing, and has a significant contribution to the qualitative improvement of many other horse breeds. Despite the importance of Thoroughbred racehorses in historical, cultural, and economical viewpoints, there was no temporal and spatial dynamics of them using the mitogenome sequences. To explore this topic, the complete mitochondrial genome sequences of 14 Thoroughbreds and two Przewalski’s horses were determined. These sequences were analyzed together along with 151 previously published horse mitochondrial genomes from a range of breeds across the globe using a Bayesian coalescent approach as well as Bayesian inference and maximum likelihood methods. The racing horses were revealed to have multiple maternal origins and to be closely related to horses from one Asian, two Middle Eastern, and five European breeds. Thoroughbred horse breed was not directly related to the Przewalski’s horse which has been regarded as the closest taxon to the all domestic horses and the only true wild horse species left in the world. Our phylogenomic analyses also supported that there was no apparent correlation between geographic origin or breed and the evolution of global horses. The most recent common ancestor of the Thoroughbreds lived approximately 8,100–111,500 years ago, which was significantly younger than the most recent common ancestor of modern horses (0.7286 My). Bayesian skyline plot revealed that the population expansion of modern horses, including Thoroughbreds, occurred approximately 5,500–11,000 years ago, which coincide with the start of domestication. This is the first phylogenomic study on the Thoroughbred racehorse in association with its spatio-temporal dynamics. The database and genetic history information of Thoroughbred mitogenomes obtained from the present study provide useful information for future horse improvement projects, as well as for the study of horse genomics, conservation, and in association with its geographical distribution.
Highlights
The Thoroughbred horse, which was developed by English aristocracy during the 17th and 18th centuries, is the world’s best-known racehorse breed [1]
The objectives of the present study were (1) to study whether the Thoroughbred racehorses have a single maternal origin; (2) to examine the phylogenetic relationships between Thoroughbred horses and other domestic horse breeds; (3) to uncover the phylogenetic relationships of the Thoroughbred and Przewalski’s horses; (4) to address the influences of breeds or countries on the heterogeneity of global horses; (5) to estimate the time of the most recent common ancestor of these racehorses; and (6) to assess divergence times of the common female ancestor and changes in the population size of modern horses
According to the BMCC tree, the most recent common ancestor of the modern horses existed approximately 0.7286 My ago (95% highest probability density (HPD) = 0.4225–1.1051); Group 5 emerged first (0.4789 My ago, 95% HPD = 0.2569–0.7563), followed by sequential segregation of Group 2 (0.3964 My ago, 95% HPD = 0.2202–0.6114), Group 1 (0.3704 My ago, 95% HPD = 0.2009–0.5805), Group 3 (0.2892 My ago, 95% HPD = 0.1253–0.5161), Group 4 (0.2599 My ago, 95% HPD = 0.1224–0.4486), and Group 6 (0.07 My ago, 95% HPD = 0.007–1.2024) (Fig 4 and Table 3)
Summary
The Thoroughbred horse, which was developed by English aristocracy during the 17th and 18th centuries, is the world’s best-known racehorse breed [1]. Most researchers support that the racehorse breed diverged from multiple maternal origins [15, 16, 18], while Bower et al [17] proposed that Thoroughbred mares were founded in the British Isles in the 17th and 18th centuries All of these studies used obscure analytical methods and dealt only with D-loop sequences that can generate ambiguous tree topologies due to the high level of recurrent mutations and comparatively short lengths (including a small number of phylogenetically informative sites). All, they all didn’t mention time of the most recent common ancestor of the Thoroughbred horses
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