Thoroughbred racehorse mitochondrial DNA demonstrates closer than expected links between maternal genetic history and pedigree records

Abstract

The potential future earnings and therefore value of Thoroughbred foals untested in the racing arena are calculated based on the performance of their forebears. Thus, lineage is of key importance. However, previous research indicates that maternally inherited mitochondrial DNA (mtDNA) does not correspond to maternal lineage according to recorded pedigree, casting doubt on the voracity of historic pedigrees. We analysed mtDNA of 296 Thoroughbred horses from 33 maternal lineages and identified an interesting trend. Subsequent to the founding of the Thoroughbred breed in the 16th century, well-populated maternal lineages were divided into sub-lineages. Only six in 10 of the Thoroughbreds sampled shared mitochondrial haplotype with other members of their maternal lineage, despite having a common maternal ancestor according to pedigree records. However, nine in 10 Thoroughbreds from the 103 sub-lineages sampled shared mtDNA with horses of their maternal pedigree sub-lineage. Thus, Thoroughbred maternal sub-lineage pedigree represents a more accurate breeding record than previously thought. Errors in pedigrees must have occurred largely, though, not exclusively, at sub-lineage foundation events, probably due to incomplete understanding of modes of inheritance in the past, where maternal sub-lineages were founded from individuals, related, but not by female descent.

A pedigree-based analysis of mitochondrial DNA diversity in a dog population on the example of German Hovawarts

“…This problem undoubtedly also concerns other breeds, and generally leads to lower mtDNA diversity in dogs in comparison to that of horses, in which dam lines play an important role in breeding. This concerns such breeds as the Thoroughbred, Lipizzan and Arabian, in populations of which 25, 37 and 74 mtDNA haplotypes, respectively, were found (Bower et al, 2013;Kavar et al, 2002;Khanshour and Cothran, 2013).…”

“…2). In this context, the results of mtDNA studies in horses, in which pedigree errors are usually identified (Hill et al, 2002;Kavar et al, 2002;Głażewska et al, 2007;Bower et al, 2013), are deserve attention. Of particular interest are the results presented by Hill et al (2000) and Bower et al (2013) on Thoroughbreds; these indicate that increased numbers of pedigree errors and haplotypes are identified when more detailed pedigree data are used.…”

“…As a rule, the error frequency resulting from mutations is determined by the mutation rate of a given mtDNA region that is estimated on a scale of thousands of years, and, thus, significantly exceeds the period described in the pedigrees of modern breeds of domestic animals. Hence, heteroplasmy or new mutations are only occasionally reported in dog or horse studies (Klütsch et al, 2011;Bower et al, 2013). This leads to the conclusion that the anticipated number of errors of this kind in mtDNA studies based on pedigree data should be small.…”

Truth in the Bones: Resolving the Identity of the Founding Elite Thoroughbred Racehorses

“…An analysis of present‐day racehorses shows that this is not always the case. In a study of 281 Thoroughbred racehorses, maternal lineages were shown to contain two or more mitochondrial genotypes (Bower et al . submitted).…”

“…submitted). The primary reasons for these discrepancies are historic errors in the General stud book , introduced early in the breed history, most of which can be traced to the foundation of maternal sub‐lineages (Bower et al . submitted).…”