Only a handful of maternal ancestry lines exist in the present-day global Cleveland Bay horse population, fresh research suggests.
The finding was based on analysis of the mitochondrial D-loop sequence in representative samples from 96 Cleveland Bay horses.
The researchers from Lincoln and Nottingham Trent universities in Britain found that four main haplotype clusters were present in the Cleveland Bay breed, covering 89% of the total sample.
“This suggests that only four principal maternal ancestry lines exist in the present-day global Cleveland Bay population,” Andy Dell and his fellow researchers reported in the open-access journal, PLOS ONE.
Comparison of the sequences with other domestic horse haplotypes showed a close association of the Cleveland Bay horse with Northern European (Clade C), Iberian (Clade A) and North African (Clade B) horse breeds.
“This indicates that the Cleveland Bay horse may not have evolved exclusively from the now-extinct Chapman horse, as previous work has suggested,” they reported.
It is possible that two of the lines, plus another almost extinct line, may not be of maternal Chapman descent, but from Iberian and Barb mares, they said.
The Cleveland Bay horse is one of the oldest equines in Britain, with pedigree data going back almost 300 years.
It one of five horse breeds listed as critical by the UK Rare Breeds Survival Trust in its annual watchlist, meaning there are fewer than 300 registered adult breeding females.
The heritage breed is thought to have been established in the 17th century from crosses between pack horse or “Chapman” mares, known to have been bred by monastic houses in the north-east of Britain, and newly imported hot-blooded Arabian, Barb or Mediterranean stallions. The results were early examples of what is now known as a warmblood.
Cleveland Bays flourished throughout the 18th century, achieving world renown as coaching and driving horses.
The Cleveland Bay Horse Society was formed in 1884 and the first stud book was produced with pedigrees dating back to 1732. The first stud book was officially closed in 1886 and the breed has maintained a closed stud book since that date.
Dell and his colleagues noted that, even when the breed society was formed, the Cleveland Bay was in decline.
“Subsequent losses in two world wars, and increasing mechanisation in transport and on land, meant that by the middle of the 20th century the breed was very close to extinction, with only four purebred stallions remaining.”
The efforts of a few dedicated breeders, including Queen Elizabeth II, brought the Cleveland Bay horse back from the brink of extinction, they said.
“Whilst studbook records dating back to the late 18th century suggest that as many as 17 different female founders contributed to the breed, the present study has identified only eleven haplotypes,” the authors said.
“Within these 11 haplotypes, four are representative of 89% of the modern day, purebred Cleveland Bay population.”
The seven minor haplotypes identified were seen in either single animals or in no more than two other individuals.
“Accepting that the seven minor haplotypes are linked to known mutational hotspots and hence can be considered minor mutations of one of the four principal haplotypes or to relatively recent introductions into the breed via the Grading Register, then it is conceivable that only four female founders contribute to the modern-day population.
“However, it is difficult to reliably deduce the timescale of this founding event or events.
“Indeed, these four females will certainly predate the formation of the studbook and most likely represent four different mares involved in early domestication of the horse,” the authors said.
The average level of inbreeding for the Cleveland Bay is estimated as 22.5%, substantially higher than the majority of domestic horse breeds, which typically range from 6.55% to 12.55%.
Dell and his colleagues said the results of their research provide important information on the origins of the Cleveland Bay horse and represent a valuable tool for conservation purposes.
The study team comprised Dell and Mark Curry, with Lincoln University; and Kelly Yarnell, Gareth Starbuck and Philippe Wilson, with Nottingham Trent University.
Dell A, Curry M, Yarnell K, Starbuck G, Wilson PB. Genetic analysis of the endangered Cleveland Bay horse: A century of breeding characterised by pedigree and microsatellite data. PLoS One Oct 29;15(10):e0240410.