The genome was sequenced from a
Thoroughbred mare named Twilight
(pictured), and is already helping scientists
to improve horse health.
Image: University of Sydney

An international project has provided the first ever complete genetic sequence of the horse, providing resources for identifying markers for horse related genetic diseases and understanding horse biology.

This week the prestigious journal Science reports on the first high-quality draft sequence of the genome of the horse in Genome sequence, comparative analysis and population genetics of the domestic horse, by Australian lead author Professor Claire Wade from the University of Sydney’s Faculty Veterinary Sciences.

“Better tests for genetic diseases such as disorders of the muscle, therapies for respiratory disease and allergic diseases of the horse are already being made possible through our increased understanding of equine biology,” says Professor Wade.

“Horses have their own special population history with important implications for how we map genetic traits in horses,” says Professor Wade. “Horses do not appear to have undergone a tight domestication bottleneck and the presence of many female ancestors in domestic horse history are evidence of this. It looks as though we basically domesticated all of the ancestral horses.” One unexpected finding was that the wild mongolian horse (Przewalski's horse) which was once thought to be the ancestor of modern horses looks very much just like another horse breed even though it has a different number of chromosomes.

Importantly, the genome sheds light on the poorly understood biology of centromere formation. Horses have a newly formed centromere that has not had time to accumulate the normal characteristics of mammalian centromeres. "This affords us an opportunity to examine how centromeres function" says Professor Wade.

To generate a high-quality draft genome sequence, the researchers from The Broad Institute of Harvard and MIT analyzed DNA from an adult Thoroughbred mare named Twilight. Twilight's DNA was decoded using conventional capillary DNA sequencing technology revealing a genome that is slightly larger than the genome of the domestic dog, and smaller than the human genome. The project was an international collaboration involving scientists from more than ten countries.

With the complete sequence a platform now exists to conduct new research and speed up current horse related projects.

Professor Dave Adelson, Chair of Bioinformatics from the University of Adelaide, also collaborated in this research project, describing the mobile genetic elements called retrotransposons that make up the bulk of the horse and other mammalian genome sequences. 

“Horses have many more active mobile genetic elements than humans or cows,” says Professor Adelson. “This indicates that the horse DNA sequence may be changing and evolving at a more rapid rate.”  

Projects are already underway to improve the health and performance of racehorses.

A University of Sydney project is currently looking at ways to improve the diagnosis of osteochondrosis, an inherited bone growth disorder which leads to a weakening of leg joints that develops in around 10 per cent of thoroughbred horses.

This new research which is being undertaken by Kao Castle from Sydney University’s Faculty of Veterinary Science will be of particular benefit to the racing industry.

Another project involving Dr Natasha Ellis, a Research Fellow with Reprogen and the Centre for Advanced Technologies in Animal Genetics and Reproduction at The University of Sydney, is using the information to look at performance related genes in horses.