Horse Genome Project--Horses

Horse Genome Project

THE HORSES IN THE HORSE GENOME PROJECT

The genome sequence for the horse comes from a small blood sample taken from a single Thoroughbred mare, Twilight. Twilight was chosen because she had the least genetic diversity amongst a group of 10 horses tested for genetic markers. This was not a surprise because the millions of modern Thoroughbred horses are descended from relatively few ancestors plus they have been highly selected for racing performance for the past 300 years. For the purposes of whole genome sequencing, limited genetic variation was necessary to simplify the process of defining the genome.

Now that we have the whole genome sequence, diversity is of great interest. What is the difference between Twilight and other Thoroughbred horses? And yet more interesting, what are the differences between Twilight and horses from other breeds? Other horses were chosen for the study to represent Arabian, Quarter Horse, American Standardbred (pacer), Tennessee Walking horse, Akal-Teke, Hokkaido horse, Mongolian horse, Selle Francais, French Trotter, Andalusian, American Saddlebred, Suffolk Punch, Shire, Norwegian Fjord, Belgian, Frisian, Trakehner, Exmoor Pony and Paso Fino. Why so many breeds? We wanted to sample the genetic diversity found among horses since domestication, 6,000 years ago. These horses come from all parts of the world, separated from each other by centuries of selection. The different breeds have been selected for different hereditary traits including behavior, size, strength and speed. What do the differences between breeds tell us about our horses? Finally, are the similarities between breeds sufficient that we can use the information from one breed to benefit another? These are some of the questions we’ll be exploring next.

	Twilight is a Thoroughbred mare bred and raised at the McConnville Barn at Cornell University. She provided the DNA for the whole genome sequence of the horse that was completed and assembled at the Broad Institute in Cambridge, MA. The Thoroughbred originated in England during the late 1600s as a race horse. One of the oldest stud books was established to maintain records and aid breeders in selecting for sound, fast race horses. Thoroughbred racing was popular and was adopted worldwide over the next 3 centuries. All modern Thoroughbred horses trace 81% of their genes to 31 ancestors.
	Bravo is a male Thoroughbred, closely related to Twilight. He lives at the McConnville Barn at Cornell University with Twilight. DNA from his blood cells was cut into large fragments and cloned into a tool called a bacterial artificial chromosome (BAC). The collect of clones is called a “BAC Library”. Scientists can use BAC libraries to find and study DNA sequences. Scientists use the information from Bravo's DNA together with Twilight's whole genome sequence to better understand the interplay of genetics, health and performance in horses.
	Hrafnhetta is an Icelandic horse mare currently living in Prospect, Kentucky. She was chosen for random sequencing for comparison to the DNA sequence of Twlight. Icelandic horses come from Iceland and are noteworthy for their fast, comfortable ambling gait called a tǿlt. The population was established from horses brought by the Norwegians in the 9th century and maintained as an isolated breed.
	Desert Heir (better known as Bisquette) is a male American Saddlebred horse. He was very important to the horse genome project. His blood cells grow very well in cell culture and it was easy for cytogeneticists to study his chromosomes. When it was time to determine the order of genes on a chromosome, the results from computer assembly of Twilight’s DNA sequences were double checked by mapping the BACs for some genes to Bisquette’s chromosomes. Agreement between the two approaches proved that the horse DNA sequence was correctly assembled. American Saddlebreds were developed in the southern United States during the late 1800s. They were prized for their smooth gaits, good disposition and intelligence. Many of the famous television horses of the 1960s were Saddlebreds, including Mr. Ed and Fury!
	Ssirena is a Lusitano mare currently living in Harrodsburg, Kentucky. She was chosen for DNA sequencing and comparison to Twilight. Lusitano horses originated on the Iberian peninsula, in Portugal, along with a sister breed, the Andalusian horse of Spain. This breed may be descended from remnant horse populations pushed onto the Iberian peninsula from northern Europe during the last ice age and later influenced by Barb and Arabian horses introduced to the area during the 8th century.