THE analysis of DNA inherited from a single parent has provided valuable insights into the history of human and animal populations. However, until recently, there was insufficient information to be able to investigate the paternal lines of the domesticated horse.
Now, Barbara Wallner and colleagues at the University of Veterinary Medicine-Vienna (Vetmeduni-Vienna) in Austria have published information on the genetic variability in the Y chromosome of the horse and show how various breeds of the modern horse are interrelated.
In mammals, sex is determined by the chromosomes an individual inherits from its parents. Two X chromosomes create a female, whereas one X and one Y create a male. Y chromosomes are only passed from fathers to sons, so each Y chromosome represents the male genealogy of the animal in question, an announcement from Vetmeduni-Vienna explained. In contrast, mothers pass on mitochondria to all their offspring. This means that an analysis of the genetic material or DNA of mitochondria can provide information on the female ancestry.
For the modern horse, it is known that mitochondrial DNA is extremely diverse, and this has been interpreted to mean that many ancestral female horses have passed their DNA on to modern horse breeds.
Until recently, though, essentially no sequence diversity had been detected on the Y chromosome of the domesticated horse. Not only does the lack of sequence markers on the Y chromosome make it difficult to trace male lineages with confidence, but it also represents a scientific paradox. How can a species with so many female lines have so few male lines?
Wallner and colleagues at the Vetmeduni-Vienna Institute of Animal Breeding & Genetics initially selected 17 horses from a range of European breeds, pooled their DNA and used modern sequencing technology to examine the level of diversity on a 200-kilobase portion of the Y chromosome Wallner had previously sequenced. The Y chromosomes were found to be highly similar: Only five positions turned out to be variable.
"The results confirmed what we had previously suspected: that the Y chromosomes of modern breeds of horses show far less variability than those of other (domesticated) animals," Wallner said.
The five variable positions, or polymorphisms, nevertheless were sufficient to enable the researchers to derive a type of "family tree" for the various breeds of modern horses that then could be investigated.
The announcement reported that an examination of more than 600 stallions from 58 (largely European) breeds showed that the animals could be grouped into six basic lines, or haplotypes, including:
* The ancestral haplotype that is distributed across almost all breeds and geographical regions;
* A second haplotype that occurs at high frequencies across a broad range of breeds, although not in Northern European breeds or in horses from the Iberian Peninsula;
* A third haplotype that is present in almost all English Thoroughbreds and in many warm-blooded breeds, and
* Three haplotypes that are found only in local Northern European breeds: one in Icelandic horses, one in Norwegian Fjord horses and one in Shetland ponies.
The pedigree of horses is very tightly controlled, with studbooks, in many cases, going as far back as the 18th century. Combining the results of the genetic analysis with pedigree data enabled the scientists to trace the paternal roots of many of the current male lines.
"The results were intriguing, for example, in the way the distribution of one haplotype reflects the widespread movement of stallions from the Middle East to Central and Western Europe in the past 200 years," Wallner noted. "Another haplotype results from a mutation that occurred in the famous English Thoroughbred stallion 'Eclipse' or in his son or grandson. It is amazing to see how much influence this line has had on modern sport horses: Almost all English Thoroughbreds and nearly half the modern sport horse breeds carry the Eclipse haplotype."
The Vetmeduni-Vienna researchers confirmed the low diversity of the horse's Y chromosome, which contrasts sharply with range of mitochondrial DNA haplotypes observed in modern horses, the announcement said. The difference is presumably due to the strong variation in male reproductive success. Wild horses have a polygynous breeding pattern (one stallion with multiple mares), while the intensive breeding practices used in domesticated horses mean that single stallions can effectively pass on their DNA to entire generations.
The senior author on the paper, Gottfried Brem, said, "Most modern breeds were established in the last two centuries, during which time the horse has undergone a transition from working and military use towards leisure and sports. This has largely been achieved through the use, in breeding, of a few selected males. The restricted genetic diversity of the modern horse Y chromosome is a reflection of what has survived the species' dynamic history."
The paper, "Identification of Genetic Variation on the Horse Y Chromosome & the Tracing of Male Founder Lineages in Modern Breeds," by Wallner, Claus Vogl, Priyank Shukla, Joerg P. Burgstaller, Thomas Druml and Brem, was recently published in the online PLOS ONE.