An international team of researchers has used ancient DNA to produce compelling evidence that the lack of genetic diversity in modern stallions is the result of the domestication process.
     The team, led by Professor Michi Hofreiter from the University of York (North Yorkshire, England), has carried out the first study on Y chromosomal DNA sequences from extinct ancient wild horses and found an abundance of diversity. The results suggest the almost complete absence of genetic diversity in modern male horses is not based on properties intrinsic to wild horses, but on the domestication process itself.
     Professor Hofreiter said: "Unlike modern female domestic horses where there is plenty of diversity, genetic diversity in male horses is practically zero."
     The Y chromosome is a valuable tool in population genetics, providing a means of directly assessing evolutionary processes that only affect the paternal lineage. So far mitochondrial DNA studies have failed to discover the origin of domestic horses. However, these new Y chromosomal markers now open the possibility of solving this issue in detail.
     As part of the study, researchers sequenced Y chromosomal DNA from eight ancient wild horses dating back from around 15,000 to more than 47,000 years and a 2,800-year-old domesticated horse. The results were compared to DNA sequences from Przewalski horses - the only surviving wild horse population - and 52 domestic horses, representing 15 modern breeds, which had been sequenced previously.
     Domestication of horses dates back approximately 5,500 years. DNA from the skeletal remains of a 2,800-year-old domesticated stallion from Siberia showed that in contrast to modern horses, Y chromosomal diversity still existed several thousand years after the initial domestication event for horses.
     Researchers had found that Przewalski's horse displays DNA haplotypes not present in modern domestic horses, suggesting they are not ancestral to modern domestic horses. However, while the Y chromosome data supported historic isolation, it also suggests a close evolutionary relationship between the domestic horse and the Przewalski's horse, since the Przewalski Y chromosomal haplotype is more similar to the two domestic ones than any of the ancient wild horse haplotypes.

Edited from Heritage Daily (24 August 2011)