Thursday, 31 December 2009 14:03
The genomes of many mammals have now been completed, including the cow, the dog, the chimpanzee and, of course, the human. Recently it was the turn of the horse to have its DNA sequence decoded. With it emerged further evidence of how horses have been close human companions and, like other mammals that share a evolutionary history with man, how they could help the understanding of hereditary diseases. But there was also a surprise: horses have a newly forming part in their genetic make-up which shows the evolutionary process in action in a way that has not been seen before.
The surprise was found on equine chromosome 11, in the form of a developing centromere. This is the nexus of a chromosome, from which its arms dangle. The appearance of a new centromere lets geneticists examine the process by which new2 chromosomes come into existence. The new horse centromere seems common to all breeds and has not yet acquired any repetitive DNA. This discovery solves one mystery: centromeres appear before repetitive DNA, rather than being caused by it.
The equine genome also indicates how extensive crossbreeding was in horse evolution. Domestication probably began 4000-6000 years ago, with wild horses initially being hunted for food, and then herded for the same purpose, before being harnessed to provide power and transport. This was unlike the domestication of dogs, where a small number of wolves probably domesticated themselves by cozying up to groups of people and acting as four-legged garbage disposal units, before being selectively in-bred into the varieties seen today. In horses, a large number of mares but few stallions appear to have been involved in the development of the genome and with the exception of thoroughbreds, there was not a lot of genetic sharing between breeds.
That pattern is partly a consequence of the fact that herds of horses usually contain many mares guarded by a single stallion. There also was the effect of Genghis Khan and his descendants, whose armies travelled with their ponies across much of Eurasia. With the use of the horse as a means of transport genetics were spread widely.
Researchers also found that 53% of horse genes appear on their chromosomes in the same order as they do in humans, as opposed to dogs where the figure is 29%. Since, even with this relatively low level of synteny, as the relationship is called, dogs have proved useful in the study of some human diseases, the researchers hope that the high-synteny horse could be similarly illuminating. The dog may be man's best friend, but his horse may thus prove to be more genetically helpful.
The Economist, November 2009
