Endogenous retrovirus story

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Earlier this week, the Washington Post printed a nice David Brown story about endogenous retroviruses and evolution.

In sheep, researchers are discovering an especially interesting story.
Sheep today sometimes develop lung or nasal tumors caused by circulating retroviruses. Ancestors of those viruses began creeping into the genome even before sheep and goats diverged from each other more than 5 million years ago.
A team led by Massimo Palmarini of the University of Glasgow Veterinary School studied two of those endogenous retroviruses. They found that wild species (such as bighorn and Dall sheep) had versions of the two retroviruses that differed slightly from the versions carried by domesticated species. The retroviral genes in those animals contained a mutation that impeded infection by the cancer-causing viruses.
In a paper published in November, the researchers argued that when people began rounding up wild sheep 9,000 years ago, the newly confined herds probably suffered epidemics of the cancer-causing viruses. Only those animals whose endogenous viruses had by chance mutated into the protective form survived.