Up to 10 percent of a mammal's genome is made up of DNA captured from retroviruses, which insert their DNA into the host genome and sometimes lose the ability to get back out of the cell. Most of this genetic material seems to be gibberish, but in humans there are signs that one viral gene is still kicking. Genetic studies of post-birth placentas spotted activity from a gene that would once have produced part of the protein envelope coating a circulating retrovirus, now extinct. Researchers identified envelope genes from other viruses active in the placentas of mice, primates and sheep. The viral gene products cause cultured cells to fuse together as they would in the placenta, suggesting they play a considerable reproductive role in these various mammals.
Pregnancies in sheep actually fail without activity from the animal's viral gene, reproductive biologist Thomas Spencer of Texas A&M University and his colleagues report in a paper published online September 11 by the Proceedings of the National Academy of Sciences. On the eighth day of pregnancy, the group injected sheep with RNA molecules designed to stick to and neutralize the retroviral RNA. By day 16 the placentas were underdeveloped, and the embryos died by day 20. Cells that would have formed the outside of the placenta failed to grow and proliferate with their normal vigor, the group found.
The result suggests that retroviral genes are equally important in primate and mouse reproduction, Spencer says. He notes that each of these groups of mammals seems to have acquired its viral gene independently. "They probably enhanced reproduction and the animals became reliant on these genes," he says. Other experts see the sheep as a good proxy for studying the viral gene's role in human reproduction. "The system provides an experimental model for testing, which we can't do in humans," says retrovirologist John Coffin of Tufts University.