Retroviruses have been infecting vertebrates for over 450 million years. Because retroviral DNA integrates into host cell DNA, the vertebrate genome is littered with remnants of these infections. Some of this retroviral DNA has been co-opted by the cell for a variety of beneficial purposes, such as development of the placenta and the control of birth timing.
On episode #193 of the science show This Week in Virology, recordedÂ at the 31st Annual Meeting of the American Society for Virology in Madison, Vincent, Rich, Carolyn, and SaraÂ discussed genetic conflict between viral and human genes, and how the placenta protects the fetus against viral infection.
You can find TWiV #193 at www.microbe.tv/twiv.