8% of Human DNA Comes from a 40 Million Year-Old Virus

A team of scientists led by Keizo Tomonaga of Osaka University determined that a virus dating from 40 million years ago is embedded in human DNA. This infection, known as the bornavirus, might be the cause of schizophrenia and passes from generation to generation inside human cellular nuclei, filling out 8% of human genetic code:

The assimilation of viral sequences into the host genome is a process referred to as endogenization. This occurs when viral DNA integrates into a chromosome of reproductive cells and is subsequently passed from parent to offspring. Until now, retroviruses were the only viruses known to generate such endogenous copies in vertebrates. But Feschotte said that scientists have found that non-retroviral viruses called bornaviruses have been endogenized repeatedly in mammals throughout evolution.

Bornavirus (BDV) owes its name to the town of Borna, Germany, where a virus epidemic in 1885 wiped out a regiment of cavalry horses. BDV infects a range of birds and mammals, including humans. It is unique because it infects only neurons, establishing a persistent infection in its host's brain, and its entire life cycle takes place in the nucleus of the infected cells.

Link via io9 | Image: US Department of Energy