In animals, infections are fought by the immune system. Studies on an unusual virus infecting wild koalas, by a team of researchers from the University of Massachusetts Medical School and the University of Queensland, reveal a new form of "genome immunity." The study was published online on October 10, 2019 in Cell. The open-access article is titled “The piRNA Response to Retroviral Invasion of the Koala Genome.” Retroviruses, including pathogens like HIV, incorporate into the chromosomes of host cells as part of their infectious lifecycle. Retroviruses don't usually infect the germ cells that produce sperm and eggs and, therefore, are not usually passed from generation to generation, but this has happened several times during evolution. Of the entire 3 billion nucleotides of the human genome, only 1.5% of the sequence forms the 20,000 genes that code for proteins - and 8% of the human genome comes from fragments of viruses. These pathogen invasions of the genome have sometimes been beneficial. For example, a gene "co-opted" from a virus is required for formation of the placenta in all mammals, including humans. Retroviral infection of germ cells has been a rare, but important, driving force in human evolution. But how the germ cells in mammals respond to pathogen invasion has not been previously described and might be quite different than what happens in other cells of the body. KoRV-A is a retrovirus sweeping through the wild koala population of Australia and it is associated with susceptibility to infection and cancer. KoRV-A spreads between individual animals, like most viruses. Surprisingly, KoRV-A also infects the germline cells, and most wild koalas are born with this pathogen as part of the genetic material of every cell in the body. The team used this system to see how germ cells respond to a retrovirus.Login Or Register To Read Full Story