Date Time Retroviruses are re-writing koala genome and causing cancer Koalas are facing multiple environmental and health issues which threaten their survival. Along with habitat loss – accelerated by last year’s devastating bush fires – domestic dog attacks and road accidents, they suffer from deadly chlamydial infections and extremely high frequency of cancer. Wild Koala | Photo: A. Gillett An international team of scientists led by the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) now demonstrate that a retrovirus invading the koala germline explains the high frequency of koala cancer. The results are reported in the journal Nature Communications. The koala retrovirus (KoRV) is a virus which, like other retroviruses such as HIV, inserts itself into the DNA of an infected cell. At some point in the past 50,000 years, KoRV has infected the egg or sperm cells of koalas, leading to offspring that carry the retrovirus in every cell in their body. The entire koala population of Queensland and New South Wales in Australia now carry copies of KoRV in their genome. All animals, including humans, have gone through similar “germ line” infections by retroviruses at some point in their evolutionary history and contain many ancient retroviruses in their genomes. These retroviruses have, over millions of years, mutated into degraded, inactive forms that are no longer harmful to the host. Since in most animal species this process occurred millions of years ago, the immediate health effects on the host at that time are unknown but it has been suspected for some time that the invasion of a genome by a retrovirus may have considerable detrimental health effects. The koala is at a very early stage of this process when the retrovirus is still active and these health effects can be studied.