Retroviral invasion of the koala genome

A dangerous inheritance Over the millennia many retroviruses — viruses that, like HIV and Rous sarcoma virus, contain RNA and a reverse transcriptase to make DNA from it — have been incorporated into mammalian genomes. Most have become inactive but some, like the koala retrovirus (KoRV), remain relatively intact and are thought to have entered the genomes more recently. A survey of KoRV prevalence in wild koalas confirms that KoRV is an inserted — and still transmissible — endogenous retrovirus. Surprisingly, it is missing completely in some individuals and in all populations on the isolated Kangaroo Island, off the South Australia coast. So KoRV seems to be in the midst of a transition between an exogenous and endogenous element. This provides a glimpse of a notable evolutionary event. But there are important conservation issues too, since KoRV is known to cause tumours in this unique and vulnerable species.