Wild koalas reveal a novel genome immune response

Viral pathogens cause devastating disease but are also major components of the genome, and in mammals, including humans, a “co-opted” viral envelope gene is required for placental development.  Viruses have rewired genome function, but it’s been millions of years since a new virus entered the human genome, and how viral pathogens are converted to stable components of the genetic material is not understood.   KoRV-A is one of the only known examples of a retrovirus that can infect the germ cells that make sperm and eggs and is currently spreading through wild koalas.  Using this novel system, Theurkauf and colleagues show that the virus is active during initial infection, but silenced after a copy is “captured” in a koala gene.  This new hybrid gene produces short “piRNAs” that identify and silence the viral genome invader, and is sweeping through wild koalas, driving evolution of the genetic blueprint for this iconic species.