Research published 10th October into a deadly infection sweeping through wild koalas has revealed that the retrovirus causing it is being fought at the genetic level, giving scientists a window into a rare evolutionary process.
The KoRV-A retrovirus first began infecting the marsupial between hundreds to a few thousand years ago, starting from the north of Australia and spreading south, and may have originated in bats. It is linked to Koala Immune Deficiency Syndrome (KIDS), which is similar but less potent to AIDS in humans, and makes the animals susceptible to fatal cancers and secondary infections like chlamydia that renders them infertile. It's feared that if left unchecked, these various misfortunes could eventually lead to the iconic animal's demise.

Retroviruses work by inserting their genome into a host genome, but unlike HIV, KoRV-A also enters the animal's germ cells that produce sperm and eggs, meaning it gets passed down through generations. Such pathogen infections of germ cells are very rare but recent research suggests they are an important driver in evolution: the last time it took place in our human ancestors was three million years ago, and a full eight percent of our genome comes from ancient viruses.

Sometimes the viruses are co-opted for useful purposes, with one responsible for the evolution of the placenta in our mammalian ancestors around 100 million years ago. In a new paper published in the journal Cell on 10th October, a team of scientists at the University of Massachusetts Medical School and the University of Queensland revealed a type of "genome immunity" in newly infected koalas.

Their findings suggest that germ cells recognize an essential step in the viral life cycle and turn it against the invader to suppress genome infection, by being able to tell the difference between its own cells and the virus.