A new study has revealed how an ancient virus in koala DNA is giving the animal additional firepower to defend against modern viral attacks.
Retroviruses are viruses which enter the body and fuse with its cells to hijack the genetic code of the host, using the way that DNA replicates itself to make more copies of the virus.
If these retroviral genes make their way into sperm and egg cells then they can be passed on to the hosts' offspring and become a permanent part of the genome, known as the germline.
The human genome has a lot of these so-called "endogenous retroviruses" – degraded and harmless pieces of retrovirus DNA that we have inherited from our ancestors.
"The koala is one of the few species known to have an ongoing invasion of the germline by a retrovirus," said Professor Alfred Roca of the University of Illinois, one of the authors of the study.
"Once the virus becomes part of the host germline, the [evolutionary] pressure is going to be for it to leave the host alone because the host is needed to reproduce the sequence.
"It has to play nice or otherwise it disappears. The question is: How does this process take place?"
With a new koala retrovirus – just 50,000 years old – it turns out that its worst affects are being managed by an even more ancient part of the germline.
The older retrovirus neuters the newer one, so that instead of making the DNA code generate new proteins which reassemble into another infectious retrovirus, it instead forces those proteins into a useless jumble.
"We might have found evidence for a molecular defence mechanism of hosts against new retroviral attacks, mediated by more ancient retroviral elements," said Dr Ulrike Lober, a researcher at the Leibniz Institute for Zoo and Wildlife Research in Berlin and the study's first author.
Dr Alex Greenwood, also of the Leibniz Institute, led the study. He said: "(It) emphasises how little we know about the diversity and reservoirs of retroviruses among wildlife.
"The koala, a species not usually associated with biomedical breakthroughs, is providing key insights into a process that has shaped 8% of the human genome, and likely shows us what happened millions of years ago when retroviruses invaded the human germline."
Despite the germline defences, the complete degradation of the koala retrovirus would take hundreds of thousands of years – exposing the maruspials to retrovirus-linked diseases.
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According to the researchers, examining how the koala retrovirus is inserted into the animal's genome will help them develop a koala retrovirus vaccine.
The study was published in Proceedings of the National Academy of Sciences.