The study of a virus infecting koalas is helping researchers better understand the accumulation of junk DNA in the human genome.

"Retroviruses insert their genome into their host's chromosome, from where they make more copies of themselves," Paul Young, a professor of virology at the University of Queensland, said in a news release. "Some can also infect what are known as germline cells, which alters the host genetic code and that of all their descendants."

Retroviruses began inserting themselves into the human genome more than five million years ago. Studying the earliest interactions between viral and human DNA is near impossible, but koalas and their genome have offered researchers a window into the history of human junk DNA.

"About a decade ago, we discovered that the wild koala population was being invaded by a retrovirus," Young said. "This isn't great news for the koala, but it has provided us with an opportunity to study what's happening to these retroviral genomes early in their association with a new host."

When retroviruses first invade, their disruptive effects are significant. Over time, however, the invader's disease-causing effects typically dissipate. The replicated viral DNA adopts new functions or becomes junk DNA, coding without a purpose.

"Until now, scientists could only guess at why and how this happened," Young said.

As they continue to monitor the retrovirus invasion among koala populations, researchers hope to gain new insights into the transformation from viral invader to inert coding.

"This means that the koala, a species not usually associated with biomedical breakthroughs, is providing key insights into a process that has shaped eight per cent of the human genome, and will likely show us what happened millions of years ago when retroviruses first invaded the human genome," said Alex Greenwood, scientist at the Leibniz Institute for Zoo and Wildlife Research in Germany.

Though junk DNA doesn't code for specific genes, several studies have shown non-coding DNA can play in important role in gene regulation and evolution.

Researchers detailed their initial investigation into the koala virus this week in the journal PNAS.

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