Marsupials have made friends of ancient enemies, incorporating viruses that once threatened their existence into their non-coding or “junk” DNA. Researchers have found several lines of evidence that suggest these stretches of DNA protect against viruses similar to the originals – probably very effectively. Although the evidence is less clear for humans and other placental mammals, the team responsible for the discovery believe it strengthens the case our ancestors did something similar.
Stretches of DNA from viruses (technically known as endogenous viral elements or EVEs) have been found in human junk DNA, as well as that of other eutherians such as rodents. In plants and invertebrates, EVES act as shields against related viruses. However, study author and University of New South Wales PhD student Emma Harding told IFLScience the possibility they do the same in eutherians has proven difficult to confirm.
Marsupials have less advanced immune systems than eutherians, and Harding thought this might make an anti-viral role for any former viruses easier to spot. In the journal Virus Evolution Harding and co-authors report the idea proved fruitful, with signs three very old viruses have done just this for a wide variety of marsupials.
“We tested representatives of 130 types of viruses, looking for their counterparts in marsupial junk DNA,” Harding told IFLScience.
The team hit on widespread matches for Filoviridae, which includes Ebola and the less prominent families Bornaviridae and Parvoviridae. “These viral fragments have been retained for a reason. Over millions of years of evolution, we would expect all DNA to change, however these fossils are preserved and kept intact,” Harding said in a statement. “The animal DNA has basically grabbed a viral sequence – which used to harm it – and ends up using it for its own benefit.” Rarer examples of EVEs from other families were also found.
The most obvious application is for these viral fossils to produce RNA that binds to the original virus, or its near relatives, destroying it and preventing new infections. “This could be a mechanism similar to vaccination, but is inherited through generations. By keeping a viral fossil, the cell is immunised against future infection,” Harding added.
Harding told IFLScience no members of these viral families are known to circulate among marsupials, while many eutherians remain affected. “It may have worked perfectly,” she said. However, this would only be for descendants of those animals that succeeded in incorporating the viruses into their genome. For other marsupials of the day, Harding suspects, the viruses in question were probably deadly.
The more sophisticated immune systems of humans and our nearer relatives mean this incorporation technique is likely to be rarer and less powerful, Harding suspects, but it may still have a role to play in our protection. However, Harding does not think COVID-19 will ever become part of our DNA, protecting us from other coronaviruses. “Coronaviruses have a large genome and mutate quite fast,” she told IFLScience, so even if one integrates it is unlikely to be passed on.
The EVEs the study identified are found among all 13 marsupial species Harding studied. The Bornaviridae' locations within the genomes are so consistent they must have been incorporated in a common ancestor that lived before Australia and South America divided, probably around 160 million years ago.
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