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Some mice have sequences in their genome from a virus that infected their fathers. We know that events like this must have happened many times in the ancient past, but this is the first time it has been observed in action. And it involved a virus that we thought couldn’t do this.

The finding means that even more of the DNA of animals derives from viruses than we thought, says Eiichi Hondo at Nagoya University in Japan. It also suggests that viral pandemics can alter the characteristics of animals by changing their genes, he says. “We believe that future pandemics of viral diseases could alter mammalian morphology or functions very quickly in their descendants,” say Hondo.

Infected males

The researchers studied the (EMCV), which circulates in rodents but can infect a wide range of animals, including humans. They first showed that EMCV can integrate into the genome of testes cells of mice growing in a dish.

Next, Hondo and his colleagues infected male mice with EMCV, then allowed those that survived to mate. They found signs of viral genetic sequences in the earlobes of the offspring, but not in those of the fathers. They are now sequencing the whole genome of the offspring to find out which viral sequences became integrated and where.

Viral genes passing down to offspring in the genome in this way was thought to happen only every few hundred thousand years. But this isn’t the only reason the team’s findings are surprising.

The EMCV virus shouldn’t readily integrate into the genome at all, says John Coffin at Tufts University School of Medicine in Massachusetts. This is because animal genomes consist of DNA, so only sequences that are also made of DNA can be added to it. However, the genomes of many viruses are made of RNA.

One group, called retroviruses, gets around this by making enzymes that turn their RNA into DNA so the virus can add its genes to the genome of a cell it infects, in order to hide for years. This is why HIV, which is a retrovirus, is so hard to eliminate from the body.

It was thought that non-retroviral RNA viruses, including EMCV, couldn’t get into animals’ genomes. Then in 2010, Coffin and other researchers showed that genes from a non-retroviral RNA virus called a bornavirus were present in some mammals, such as people, rodents and elephants.

But bornavirus replicates in the nucleus of cells, says Coffin, where there is a chance of its RNA being turned into DNA and splicing into a host genome. EMCV, by contrast, replicates outside the nucleus,
so shouldn’t be integrated.

Hondo says bits of DNA in mammals often resemble viral sequences. This is often dismissed as coincidence if those sequences are in viruses we didn’t think could invade genomes. His team plans to see what other non-retroviral RNA viruses – including Ebola and Zika – have genetic sequences that have been or could be passed to animals in this way

Reference: bioRxiv,