Viruses are quietly changing us

 　　Viruses are an old adversary of mankind. With this new coronavirus rampage around the world, viruses are once again on our nerves.

　　But when I say "viruses are irritating our nerves," they are in fact already in our nerves. Well, they are hiding in the DNA of every cell in our body - including nerve cells. These viruses are called "endogenous viruses". Endogenous viruses, which are technically just a segment of the virus' DNA, are contrasted with exogenous viruses, which are what we usually call viruses. Exogenous viruses are generally composed of two parts: the genetic material of the virus (DNA or RNA), and the protein shell that encases it.

　　Endogenous viruses are left over from exogenous viruses after they infected our ancient ancestors (probably since the time when mammals first evolved on Earth). Viruses embed their DNA into the DNA of the host animal, and if the host animal is lucky enough to survive and reproduce, they reproduce on the host animal's coattails, passing it on from generation to generation, and the host animal can't get rid of it even if it wants to. It's not just us humans who are so unlucky, it's almost all animals.

　　However, it is rare for an animal's DNA to be genetically embedded in a virus, and it may only happen once every few hundred thousand years. Why is it rare? On the one hand, because animals have a particularly strong set of "fortifications", including cell membranes and nuclear membranes; on the other hand, because even in the very rare cases where the cell membranes and nuclear membranes of animals are occasionally embedded by viruses, they will die together, and the embedded viruses will not survive.

　　But such a rare thing happened right now - only, not in humans, but in mice.

　　A group in Japan has been studying the encephalomyocarditis virus (EMCV) for a long time. This virus is often prevalent in rodents such as mice, but can also infect humans and other mammals. Recently, they observed in the laboratory that EMCV, after infecting male mice, embeds its viral genes into the genome of mouse testicular cells. After the infected mice were mated, the researchers found that their offspring had slightly altered ears - apparently caused by the embedded viral gene. This suggests that the embedded EMCV virus gene has been passed on to the next generation of mice and has influenced their trait characteristics.

　　It was unexpected that viral genes embedded in animal genomes and passed on to offspring would happen so easily, but the surprise did not stop there.

　　As virologists understand it, the EMCV virus should not be so easily embedded in the animal genome. Because the animal genome is composed of DNA, to be embedded in it, it has to be a DNA-containing virus, and EMCV viruses are RNA viruses, which carry genetic material that is RNA.

　　Of course, there is a class of RNA viruses called retroviruses, which can also embed their genetic material into the animal genome. The mechanism is that they transcribe their RNA into DNA first and then embed the DNA into the animal genome. HIV belongs to this group of retroviruses. But again, EMCV viruses are not retroviruses. Therefore, it is not the turn of the EMCV virus to embed its own genes into the animal genome, but this unbelievable thing has happened.

　　Although the reason for this remains to be further investigated, this finding tells us at least two things: First, it is much easier for viruses to embed their genes into animal genomes than we thought, and there may be more genes in our genome that come from viruses. The second point is that a future viral pandemic could permanently change the form or function of many animals. Some of the animals around us, and even ourselves, may look very different in a few tens of millions of years than they do today.