Vetted viruses
Because these pieces have been present in vertebrate genomes for some 40 million years, "there might be some selective advantage to having them," Skalka says.For bornaviruses and filoviruses in particular, she notes, "there must be something special about these viruses," to have kept them around for so long.
Skalka speculates that these two virus groups might also be evolving more slowly. One reason for that might be because they have found a happy equilibrium with a reservoir species—such as bats—that has fostered the relative stasis.
Such a theory is "very tantalizing," Coffin says. "I think it makes perfectly good sense [but] it obviously requires some experimental verification."
Other possibilities for their more frequent appearance in vertebrate genomes are that they might have a special relationship with germ line cells or have RNA that is more recognized by LINE elements—and thus more prone to get copied and spliced into the host's genetic code.
Genetic immunity
Although strains of hemorrhagic fever can be fatal for many humans and animals, these viral genetics might also be conferring some protection on their hosts. Skalka explains that RNA from these integrated viral sequences could bind with RNA of the incoming virus and destroy it or that proteins from these code segments could be similar, albeit different enough to intruding viruses to "muck up the whole replication cycle," she says.
Some of the next steps will be to try to find more of these viral fossils in animal genomes. And as more genomes are sequenced and analysis tools become even more efficient, Coffin expects that "things that are older—and thus more diverged—will become easier to find."
But the real trick will be trying to figure out just what these genetic relics are doing in the genomes. "In the case of retroviruses" in the genome, "they have conferred benefits that have nothing to do with viruses," Coffin says, noting one retroviral gene that has been found to help with placental growth. "They are just genes that the host has found useful for one function or another."
Skalka and her team hope to uncover if the same is true for these non-retroviral genes, she says. "We would like to know what the significance is in human beings."
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8 Comments
Add CommentIt sounds like the viruses are allowing us to live on this planet and making sure we can change when the cycles change. A person can't help wondering... ...after all these years, how much of us is actually us? If, over time, we have become a virus, then viruses are pretty darn good looking and nice to snuggle up to during cold weather. Maybe that fever virus allows us to keep warm in cold weather...nice guy to have on our side.
Reply | Report Abuse | Link to thisI think it's cool and shows how all life on Earth is rather intimately related.
Reply | Report Abuse | Link to thisWe consider ourselves independent from our environment, however it seems that we are not individual units so much as nodes of life mass swimming through a veritable pea soup, churning with our surroundings.
Reply | Report Abuse | Link to thisIs it possible that some sequences in DNA are used, not for coding proteins, nor regulating the expression of other genes, but as a library of known pathogens for priming the immune system. There would be selective pressure to keep these sequences the same as the pathogen, even tracking the evolution of the pathogen - so even if the sequence got into the genome a long time ago, it would have been updated to match the changes in the pathogen.
Reply | Report Abuse | Link to thisI suspect that, once inserted into our predecessors' genome, DNA need not infer any survival benefit as long as it not be a short term detriment to an individual's survival.
Reply | Report Abuse | Link to thisPerhaps our DNA not only acts as a reservoir for viral DNA but under some conditions our own DNA spawn our own pathogens.
How are you sure that viral DNA is inserting itself into the host genome and not that the host genome is splitting off into viral DNA particles? Perhaps we are making our own viruses?
Reply | Report Abuse | Link to thisrbinanth - Good question: I independently had a similar thought preceding your comment...
Reply | Report Abuse | Link to thisI'm not well read on the subject, but if I recall normal viral processes insert their DNA into the host's DNA, utilizing its additional function to replicate copies of the virus. But I think the question is still valid: under some conditions could the viral DNA snippet be reactivated, generating new viruses?
The article repeatedly states that there must be some benefit imparted to a host for it to retain DNA for millions of years. As I understand, most of human DNA, for example, is nonfunctional, inferring no characteristic, beneficial or not, to the host. It seems that DNA, once inserted into a genome, must merely not be detrimental to its reproduction in order to be retained. In this way many viruses' RNA could be inserted into a host's genome, accumulating over millions of years unless some process specifically removed it.
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