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.
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."