Huge strips of our DNA library are made up of genes that do not encode and have long been considered “junk DNA.” However, recent findings have shown that these pieces of DNA have many purposes in mammals.
Some help shape the structure of our DNA molecules so that they can pack perfectly within cell nuclei, while others are involved in gene regulation. Now, researchers at the University of New South Wales, Australia, have discovered another potential purpose for these non-coding instructions, within the genome of marsupials.
Some of the gene sequences that were once considered “junk” are actually fragments of viruses left buried in our DNA from an infection in a forgotten ancestor.
Whenever a virus infects you, there is a chance that it will leave behind a piece of itself within your DNA and, if this happens in an egg or sperm, it will be passed down through the generations. They are known as endogenous viral elements (EVE).
In humans, viral DNA fragments make up about 8 percent of our genome. They can provide a record of viral infections throughout our evolutionary history, such as genetic memory.
“These viral fragments have been preserved for a reason,” said paleovirologist Emma Harding. “Over millions of years of evolution, we would expect all DNA to change, but these fossils are preserved and kept intact.”
To try to find out why, Harding and colleagues looked for EVEs in the genomes of 13 species of marsupials, including the wallaby tammar (Macropus eugeni), Devil of Tasmania (Sarcophilus harrisii), and fat-tailed dunnarts (Sminthopsis crassicaudata).
They found EVEs from three viral groups: Bornaviridae, Filoviridae, i Parvoviridae – in all sampled animals.
“One of the EVEs I found was from Bornaviridae a family of viruses, which first entered the DNA of animals during the time of the dinosaurs when the South American and Australian land masses were still united, ”Harding said. Bornaviridae it is present in the opossums of America and the marsupials of Australia.
He Bornaviridae EVEs were particularly common and were more related to similar viral fossils found in birds and reptiles rather than those seen in placental mammals like us.
“Bornaviridae “Viruses were thought to have evolved previously 100 million years ago,” Harding explained. “But what I found in almost every marsupial DNA we examined is 160 million years old.”
Surprisingly, some of these ancient viral fragments were still being transcribed into RNA. Often in cells, RNA transcripts act as protein templates. But in this case they were not being translated, effectively turning them into non-coding RNAs.
That doesn’t make them useless. Non-coding RNA is used in a variety of cellular functions, including the regulation of RNA transcription among other genes.
A tamarind wallaby, one of the species under study. (Hossein Anv / Unsplash)
Significantly, this type of RNA is also known to be used for many cellular functions, including regulating RNA creation, and is also known to contribute to immune defense against viruses in plants and invertebrates. Bats also have a large cache of these fossil viral fragments and are well known for their unfortunate ability to survive while carrying deadly viruses that most other mammals do.
Observing the koalas in more detail, the researchers found that some of the EVEs were being transcribed into small RNA molecules known to be antiviral in invertebrates.
“This suggests the tempting possibility of this RNA defense system, which was previously thought to have been abandoned in mammals in favor of the interferon system, although it was active and protected marsupial cells,” they said. write Harding and colleagues a Microbiology Australia.
As marsupials spend most of their development time inside their mother’s pouch, some are born before they have even developed bones let alone a fully functioning immune system. Therefore, this type of antiviral defense could be critical for the young bag, the team suspects.
“This could be a mechanism similar to vaccination, but it is inherited for generations. By maintaining a viral fossil, the cell is immunized against future infection,” Harding said.
“If we can show that it occurs in marsupials, it can also occur in other animals, including humans.”
This researcher was published in Evolution of the virus.