[ad_1]
Huge swathes of our DNA library are made up of non-coding genes that have long been considered “unwanted DNA”. Recent findings, however, have shown that these pieces of DNA actually serve many purposes in mammals.
Some help shape the structure of our DNA molecules so that they can be neatly packaged in our cell nuclei while others are involved in the regulation of genes. Now, researchers at the University of New South Wales in Australia have discovered another potential target for these non-coding instructions, in the genomes of marsupials.
Some of the gene sequences once thought of as “junk” are actually virus fragments left buried in our DNA as a result of infection in a long forgotten ancestor.
Whenever a virus infects you, there is a chance that it will leave a piece of itself in your DNA, and if this happens in an egg or sperm, it will be passed from generation to generation. These are known as endogenous viral elements (EVE).
In humans, fragments of viral DNA make up about 8% of our genome. They can provide a record of viral infections throughout our evolutionary history, such as genetic memory.
“These viral fragments were kept for a reason,” paleovirologist Emma Harding said. “Over millions of years of evolution we would expect all DNA to change, however, these fossils are preserved and kept intact.”
To try to understand why, Harding and his colleagues searched for EVEs in the genomes of 13 species of marsupials, including the wallaby tammar (Macropus eugeni), Tasmanian devil (Sarcophilus harrisii) and fat-tailed dunnarts (Sminthopsis crassicaudata).
They found EVEs from three viral groups – Bornaviridae, Filoviridae, and Parvoviridae – in all animals sampled.
“One of the EVEs that I found was from Bornaviridae family of viruses, which first entered the DNA of animals during the dinosaur era, when the South American and Australian mainland masses were still united, ”said Harding. Bornaviridae is present in opossums of America as well as marsupials of Australia.
The Bornaviridae EVEs were particularly prevalent and more closely related to similar viral fossils found in birds and reptiles rather than those seen in placental mammals like us.
“Bornaviridae Viruses were previously thought to have evolved 100 million years ago, ”said Harding. “But the one I’ve found in almost every marsupial DNA we’ve looked at places it 160 million years old. “
Surprisingly, some of these old viral fragments were still being transcribed into RNA. Often in cells, RNA transcripts act as protein templates. But in this case, they weren’t translated, making them non-coding RNA.
This does not make them unnecessary. Non-coding RNA is used in a number of cellular functions, including the regulation of RNA transcription among other genes.
A tammar wallaby, one of the species studied. (Hossein Anv / Unsplash)
Significantly, this type of RNA is also known to be used for many cellular functions, including the regulation of RNA creation, and it is also known to aid immune defense against viruses in plants and invertebrates. Bats also have a particularly large cache of these fossil viral fragments, and they are well known for their unfortunate ability to survive while carrying deadly viruses that most other mammals do.
Looking at the koalas in more detail, the researchers found that some of the EVEs were indeed transcribed into small RNA molecules known to be antiviral in invertebrates.
“This suggests the tantalizing possibility of this RNA defense system, previously thought to have been abandoned in mammals in favor of the interferon system, still being active and protecting marsupial cells,” wrote Harding and colleagues in Microbiology Australia.
Since marsupials spend most of their developmental time in their mother’s pouch, some are born before they have even developed any bones, let alone a fully functioning immune system. So this type of antiviral defense could be essential for pocket youth, the team suspects.
“It could be a mechanism similar to vaccination, but inherited from generation to generation. By retaining a viral fossil, the cell is immune to future infection,” said Harding.
“If we can show that this happens in marsupials, it can also happen in other animals, including humans.”
This researcher was published in Evolution of viruses.
[ad_2]
Source link