DNA is in some ways a mystery to scientists and there are still many studies that try to unveil all the secrets. Large portions of our genetic code are made up of “non-coding genes,” also called “junk DNA.” Studies, however, have reconsidered the role of these fragments of genetic code.
They may have many purposes within mammal DNA: some help to form the structure in our DNA molecules, while others are involved in gene regulation. Recently, researchers at the University of New South Wales in Australia have discovered another potential purpose of this “junk DNA.”
Some of these “junk” gene sequences are actually fragments of viruses left buried in DNA by an infection of an ancestor. Whenever a virus infects a subject, there is a possibility that it will leave a piece of itself inside the host’s DNA and, if this happens in an egg or sperm, it will be transmitted across generations. These are called endogenous retroviruses (ERVs, Endogenous Retroviruses).
Experts have searched for these ERVs in genomes of 13 species of marsupials. In all sampled animals, they found ERV Bornaviridae, Filoviridae and Parvoviridae. “The Bornaviridae first entered the animal DNA during the dinosaur period, when the South American and Australian land masses were still united,” said paleovirologist Emma Harding.
“Previously it was thought that the Bornaviridae viruses had evolved 100 million years ago,” the expert continues. “What I found in almost every marsupial DNA we tested puts it at 160 million years.” Surprisingly, some of these ancient viral fragments were still transcribed into non-coding RNA.
The latter is used in a series of cellular functions, including adjusting RNA transcription between other genes. In koalas (which have a really bizarre way to drink), ERVs were actually transcribed into small RNA molecules known to be antivirals in invertebrates. “This could be a similar mechanism to vaccination, but it is inherited through generations. By keeping a viral fossil, the cell is immunized against future infections,” Harding finally says. “If we can show that it occurs in marsupials, it could also occur in other animals, including humans.”