The world beneath the ice of Antarctica may hold the answer to many mysteries, including one that has been vexing biologists for a long time: the origin of viruses.
Researchers have found a microorganism in a hyper-saline Antarctic lake that contains a piece of DNA that can build itself a hardy capsule. It turns out that this then allows the DNA to escape from its cell and invade other microbes of the same species, before beginning to replicate in the new host. The team thinks this might hint at the origin of at least some viruses, before life looked anything like it does today.
The microbe in question was discovered in the salty lakes of Rauer Islands, off the coast of Antarctica. Known as Halorubrum lacusprofundi, they are a group of single-celled organisms known as archaea, meaning they have no cell nucleus. Within the mess of DNA floating around the cell, however, it does have what are known as plasmids. These small circles of DNA are known to move from one cell to the next, passing on small bits of genetic information.
“Unlike viruses, which encase themselves in a protective protein coat, plasmids usually move around by cell to cell contact, or as a piece of naked DNA,” explains Professor Rick Cavicchioli, co-author of the new paper in Nature Microbiology, in a statement. “But the plasmids that we found in the Antarctic microbes were masquerading as viruses. They produced proteins which went into the host’s membrane, which then allowed the membrane to bud off containing the plasmid DNA.”
Not only were the tiny bits of DNA able to “infect” cells of H. lacusprofundi that did not have any plasmids already within them, but they also found that the circles of genetic material took some of the new host's DNA and integrated it into its own, before once again making a new membrane and then going off to infect more cells.
This may be a crucial step in the evolution of the category-defying viruses.
It is generally accepted that viruses are ancient agents. It is thought that they probably evolved before the three main domains of life – bacteria, eukaryotes, and archaea. They then split, infecting the common ancestor to all three. But where, or more importantly how, viruses first came about is anything but certain.
This latest finding suggests that one such theory, that viruses evolved from plasmids, may actually have some support. Previously, it was thought that this might not be possible, because plasmids are usually far more basic – being made from just naked circular DNA – while viruses have a complex protein case. Many, therefore, thought that there was too big a leap between the two. Now, however, it seems there may be a plausible pathway between plasmids and at least some viruses.
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