Evolution: posible origin of viruses (Introduction)

by David Turell , Monday, August 21, 2017, 17:34 (2652 days ago) @ David Turell

A strange form of plasmid life in Antarctica suggests the origin of viruses when life appeared:

https://www.newscientist.com/article/2144518-antarctic-mystery-microbe-could-tell-us-wh...

"Biologists have puzzled for decades about where viruses come from. Are they an older, simpler form of life – or are they parasites that arose only once cells had evolved?

"Ricardo Cavicchioli of the University of New South Wales in Australia and his colleagues have found a microorganism in the lakes of the Rauer Islands off the coast of Antarctica that might shed some light on the question. The organism, which they named Halorubrum lacusprofundi R1S1, is an archaean: a kind of single-celled organism that looks like a bacterium, but actually belongs to a separate domain of life.

"The group knew that viruses often play an important role in Antarctic ecosystems, so team member Susanne Erdmann searched for viruses inside the organism’s cells. She found something unexpected: a plasmid.

"Plasmids are small fragments of DNA, often circular, that reside in living cells. They are not part of the cell’s main genome, and can replicate themselves independently. Often, a plasmid will carry a gene that is somehow useful to the cell: for instance, antibiotic resistance genes are sometimes found on plasmids.

"The plasmid Erdmann found, which the team calls “pR1SE”, is unusual. The genes it carries allow it to make vesicles – essentially bubbles made of lipids – that enclose it in a protective layer. Encased in its protective bubble, pR1SE can leave its host cell to seek out new hosts.

"In other words, pR1SE looks and acts a lot like a virus. But it carries genes that are found only on plasmids, and lacks any telltale virus genes. It is a plasmid with the attributes of a virus. “There really are no major distinctions left between plasmids and viruses,” says Cavicchioli.

"He suggests that viruses could have evolved from plasmids like pR1SE, by acquiring genes from their host that allowed them to make a hard capsid shell rather than a soft vesicle.
This lines up with existing evidence on the origin of viruses.

"There have been three leading ideas: either viruses originated before cells, or some cells evolved simpler forms and became viruses, or genes “escaped” from cells and became viruses. This third escape hypothesis has gathered support in recent years: in March 2017, a study suggested that many capsid proteins can be traced back to proteins found in cells.

"The evidence implies that such escapes began early in the history of life, says Patrick Forterre of the Pasteur Institute in Paris. “Traditionally the escape hypothesis has been associated with the idea that viruses are recent,” he says. “Now the escape hypothesis should be viewed in a broader context.” The first viruses may have escaped from some of the first cells on Earth."

Comment: Interesting. An other domain of early life. Doesn't help explain the origin of life but does point to an origin for viruses.