Friday, 25 August 2017

Filling The Gaps - How Viruses Might Have Evolved

Virus particles

Credit: Sebastian Kaulitzki/SPL/Getty
Antarctic mystery microbe could tell us where viruses came from | New Scientist

A recent discovery in Antarctica could shed some light on the origin of viruses.

Ricardo Cavicchioli of the University of New South Wales in Australia and his colleagues found a microorganism, an archaean, Halorubrum lacusprofundi R1S1, in lakes on the Rauer Islands, Antarctica. That's not unusual in itself; new organisms are discovered daily. What was slightly unusual with this particular one is that it contains an independent plasmid that the team have names "pR1SE". Plasmids are small fragments of DNA, often circular, that are self-replicating and often carry a gene that is beneficial to the organism. Bacterial antibiotic resistance is often carried on a plasmid and, as such, can be transferred to other bacteria.

So a plasmid itself in the cells of H. lacusprofundi R1S1 is nothing unusual either. What is unusual about pR1SE is that, like some viruses, it also has genes for making a lipid capsule around itself. This enables pR1SE to leave its host and find new ones. It that respect it behaves very much like a virus.

The major difference between viruses and plasmids is the mechanism of transferring their genomic information between host cells. Here, we describe the archaeal plasmid pR1SE from an Antarctic species of haloarchaea that transfers via a mechanism similar to a virus. pR1SE encodes proteins that are found in regularly shaped membrane vesicles, and the vesicles enclose the plasmid DNA. The released vesicles are capable of infecting a plasmid-free strain, which then gains the ability to produce plasmid-containing vesicles. pR1SE can integrate and replicate as part of the host genome, resolve out with fragments of host DNA incorporated or portions of the plasmid left behind, form vesicles and transfer to new hosts. The pR1SE mechanism of transfer of DNA could represent the predecessor of a strategy used by viruses to pass on their genomic DNA and fulfil roles in gene exchange, supporting a strong evolutionary connection between plasmids and viruses.

Currently, there are three competing hypotheses about the origin of viruses although they aren't necessarily mutually exclusive, as there could be more than a single origin:

  • An independent origin separate to that of bacteria and arcaea (which could also have different origins).
  • A highly evolved or degenerate cell, stripped down to the barest essentials.
  • An 'escaped' gene originating in a cell but which broke free.

This discovery adds some weight to the latter hypothesis by suggesting a mechanism by which this could have happened.

Although the pR1SE only has genes normally found only in plasmids and none of the usual virus genes, there is no biological reason to distinguish it from a virus. The lipid capsule is soft, not rigid as with other viruses, but this could evolve by natural selection. This plasmid suggests that viruses could have evolved from some of the earliest cells.

It's probably hardly worth asking because these sorts of questions are routinely ignored by creationists, but, since creationism includes the view that everything on Earth was either made for humans or was added later to harm us for being sinful, how can this plasmid be explained in that model? It seems to spend its entire existence looking for uninfected archaea to infect, and producing copies of itself, in a part of the world that is normally uninhabitable by humans.

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