Journal club: Did giant viruses shrink, or did small viruses grow?
Friday, April 7, 2017 at 11:18AM The smallest viruses can be just a few nanometres wide and contain two genes. Viroids can get even smaller. Recently, however, giant viruses have been found, 1000-fold larger and with more genetic content than some bacteria.
This raises a fascinating question as to the origin of viruses. The first model is that viruses were spawned out from some proto-bacteria-like cells. A bit of bacterial DNA, like a plasmid, that was able to survive cell-free and move from cell to cell. In this view, viruses don't really belong on the tree of life, and giant viruses are just abnormally large viruses that have captured more and more DNA from hosts.
Giant viruses raise a second model. What if the tree of life had an original fourth strand (bacteria, eukaryotes, Archaea and proto-viruses). The first three strands are still with us today, while the proto-viruses have devolved from a free-living cell to a dependent virus. Under this alternative view, giant viruses are ancient relics closest to the original proto-viruses, with the smaller viruses having gone further down the evolutionary pathway of becoming highly efficient replicators.
A paper published in Science this week argues for the first model, with evidence that some key machinery in giant viruses is directly stolen from other lineages of the tree of life. But the argument is not closed, and as more and more giant viruses are found, I look forward to seeing which direction the evidence moves.
evolution
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