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Melbourne’s monster virus could redraw the tree of life

Felicity Nelson went for a swim in the La Trobe University pond and opened a Pandora’s Box of scientific discovery

The serendipitous discovery of a giant virus, the pandoravirus, in a Melbourne pond could drastically alter our understanding of the history of life. Biologists have long held that viruses are little more than biological robots that rely on host cell machinery for protein manufacture and reproduction. This new monster virus may overturn this assumption and finally give viruses (or their ancestors) a place on the tree of life.

The pandoravirus is the largest virus ever found. It is a hundred times larger than an average virus and, at one micrometer in length, it is easily observed under a light microscope.  While a typical virus (such as HIV) has around 10 genes, Pandoravirus salinus from Chile has up to 2556 genes and Pandoravirus dulcis from Melbourne has 1502 genes. P. salinus is larger than some parasitic eukaryotes and almost twice as large as the previous record holder, the megavirus.

Perhaps even more compelling than its enormous size is the uniqueness of this virus. Ninety-three percent of the pandoravirus genome is completely new to science! This strongly suggests that it evolved from a completely separate line, instead of acquiring its genetic material from hosts or losing part of its genome as it evolved into a parasite over time from a free-living organism. The three domains of life (bacteria, achaea and eukaryotes) may soon become four, with the ancestors of the pandoraviruses being included as another branch of living microorganisms.

Jean-Michel Claverie and Chantal Abergel from Aix-Marseille University in France stumbled across the virus while collecting samples from water bodies in Chile in 2009 and at La Trobe University in Melbourne in 2011. Their team published a paper in Science last month proving the unusually massive amoeba parasites are in fact viruses and not, as was supposed, peculiar bacteria. Their team used light and electron microscopy to show that replication was virus-like and studied the genome to show that there was not enough information to make proteins. The paper suggests the name “pandoravirus” for the new group, reflecting both the amphora shape of the virus and its unforeseen, far-reaching consequences for biological science.

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