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The Pandoravirus and the Tree of Life

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By Ben Rush, Scientist, Cell Biology

In classical Greek mythology, Pandora’s Box was responsible for unleashing all forms of evil upon the world. 

The Pandoravirus, named by researchers in honor of that ominous mythological symbol, is the largest virus ever discovered. However, unlike the Pandora’s Box of mythology and popular culture, the Pandoravirus does not appear to be a threat to human health and safety. Samples of Pandoravirus were extracted from underwater sediments off the coast of Chile by French virologist Jean-Michel Claverie, who, in 2010, along with his wife and colleague, Chantal Abergel, also discovered Megavirus off the coast of Chile. A second variation of Pandoravirus was unearthed in the muck of a freshwater pond near Melbourne, Australia. The discovery of this new, mysterious “jumbo??? virus family has not led to hysteria or an outbreak of incurable disease, but has sparked larger, more philosophical questions about the origins of life on earth.

Since their discovery, viruses have been considered unqualified to share the “living??? status of other organisms. They do not have the ability to produce proteins or energy on their own and rely on living cells for replication. Viruses are inactive particles drifting through the environment without intent or will of any kind. They remain inert, until, by chance, the virus particle comes into contact with a cell that causes it to become activated and infect that cell. We have had a rather defined understanding of viruses for some time now.

Then, in 2003, Didier Raoult and his team published the discovery of a large amoeba virus, the Mimivirus (Science 28 March 2003: Vol. 299, pg 2033). Raoult, et al showed that the Mimivirus contained some genes that resembled those in living organisms and play a role in protein synthesis. These genes differed enough from any known host that the acquisition of these protein genes from its host was eliminated.  This led Raoult and his team to conclude that the virus evolved from a cellular ancestor, which lost a large portion of its genes as it morphed into its parasitic form. They also suggest that this cellular ancestor represents a new branch on the tree of life that predates the known branches as we understand them today.

The Mimivirus was the largest virus ever discovered, with 1.18 million base pairs and 900 genes. Then, in 2010, the Megavirus topped this with 1.25 million base pairs.  However, in July, a team of scientists published their discovery of the Pandoravirus (Science 19 July 2013: Vol 341, pg 281). This virus contains an astounding minimum of 2.5 million bases, larger than some bacteria and eukaryotic cells. These 2.5 million bases encode for 2,556 genes – only 7% of which match genes known to exist. This means that 97% of its genome has never been identified before. The massive Pandoravirus is 1 µm long and 0.5 µm wide, meaning that it is visible with a light microscope. Even more unique is its method of replication: typically, viral components are synthesized individually and later combine to form mature virions. In the case of the Pandoravirus, new virions are formed from one end of the virus to the other, in a “knitting??? fashion as described by the authors.

Many of the characteristics seen in the Pandoraviruses are unique and have never been observed before. It is the largest known virus to date, in both genome size and physical size. The genome size of viruses are no longer universally smaller than eukaryotes and bacteria. The Pandoravirus also replicates differently from other viruses. This discovery forces us to question our understanding of the three domains of life. This doesn’t become more clear then when observing the computer analysis seen in the Pandoravirus discovery publication comparing the DNA polymerase sequences that includes samples from each of the three domains (Archaea, Bacteria, and Eukaryota) and the six largest DNA viruses. This analysis sorts the three domains and the viral DNA sequences into four distinct groups, indicating a fourth domain.  

The recent discovery of the Pandoravirus, the largest known virus to date, reminds us just how dynamic the scientific world remains. Our scientific understandings are constantly evolving with each new discovery. The Pandoravirus questions our current definition of viruses, and allows scientists to further study the origin and classification of viruses. Will viruses one day find themselves on a new branch in the tree of life? 

According to Claverie, “Huge discoveries remain to be made at the most fundamental level that may change our present conception about the origin of life and evolution.???