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Science: Supersized Viruses Break Records, Raise Questions

The discovery of two viruses that are huge — both physically and genetically speaking — is forcing researchers to reconsider what it means to be "viral" in the first place. The newly identified viruses are a full micrometer (one millionth of a meter) in size, whereas the majority of known viruses don't exceed 0.3 micrometers. (For comparison's sake, the diameter of a human hair is about 80 micrometers wide.)

As such, the new huge viruses defy comparison to any virus families on record and their origins cannot be traced back to any known lineage.

Along with Matthieu Legendre and colleagues from the Structural and Genomic Information Laboratory in France, Nadège Philippe, partially supported by Uppsala University in Sweden, worked with collaborators from Université Joseph Fourier in Grenoble, France, and located these giant viruses in sediment-dwelling amoebae. The researchers dubbed them Pandoraviruses, due to their amphora-like shapes and surprising genomic contents.

"The lack of similarity of most of their genes with other life forms might be an indication that they originated from a totally different primitive cellular lineage — a different tree of life altogether — than bacteria, archaea and eukarya," explained Jean-Michel Claverie and Chantal Abergel, co-authors of the report that describes these Pandoraviruses.

Their full report appears in the 19 July issue of the journal Science.

Claverie and Abergel were part of a team that discovered viruses on the order of 0.4 micrometers, known as Mimiviruses, about 10 years ago. At the time, the discovery prompted the scientific community to reconsider the upper limits of the viral world. Since then, the two researchers helped to discover Megavirus chilensis, which held the record for biggest virus — until now.

The researchers had noted that Mimiviruses have an affinity for a particular sediment-dwelling amoeba, known as Acanthamoeba, so they had been digging around in marine sediments, looking for similarly huge viruses. They found Pandoravirus salinus, which they identified at the mouth of the Tunquen River in Chile, and Pandoravirus dulcis, which they discovered at the bottom of a freshwater pond near Melbourne, Australia.


Electron microscopy image of a Pandoravirus particle (edited using Adobe Photoshop artistic filters). [Courtesy of Chantal Abergel / Jean-Michel Claverie]

Both viruses are larger than parasitic bacteria, and each is visible with traditional light microscopy alone. Compared to Mimivirus genomes, which have approximately 1.0 megabase pairs in their genomic sequences, the researchers say that P. salinus and P. dulcis respectively have 1.9 and 2.5 megabase pairs.

"The findings really indicate that we don't know what the possible limits [of viruses] are anymore," Claverie and Abergel said. The researchers also note that Pandoravirus-like particles have been observed before; they just weren't interpreted as viruses at the time (as was the case for Mimiviruses 20 years ago).

The newly identified viruses may help researchers unravel the origins of life on Earth, and they might even have some clinical applications one day. But, Philippe and this team of researchers also hope their discovery will inspire more funding for basic research so that they and others can continue hunting for new viruses around the world.

"We hope that the discovery of these viruses will promote basic research and not just the innovation and translational disease-linked research," Claverie and Abergel said.

"On the other hand," they said, "the genes of these new viruses might point to alternative metabolic pathways that could revolutionize the biotech and pharmaceutical industries."

Read the abstract, "Pandoraviruses: Amoeba Viruses with Genomes up to 2.5 Mb Reaching that of Parasitic Eukaryotes," by Nadége Philippe et al.