Largest Viruses Ever Revealed

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Electron microscopy provides an image of a Pandoravirus particle.
Courtesy of Chantal Abergel/Jean-Michel Claverie

More than 93 percent of pandoravirus genes resemble nothing known. This makes their origins a mystery — analysis of their genomes suggests pandoraviruses are not related to any known virus family.

"These viruses have more than 2,000 new genes coding for proteins and enzymes that do unknown things," Abergel said. "Elucidating their biochemical and regulatory functions might be of tremendous interest for biotech and biomedical applications. We want to propose a full large-scale functional genomics project on the pandoravirus genomes."

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The fact that pandoraviruses are totally different from the previously known family of giant viruses may suggest even more families of giant viruses remain to be discovered, said researcher Jean-Michel Claverie, head of the Structural and Genomic Information Laboratory in Marseille, France.

"Our knowledge of the microbial biodiversity on this planet is still very partial," Claverie said. "Huge discoveries remain to be made at the most fundamental level that may change our present scenario about the origin of life and its evolution."

It remains a mystery why pandoraviruses have more than 2,500 genes while most viruseshave far less, the researchers said. One controversial suggestion the researchers make is that giant viruses and other viruses that depend on DNA as their genetic material may be the shrunken descendants of living, cellular ancestors.

"Parasites of any kind are submitted to the universal process of 'genome reduction' — that is, they may lose genes without harm, because the host can always provide the missing function," Claverie said. DNA viruses small and giant may all have degenerated from the same or similar cellular ancestors, "but only differ by the rate by which they lost genes from the starting ancestral genome," he said.

Future research could turn up "even more intermediary life forms between viruses and cells, establishing a continuity between the two," Abergel said. "How should we define the boundaries between cells and viruses?"

The scientists detailed their findings in the July 19 issue of the journal Science.

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