Giant viruses to open ‘Pandora’s box’?


These viruses are so big they might just be your ancestors.

Two newly discovered viruses are twice as large as the previous record-holders and may represent a completely new life form, French scientists reported in the U.S. journal Science. Researchers say they were “extremely surprised” by the discovery of what they are calling “Pandoraviruses,” which are not believed to be the type that make people sick.

Instead, what is most interesting about them is their giant genome — from 1,900 to 2,500 genes — way more than viruses such as influenza, which has just 10. Humans, by comparison, have around 24,000.

The previous record was held by the Megavirus chilensis virus, at 1,200 genes. Before that came the Mimivirus with around 1,000 genes, discovered by the same team of scientists a decade ago.

Though not typically deemed to qualify as a form of life, some scientists say these giant viruses merit consideration as a new kind of living object.

One, Pandoravirus salinus, was found on sediment off the coast of Las Cruces, Chile. The other, Pandoravirus dulcis, was found in the muck of a pond in Melbourne, Australia. They are visible under a light microscope and appear to have more in common with cells than other known viruses.

Pandoraviruses come from a different family than previously known giant viruses, said researchers Jean-Michel Claverie, a professor at the school of medicine of Aix-Marseille University, and Chantal Abergel, director of research at France’s National Center for Scientific Research.

Their container-like shape and unique set of genes “made us associate them to the Pandora’s box. The opening of the box will definitively break the foundations of what we thought viruses were,” the researchers said in an email. Most of their genes appear unfamiliar to scientists, and they contain code for proteins and enzymes that “do unknown things,” the authors said.

“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,” they wrote.

That means, according to the researchers, that Pandoraviruses may come from a “different tree of life altogether” than the three domains of life known to science: bacteria; single-celled micro-organisms known as archaea; and Eukarya, which include fungi, plants and animals.

Gustavo Caetano-Anolles, a professor of bioinformatics at the University of Illinois who studies giant viruses, theorizes that they descended from a cell. If true, “then we will have two kinds of ancestors — an ancestor that is shared between viruses and cells, and an ancestor shared by all the cellular super kingdoms,” he said.

“The problem here is more from an evolutionary point of view. Where do these viruses come from? They are definitely part of something that we do not understand very well and that has the same complexity as cells,” he said, adding it may be that viruses that make people sick are “part of a lineage that go rogue.”