Newly Discovered Jumbo Viruses May Point to New Type of Life

Scientists say that two newly discovered jumbo-sized viruses, which are now blurring the line between viral and cellular life, may point to the existence of a new type of life.

The two large viruses have been dubbed "Pandoraviruses" by researchers because of the surprises that they could have in store for biologists. The name also references the mythical Greek figure who opened a box and consequently released evil into the world. The viruses are detailed in this week's issue of the journal Science.

Study co-author Jean-Michel Claverie, a virologist at the French National Research Agency at Aix-Marseille University, explains that the discovery of Pandoraviruses indicate that our knowledge of Earth's microbial biodiversity is still incomplete. Claverie added:

"Huge discoveries remain to be made at the most fundamental level that may change our present conception about the origin of life and its evolution."

Claverie's lab discovered one of the viruses, Pandoravirus salinus, in sediments collected off the coast of Chile. The second virus, Pandoravirus dulcis, was found lurking in mud in a freshwater pond near Melbourne, Australia.

Study co-author Chantel Abergel added:

"These viruses have more than 2,000 new genes coding for proteins and enzymes that do unknown things."

The two new viruses are so large that they can be seen using a traditional light microscope. Their genomes are also massive. P. salinus is 1.91 million DNA bases long while P. dulcis is 2.47 million DNA bases. The previous viral genome record holder was Megavirus chilensis, which is 1.18 million bases.

At first the scientists thought that the two new large viruses were the same virus, but compared the two genome sequences and their encoded proteins and realized that they represented a new virus family. The researchers also conducted several experiments to confirm that Pandoraviruses were in fact viruses.

The researchers used light and electron microscopes and followed the newly found viruses through a complete replication cycle. They met all three criteria to be labeled viruses:

  • Instead of splitting in two like bacterium or cells, the Pandoraviruses spawned hundreds of new copies in one cycle.
  • Both lacked the genes needed for energy production.
  • They could not produce proteins without infecting single-celled organisms known as amoebas, which appeared to be the Pandoraviruses' preferred hosts.

There are several irregularities that raise questions about the origins of Pandoraviruses. Claverie and his team prefer the theory that ancient ancestors of Pandoraviruses were once free-living cells that gradually lost most of their genes as they became parasites. Some researchers believe that this hypothetical cell could have constituted a "fourth domain" of life, which is a previously unknown branch of life distinct from the accepted three domains: Bacteria, Archaea, and Eukaryotes (the domain animals and plants belong to).

The researchers are now looking for other Pandoraviruses to discover their evolutionary origins and better study their genes.