The organisms were initially called NLF for ‘new life form’ when first discovered by evolutionary biologists at Aix-Marseille University in France. Pandoravirus salinus was discovered on the coast of Chile and appeared to be infecting and killing amoebae. When viewed under a microscope, it seemed the size of a small bacterial cell. Pandoravirus dulcis was later found in a freshwater pond in Melbourne, Australia. It was the discovery of the second similar organism that led the researchers to conclude that both were viruses, and the largest viruses ever found.
The viruses are each around 1 micrometer long and are 0.5 micrometers across. Their respective genomes are at 1.9 million and 2.5 million bases of DNA, which make the viruses larger than many bacteria and even some eukaryotic cells. The viruses are even larger than Mimivirus, a nucleocytoplasmic large DNA virus with a genome size of about 1.1 megabases, and Megavirus, which has a genome size of approximately 1.2 megabases. Mimivirus was discovered in 1992 but not recognized as a virus until 2003 while Megavirus was discovered in seawater off the coast of Chile in 2011. The Pandoraviruses are also larger than and bigger than the genomes of intracellular bacteria such as Tremblaya (138,927 base pairs) and Rickettsia (1,111,523 bp), as well as some free living bacteria, and many free living Archaea.
Detailed analysis has shown that two Pandoravirus giant viruses have almost nothing in common with previously identified giant viruses. Only 7% of their genes match those in existing databases. The name Pandovirus was coined by the discoverers in memory of the Greek myth about Pandora, as they anticipate many surprises from their future study.
As the two Pandoraviruses were discovered in sediments on continents located 15,000 km apart, this confirmed that they were not artifacts of known cells. It also indicates the viruses may be widespread. One such example is that of an organism identified as bacteria in 2008 in Germany. It was found in an amoeba living in the contact lens of a woman with keratitis. The parasitologist who found it, Rolf Michel, recognised that both P. salinus and P. dulcis were almost identical to his description of the bacterium.
Pandoraviruses lack many features of cellular organisms such as bacteria. Unlike bacteria, the viruses do not make their own proteins, produce energy via ATP or reproduce by dividing. Pandoraviruses also have no gene allowing them to build a protein like the capsid protein, which is the basic building block of traditional viruses.
The viruses do however contain many core genes common to the other giant viruses, and have a viral life cycle. The researchers were able to observe the viruses being taken up by amoeba hosts and emptying their proteins and DNA into the host cells. The viruses then took control of the host-cell nuclei and produced hundreds of new viral particles, eventually splitting the host cells open. Pandoraviruses also display the essential characteristics of other viruses in that they contain no ribosome, produce no energy and do not divide.
The next step for the researchers is to determine the origin of the Pandoviruses by characterising the unknown genes and the proteins they encode. Giant viruses may well have evolved from cells, which would mean the ancestors of Pandoraviruses must be distinctly different from the bacteria, archaea and eukaryotes we have today.