For all that scientists can tell from their genes, they just landed from planet Mars.
Pandoraviruses have massive DNA consisting of over 2,500 genes compared with about 13 genes found in influenza viruses. Measuring about one micrometer in length, pandoraviruses are tens to hundreds of times bigger than other viruses.
The discovery of the new genus follows the surprising discovery of viruses of the genus Mimivirus
, about 0.7 micron, and Megavirus chilensis
reports that according to Chantal Abergel
, one of the authors of the new study, when scientists first discovered giant viruses they were so baffled that they simply tagged them NLF, that is, "New Life Form."
According to the authors Claverie, Abergel et al.
, in a paper entitled "Pandoraviruses: Amoeba Viruses with Genomes Up to 2.5 Mb Reaching That of Parasitic Eukaryotes,"
published in journal Science
, July 19: "Ten years ago, the discovery of Mimivirus
, a virus infecting Acanthamoeba, initiated a reappraisal of the upper limits of the viral world, both in terms of particle size (>0.7 micrometers) and genome complexity (>1000 genes)... We report the isolation of two giant viruses, one off the coast of central Chile, the other from a freshwater pond near Melbourne (Australia), without morphological or genomic resemblance to any previously defined virus families... These viruses are the first members of the proposed 'Pandoravirus' genus, a term reflecting their lack of similarity with previously described microorganisms and the surprises expected from their future study."
Biologists have always viewed viruses with suspicion and puzzlement as the weirdos of the biological scheme of things. What do you call a strand of DNA-in-protein-envelope that must hijack the cellular apparatus of other living things to perform even the most basic of life defining tasks such as reproduction? Then Pandoravirus
appears with a new set of riddles to add to the unsolved: Expressing the confusion biologists have always felt about the status of viruses in the hierarchy of life to which pandoraviruses now add new complications, Abergel said: "The question is whether Pandoravirus
might have evolved from a bacterium. And the answer is, maybe it could. We don’t really understand where viruses come from, and we don’t really understand what they are."
When scientists finally overcame their first shock at the discovery of the first giant viruses and convinced themselves that the weird "Martian" NLFs were actually giant viruses, they began a search for others.
Study co-author Jean-Michel Claverie
, a microbiologist at Aix-Marseille Universite in France and Chantal Abergel
, also of Aix-Marseille Universite, joined the search for other new giant viruses in amoeba rich water sediments where the first big viruses were found.
They eventually found two species of genus Pandoravirus
which are amoeba parasites: Pandoravirus salinus
in Chile's Tunquen River, and Pandoravirus dulcis
from a freshwater pond near Melbourne in Australia.
: "We have been thinking deeply into the limits of viruses, and this is why we're open more than other labs to finding exotic things—we push the envelope of what we would consider possible."
According to the study authors: "Finding such a new type of virus that is so different happens once every 50 years—it's a major discovery."
The authors pointed out that the reason why pandoraviruses remained unknown for so long could be that scientists, working on certain assumptions about viruses, missed them. Apart from the fact that most viruses are much smaller, pandoraviruses lack key morphological features characteristic of other viruses which means that a researcher who chances on them would tend to assume they are some kind of bacterium.
reports the authors said: "When people look into cells and when they see things that don't have the right dimension or don't have regular assets or geometries, they don't think of viruses—they think its some kind of bacteria."
They explained further that scientists who discovered them earlier might have ignored them when they failed to cultivate them in the lab: It is known that many naturally occurring bacteria cannot be grown in the lab.
Pandovirus: Life from Mars
What is probably most remarkable about pandoraviruses
is that they share very little in common with other viruses. Not only is their mode of reproduction unusual, more than 93 percent of their 2,500 genes have no evolutionary links in nature: The startling implication of this is that pandoraviruses
could have dropped recently from Mars for all scientists can tell at the moment.
The startling novelty has lead to the suggestion that they represent a "fourth domain"
of life in addition to the three previously recognized: bacteria, archaea (formerly grouped together as prokaryotae
) and eukaryotae
to which humans also belong.
Claverie and Abergel explain
: "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."
So novel are the new genes found in pandoraviruses that scientists have launched into research work to determine exactly what the genes do as the first step to figuring out whether the new viruses warrant the creation of a "fourth domain" of life.
reports Claverie and Abergel said that the three-domain system is "probably pretty wrong—we are missing some part of the puzzle here."
However, some experts are saying there isn't any evidence that Pandoravirus
represents a "fourth domain of life". According to The New York Times
, professor of evolutionary molecular biology at Newcastle University, T. Martin Embley, said: "They provide no evidence for that notion, so it seems a distraction to me."
also reports that Eugene Koonin, evolutionary biologist at the National Institutes of Health and a specialist in viruses, said: "These viruses, unusual as they might be, are still related to other smaller viruses. The internal environment of the amoeba cell provides a very good playground for acquiring various kinds of genes from different sources." He predicted: "We are going to see many, many more giant viruses discovered around the world, some of which, probably will be bigger than pandoraviruses."
However, with its novel morphological features and genetic code twice the size of Megavirus
, pandoraviruses will eventually add a new range of questions to the abiding question of the origins of life on Earth. Claverie told NPR
: "We believe that these new pandoraviruses have emerged from a new ancestral cellular type that no longer exists."
What should come as a relief is that pandoraviruses are not harmful to humans. The New York Times
reports that scientists say the giant viruses are very widely distributed, so widely distributed that we have been carrying them around in our bodies all the time without knowing. Recently, a team of French researchers isolated a giant virus from blood donated by a healthy volunteer.
Claverie said: “I don’t believe we have the proof at the moment that these viruses could infect humans," but adds cautiously: "never say never."