They are so different from existing forms of life, they might just as well have come from outer space. Called Pandoravirus because "opening" it has released so many questions about life ? this virus is unlike anything ever seen before. Plus it is twice as large as any viruses previously discovered.
More than 90 per cent of the Pandoravirus genes are new to science and have no known counterparts in other viruses, bacteria or higher forms of life such as animals.
Two species have been discovered. Both are egg-shaped and so big, at a micrometre long, and half that width, that they can be seen through a standard lab microscope.
The one with the largest genome, Pandoravirus salinus, has 2.47 million DNA base pairs and came from sediments in the mouth of the Tunquen river in Chile. The other, Pandoravirus dulcis, boasts 1.9 million base pairs and came from a shallow pond near Melbourne, Australia.
The genome of P. salinus is twice the size of that belonging to the previous record-holder, Megavirus chilensis, or mimivirus. The Pandoraviruses are also larger than many bacteria, and even some cells of plants and animals.
Most common viruses have fewer than 10 genes, but P. salinus has 2556 genes, 93 per cent of which have no known counterparts in any other sequenced organisms. P. dulcis has 1502 genes.
"No microorganism closely related to P. salinus has ever been sequenced," say the discoverers of the Pandoraviruses, Jean-Michel Claverie and Chantal Abergel of the CNRS, the French national research agency, at Aix-Marseille University in France. "One of our jokes is that either they are from outer space, or from a cellular ancestor that's now disappeared," says Abergel.
My viral life
As part of their investigation, Claverie, Abergel and their colleagues examined the life cycle of P. salinus. First, virus particles invade host amoebas. DNA from the virus then takes control of the amoeba nucleus, ordering it to make hundreds of new viral particles. After 10 to 15 hours, the host cell bursts, releasing the particles to seek out and infect new cells.
"Pandoraviruses appear to be built through a mysterious continuous process where the core and the shell are assembled simultaneously," says Claverie.
The Pandoraviruses qualify as viruses because they can't replicate or process their own DNA and have to rely on a host to do that for them. They have no genes for making their own energy-storing molecules, and can't make proteins. There is also no sign of the cell-like division that happens in all bacteria, eukaryotes and Archaea.
Being able to make your own energy is a definition of life, so, like all viruses, Pandoraviruses are not strictly alive, points out Gary Foster of the University of Bristol, UK, who studies viruses that infect fungi. "It fulfils all the criteria for being a virus, except the sheer size, and that's what's blowing people's minds away," says Foster.
Virus hunters
Claverie and his colleagues, who are veteran hunters of giant viruses suggest that the Pandoraviruses are unique and deserve their own, new domain of life, beyond the three existing domains of bacteria, eukaryotes and Archaea.
But Foster, who is involved in the International Committee for Virus Taxonomy, thinks it's too early for that. "If you change the domains for every new weird thing, it would be an absolute mess," he says. So for now, Foster thinks the viral definition is suitable.
Foster believes that there may be many more weird organisms in the world, with equally weird genomes. "We just haven't looked hard enough," he says. "As we look at the extremes of life, we will find many more of these weird things."
Claverie believes Pandoraviruses might be abundant in sediments, but haven't been found before because sediments are seldom explored. "As far as we know, this is the first recovery of viruses from sediments," he says. "They're probably everywhere, and we're actively looking for them."
As to the evolution of the Pandoraviruses, Claverie thinks the most likely explanation is that they were once self-supporting cells, but downsized themselves to viruses by becoming parasites, jettisoning all the genes they formerly needed to process DNA and make their own energy and relying instead on the corresponding machinery in their new hosts.
Journal reference: Science, DOI: 10.1126/science.1239181
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