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Giant viruses raise questions about life

Electron microscopy image of a Pandoravirus particle

Electron microscopy image of a Pandoravirus particle (Source: Chantal Abergel/Jean-Michel Claverie)

Mega virus The discovery of two giant new viruses, including one discovered by chance in a pond near Melbourne, has made scientists ponder the origins of life.

The so-called Pandoraviruses are at least twice the size, both in terms of their physical scale and genome complexity, of the previous record holder Mimivirus.

The finding was made by a team of French researchers led by environmental virologist Nadège Philippe from Aix-Marseille University, and was reported today in Science.

They unearthed one of the giant new viruses; one named Pandoravirus salinus was trawled from marine sediments at the mouth of the Tunquen River in Chile and another, P. dulcis, was found by chance at the bottom of a freshwater pond near Melbourne.

Both Pandoraviruses parasitise sediment-dwelling, single-cell amoebas, known as Acanthamoeba and are so large that they are able to be viewed under a light microscope.

No limits?

Because of their large size¬ -- about a full micrometre -- they were initially thought to be bacteria, however lab tests revealed them to be unaffected by antibiotics, leading to further study.

When their genomes were sequenced, P. salinus and P. dulcis have 1.9 and 2.5 mega base pairs (Mb), respectively, in their genomic sequences. Mimivirus genomes, by comparison, have approximately 1.0 Mb.

"As we already discovered the two previously largest viruses (in terms of genome and particle sizes), we were prepared to find new viruses in the 1000 gene ranges, but not to more than double that figure, and now reach 2500," say the researchers, who were also involved in the identification of Mimivirus.

"This really indicates that we don't know what are the possible limits anymore."

Unknown origins

With more than 93 per cent of their genes unknown, Pandoraviruses defy comparison to any other virus families and their origins cannot be traced back to any known lineage.

"It indicates that our knowledge of the microbial biodiversity on this planet is still very limited, and that huge discoveries remain to be made at the most fundamental level. These may change our present understanding about the origin of life and its evolution."

They say that the finding supports their preferred evolutionary scenario, where viruses closer to their ancestral cellular forms are expected to exhibit larger genomes.

Parasites typically experience 'genome reduction'. That is, they may lose genes without harm because the host provides the missing function. Depending on the rate of evolution, over time parasite genomes shrink.

"Looking for even bigger viruses with bigger genome was a way to go back in time, and take a closer (earlier) look at this postulated ancestor that we believe predated the emergence of the eukaryotic [cellular] lineage," they say.

Hiding in plain sight

The researchers admit that Pandoravirus-like particles were first observed about 13 years ago; they just weren't interpreted as viruses at the time.

"The fact that two of them were found almost simultaneously from very distant locations either indicate that we were incredibly lucky, or that they are not rare," they say.

"One explanation for their late discovery would be that they are only found in sediments, an environment that has not been well explored, and that is also difficult to work with. As far as we know, this is the first recovery of viruses from sediments."

"They are probably everywhere. We are actively looking for them."

Tags: biology, weird-and-wonderful