Pandoravirus: Neither fish nor flesh

5. November 2013
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In the muddy swamp off the coast of Chile, Pandora's Box awaited its discovery. Researchers have now found a virus that puts time-honoured theories in question: Are there really only three cellular kingdoms?

Right up until preliminary examinations, medical students learn that archaea, bacteria and eukaryotes dominate our planet as its cellular life forms. Whoever looks up “virus” reads about small particles only visible to an electron microscope which need host cells in order to multiply, and that they carry only a handful of genetic components – for instance in influenza virions there are seven to eight RNA such sections. Recent work puts some of this dogma increasingly in question. A chronology of events:

Giant among dwarfs

In the search for triggering factors leading to a pneumonia wave in 1992, researchers isolated from a cooling tower what were supposedly Gram-positive bacteria. They were residing in Acanthamoeba polyphaga, an amoeba. The supposedly new germs were to be categorised in nomenclature as Bradfordcoccus after their discovery site in Bradford. Eleven years later French scientists surprised the field: What bacteria? Because – this was a matter of a previously unknown giant virus, given the name acanthamoeba polyphaga mimivirus. The capsid has a diameter of 400 nanometers, and there are in the genome around 1.2 mega base pairs to be found. Whether mimivirus human pathogens occur is something disputable. Doctors consider there to be a possible link to pneumonia, there are indirect pieces of evidence in the form of antibodies against the virus. However, there is as yet a lack of hard and sound arguments.

Really a virus?

From a developmental biologist’s perspective, mimivirus aggressively macerate the borders of definitions: they are similar in size to bacteria such as Rickettsia conorii and also have a genome that can measure comparably with that of bacteria. Genes for synthesis of amino acids or nucleotides are also carried on board. In comparison to known viruses it is striking that as many as four aminoacyl-tRNA synthetases can be formed: for arginine, cysteine, methionine and for tyrosine. Without a host life still looks bad for them, both with regard to protein as well as energy metabolism. Mimivirus is not just one singularly individual case, in mid-2010 Jean-Michel Claverie and Chantal Abergel discovered the next giant: Megavirus chilensis. Its capsid measures 440 nanometers in diameter, and molecular biologists found 1.3 million base pairs. The mega virus even expressed seven aminoacyl-tRNA synthetases.

Pandora’s Box opens itself up

Now, Jean-Michel Claverie and Chantal Abergel, of Marseille, have landed the next hit. They discovered a previously unknown species off the coast of Chile and later in a body of still water in Melbourne. Pandora viruses, as the new name goes, are oval shaped and live parasitically in amoebae. Like their mythological namesakes, they bring disaster – not exactly for patients, but for established theories. The hope of reaching a deeper understanding of life remains alive (taking the Greek metaphor further). It all starts by looking at their size – pandoraviruses are half a micron wide and a micron long. These are dimensions that are in the light microscopic range; some bacteria have significantly less to show. Claverie and Abergel even speak of new life forms: with 2.5 million base pairs and 2,556 genes, the saltwater-habitat representative Pandoravirus salinus exceeds viral size limits by a factor of two. Pandoravirus dulcis found in fresh water can also be described as no slouch in this regard, here 1.9 million base pairs and 1,500 genes are to be found. According to current knowledge, more than 93 percent of the genome is enigmatic, without relationship to other mega viruses or even to eukaryotes. A mere seven percent has yielded useful information in comparison with databases. “What the hell is going on with the other genes?” Claverie wants to answer this question. Currently, scientists are raising a lot of questions, including the status of these viruses as non-autonomous forms of life.

131105_Pandora

 Particles of pandoravirus under the electron microscope
© Chantal Abergel / Jean-Michel Claverie

Living in four dimensions

It remains to be ascertained how pandoraviruses could be included in the evolution process because of the unusual genetic material alone. Abergel believed that in earlier times there was more diversity than today – there were not just the archaea, bacteria and eukaryotes which have ended up dominating events, but a fourth, previously unknown dimension of life. Common precursor forms, ie primordial cells could have developed into bacteria and eukaryotic cells. In others, the typical functions were lost, and viruses emerged. The aforementioned researchers hold pandoraviruses to be a relic of this era. They may resemble primordial cells more strongly than previously known forms. There are many hypotheses – about one thing, however, virologists are certain: as already seen with mimivirus from Bradford, pandoraviruses explain so many phenomena, including what seem to be scientific misinterpretations. In Nature magazine Chantal Abergel gives us food for thought – that when considering many such related publications, there may be a giant virus in the background. We’ll just have to wait and see.

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