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February 21, 2024

Complex mega viruses befuddle researchers

By MELANIE LEVINE | October 26, 2013

Sometimes a new discovery presents such a mystery that scientists have no choice but to choose a name that reflects the intrigue.

In an article published recently in Science, researchers in France and Sweden announced they had discovered two new giant viruses, called Pandoraviruses, that are more dissimilar to any virus previously described. Pandoravirus salinus, found on the coast of Chile, and Pandoravirus dulcis, from a freshwater pond in Melbourne, Australia, are the latest additions to an increasingly diverse understanding of the microbial world.

The Pandoraviruses have very little in common with other viruses, and in fact, their unusual qualities make them appear more like bacteria. At about one micrometer in diameter, they are enormous on the scale of microorganisms, and genomes of more than 1,100 genes make them more complex than some eukaryotic cells. A typical virus such as HIV, for comparison, is about .12 micrometers in diameters and has only nine genes.

A larger genome means Pandoraviruses can produce a larger number of proteins, but analysis found that only 6 percent of the proteins that Pandoravirus salinus encodes are similar to those in other viruses or cells, meaning that most of the functional operations of Pandoraviruses are not yet understood. Furthermore, Pardoraviruses contain no gene at all in their large genomes that corresponds to a capsid protein, one of the building blocks of traditional viruses.

Despite these differences, analysis showed that Pandoraviruses are indeed classifiable as viruses: they have no ribosomes, do not produce energy and do not divide. These characteristics definitively distinguish viruses from cells, but the unusual properties of Pandoraviruses may reveal an important link between the two groups. Cellular life is believed to have evolved billions of years ago from simpler biological forms like viruses, which had emerged from yet simpler organic compounds.

The first virus of this size, Mimivirus, was discovered ten years ago, but the level of complexity of Pandoravirus is novel. The discovery of the two Pandoravirus species simultaneously in locations 15,000 km apart suggests that despite having only been recently discovered, they might not be rare at all, and scientists may soon discover more complex viral species.

The enormous complexity of these viruses is puzzling because it doesn’t fit in with our current understanding of viruses. Pandoraviruses are known to infect amoebas of the Acanthamoeba genus, a typical behavior of viruses and an act that shouldn’t necessitate such a large size and genome. However, we know from our own situations that genome size and complexity of the organism aren’t perfectly correlated; humans, after all, have about the same number of genes as Tetraodon nigroviridi, a pufferfish.


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