Scientists have discovered a pair of viruses that defy classification. Bigger and more genetically complex than any viral genus known to science, these so-called "pandoraviruses" could reignite a longstanding debate over the classification of life itself.
The Largest Virus Ever
There was a time when scientists regarded viruses as small and simple. Then along came mimivirus. When the viral juggernaut was discovered ten years ago, it was far and away the biggest, most genetically complex virus scientists had ever encountered. At 600 nanometers in diameter (comparable in size to some bacteria), and with a genome containing roughly 1,000 genes, it in many ways bears a closer resemblance to a parasitic bacterium than a virus.(Consider HIV, which measures just 120 nanometers across, and packs a total of just nine genes.) Hence the name mimivirus – short for "microbe mimicking virus."
Pandoravirus genome size, relative to eukaryotes, bacteria, archaea and other viruses – via Elizabeth Pennisi's analysis of the newly announced pandoraviruses
A handful of other large viruses have been discovered since – the mamavirus, for example, and the aptly named megavirus. Today, however, researchers led by Aix-Marseille University biologists Jean-Michel Claverie and Chantal Abergel (who helped discover mimivirus) have announced a discovery that blows them all out of the water. The pandoraviruses (recovered from mud in the coastal waters of Melbourne, Australia and Central Chile) are twice as big as any virus known to science. Each is also in possession of around 2,500 genes – 2,300 of which are entirely new to biology. Pandoraviruses, in other words, are more than just huge – they're also unmistakably distinct from any known virus on Earth – including other giant viruses.
That these pandoraviruses are as genomically unique as they are (not to mention the fact that they were recovered from entirely different continents) suggests that giant viral genera may be more common than previously believed, while raising compelling questions about the boundaries between living cells and inanimate viruses.
Are Viruses Alive?
Here's a fun game to play the next time you find yourself in the company of a bunch of biologists. First, wait for lull in the conversation. Next, shift forward slightly in your chair and casually ask whether or not viruses are "alive."
Well done! You've just stoked the flames of a a debate that's been raging off and on for about century. Sit back and enjoy. Preferably with some popcorn.
"First seen as poisons, then as life-forms, then biological chemicals, viruses today are thought of as being in a gray area between living and nonliving," writes molecular biologist Luis Villarreal in this in-depth feature for Scientific American. "They cannot replicate on their own but can do so in truly living cells and can also affect the behavior of their hosts profoundly."
The ambiguity stems in large part from viruses' paradoxical nature. On one hand, they posses many features that we associate with life – DNA and RNA, for example, and the capacity to evolve. They're also freakishly adept at hijacking the cellular machinery of their hosts, which they exploit to reproduce, spread and infect, at times with lethal efficiency. It's an impressive, if frightening, feat – especially for something that lacks the ability to produce its own proteins; because as devious as they are, viruses were long seen as relatively simple. They're quite small, physically, scientists thought, and their genomes are tiny. They have no cell wall. They don't even undergo metabolic processes.
It's reasons like these that have seen viruses denied so much as a humble sprig on the tree of life. Ten years ago, however, the accidental discovery of mimivirus caused many scientists to reconsider the qualifications for inclusion in that tree's hallowed branches. Pandoraviruses, with their hulking size and unprecedented genomic composition, are sure to do the same.
"Because more than 93% of Pandoraviruses genes resemble nothing known," write the researchers in the latest issue of Science, "their origin cannot be traced back to any known cellular lineage."
"However, their DNA polymerase does cluster with those of other giant DNA viruses, suggesting the controversial existence of a fourth domain of life."
Images via Abergel et al. unless otherwise indicated