By most standards, viruses aren’t alive. They have genetic material, yes, and they reproduce, but a true living organism must have some form of metabolism – it must be able to build up and break down chemicals on its own. Viruses have no metabolism. They don’t make their own proteins, and they can only reproduce by enslaving host cells into manufacturing copies of themselves, and then forcing the host cell to self-destruct and unleash the new viruses. However, researchers Nadége Philippe and colleagues have recently discovered massive, bacteria-sized viruses with genomes larger than those of any viruses known heretofore, and this discovery could change our understanding not only of viruses, but of the tree of life.
These newly discovered viruses are the largest found to date, but they are not the first giant viruses ever found. A decade ago, researchers Bernard La Scola and colleagues discovered the Mimivirus, which has a genome of 1.18 million bases. Since then, researchers have found similar “megaviruses,” which seem to share certain structural and functional peculiarities. Now, Philippe and colleagues have found two huge viruses, so very unique that they have been dubbed the “Pandoraviruses.”
“It’s like finding a sasquatch.” – Elodie Ghedin, virologist at University of Pittsburg, PA1
Pandoravirus salinus and Pandoravirus dulcis weigh in at 2.47 and 1.91 million base pairs long, respectively, and P. salinus is 1 micron long – one hundred times the size of most viruses! So, if these viruses are so large, why haven’t researchers seen them before now? Actually, they may have done so unknowingly. The discoverers suspect that people have seen Pandoraviruses before, but assumed that they were bacteria, since they were much larger than any known viruses. In fact, Pandoraviruses may be quite common – the researchers found one in Chile and one in Australia, and they could well exist in many other places.
Pandoraviruses are viruses, without a doubt: they do not produce their own proteins, they depend on living cells for replication, and they share some genes with other large viruses. Morphologically and genetically, however, Pandoraviruses are unique. Unlike most large viruses, they require activities in the host cell’s nucleus, lack many core viral genes, and have relatively few introns (pieces of a gene that are spliced out of the final gene product) in their genomes.
As the researchers stated in their paper, “93% of Pandoraviruses genes resemble nothing known, [and] their origin cannot be traced back to any known cellular lineage.”2 Their almost alien genome, coupled with the huge size of the genome, may even suggest the existence of a fourth domain of life, in addition to Bacteria, Archaea, and Eukaryota. Perhaps, the researchers suggest, early life was extraordinarily diverse, consisting of many different domains that evolved, existed, and eventually died out, leaving only the three that we know today – and maybe a fourth that we are just on the frontier of discovering.
1Pennisi, Elizabeth. “Ever-Bigger Viruses Shake Tree of Life.” Science Vol. 341, Issue 6143, pp. 226-227 (2013). DOI: 10.1126/science.341.6143.226. http://www.sciencemag.org/content/341/6143/226
2Philippe, Nadége, Matthieu Legendre, Gabriel Doutre, Yohann Couté, Olivier Poirot, Magali Lescot, Defne Arslan, Virginie Seltzer, Lionel Bertaux, Christophe Bruley, Jérome Garin, Jean-Michel Claverie, Chantal Abergel. “Pandoraviruses: Amoeba Viruses with Genomes Up to 2.5 Mb Reaching That of Parasitic Eukaryotes.” Science Vol. 341, Issue 6143, pp. 281-286 (2013). DOI: 10.1126/science.1239181. http://www.sciencemag.org/content/341/6143/281