In 2013, the discovery of two giant viruses unlike anything seen before blurred the line between the viral and cellular world. Pandoraviruses are as big as bacteria, and contain genomes that are more complex than those found in some eukaryotic organisms (1). Their strange amphora shape and enormous, atypical genome (2) led scientists to wonder where they came from.
The same team has since isolated three new members of the family in Marseille, continental France, Nouméa, New Caledonia, and Melbourne, Australia. With another virus found in Germany, the team compared those six known cases using different approaches. Analyses showed that despite having very similar shapes and functions, these viruses only share half of their genes coding for proteins. Usually, however, members of the same family have more genes in common.
Furthermore, these new members contain a large number of orphan genes, i.e. genes which encode proteins that have no equivalent in other living organisms (this was already the case for the two previously discovered pandoraviruses). This unexplained characteristic is at the heart of many a debate over the origin of viruses. What most surprised researchers was that the orphan genes differed from one pandoravirus to another, making it less and less likely that they were inherited from a common ancestor!
Bioinformatic analysis showed that these orphan genes exhibit features very similar to those of non-coding (or intergenic) regions in the pandoravirus genome. Findings indicate the only possible explanation for the gigantic size of pandoravirus genomes, their diversity and the large proportion of orphan genes they contain: most of these viruses’ genes may originate spontaneously and randomly in intergenic regions. In this scenario, genes “appear” in different locations from one strain to another, thus explaining their unique nature.
If confirmed, this groundbreaking hypothesis would make these giant viruses craftsmen of genetic creativity — a central, but still poorly explained component of any understanding of the source of life and its evolution. Paper. (public access) – Matthieu Legendre, Elisabeth Fabre, Olivier Poirot, Sandra Jeudy, Audrey Lartigue, Jean-Marie Alempic, Laure Beucher, Nadège Philippe, Lionel Bertaux, Eugène Christo-Foroux, Karine Labadie, Yohann Couté, Chantal Abergel, Jean-Michel Claverie. Diversity and evolution of the emerging Pandoraviridae family. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-04698-4 More.
No wonder that French virologist got frozen out for dissing Darwin.
See also: Why viruses are not considered to be alive
Another stab at whether viruses are alive
Phil Sci journal: Special section on understanding viruses
Should NASA look for viruses in space? Actually, it’s not clear that RNA came first. Nor is it clear that viruses precede life. A good case can doubtless be made for viruses being part of the scrap heap of existing life. But no matter. If you think you can find viruses in space, boldly go.
Why “evolution” is changing? Consider viruses
The Scientist asks, Should giant viruses be the fourth domain of life? Eukaryotes, prokaryotes, archaea… and viruses?
Are viruses nature’s perfect machine? Or alive?