Giant viruses over the dike
“We cracked the DNA-code of a giant algae virus; this is the first algae virus that belongs to the ‘Giant Viruses’”, concludes prof. Dr Corina Brussaard of the Royal Netherlands Institute for Sea Research and the University of Amsterdam in the 10 June issue of the journal PNAS. Until now the few known giant viruses all had an animal host, but now it appears that giant viruses with a plant host also exist.
Moreover, a host that is very common to sea water. The algae Phaeocystis globosa is well known for the enormous accumulations of foam on the beach formed by dead algae. The newly discovered giant virus ‘PgV-16T’ is also very common and capable of playing an important role in regulating this algae species that is common all over the world.
Giant viruses were first discovered only a few years ago. Most of them contain much more DNA than ‘ordinary’ viruses, although the currently discovered giant algae virus is quite modest in this respect. A more important conclusion is that the evolution of this type of virus most likely has been very different than that of the ‘ordinary’ smaller viruses.
“This is a very exciting discovery that changes our view of viruses”, says Corina Brussaard. “It is the largest algae virus of which we completely unravelled its DNA-code. Moreover, it’s the first ever giant virus that uses algae as host, which means they are much more common than we thought. But that is not all, within this virus we found a virophage; a virus that infects the giant virus. This implies that virophages are not restricted to the largest viruses, but can also infect smaller giant viruses like the one we discovered. These virophages are responsible for an exchange of genetic material between the different kinds of viruses.
These discoveries have prompted a discussion about the origin of viruses. The giant viruses have characteristics that until now were unique to living cells, which puts them far back in the evolution. It appears that the giant viruses originally were independent cells that existed inside other organisms as symbiont or parasite, but later lost genetic material making them dependent on their host. Probably, the giant viruses represent an extinct form of life that existed at the time of the last universal common ancestor, 3-4 billion years ago, and contributed to the development of our modern cells.
Brussaard: “If this is really the case, it means a potentially new pathway for the formation of viruses and the exchange of genetic material, with all sorts of possible unforeseen implications for our perception of viruses as pathogens.”
The study was conducted together with the research group of prof. dr Jean-Michel Claverie of the University of Marseille, France, also the discoverer of the first giant virus ‘Mimivirus’. The Dutch study was funded by the NIOZ.
Sebastien Santini, Sandra Jeudy, Julia Bartoli, Olivier Poirot, Magali Lescot, Chantal Abergel, Valérie Barbe, K. Eric Wommack, Anna A.M. Noordeloos, Croina P.D. Brussaard, Jean-Michel Claverie. The genome of Phaeocystis globosa virus PgV-16T highlights the common ancestry of the largest known DNA viruses infecting eukaryotes. Proceedings of the National Academy of Sciences of the United States of America (PNAS). doi:10.1073/pnas.1303251110