One of the biggest debates in biology today is over whether or not viruses count as life. Most scientists would contend that viruses are not alive and for good reasons: they cannot replicate on their own and require hijacking of their host's replication machinery in order to reproduce, they do not have any metabolism, they have no cellular structure and their replication is driven by spontaneous assembly or virus particles rather than by cellular division. Classifying viruses as "alive" would mean that cellular organization would not be included as a requirement for life (and opening the category up to things like prions). But according to a new article published in Nature, new findings are beginning to challenge the notion that viruses are not alive.
In 2003, a new family of viruses was discovered by researchers in France. From a sample taken from a cooling tower in the UK, researchers found a new type of virus that was infecting ameobae in the sample.1 This virus was way bigger than any other viruses known at the time, both in physical size and genome size. Having about 900 protein-coding genes - 3 times that of the next biggest virus and larger than some bacterial species - the virus is a behemoth. It was called mimivrus, for "mimicing microbe".
But that isnt the interesting part.
The same research team recently isolated a new strain of this giant virus, again from a cooling tower - this time in Paris. Electron micrographs of the virus showed that this strain, called mamavirus, had a friend. They discovered another, tiny virus that was closely associated with the mamavirus. They nicknamed this one Sputnik because it seemed to be a satellite of its larger companion. The team noticed that the mamavirus infected ameobae, just like its mimivirus cousin, and turned the cells into a virus assembly line to pump out more mamaviruses. However, if the amoebae were coinfected with Sputnik, the little satellite virus would hijack - not the amoeba's machinery - but the viral factory set up by the mamavirus! The mamavirus particles would turn out all screwed up and less infectious. It would seem that Sputkik is a viral virus! The researchers had found the first known virophage.2
Just like bacteriophages prey on bacteria, virophages prey on bacteria. They hijack the virus' machinery (that the virus itself has hijacked from the host cell), and this causes the virus to become ill. And that is what has rekindled the debate over viral "life". How can a virus - something that supposedly isnt alive - become sick?
I dont belive that this really throws a monkey wrench in the current classification of viruses as "nonlife". What the virophage is doing is interfering with the mamavirus' "virus factory" for its own use, and the mamavirus progeny turn out sickly. This is not a whole lot different from mutating a step in the assembly line so the virus churns out avirulant progeny. Mutate gene A so that protein B isn't incorpreted into the viral capsid, and you get viruses that are less infectous. Does that mean that the virus is alive? Not really. In both cases, the same sort of assembly line is being interrupted. A virus falling prey to another virus makes neither of them any more "alive" than viruses that have been mutated in a lab. The only difference here is that the changes to the viral factory are being carried out by another virus instead of a mutagen. Does that mean that the virophages are "alive"? No more than bacteriophage are alive. The new findings, I feel, do not suggest that viruses are alive. They suggest that viruses can fall prey to their own devious schemes.
But, draw your own conclusions. The debate on whether or not viruses are alive will not end with these new findings, and is likely to only become more intense. But, regardless of whether you think viruses are alive or not, you have to admit: mimi/mamaviruses and virophages are pretty frickin' cool.
1. La Scola, B. et al. Science 299, 2033 (2003)
2. La Scola, B. et al. Nature doi:10.1038/nature07218 (2008)