A Sputnik moment for virus-infecting viruses

The vermin only teaze and pinch
Their foes superior by an inch.
So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite 'em,
And so proceed ad infinitum.

Jonathan Swift, "On Poetry: A Rhapsody"

There's so much to love about this story from Nature News. While surveying genetic snippets from a hypersaline lake in Antarctica, an Australian research team found a virus that preys on other viruses. And they sorted out the ecology of the lake well enough to figure out that the virus infected by the virus-infecting virus is a major predator on the lake's algae. Without a parasite to control the algal parasite, algal populations would decline and the lake's ability to sustain life at all would decline.

An Australian research team found the virophage while surveying the extremely salty Organic Lake in eastern Antarctica. While sequencing the collective genome of microbes living in the surface waters, they discovered the virus, which they dubbed the Organic Lake Virophage (OLV).

The OLV genome was identified nestling within the sequences of phycodnaviruses -- a group of giant viruses that attack algae. Evidence of gene exchange, and possible co-evolution, between the two suggests that OLV preys on the phycodnavirus. Although OLV is the dominant virophage in the lake, the work suggests others might be present.

By killing phycodnaviruses, the OLV might allow algae to thrive. Ricardo Cavicchioli, a microbiologist at the University of New South Wales in Sydney, Australia, and his colleagues found that mathematical models of the Organic Lake system that took account of the virophage's toll on its host showed lower algal mortality and more blooms during the lake's two ice-free summer months.

i-1461957a6aac88c91a0d5183e3be64d7-virophage.jpgVirophage means virus-eater, just as bacteriophage (a term you may recall from high school biology classes) means bacterium-eater. This is the first virophage detected in the wild, but other virophages have been shown to regulate algal populations as well. Another paper coming out this week finds that a widespread zooplankton called Cafeteria roenbergensis are attacked by a virus, and that infections of that virus are also regulated by a virophage called Mavirus:

"The Mavirus is able to rescue the infected zooplankton -- which, in a way, confers immunity from infection," says Curtis Suttle, a marine microbiologist at the University of British Columbia in Vancouver, Canada, and leader of the team that discovered the Mavirus.

"We unknowingly had Mavirus in culture with our Cafeteria system since the early 1990s," says Suttle. But the virophage was not identified until the Cafeteria genome was sequenced.

These viruses could be detected because their genes integrated into the plankton or algal genome, and were detected as researchers tried to match chunks of those genomes against other known genes. Both new virophages matched sequences from the first virophage ever detected, a virus called Sputnik detected in a French water-cooling tower. Similar gene snippets have been found elsewhere but not traced to a functional virus, and the researchers think there are lots of virophages waiting to be discovered. It'll be interesting to see how strong an effect they have on other host populations, and to explore whether virophagy evolved more than once. The three virus-infecting viruses known so far are all related, but the latest two were identified because we had Sputnik's gene sequences to compare new samples against.

Other lineages of virophages will have to be detected the way Sputnik was, by observing tiny viruses being spun off from a virus factory inside a host cell, and then filtering out the viruses and sequencing their genes.

Image: From the first paper describing a virophage, small particles of the virophage Sputnik inside the capsule of a mamavirus. Scale bar = 200 nm, Sputnik particles are roughly 50 nm across. From figure 1 of La Scola, et. al (2008) "The virophage as a unique parasite of the giant mimivirus," Nature 455:100-104.

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Since a "free" virus has virtually no metabolism, what's the mechanism of a virus' virus? Is it just hitchhiking, and then competing in the algae cell, or is it somehow doing something inside the larger virus?

Could these discoveries lead to the development of virophages that attack the viruses that infect humans?

By Beachcreature (not verified) on 29 Mar 2011 #permalink

Viruses "hijack" enzymes from their host cell to replicate. Some viruses depend more on their hosts' machinery than others. For example, HIV only has 9 genes while Mimivirus has over 1000. So giant viruses like Mimivirus and Cafeteria virus can make a lot of their own enzymes. It looks like virophages depend not so much on the cell but rather "hijack" enzymes from the giant virus that they associate with.