CC BY-SA 3.0
By Michael Le Page
Giants, self-sacrifice, biological warfare: this story has them all. A voracious marine predator plagued by a giant virus has a defence system we’ve never seen before – it fights back by making its very own virus.
The individuals that make these bioweapons sacrifice themselves for the greater good, saving their fellow predators in the process.
The single-celled predator, Cafeteria roenbergensis, is common in coastal waters around the world, where it snacks on bacteria (the biologists who discovered it in 1988 near the Danish town of Roenbjerg sat discussing their find in the local… yes, you guessed it).
But Cafeteria has a deadly enemy of its own, the giant CroV virus.
Most viruses are little more than a protein shell encapsulating a handful of genes. They depend entirely on the machinery of the cells they infect to make more copies of themselves.
But giant viruses, discovered only in 2003, are more like living cells than normal viruses. They have the machinery to make proteins, which means they are vulnerable to viral attack themselves. For example, maviruses infect CroVs, forcing them to make more maviruses instead of CroVs, as Matthias Fischer, now at the Max Planck Institute in Germany, discovered in 2011.
That, of course, is good news for Cafeteria, because mavirus halts the spread of CroV.
And Cafeteria has evolved to exploit the concept that the enemy of my enemy is my friend. Rather than waiting for maviruses to arrive by chance when CroVs attack, it actually carries the genes that code for mavirus inside its own genome.
These genes are usually dormant, but they get turned on when Cafeteria is invaded by CroV. “It acts as an inducible antiviral defence system,” write Fischer and his colleague Thomas Hackl in a new preprint paper.
The infected Cafeteria cell still dies – but when it breaks apart it releases maviruses rather than CroVs, preventing the spread of the infection. This, then, is altruistic behaviour, which turns out to be surprisingly common among microbes. For instance, some bacteria kill themselves as soon as they are infected by viruses to prevent the infection spreading.
Other microbes form spore-bearing structures, with the cells making the stalk sacrificing themselves to give the spore-forming cells at the top a chance of surviving.
Bioweapons at the ready
Cafeteria may not be the only animal to use living bioweapons to defend itself. A wide range of animals, from sea anemones to crocodiles, harbour genetic elements called Maverick transposons that closely resemble the mavirus genes. It’s possible that some of these organisms can also unleash viruses that attack giant viruses.
It is common for viral genes to end up inside the genomes of animals. In fact, our genomes are littered with the mutant remains of viruses and genetic parasites.
Many viruses deliberately insert their genes into the genomes of the animals they attack, so they can lie dormant and emerge when conditions are favourable. In response, most animals have evolved ways of shutting down genes that code for viruses.
It is, however, extremely unusual for an animal to deliberately trigger virus production, as Cafeteria does – but then mavirus is unusual, too, because it targets another virus rather than Cafeteria itself.
What is common is for genes that originally came from viruses to be co-opted for new purposes. Genes of viral origin play a key role during pregnancy, for instance.
And some bacteria have “spearguns” that they use to attack other bacteria. These spearguns evolved from the apparatus that bacteria-attacking viruses use to inject their genes into their victims.
Journal reference: Biorxiv, DOI: 10.1101/068312
Read more: How giant viruses could rewrite the story of __life on Earth
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