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Scientists have discovered viruses that infect viruses... virusception, if you will. Does this mean viruses qualify as life yet? Or are they still hanging out in the misfits drawer with sporks and Pluto?

Hosted by: Hank Green

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Use the code “SCISHOW” at [♪INTRO]. Viruses make so many people sick and miserable that it seems like turnabout—it’s only fair play! Make the viruses get sick for once!

But that’s ridiculous… right? Well, maybe not. Virus-infecting viruses, or virophages, are totally real, because evolution just doesn’t know when to quit.

The first virophage, named Sputnik, was discovered in 2008 by a team of French researchers in a Parisian water tower. But they didn’t discover it alone. Pretty much everything we learn about these extremely small viruses is linked to the extremely big viruses they infect, called giant viruses.

And since giant viruses, like mimiviruses and pandoraviruses, were only discovered in 2003, this whole field is brand new territory. Giant viruses were discovered infecting amoebas, and have been found all over the world. It’s not entirely clear whether they can make humans sick or not.

They’ve been isolated within humans a few times, including people sick with pneumonia, but it’s not 100% clear if they were causing the illness or just hanging out. And they’re huge… relatively speaking. In terms of their size and genetic complexity, giant viruses are more like bacteria than most other viruses.

Like, mimiviruses are about 500 to 800 nanometers across and have genomes of around 12 million base pairs. And pandoraviruses are even bigger. Now, mimiviruses are preyed upon by the Sputnik virophage, which is only 50 nanometers across and has a genome of about 18,000 base pairs.

That’s actually not super small for a virus. The bacteria-infecting virus lambda forms particles of a similar size, and the flu virus actually has a smaller genome. But what’s weird is that even though giant viruses are big, it doesn’t seem like they should be able to support invaders.

Especially since virophages are still…viruses. They’re made of DNA and protein, but they lack all the cellular replication tools needed to copy DNA, and the translation tools needed to make proteins. That’s why viruses need hosts.

They enter host cells and take over cellular equipment to make little virus factories. Without a host cell, viruses would never reproduce. And, in this case, the hosts are...also viruses.

Mimiviruses can’t copy DNA and protein on their own either, and have to infect an amoeba to reproduce. That’s where the virophage sneaks in. It infects the mimivirus while it’s infecting the amoeba and borrows the giant virus factory.

So instead of making more giant viruses, the amoeba churns out the virophage copies instead. This directly harms the giant virus and its efforts to reproduce, which is what leads researchers to say that virophages are parasites of giant viruses, not of amoebas. Now, giant viruses are kind of special because they seem to have some of that cellular equipment, like a few genes involved in protein translation.

But they don’t have ribosomes, the big factories that actually string proteins together. So they can’t get the job done all by themselves. That’s why many biologists exclude viruses from the tree of life that includes bacteria, archaea, and eukaryotes.

If viruses can’t replicate on their own, they’re not really alive. Scientists can compare various genes to determine evolutionary relationships, but if you want to go back to the very beginning, you have to compare something that all cellular life has. That’s ribosomes.

Scientists line up the gene sequences of ribosomes to determine who’s most closely related to whom. Since viruses don’t have ribosomes, that means there’s no way to fit them into that paradigm. Viruses have historically just not been considered enough to count as life.

Not enough complexity, not a big enough genome to fit in the same camp as life as we know it. But there’s still a debate going on. Like, maybe we’re being cell chauvinists.

After all, viruses don’t have ribosomes, but we cellular organisms don’t have capsid proteins, which form the coats of many viruses. And giant viruses are supercharging that debate because they are more complex and they do have big genomes. Not to mention, they have some of that protein-making machinery that was thought to be unique to us proper protein-making cellular creatures.

And the discovery of virophages has fired things up, too. Virophage DNA can become integrated into the DNA of its host virus, just like other viral genomes can integrate into cellular hosts. So if giant viruses can be preyed on like cells... maybe that’s an argument that we should think of them as being more alive.

At least, according to some scientists. We’ve only discovered 18 types of virophages so far, in places as diverse as lakes in Antarctica to contact lens fluid in France. Many of those are just hits in genome searches, meaning sequences similar to known virophages have turned up in genomic studies.

Nothing about them has actually been studied in detail yet, so we don’t know what they look like or what they infect. Which only sort of counts as discovering them. But it seems like there could be plenty more of them out there, and virophages and giant viruses might be more common than we expect.

The question of whether viruses are alive isn’t something we’re going to answer in a 4 minute video… because it’s not something that biologists are going to settle any time soon. The more we try to categorize evolution in neat boxes, the more curveballs it throws us. It’s a virus-eat-virus world out there.

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