Please welcome Megavirus, the world's most ginormous virus

By Carl Zimmer | October 10, 2011 3:00 pm

There are many weird viruses on this planet, but none weirder–in a fundamentally important way–than a group known as the giant viruses.

For years, they were hiding in plain sight. They were so big–about a hundred times bigger than typical viruses–that scientists mistook them for bacteria. But a close look revealed that they infected amoebae and built new copies of themselves, as all viruses do. And yet, as I point out in A Planet of Viruses, giant viruses certainly straddle the boundary between viruses and cellular life. Flu viruses may only have ten genes, but giant viruses may have 1,000 or more. When giant viruses invade a host cell, they don’t burst open like other viruses, so that their genes and proteins can disperse to do their different jobs. Instead, they assemble into a “virus factory” that sucks in building blocks and spits out large pieces of future giant viruses. Giant viruses even get infected with their own viruses. People often ask me if I think viruses are alive. If giant viruses aren’t alive, they sure are close.

Ever since giant viruses were first unveiled seven years ago, scientists have argued about the origins of these not-so-wee beasties. Many of their genes are different from those found in cellular life forms, or even other viruses. It’s possible that giant viruses amassed their enormous genetic armamentarium over billions of years, picking up genes from long-extinct host or swapping them with other viruses we have yet to find. Other scientists have suggested that giant viruses started out giant–or even bigger than they are today. Some have even argued that they represent a new domain of life, although others aren’t so sure.

A new study suggests that giant viruses are indeed ancient. It is the work of a team of French researchers led by Jean-Michel Claverie, who went searching for new giant viruses in the waters near a marine biology station in Chile. They found a new kind so different from other giant viruses that they gave it a name of its own:


It’s astonishing to me that such a glorious name wasn’t already taken. In the dinosaur world, people are always hunting for Latin ways to say, “I’ve got a really big dinosaur.” Supersaurus, Ultrasaurus, Megalosaurus, Truckasaurus. The name Megavirus is truth in advertising. Its genome is 1.259 million base pairs long, which is 6.5 percent longer than the previous record holder among giant viruses. In that abundance of DNA are 1120 genes. That’s hundreds more genes than found in a lot of bacteria. (You can browse its genome for yourself here.)

Claverie and his colleagues compared the genes in Megavirus to the best-studied of the giant viruses, Mimivirus. They could not find matches for 258 Megavirus genes in Mimivirus. But they found counterparts to most of its genes, including genes for distinctively giant-virus features such as the viral factory. (The inset in the picture above shows the portal of a Megavirus viral factory, called a “stargate.” It’s similar to the stargate found in Mimivirus.) Mimiviruses have some genes for building and folding proteins, and so do Megavirus.

These results lead Claverie and his colleagues to conclude that giant viruses started out giant. They might have even been some full-blown cellular life form. In the Mimivirus and Megavirus lineage, the genes mutated in different trajectories, and new copies of genes arose, producing different gene families. The lack of 258 Megavirus genes in Mimivirus might not mean that Megavirus picked up those genes from other sources. It’s possible that Mimivirus lost those genes. Likewise, Megavirus may have lost hundreds of genes as well. Giant viruses might thus be relicts of the first chapters of the history of life. (You can read more about this scenario in this 2010 review by Claverie: pdf.)

Fortunately, there’s a straightforward way to test this hypothesis: find more giant viruses and see if they fit the pattern. Giant viruses seem to thrive in all sorts of habitats, so there should be no end of new species to find. And given that it didn’t take long to trump the old genome-size record with Megavirus, you can expect scientists to find even bigger viruses somewhere on Earth.

Whether they should call these new species Truckavirus, I leave to greater minds.

Reference: “Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae” Defne Arslan, Matthieu Legendre, Virginie Seltzer, Chantal Abergel, and Jean-Michel Claverie. PNAS, in press. Link [should work by the end of this week]

CATEGORIZED UNDER: A Planet of Viruses, Top posts

Comments (19)

  1. aupaathletic

    It’s astonishing to me that such a glorious name wasn’t already taken. In the dinosaur world, people are always hunting for Latin ways to say, “I’ve got a really big dinosaur.”

    hahaha. Great article!

  2. megan

    They creep me out. It looks like a huge eye of Sauron.

  3. jackson

    I’m also surprised that the name megavirus wasn’t taken. lol Maybe the next one they find will be named Superman virus or King Kong virus. lol

  4. Torbjorn Larsson, OM

    Megathanks, I love seeing deep lineage hypothesized or confirmed. The more we know about our roots, the better.

  5. Great discovery – and nice article. I really think we are only just starting to uncover the true diversity of viruses, and that the best is yet to come. The possibility that viruses provided the eukaryotic nucleus becomes more likely with the continuing discoveries of very big viruses representing very ancient genetic lineages, which do not appear to have been assembled by horizontal gene transfer out of cells.

  6. Torbjorn Larsson, OM

    From Claverie et al: “virocell”.

    Ha, when this layman was trying to place viruses as life (after all, they do participate in the process of life, evolution) years ago, I concluded that the cell + virus system was more complex than the healthy cell in some senses (say, genetically).

    I didn’t realize at the time the correct conclusion was to invert the virion factory as the phylogenetic ancestor and the virion as the not-so-constrained “seed”. It was but later I heard of the hourglass developmental model for animals, and I never went over my old ideas and connected the two.

    Maybe that model _is_ generalizable over domains. In any case, looking for ancestry also in the virion factory seems to make so much more sense now.

    A viral article, indeed.

    The possibility that viruses provided the eukaryotic nucleus becomes more likely

    Yes, all of these models becomes more likely, including the model that these viruses are cellular descendants independent of eukaryotes or the nucleus.

    What we need would be testing.

    [Actually, since planctomycetes independently use membrane assembly like eukaryotes, with related proteins that then seems to have deep ancestry, the idea that the mitochondrion and/or archaean ancestors of eukaryotes had access to this core functionality [sic!] seems testable.

    That would be simpler than assuming viral transfer later, wouldn’t it? I am no biologist, but I think that, simplest phylogeny, constitutes the test we are looking for.]

  7. Torbjorn Larsson, OM

    So I don’t have access to the paper yet, but apparently the authors _do_ find relationship to eukaryotes.

    But they argue, from whatever support (maybe age combined with the giant virus lacking nucleus as the eukaryote/megavirus ancestor would), that it is right after the eukaryote/archaean split and before eukaryotes evolved nucleus. This research gets more and more interesting. [/paper abstinence]

    I still stand with the idea of testing phylogeny of this type of membrane assembly, it can’t hurt.

  8. JM Claverie

    Very good article. Perfect understanding of the science, great sense of humor. To my opinion the best Megavirus article I read on the net, with the one by BBC-News a close second.
    Thanks for your interest in our work.

  9. Chris

    All this talk about being the largest virus and most genes but you never give the actual size!

  10. lqd

    I still don’t understand how giant viruses can be grouped into the fourth domain of LIFE, aren’t viruses considered nonliving? Or has the concensus changed? Mr. Zimmer, can you tell me if most biologists still consider viruses nonliving? And if they do, why did several biologists suddenly decide to group them into the fourth domain?

  11. David B. Benson

    What Chris wrote.

    How big are these monsters? Should I now be afraid to walk in the dark?

  12. Chris

    @10 David
    From an episode of Star Trek: Voyager

  13. Rolf

    The wonders of the Creator.

  14. Dave in Calif

    We might be megaviruses ourselves…superdupermonstertruck virus.

  15. Margo Tannenbaum

    EXCELLENT and very thought provoking article! Please – continue to publish updates and news about this Megavirus and it’s effects on our world. Thank you, most sincerely.

  16. Kim

    I wonder at the line of thought …that indicates the virus may have been another creature at one time… and lost so much of it’s genetic information… that all that is left is a virus… the implications of that thought are endless and very enertaining…. What was it? Why did it change? Was it a spare part or much more? Will it ever be apart of another creature? How could this come about? Some day gene study may tell us how we evolved.

  17. Does make one wonder at the networking amongst the microbes all around us. We seem more like colonial groupings of bacteria/viruses and less like dominant organisms.


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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.


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