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Megavirus claims ‘biggest genome’ crown

Move over Mimi, there’s a new big virus in town. Harvested from the guts of amoebae pseudopodding their way around the coast of Las Cruces, Chile, the virus Megavirus chilensis has the largest fully sequenced viral genome to date, researchers report this week in the Proceedings of the National Academy of Sciences. This genomic extravagance grants Megavirus membership in the exclusive pantheon of freakishly large, big-genome viruses that includes the celebrities Mimivirus and Mamavirus.

The gargantuan Mimivirus, so large it was initially mistaken for a bacterium, had already shattered expectations of what a proper virus should and should not be (for more, see ‘The challenge of microbial diversity: Out on a limb’). The closely related and nearly identical Mamavirus further challenged dogma by becoming infected by another virus, resurrecting old debates about what it means to be alive (see ‘‘Virophage’ suggests viruses are alive’).

Now, a team of French researchers led by Chantal Abergel and Jean-Michel Claverie report the discovery of the latest member of the giant virus club: Megavirus chilensis. Its equally massive (by viral standards) genome boasts 1,259,197 base pairs and 1,120 genes, 23% of which do not have any counterparts in the Mimivirus genome. That makes it the biggest of any fully sequenced viral genome to date, besting Mimivirus by 6.5%, the authors say.

So all-in-all, one more addition to the glorious microbial freakshow that never ceases to astound. (One of my other favorite attractions: Epulopiscium, the monstrous bacterium with 200,000 copies of its own genome.) You’ve just got to love this stuff not just for the sheer spectacle of it all, but especially for the reminder that there are still whole worlds out there waiting to turn dogma on its ear – all we have to do is figure out where to look.

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