by Marcia Stone
Viruses are supposed to be small and simple—not even alive, just mobile genetic material after all. So what do we make of giant double-stranded DNA (dsDNA) viruses, one of which—the newly discovered Pandoravirus salinus—has an even larger genome than a hunky parasitic eukaryote called Encephalitozoon? The recent identification of P. salinus adds evidence to growing speculation that it and other mammoth viruses evolved from cellular ancestors and represent domains of life that likely existed on Earth before the last universal common ancestor (LUCA). Increasing numbers of scientists are coming around to this point of view; some of whom like Gustavo Caetano-Anollés at the University of Illinois at Urbana-Champaign go even further, asserting in Microbe magazine that “giant viruses not only existed at the same time as the LUCA of cellular life, they’re direct descendants of the lineage that gave rise to it.” Caetano-Anollés does not say this lightly—he has protein data to substantiate the claim.
Mimiviruses and family
When they were initially spotted making life miserable for amoebas in a British water tower by Tim Rowbotham in 1993, the first-reported giant viruses were thought to be (yawn) bacteria and largely ignored. New-generation sequencing a decade later revealed their viral nature and because they’re as big as small cocci and stain Gram-positive the identifying scientists Didier Raoult, Jean-Michel Claverie and colleagues at Aix-Marseille Université in France named them “Mimiviruses” for “Mimicking Microbes.”
Mining the Global Ocean Sampling database searching for more giant viruses hinted that Mimiviruses had an extended seafaring family. However, the predicted marine relatives of Mimivirus remained elusive until 2010 when the Cafeteria roenbergenis virus, CroV for short, was discovered infecting the flagellate grazer Cafeteria roenbergenis. Then Megavirus chilensis was isolated from near-shore sediment off the coast of Chile and it, like Mimivirus, happily preys on several species of Acanthamoeba in the laboratory. Which eukaryotes M. Chilensis terrorize in the wild are still unknown.