For many years, researchers mistook viruses known as giant viruses — which are 100 times larger than typical viruses — for bacteria, Carl Zimmer says at Discover's The Loom blog. But on closer examination, researchers found that despite their unusual size, these large viruses work much like typical viruses, infecting hosts and building new copies of themselves. Since they were "unveiled" seven years ago, researchers have debated the origins of these giant viruses, Zimmer says. "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," he says. Others have even suggested these giant viruses may represent a new form of life. Now, a new study published in PNAS reveals that giant viruses are indeed very old, and the researchers even found a new kind of giant virus so different from others that they gave it a new name, Zimmer says: "Megavirus." Its genome is nearly 1.3 million base pairs long, and contains 1,120 genes. When they compared it to the Mimivirus, the researchers found that the two share no genes in common, leading the team to conclude "that giant viruses started out giant," Zimmer adds. "In the Mimivirus and Megavirus lineage, the genes mutated in different trajectories, and new copies of genes arose, producing different gene families. … It's possible that Mimivirus lost 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."
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