Cellular Origins of Giant Viruses?

The largest virus to be sequenced prompts researchers to consider whether giant viruses were once full-fledged living organisms.

Oct 16, 2011
Cristina Luiggi

Megavirus particleWIKIMEDIA, CHANTAL ABERGEL

Floating in the coastal waters of Chile, a team of French researchers has discovered and isolated yet another giant virus: Megavirus chilensis. Ringing in at a whopping 1.3 million base pairs, the Megavirus genome is the largest viral genome to be sequenced to date. Encoding around 1,120 putative proteins—the viral genome lies well in the range of many bacterial genomes and has researchers once again scratching their heads over the evolutionary origins of such mega-scale viruses.

A comparison of the genomes of Megavirus and Mimivirus—the first giant virus discovered—led researchers to conclude that not only are they distant relatives that share a wide range of genes including those for DNA repair, transcription, and viral factory genes, but that they may have both descended from an ancestor with a much larger genome. In fact, the authors suggest that the common ancestor may have been a free-living eukaryotic cell—one of the very first eukaryotic organisms—evidenced by the similarity of some giant virus genes with their living eukaryotic counterparts, Ars Technica reports. “Giant viruses might thus be relicts of the first chapters of the history of life,” Carl Zimmer wrote in a Discover blog post.