A giant virus that infests microscopic sea creatures has the largest genome of any marine virus, and the second largest of any virus. Its genome includes a host of genes not normally found in viruses, lending support to claims that viruses had a critical role in the evolution of complex life.

The virus was found infesting a single-celled organism called Cafeteria roenbergensis, which lives in oceans around the world and eats other single cells, mostly bacteria. The virus is named after its host, going by the name of Cafeteria roenbergensis virus strain BV-PW1, or CroV.

As viruses go CroV is a whopper, with a protein shell 300 nanometres across – though the record held by the mimivirus, 500nm across, is in no danger. CroV was first discovered in 1995 by Curtis Suttle of the University of British Columbia in Vancouver, Canada, and his colleague Randy Garza. Now a team headed by Suttle has analysed its DNA.

CroV’s genome is 730,000 bases long; among viruses, only the mimivirus has a larger genome. CroV’s contains 544 genes, including some that code for a series of proteins used in metabolism. Viruses rarely carry the genes for such things, because they normally hijack the molecular machinery of cellular organisms.


CroV’s advanced genetic baggage marks it as similar to the mimivirus, which also carries a lot of genes that viruses do not normally have.

“They really blur the lines between cellular and viral life,” says Suttle.

No thief

It has often been assumed that such giant viruses “steal” their genes from their hosts. However, many of CroV’s genes are completely different to anything found in cellular organisms, suggesting that they are unique to viruses.

CroV’s genome is remarkably similar to that of the mimivirus, with 12 per cent of the genes apparently matching up. This implies that all giant viruses have a single common ancestor, says Suttle. Jean-Michel Claverie of the Genomic and Structural Information Laboratory in Marseille, France, who was not involved in the study, agrees.

Claverie is one of a growing band of researchers who think that viruses may have shaped the course of evolution. Around 2 billion years ago, he suggests, a giant virus became trapped in a bacterium. The two formed a cooperative relationship, and, so the controversial theory goes, the virus eventually developed into the nucleus found in modern eukaryotic cells.

“Suttle’s work is very important for the debate on the evolution of viruses,” he says.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1007615107