Which genus of giant viruses has the largest known genome?

Pandoravirus is the genus of giant viruses with the largest known viral genome. Members of the genus have genomes reaching roughly 2.5 million base pairs, a remarkable size for a virus and comparable to the genomes of some small cellular organisms.

Pandoraviruses were announced in 2013 after researchers identified Pandoravirus salinus in seawater collected off the coast of Chile and Pandoravirus dulcis in a freshwater pond in Australia. They infect amoebae rather than humans. Their oval particles can approach about 1 micrometer in length, large enough that early giant viruses sometimes confused researchers examining environmental samples.

The largest genome does not mean the largest virus particle. Pithovirus particles are longer, reaching about 1.5 micrometers, while Pandoravirus is generally described as the second-largest known virus by particle length. Mimivirus and related giant viruses also have large genomes, but typically below the Pandoravirus range.

A striking feature is that many Pandoravirus genes have no obvious counterparts in known organisms. This once encouraged speculation about a possible “fourth domain” of life, but viruses are not generally classified as a fourth cellular domain, and the evolutionary origin of giant viruses remains debated.