Viruses have always been inconvenient to categorize. In 2003, classifying these quasi-organisms got way more complicated with the discovery of a viral behemoth dubbed Mimivirus.

Scientists are so puzzled by Mimivirus's enormous physical size and staggering number of genes that they are throwing around the idea that the tree of life has a fourth domain that we are only just now finding out about.

Genes encode protein molecules, which in turn carry out a huge variety of functions. Humans have more than 20,000 protein-encoding genes. Baker's yeast has around 6,000. The various flu viruses have somewhere on the order of a dozen. Ebola virus has seven.

Mimivirus has an astonishing 979 protein-encoding genes, and it's not even the largest virus of all. In 2013, a paper in the journal Science reported that a species of Pandoravirus has 2,541 protein-encoding genes.

Viruses straddle the line between living and nonliving, precisely because they have very few protein-encoding genes in their extremely tiny "bodies." The genes they do have essentially provide the bare minimum needed to create copies of the virus — instructions for hijacking the cellular machinery of some other creature to do pretty much everything for them. The ability to reproduce is central to the definition of life, yet, viruses lack the hardware necessary to perform even this basic task on their own.

Evolution is a thrifty process, and many genes and the proteins they encode get repurposed as time goes on. As a result, many genes, especially those in more primitive organisms, show up elsewhere in the tree of life.

These giant viruses don't seem to follow that rule, either. Only 7 percent of their genes match those of other organisms that have been sequenced, which raises an important question.

"What the hell is going on with the other genes?" Jean-Michel Claverie of Aix-Marseille University in France, one of the first people to identify giant viruses, asked in a piece from Nature. "This opens a Pandora's box. What kinds of discoveries are going to come from studying the contents?"

Below is an excellent episode of RadioLab that delves into these questions and some fast facts about the five largest known viruses.

Pandoravirus (salinus and dulcis)

Genes: 2,541 (Pandoravirus salinus) and 1,487 (Pandoravirus dulcis)

Year Discovered: 2013

Megavirus

Genes: 1,120 (Megavirus chilensis)

Year Discovered: 2010

Mamavirus

Genes: 1,023

Year Discovered: 2008

Mimivirus

Genes: 979

Year Discovered: 2003

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Tags: Science Virus