The world's largest virus has been discovered off the coast of Las Cruces, Chile. Appropriately named Megavirus chilensis, it is about 10 to 20 times larger than the average virus and 6.5% bigger than its nearest competitor, the Mimivirus. The Megavirus measures about .7 micrometers (thousandths of a millimeter) in diameter and is an oddity among its virus brethren, which are usually smaller than bacteria.

It is bigger than some bacteria, said Professor Jean-Michel Claverie of Aix-Marseille University in Marseille, France to BBC News. You don't need an electron microscope to see it; you can see it with an ordinary light microscope.

Viruses, unlike bacteria, are unable to copy themselves outside a host cell. In the wild, Megavirus probably infects ocean-inhabiting amoebas, free-floating single-celled organisms that prey on bacteria. Like that of the Mimivirus, the exterior of the Megavirus is lined with fibrils, hair-like structures that make the virus appear to be a bacterium. Scientists believe that this bacteria-like appearance attracts amoebae, which then play host to the Megavirus as it replicates itself.

Aside from its size, Megavirus is also set apart from other viruses by the complexity of its genome. Most viruses are extremely simple in construction, carrying only the DNA or RNA required - a skeleton crew, as it were, of genetic code. The Megavirus genome - the complete set of the virus' DNA - is unusually complex, with 1.26 million base pairs of DNA, or megabases. For reasons of comparison, the most basic viral genomes have a genome size of only 2 kilobases, or 2000 base pairs of DNA or RNA.

Both the Megavirus and the Mimivirus possess DNA genes, making them DNA viruses. Further similarities between the two behemoths lead scientists to conclude that they are genetic cousins. According to the abstract of the study published by Professor Claverie and his team, previous discoveries that some of the many genes found in the Mimivirus coded for functions once thought of as unique to cellular organisms led to further research about the relationship between large DNA viruses and early eukaryotes, organisms with cells whose membranes contain other structures.

The significance of the findings of Professor Claverie and his team lies in the support they lend to the theory that these large DNA viruses may actually be the descendants of these more complex eukaryotes. Having shed the various attributes of their ancient ancestors, perhaps the Megavirus, the Mimivirus and their ilk acquired the tendencies of other viruses despite having evolved from a different lineage.