Pithovirus

Pithovirus is a genus of giant virus known from one species, Pithovirus sibericum which infectsamoebas.^[1][2] It is a double stranded DNA virus, and is member of the nucleocytoplasmic large DNA viruses clade. It was first described in 2014 after a specimen was revived from a 30,000-year-old ice core harvested from Siberia's permafrost.

Description

A specimen of Pithovirus measures approximately 1.5 µm in length and 0.5 µm in diameter, making it the largest virus yet found. It is 50% larger than the pandoraviruses, the previous largest known viruses.^[3] The species has a thick, oval wall with an opening at one end. Internally, its structure resembles honeycomb.^[1]

The genome of Pithovirus contains approximately 500 distinct genes, more than a typical virus but an order of magnitude less than found in Pandoravirus.^[3] Thus, its genome is far less densely packed than any other known virus. Two-thirds of its proteins are unlike those of other viruses. Despite the physical similarity with Pandoravirus, the Pithovirus genome sequence reveals that it is barely related to that virus, but more closely resembles members of the Marseilleviridae, Megaviridae and Iridoviridae.^[4] These families all contain large icosahedral viruses with DNA genomes.

Jean-Michel Claverie and Chantal Abergel, discoverers of the virus, speculated that Pithovirus may be a remnant of a large group of parasites that preyed on lifeforms common in early Earth history.^[5]

Replication

The viral genome encodes the genes needed to produce mRNA; these molecules are present in the purified Pithovirus particle.Pithovirus therefore undergoes its entire replication cycle in its host's cytoplasm, rather than the more typical method of taking over the host's nucleus.

Pithovirus sibericum was discovered in a 30,000 year old sample of Siberian permafrost by Claverie and Abergel of Aix-Marseille University.^[1] The virus was discovered buried 30 m (98 ft) below the surface of a late Pleistocene sediment.^[2][4] It was found when riverbank samples harvested in 2000 were exposed to amoebas.^[5] The amoebas started dying and when examined were found to contain giant virus specimens. The genus name Pithovirus, a reference to large storage containers of ancient Greece known as pithos, was chosen to describe the new species. The authors said they got the idea to probe permafrost samples for new viruses after reading about an experiment that revived a similar aged seed of Silene stenophylla two years earlier.^[1] The Pithovirus findings were published in the Proceedings of the National Academy of Sciences in March 2014.

Although the virus is harmless to humans, its viability after being frozen for millennia raised concerns that global climate change and tundra drilling operations could lead to new, deadly viruses being unearthed