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| Imaging of Mollivirus particles. (A) Scanning electron microscopy of two isolated particles showing the apex structure. (B) Transmission electron microscopy (TEM) imaging of an ultrathin section of an open particle after fusion of its internal lipid membrane with that of a phagosome. (C) Enlarged view of the viral tegument of a Mollivirus particle highlighting the layer made of a mesh of fibrils (black arrow), resembling Pandoraviruses’ intermediate layer, and the underneath internal membrane (white arrow). Three ∼25-nm interspaced rings are visible around the mature particle. (D) Light microscopy (Nomarski optics 63×) imaging of a lawn of Mollivirus particles, some of them (black arrow) exhibiting a depression at the apex. Figure via Legendre et al., 2016. |
A new study details the discovery of another giant DNA virus found in the Siberian permafrost. The new virus, called Mollivirus, is similar to another giant DNA virus that was also discovered in the permafrost, Pithovirus. However, although the pithovirus is oblong, the mollivirus is more round. It is also similar is structure to pandoraviruses, another set of large DNA virus. The mollivirus is smaller than some of the other large DNA viruses at 500-600 nm in diamter; however, it is still large enough to be seen with a light microscope (as seen by panel D in the picture above). Like the pithovirus, the researchers were able to infect an amoeba with the mollivirus and revive it. The genome is 651 kb long encoding for 523 proteins; 16% of the genes have their nearest orthologs to genes from pandoravirus and 10% to Acanthamoeba castellanii, most likely through horizontal gene transfer. As with these other large DNA viruses, most of the genes have no known orthologs. One of the more surprising findings is that ribosomes from the host are packaged into the virions.
These large DNA viruses have changed the way we think about viruses. With genome sizes in these viruses ranging from 0.6 to 2.8 Mb, they are comparable in size to the smallest eukaryote parasites. Theses viruses have diverse morphologies as seen below.
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| A Pithovirus. Credit: ulia Bartoli & Chantal Abergel; Information Génomique et Structurale, CNRS-AMU via Nature. |
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| Marseillevirus at different stages of its formation in an amoeba. Credit: Copyright Raoult / URMITE via Science Daily. |
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| The complex interior of a Mimivirus. Electron microscopy at magnification of about 200.Credit: Didier Raoult, picture by N. Aldrovandi via Live Science. |
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| Megavirus virion via Virology Blog. |
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| Pandoravirus virion via Virology Blog. |
Hopefully the reporting on the discovery of the mollivirus will be better than that of the pithovirus. When the pithovirus sample from Siberia was revived in amoebas, many science news sites proclaimed the scientists had resurrected a zombie virus and that unknown dangers lay in wait. The headlines from other news sources were much worse and I won't bother linking to them. Some news reports were much better and specifically mentioned that this virus posed no risk to humans. These viruses pose a risk to some organisms, but if you aren't an amoeba, then you'll be fine. These discoveries are offering vital clues into the evolution of viruses. Their discovery has also helped restart a conversation of if viruses are alive or not and what the definition of life should be.
