Discovery of New Mega Viruses Suggests Possibility of New Domain

Following initial work in isolating Megaviridae i.e. Mimivirus (Megavirus chilensis), a further study involving world wide sampling of aquatic environments assessing diversity of Megaviridae has resulted in the classification of two new megaviruses. These have drastically dissimilar morphological resemblance to previous mega viruses, and 93% of Pandoravirus genes are unknown and cannot be traced back to any known lineage. The authors have classified these as a proposed genus – Pandoravirus.

The two organisms were first identified as parasites – The first was found in superficial marine sediment at the mouth of the Tunquen river (coast of central Chile). The second was extracted from mud taken at the bottom of a shallow freshwater pond near Melbourne, Australia.

They appear as ovoid particles, and both have characteristic ultrastructural features. Assuming viral nature, they were named Pandoravirus salinus and Pandoravirus dulcis

This was confirmed through observations of the parasites’ replication cycles in Acanthamoeba castellanii. Both species follow the same stage characteristics which are reminiscent of M. chilensis. The tegument and internal compartment of the particles are created simultaneously, unlike eukaryotic DNA viruses/phages. They also demonstrate no binary fission.

Sequencing their genomes resulted in a 2,473,870–bp sequence for P. salinus (2.47Mb) and 1,908,524-bp sequence for P. dulcis. (1.91Mb). Both were GC rich and had large variance in Protein-coding and non-coding regions, setting them aside from previously sequenced giant viruses.

Aligning the genomes showed them to be near identical, interrupted only by 4 sequences specific to P. salinus indicating the global gene content of P. dulcis is a subset of P. salinus.

Subsequent analysis comparing masked predicted protein sequences of P. salinus to National Center for Biotechnology Information nonredundant database entries yielded wide phylogenetic distribution between eukaryotes, bacteria and viruses. This coupled with low similarity levels, indicated no relation to previously sequenced micro-organisms. A lack in similarity of putitative protein coding sequences within the Megaviridae indicated they aren’t associated with the clade either.

The issue that the irregular morphology, replication and dissimilarity to sequenced genes may mean gene translation didn’t follow the standard genetic code and obscuring similarities. This possibility was investigated via a proteomic analysis.

This found P. salinus thus lacks most of the hallmark components of cellular organisms, including reduced intracellular parasites. It does posses half the genes for double stranded DNA viruses, but lacks a gene for major capsid protein and also genes for typical icosahedral symmetry. Unlike most Megaviridae, A lack of some core DNA genes confirmed that replication requires host functions normally segregated in the nucleus. Presence of DNA processing proteins keep it typically virus-like though.

A defining link for Megaviridae is thought to be virally encoded amino acid–tRNA ligases, but in Pandoraviruses these are closer to Acanthamoeba homologs.

As similar organisms were observed (though misinterpreted) 13 years ago and from their wide distribution, Pandoraviruses may not be rare but unobserved, due to unidentified niche and misinterpretation of organism.

Questioning similar misinterpreted organisms and exploring these niches with the faster and cheaper techniques available now could lead to more discoveries similar to Pandoravirus.

The dissimilarity to other domains and Megaviridae represents a possible new domain, forming link between viruses and cells. Though as so much of Pandoraviruses are not previously sequenced this could only be said in confidence with further research and understanding.

Philippe, N., Legendre, M., Doutre, G., Couté, Y., Poirot, O., Lescot, M., & Abergel, C. (2013). Pandoraviruses: amoeba viruses with genomes up to 2.5 Mb reaching that of parasitic eukaryotes. Science, 341(6143), 281-286.