Ultrastructure of Meelsvirus: A nuclear virus of arrow worms (phylum Chaetognatha) producing giant "tailed" virions.

George L Shinn,Brianna L Bullard,Carmen San Martin

doi:10.1371/journal.pone.0203282

Abstract

Most known giant viruses, i.e., viruses producing giant virions, parasitize amoebae and other unicellular eukaryotes. Although they vary in the level of dependence on host nuclear functions, their virions self-assemble in the host cell’s cytoplasm. Here we report the discovery of a new prototype of giant virus infecting epidermal cells of the marine arrow worm Adhesisagitta hispida. Its 1.25 μm-long virions self-assemble and accumulate in the host cell’s nucleus. Conventional transmission electron microscopy reveals that the virions have a unique bipartite structure. An ovoid nucleocapsid, situated in a broad “head” end of the virion is surrounded by a thin envelope. The latter extends away from the head to form a voluminous conical “tail” filled with electron-dense extracapsidular material. The 31nm-thick capsid wall has a distinctive substructure resulting from a patterned arrangement of subunits; it bears no ultrastructural resemblance to the virion walls of other known giant viruses. The envelope self-assembles coincident with the capsid and remotely from all host membranes. We postulate that transmission to new hosts occurs by rupture of protruding virion-filled nuclei when infected arrow worms mate. Future genomic work is needed to determine the phylogenetic position of this new virus, which we have provisionally named Meelsvirus.

Highlights

“Giant viruses” are distinguished by the large physical size and/or large genome content of their virions [1, 2]
Based on morphological and molecular differences between them, Abergel, Legendre & Claverie [12] concluded that Mimivirus, Pandoravirus, Pithovirus, and Mollivirus are no more closely related to each other than any of them is to the last common ancestor of cellular organisms
This paper reports the serendipitous discovery of a new prototype of giant virus infecting epidermal cells of the marine arrow worm Adhesisagitta hispida

Summary

Introduction

“Giant viruses” are distinguished by the large physical size and/or large genome content of their virions (infective virus particles) [1, 2]. Viruses producing giant virions do not constitute a monophyletic group [1, 2, 6, 8,9,10,11,12]. Based on morphological and molecular differences between them, Abergel, Legendre & Claverie [12] concluded that Mimivirus, Pandoravirus, Pithovirus, and Mollivirus are no more closely related to each other than any of them is to the last common ancestor of cellular organisms. They may have originated separately from different ancestral protocells [12]. A few dozen genera of giant and near-giant viruses have been reported t

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Conclusion