Virology: Current Research

Serendipitous Discovery in a Marine Invertebrate (Phylum Chaetognatha) of the Longest Giant Viruses Reported to Date

Abstract

Roxane-Marie Barthélémy, Eric Faure and Taichiro Goto

The recent discovery of a nuclear giant virus that infects chaetognaths (small marine invertebrates) led us to reanalyze the surprising structures previously observed in this taxon. These elements, initially thought to be bristles and, later, bacteria, have been observed in two species and are in fact viral particles that likely correspond to two host-specific species. All of these viral particles have a spindle (fusiform) shape, an outer envelope and a tegument-like structure surrounding one internal membrane delimiting a compartment containing the genome and proteins. Electron photographs have provided a view of the sequential viral assembly and egress processes, which are concomitant and occur through the cytoplasmic membrane. During viral budding, the tegument-like wall self-assembles from a ring structure. Moreover, in the cell cytoplasm, the viral nucleoid is surrounded by two membranes. The virions that infect Paraspadella gotoi have a length range of ~2.5-3.1 μm and are not completely covered by the envelope revealing a kind of small "paintbrush" that is probably protein in nature. This structure does not appear in the viral particles infecting Spadella cephaloptera, who’s the size of a virus exceeds 4 μm, which is approximately twice the length of the bacterium E. coli and represents the longest known length of a virus. Moreover, they are perhaps the first known photographs of giant viruses (1967). Future genomic studies and further ultra-structural investigations are needed to improve knowledge of these viruses, which may represent a novel virus family for we provisionally propose the name Klothoviridae and the type species Klothovirus casanovai.