Squashed ball-like dsDNA virus infecting a marine fungoid protist Sicyoidochytrium minutum (Thraustochytriaceae, Labyrinthulomycetes)

Abstract

Thraustochytrids are cosmopolitan osmoterotrophic microorganisms that play imtrophic or heterotrophic microorganisms that play important roles as decomposers, producers of polyunsaturated fatty acids, and pathogens of mollusks, especially in coastal ecosystems. Sicyoidochytrium minutum DNA virus (SmDNAV), a novel double-stranded DNA (dsDNA) virus infecting a marine eukaryotic decomposer, S. minutum (formerly Ulkenia minuta), was isolated from the estuary of the Shukugawa River, Hyogo Prefecture, Japan in July 2003, and its basic characteristics were examined. The morphology of the virus particles is 'squashed ball-like' (ca. 146 and 112 nm in length and width, respectively) and lacking a tail, distinctive from any other previously known viruses. Virions are formed in the cytoplasm of host cells, frequently accompanied by the disappearance of the nucleus. The lytic cycle and the burst size were estimated at <8 h and 3.6 x 10 2 to 1.1 x 10 3 infectious units per host cell, respectively. SmDNAV harbored a single molecule of dsDNA approximately 200 kbp in length. This is the first report of a marine fungoid protist-infecting DNA virus that has been isolated and characterized.