Morphology and ultrastructure of Lymphocystis disease virus, a fish iridovirus, grown in tissue culture.

The morphology and the ultrastructure of the Lymphocystis disease virus (LDV) strain Leetown , a fish iridovirus , was studied by electron microscopy. The virus was grown on bluegill fry (BF-2) cells at 21 degrees. LDV showed an icosahedral shape by ultrathin sections and negative staining, with a diameter of about 200 nm. The shell of the virion seemed to be composed of two unitary membranes with a total diameter of 16 nm. The outer membrane demonstrated swelling in negative staining, exhibiting a central core and the presence of globular subunits at its external surface organized in geometrical arrays of 60 and 90 degrees. Glutaraldehyde fixation preserved very effectively the icosahedral structure with the subunits remaining invisible. The internal structure of the virions was composed of osmiophilic threads or granules of 6 to 8 nm in diameter surrounded by an amorphous material of 10 to 20 nm in thickness. External filaments were observed at the surface of the particles in ultrathin sections, giving the appearance of a halo surrounding the shell. In negative staining these filaments were rarely observed; in one virus preparation, they appeared in bundles.