Iridoviruses are large (120 to 300 nm in diameter) non-occluded viruses with icosahedral symmetry. An iridovirus virion is composed of three concentric domains; an outer proteinaceous capsid, an intermediate lipid membrane with associated polypeptides, and a central core containing DNA-protein complexes. Some, but not all, viruses possess an outer envelope acquired by budding through host membranes. Fibrillar structures have also been observed protruding from capsid subunits of LCDV-1, MIV, and CIV, but not from FV3. Depending on the detection method used iridoviruses possess between 25 to 75 structural proteins ranging in molecular weight from 12,000 to 150,000 kDa. A common feature of all iridoviruses is the presence of a major capsid protein of around 50 kDa that accounts for up to 45% of total virion protein.

Prolonged storage of Sericesthis iridescent virus (SIV) in distilled water at 4°C led to the disintegration of virions into triangular, pentagonal and linear fragments consisting of 55, 16, and 9 subunits respectively. These observations led Wrigley to propose the following 1562 morphological subunit structure for SIV. Later studies on TIV suggested that a structure represented by 1472 subunits may be more appropriate. Larger iridoviruses break down into larger but morphologically similar subunits.

 

Schematic diagram adapted from N.G. Wrigley 1969 (J. Gen. Virol. 5:123-134)

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