| Summary: | The genus Megalocytivirus is the newest member of the family Iridoviridae, and as such, little is known about the genetic diversity of these globally emerging fish pathogens. Using an Illumina MiSeq sequencer, we sequenced the genomes of two megalocytiviruses (MCVs) isolated from epizootics involving South American cichlids (keyhole cichlid, Cleithracara maronii and oscar, Astronotus ocellatus) and three spot gourami Trichopodus trichopterus circulating in the ornamental fish trade in the early 1990s. Phylogenomic analyses revealed South American cichlid iridovirus (SACIV) and three spot gourami iridovirus (TSGIV) possess nearly identical genomes and form a novel clade within the turbot reddish body iridovirus genotype (TRBIV Clade 2) previously reported from flatfish species reared for food in Asia (TRBIV Clade 1). The SACIV and TSGIV genomes were similar in size (111,347 and 111,591 bps, respectively), gene content (116 open reading frames for both), and %GC content (56.3 and 56.5, respectively) compared to other MCVs. However, both possess a unique truncated paralog of the major capsid protein (MCP) gene located immediately upstream of the full length parent gene. The MCP paralog likely arose through a gene duplication event and its function could be to increase antigenic diversity. Histopathological examination of archived oscar tissue sections revealed abundant cytomegalic cells characterized by basophilic granular cytoplasmic inclusions within various organs, particularly the anterior kidney, spleen and intestinal submucosa. A conventional PCR assay, designed to amplify and distinguish through Sanger sequencing all MCV genotypes, was partially validated and used to confirm the presence of SACIV DNA within archived formalin-fixed paraffin-embedded (FFPE) oscar tissues. TSGIV-infected grunt fin cells (GF) displayed cytopathic effect (e.g., cytomegaly, rounding, and refractility) as early as 96 hr post infection (pi). Ultrastructural examination revealed non-enveloped virus particles displaying hexagonal symmetry (120-144 nm) and an electron-dense core within the cytoplasm of infected GF cells, consistent with the ultrastructural morphology of a MCV. The sequencing of SACIV and TSGIV provides the first complete TRBIV Clade 2 genome sequences and expands the known host and geographic range of the TRBIV genotype to include freshwater ornamental fishes traded in North America.
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