Isolation, identification and genomic analysis of an ISKNV-type megalocytivirus from spotted knifejaw (Oplegnathus punctatus)

The spotted knifejaw (Oplegnathus punctatus) is an emerging economical aquaculture species in China in recent years. In the present study, we characterized an iridovirus-like disease outbreak in spotted knifejaw aquaculture industry in Shandong Province (SKIV-SD), China. SKIV-SD induced severe cytopathic effect (CPE) in spotted knifejaw brain (SKB) cells as the infection proceeded. Moreover, electron microscopic observation showed the abundant hexagonal virus particles of ~140 nm in diameter in the cytoplasm of infected SKB cells. After challenge with SKIV-SD, the cumulative mortality rate was up to 100% at 9 days post-infection. And the diseased spotted knifejaw displayed pathognomonic microscopic lesions, demonstrated by basophilic cytoplasmic inclusions in spleen and kidney which was a general sign of megalocytivirus infection. Using Illumina NovaSeq system, we assembled the sequence data, and annotated the full genome of SKIV-SD, which was composed of 111,198 bp and contained 116 ORFs. Phylogenetic analyses based on 4 iridovirus core genes, including major capsid protein (MCP), myristylated membrane protein (MMP), DNA polymerase and ribonuclease III genes indicated that SKIV-SD shared the nearest relationship to infectious spleen and kidney necrosis virus (ISKNV). Moreover, the genome co-linearity analysis indicated that the gene orders and gene spacing of 25 iridovirus core genes were conserved between SKIV-SD and ISKNV. Together, our data speculated that SKIV-SD, an ISKNV-type megalocytivirus, was a lethal viral pathogen to cultured spotted knifejaw. •An ISKNV-type megalocytivirus, SKIV-SD was isolated from diseased spotted knifejaw.•SKIV-SD replicated well in spotted knifejaw brain (SKB) cells.•SKIV-SD genome was composed of 111,198 bp and contained 116 ORFs.•Phylogenetic analyses indicated that SKIV-SD shared the nearest relationship to ISKNV.•SKIV-SD genome shared the conserved co-linearity with ISKNV.