Polymorphism in an open reading frame, ORF-2, in the megalocytivirus of freshwater ornamental fishes

We examined the multi-length amplicons by PCR with a single set of primers to a putative open reading frame, the ORF-2 genomic region of megalocytivirus isolates obtained from freshwater ornamental fish. From sequence analysis of the multi-amplicons, we demonstrated the presence of distinct genomic polymorphisms in the ORF-2 region with differing numbers of repetitive element (RE) of 32 aa and 17 aa. SMPV (single major amplicon-producing megalocytiviruses) and MMPV (multiple major amplicon-producing megalocytiviruses) after challenge experiments in two different host species, the pearl gourami (Trichogaster leeri), the original ornamental species, and rock bream (Oplegnathus fasciatus), a cross-infective marine species, displayed the same feature of polymorphism in the ORF-2 region as those in the original sample. No significant difference in pathogenicity was found between SMPV and MMPV. Additionally, viral DNA extracted from the infected tissue homogenates incubated even for two days can produce distinct polymorphism in ORF-2 region is implying that each RE-subtype corresponding to each different amplicon is present in the capsid of viral particles rather than in the form of a naked nucleic acid. This is the first report elucidating the presence of various RE subtypes of megalocytivirus and possible simultaneous infection with different RE subtypes of virus in freshwater ornamental fish.