Abstract
As a new type of shrimp lethal virus, decapod iridescent virus 1 (DIV1) has caused huge economic losses to shrimp farmers in China. Up to now, DIV1 has been detected in a variety of shrimps, but there is no report in Marsupenaeus japonicus. In the current study, we calculated the LC50 to evaluate the toxicity of DIV1 to M. japonicus and determined through nested PCR that M. japonicus can be the host of DIV1. Through enzyme activity study, it was found that DIV1 can inhibit the activities of superoxide dismutase, catalase, lysozyme, and phenoloxidase, which could be a way for DIV1 to achieve immune evasion. In a comprehensive study on the transcriptomic changes of M. japonicus in response to DIV1 infection, a total of 52,287 unigenes were de novo assembled, and 20,342 SSR markers associated with these unigenes were obtained. Through a comparative transcriptomic analysis, 6,900 differentially expressed genes were identified, including 3,882 upregulated genes and 3,018 downregulated genes. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that some GO terms related to virus invasion, replication, and host antiviral infection were promoted under DIV1 infection, such as carbohydrate binding, chitin binding, chitin metabolic process, and DNA replication initiation, and some KEGG pathways related to immune response were significantly influenced by DIV1 infection, including Toll and IMD signaling pathway, JAK-STAT signaling pathway, IL-17 signaling pathway, C-type lectin receptor signaling pathway, complement and coagulation cascades, antigen processing and presentation, necroptosis, apoptosis, NOD-like receptor signaling pathway, apoptosis—multiple species, and TNF signaling pathway. Further analysis showed that STAT, Dorsal, Relish, heat shock protein 70 (HSP70), C-type lectins, and caspase play an important role in DIV1 infection. This is the first detailed study of DIV1 infection in M. japonicus, which initially reveals the molecular mechanism of DIV1 infection in M. japonicus by using the transcriptome analysis of hemocytes combined with enzyme activity study.
Highlights
Iridoviridae is a member of a monophyletic clade of large, nucleocytoplasmic DNA viruses with double-stranded DNA genomes ( ̇Ince et al, 2018)
Probit analysis showed that the LC50 of decapod iridescent virus 1 (DIV1) infection in M. japonicus is 2.64 × 109, 3.61 × 106, 2.69 × 105, 1.05 × 105, and 6.24 × 104 copies/μg DNA at 36, 48, 60, 72, and 84 hpi, respectively (Figure 1E)
To validate the sequencing results, five upregulated genes and seven downregulated genes were chosen for the qRTPCR analysis, including trypsin (TPS), C-type lectin 2 (CTL2), E3 ubiquitin-protein ligase RNF152-like (RNF152), heat shock protein 70 (HSP70), prophenoloxidase, crustin-like peptide type 4 (Crus4), copper/zinc superoxide dismutase isoform 5 (Cu/Zn-SODi5), FIGURE 4 | Gene ontology (GO) terms and annotation of the integrated transcriptome assembly
Summary
Iridoviridae is a member of a monophyletic clade of large, nucleocytoplasmic DNA viruses with double-stranded DNA genomes ( ̇Ince et al, 2018) According to their particle sizes, host range, DNA cross-hybridization, the presence of a methyltransferase, and the sequence of major capsid protein, family Iridoviridae is subclassified into five genera, including Iridovirus, Chloriridovirus, Ranavirus, Lymphocystivirus, and Megalocytivirus (Chinchar et al, 2017). Because the genome similarity between these two original isolations was 99%, the Executive Committee of the The International Committee on Taxonomy of Viruses (ICTV) (2019) identified SHIV 20141215 and CQIV CN01 as two virus isolates of decapod iridescent virus 1 (DIV1). This is the only species of the new genus Decapodiridovirus within the family Iridoviridae. Z. et al, 2020; He et al, 2021)
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