First Report on Natural Infection of Nodavirus in an Echinodermata, Sea Cucumber (Apostichopus japonicas)

Cross-species transmission of emerging viruses happens occasionally due to epidemiological, biological, and ecological factors, and it has caused more concern recently. Covert mortality nodavirus (CMNV) was revealed to be a unique shrimp virus that could cross species barrier to infect vertebrate fish. In the present study, CMNV reverse transcription-nested PCR (RT-nPCR)-positive samples were identified from farmed sea cucumber (Apostichopus japonicas) in the CMNV host range investigation. The amplicons of RT-nPCR from sea cucumber were sequenced, and its sequences showed 100% identity with the RNA-dependent RNA polymerase gene of the original CMNV isolate. Histopathological analysis revealed pathologic changes, including karyopyknosis and vacuolation of the epithelial cells, in the sea cucumber intestinal tissue. The extensive positive hybridization signals with CMNV probe were shown in the damaged epithelial cells in the in situ hybridization assay. Meanwhile, transmission electron microscopy analysis revealed CMNV-like virus particles in the intestine epithelium. All the results indicated that the sea cucumber, an Echinodermata, is a new host of CMNV. This study supplied further evidence of the wide host range of CMNV and also reminded us to pay close attention to its potential risk to threaten different aquaculture animal species.

Transcriptome analysis reveals differentially expressed genes for improving survival of dormant Apostichopus japonicus in response to high temperature

Background: Aestivation is one of the strategies used by sea cucumbers (Apostichopus japonicas) in order to improve survival in response to the high-temperature and droughty conditions. Previous studies have carried out to investigate the immune or physiological alterations at the aestivation stage. However, it lacks information on the relationship between immunity and physiology. Herein, transcriptome sequencing was used to study gene expression during the aestivation stage. The results of this study provide a comprehensive understanding of the molecular mechanisms that protect sea cucumbers from the high-temperature condition, which favors improving survival in cultured sea cucumbers. Results: The transcriptome analysis of dormant (aestivation) and revival sea cucumbers generated 2,368 differentially expressed genes (down-regulation: 927; up-regulation: 1,441) and 39,081 unchanged genes. Basing on Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses, the down-regulated genes of dormant group were identified to be involved in DNA replication, RNA metabolic process, protein modification and biosynthesis, macromolecule metabolism, and cellular metabolism, which resulted in the inhibition of motility, skeletal development, neural activity, cell proliferation and development of A. japonicas. In contrast, the up-regulated genes were associated with fatty acid metabolism, carbohydrate hydrolysis, and phagocytosis. Protein-protein interaction network analysis further revealed that carbohydrate hydrolysis promoted the phagocytosis activity in the dormant group. Furthermore, the expression pattern of all tested genes in qRT-PCR analysis fitted well with those in RNA-Seq, with the exception of FASNL, which was unchanged in the qRT-PCR but up-regulated in RNA-seq. Conclusions: During the dormant stage, sea cucumbers decreased DNA replication, transcription and translation to achieve a hypometabolic state beneficial for reducing energy consumption. On the contrary, fatty acid metabolism and carbohydrate hydrolysis were increased for energy supply. Moreover, high levels of carbohydrate hydrolysis promoted phagocytosis, which is a crucial innate immune response to infection by pathogens. These results provided new insight into potential molecular mechanisms that enable the sea cucumbers to respond to high temperatures. Keywords: Aestivation, Apostichopus japonicas, hypometabolism, fatty acid metabolism, carbohydrate hydrolysis, phagocytosis