Mandarin fish ranavirus (MRV) infection poses a substantial challenge to the mandarin fish culture industry as no effective preventive or therapeutic measures currently exist. The creation of a highly permissive cell line from a natural host is crucial for developing a vaccine for MRV and understanding its pathogenic mechanisms. In this research, the mandarin fish (Siniperca chuatsi) kidney cell line (SCK) was isolated from mandarin fish kidneys. Subsequently, SCK-a to SCK-g monoclonal cell lines were derived from the SCK cell population, distinguished by morphological variations. Notably, MRV infection induced an advanced cytopathic effect (CPE) in almost all cells of the SCK-f clone. Further tests showed that MRV achieved a peak viral titer of 10^10.7 50% tissue culture infectious dose (TCID₅₀)/mL and consistently exceeded 10¹⁰ TCID₅₀/mL across nine passages in SCK-f cells. Electron microscopy verified the MRV virion integrity within SCK-f. In vivo experiments revealed that MRV infections led to cumulative mortality rates of 86.9% in mandarin fish and 88.9% in largemouth bass (Micropterus salmoides). Such results suggest that SCK-f is highly permissive to MRV. This study underscores the importance of cellular diversity in developing viral permissive cell lines. The SCK monoclonal cell line pool may offer potential for generating highly permissive cell lines for other mandarin fish viruses.

Background: Birnaviruses infect a wide range of aquatic and terrestrial hosts, including several economically important fish species. This study aimed to isolate and characterize a novel birnavirus strain from mandarin fish (Siniperca chuatsi), a high-value freshwater species in Chinese aquaculture. Methods: A novel strain, designated mandarin fish birnavirus (MFBV), was isolated from diseased fish and propagated in SCK cells. The complete genome was determined using high-throughput sequencing and RACE. Viral replication kinetics, tissue distribution, and pathogenicity were assessed through in vitro infection, RT-qPCR, histopathology, and experimental challenges. In addition, disinfectant sensitivity and environmental stability were evaluated. Results: The MFBV genome comprises two segments (A: 3539 bp; B: 2719 bp), and phylogenetic analysis revealed close relatedness to largemouth bass birnavirus (LBBV) and Lates calcarifer birnavirus (LCBV). MFBV displayed rapid replication in SCK cells, completing a replication cycle in 8–10 h. In juvenile and fry fish, an experimental infection caused acute disease with cumulative mortality ranging from 41.8% to 83.6%, with fry showing higher susceptibility. Viral RNA was detected in multiple tissues (7.9 × 10⁶–7.9 × 10⁷ copies/μg RNA), and histopathological lesions were observed in the intestine, spleen, and kidney. MFBV was highly sensitive to glutaraldehyde (20 ppm), while other disinfectants showed reduced efficacy. Viral half-life ranged from 36.5 to 144.5 h at room temperature. Conclusions: These findings demonstrate that MFBV can induce acute systemic infection in mandarin fish. The results offer new insights into the genomic and biological features of birnaviruses, contributing to improved disease management and viral taxonomy.

Siniperca chuatsi is an important high-quality freshwater aquaculture species in China. In nature, it feeds exclusively on live food. In this study, domesticated juvenile S. chuatsi were divided into three groups and fed live food (group L), compound feed (group C), or a mixed diet (group M) for three months to investigate the molecular mechanisms underlying adaptation to compound feed. Histopathological examination revealed that compound feed consumption induced looser liver cell arrangement, hepatocyte morphological irregularities, and vacuolization. A total of 1033 and 1428 differentially expressed genes (DEGs), and 187 and 184 differential metabolites (DMs), were identified in the C vs. L and C vs. M groups, respectively. Transcriptomic analysis revealed that the significantly and commonly enriched metabolic pathways shared by both comparison groups were predominantly involved in amino acid, carbohydrate, and lipid metabolisms. Metabolomic analysis demonstrated that the significantly and commonly enriched metabolic pathways shared by both comparison groups were the arachidonic acid metabolism, linoleic acid metabolism, oxidative phosphorylation, and PPAR signalling pathways. Integrated omics analysis showed that the PPAR signalling pathway was the only significantly co-enriched pathway across both omics datasets. This study provides new insights into the molecular mechanisms of compound feed adaptation and provides theoretical support for selecting feed traits in S. chuatsi.

Neuropeptide Q (spexin, spx) is a pleiotropic signalling molecule that regulates appetite and metabolism primarily via activation of galanin and melanocortin receptors. Here, we cloned the open reading frame (ORF) of spx from Siniperca chuatsi (Scspx), characterised its spatiotemporal expression, elucidated spx regulatory features during starvation and feed adaptation, and identified SPX-interacting proteins using glutathione S-transferase pull-down and mass spectrometry. The Scspx ORF was 312 bp, encoding 103 amino acids. The predominant expression of spx was found in the liver of feed-trained S. chuatsi, where it was 17.36-fold greater than in muscle. During fasting (0, 3, 5, and 7 d), spx expression in the muscle, liver, and intestine initially increased and then declined, whereas brain and stomach tissues exhibited the opposite tendency. Compared to the smallest individuals, hepatic and brain spx expression was substantially lower in the largest individuals, whereas stomach expression was higher (p < 0.05). Fatty acid binding protein 2 was identified as a novel SPX-interacting partner, implicating SPX in feed adaptation through lipid metabolic regulation via the peroxisome proliferator-activated receptor signalling pathway. Our results provide the first evidence of a direct SPX-FABP2 interaction in fish, pointing to a coordinated role in downstream gene regulation. This work hereby uncovers a novel regulatory axis within the piscine energy metabolism network. These findings provide new insight into the regulatory role of SPX in feed adaptation in S. chuatsi, offering a foundation for genetic analysis.

Birnaviruses infect a wide range of hosts, including economically important fish species. In this study, we report the isolation and characterization of a novel birnavirus, named mandarin fish birnavirus (MFBV). We obtained the full-length sequence of the MFBV genome. The segment A is 3539 bp, and the segment B is 2719 bp in length. Phylogenetic analyses revealed its close relation-ship with Largemouth bass birnavirus (LBBV) and Lates calcarifer birnavirus (LCBV) of the Birnaviridae family. Further, characterization of MFBV revealed its rapid replication kinetics that replication cycle is about 8-10 hours in mandarin fish kidney cells. Both intraperitoneal in-jection and immersion infection routes resulted in severe symptoms in juvenile or fry fish. Mortal-ity was occured within one day post-infection (dpi), with cumulative mortality rates ranging from 41.8% to 83.6% depending on the challenge method. Notably, compared to juvenile fish, fry fish exhibited higher susceptibility to infection. Viral RNA copies in multiple tissues ranged from the highest of 107.9 to the lowest of 106.9 per μg total RNA revealed that MFBV has a broad tissue tropism. Histopathological examination showed characteristic lesions, including villus damage in the intestine, necrosis in the spleen and kidney cells. MFBV was most sensitive to glutaralde-hyde, with a final concentration of 20 ppm resulting in an 8 log-unit reduction in titer. While povidone-iodine, formaldehyde, sodium hypochlorite, hydrogen peroxide, and potassium per-manganate required concentrations in the 10³ ppm range to achieve similar efficacy. The half-life of the virus during different phase was 36.5 hours to 144.5 hours.