Interferon Regulatory Factor 3 (IRF3) is a member of interferon-regulated transcription factor family and is known to play an important role in the innate immune response against viral infections. In this study, the expression of IRF3 in different tissues, developmental stages, and stocking densities of olive flounder was investigated. The expression of IRF3 was observed to gradually increase in early-stage juvenile fish. The highest expression was observed in later-stage juvenile fish when immune tissues were formed. High IRF3 expression was observed in the muscles and the brain tissues. The expression of IRF3 was observed in fish at different stocking densities after viral hemorrhagic septicemia virus (VHSV) infection. It yielded an interesting expression pattern in the muscles and the brain tissues of fish stocked at low density. These observations can be used as basic data for the study of the expression of immune response-related genes against viruses based on stocking density and immune systems in other fish species.

Olive flounder (Paralichthys olivaceus) is a vital aquaculture species in East Asia. However, few studies that estimate the genetic parameters of this species have been conducted. We estimated the genetic parameters of growth traits and designed an optimum breeding programme for this species. Heritability, genetic and phenotypic correlations, and breeding values were estimated for growth traits: body weight (BW), total length (TL), and condition factor (CF). A linear mixed animal model using the restricted maximum likelihood (REML) algorithm was applied to the statistical analysis of 9 traits (BW, TL, and CF at 11, 18, and 22 months of age) for a total of 54,159 animals from 7 generations. Increases of 13%, 8%, and 6.5% in BW, TL, and CF at the harvest stage were observed, respectively, after 7 generations of selection. The heritabilities of all growth traits were moderate, ranging from 0.35 to 0.46. The phenotypic and genetic correlations between BW and TL were high and positive in all three stages (0.91 and 0.92, 0.91 and 0.93, and 0.88 and 0.91). The estimated breeding values of BW and TL increased over the generations; however, the estimated breeding value of CF fluctuated. The optimum progeny number within full-sib families for an accuracy of 0.632 is suggested to be between 10 and 25. Findings indicated that a considerable response to selection and single-trait selection based on BW would be effective in olive flounder.

Ubiquitin-40S ribosomal protein S27a (RPS27A), ubiquitin-like protein Fubi, and ribosomal protein (S30FAU) are ubiquitin-related proteins that are involved in the regulation of immune-related functions such as cell cycle, protein expression, and apoptosis. This study aimed to confirm the molecular characteristics, gene expression analysis, and antibacterial activity of RPS27A and S30FAU identified from the starry flounder (15 starry flounders of 128.7 ± 18.2 g). An expression analysis using a normal fish showed that RPS27A was highly expressed in the head kidney, heart, and stomach. In contrast, S30FAU exhibited high expression in the stomach, heart, and head kidney. Upon simulating an artificial pathogen infection, RPS27A was highly expressed in the heart at 1 h and 3 days post-viral hemorrhagic septicemia (VHSV) infection, and had a high expression in the kidney, liver, and heart at 7 days post-Streptococcus parauberis (S. parauberis) infection. S30FAU was highly expressed in the spleen and gills at 1 day and 12 h post-VHSV infection, respectively, and exhibited a high expression in the kidney at 7 days post-S. parauberis infection. In an MIC analysis, RPS27A and S30FAU showed antimicrobial activity against all bacteria used in this study. In the biofilm assay, S30FAU was removed from S. parauberis in a concentration-dependent manner, and the cytotoxicity test showed no hemolytic activity in both RPS27A and S30FAU. Therefore, RPS27A and S30FAU of the starry flounder were confirmed to possess antimicrobial peptide abilities without limitations of cytotoxicity. This study provides valuable information on the antibacterial ability and molecular biology of the ubiquitin family isolated from the starry flounder.

Polymeric immunoglobulin receptor (pIgR) mediates the transfer of polymeric immunoglobulin to protect organisms and is one of the most important mucosal effectors. In this study, the developmental stage- and tissue-specific expression of pIgR were observed before virus inoculation in olive flounder. pIgR was gradually expressed until the formation of immune tissue, exhibiting high expression in the late juvenile period; thereafter, pIgR expression gradually decreased and exhibited high expression in the spleen and skin. Moreover, pIgR expression after viral hemorrhagic septicemia virus infection was high in the kidney and spleen tissues at high density and low at low density. The results of this study can provide a basis for future studies on breeding density, virus expression, and immune system studies in fish.