Largemouth bass iridovirus (LMBV) can cause high mortality and lead to heavy economic loss in the cultivation of largemouth bass, but there was no effective treatment. Here, the present study constructed a recombinant Pichia pastoris expressing LMBV major capsid protein (MCPD). The recombinant GS115-pW317-MCPD was then used to immunize largemouth bass via oral administration, and mucosal immune response mediated by immunoglobulins (Igs) was measured after oral immunization. Serum antibody levels were measured by ELISA, neutralizing antibody titers were determined by serum neutralization test (SNT), antigen presentation-related gene expressions were detected by RT-PCR, and the histopathological characteristics of immunized fish were assessed after challenging with 0.1 ml 10^7.19 TCID₅₀/ml LMBV. The relative percentage survival (RPS) was also determined. Our results showed that the serum antibody titers of immunized fish were significantly higher than that of control groups (P < 0.05). IgT and IgM expressions in gut were increased significantly after vaccination with GS115-pW317-MCPD; however, much stronger response in gut was observed as compared with gill. The expression levels of major histocompatibility complex (MHC) II, CD8, and T-cell receptor (TCR) were significantly elevated in GS115-pW317-MCPD group (P < 0.05), while CD4 and MHC I transcription levels remained unchanged after oral immunization (P > 0.05). The RPS of fish orally immunized with 1.0 × 10⁸ CFU/g GS115-pW317-MCPD was reached up to 41.6% after challenge with 0.1 ml 10^9.46 TCID₅₀/ml LMBV. Moreover, orally immunizing with GS115-pW317-MCPD can relieve the pathological damage caused by LMBV. Therefore, GS115-pW317-MCPD showed a promising potential against LMBV.

Alkalinity stress is a significant abiotic factor influencing fish growth and productivity. Understanding the mechanisms underlying fish tolerance to saline–alkaline stress is essential for developing highly resistant fish strains and expanding the potential for freshwater aquaculture. This study examined the impact of alkaline stress on the survival rate, digestive enzyme activities, tissue morphology, and gene expression profiles in juvenile grass carp. Acute stress experiments revealed that the LC₅₀ value for juvenile grass carp (31.72 ± 8.88 g) exposed to alkalinity for 96 h was 44.11 mM, indicating relatively high tolerance to alkalinity. Furthermore, NaHCO₃-induced alkaline stress significantly affected digestive enzyme activities and tissue morphology. A tissue-specific transcriptome analysis (gill, liver, and kidney) was conducted at various time points during alkalinity stress exposure, identifying numerous differentially expressed genes and pathways associated with alkaline stress. Notably, several alkali-responsive genes, including serpinh1b, aqp10a, cbr11, scos3b, and mycb, were differentially regulated across the compared groups in PCR Array. The findings provide valuable insights into the mechanisms of alkaline stress tolerance.

The Chinese soft-shelled turtle (Pelodiscus sinensis) is an important aquaculture reptile with rich nutritional and medicinal values. In recent decades, the wild resources of P. sinensis have been depleting due to natural and artificial factors. Herein, we report the complete mitochondrial genome of four P. sinensis strains, including the Japanese (RB) strain, Qingxi Huabie (HB) strain, Jiangxi (JB) strain, and Qingxi Wubie (WB) strain. The nucleotide composition within the complete mitogenomes was biased towards A + T with a variable frequency ranging from 59.28% (cox3) to 70.31% (atp8). The mitogenomes of all four strains contained 13 protein-coding genes (PCGs), 22 tRNAs, 2 rRNAs, 1 control region, and a replication origin region of the L-strand replication (OL), which was consistent with most vertebrates. Additionally, the atp8, nad4l, nad6, and nad3 genes possessed high genetic variation and can be used as potential markers for the identification of these P. sinensis strains. Additionally, all PCGs genes were evolving primarily under purifying selection. Through comparative analysis, it was revealed that most of the tRNAs were structurally different in the TψC stem, DHU stem, and acceptor stem. The length of the tandem repeats in the control region was variable in the four P. sinensis strains, ranging from 2 bp to 50 bp. Phylogenetic analysis indicated that all P. sinensis strains clustered into one branch and were closely related to other Trionychinae species. Overall, this study provides mitochondrial genome information for different P. sinensis strains to support further species identification and germplasm resource conservation.