Liver transcriptome analysis reveal the metabolic and apoptotic responses of Trachinotus ovatus under acute cold stress.

Trachinotus ovatus is an economically important fish and has been recommended as a high-quality aquaculture fish breed for the high-quality development of sea ranches in the South China Sea. However, T. ovatus shows intolerance to low temperature, greatly limiting the extension of farming scale, reducing production efficiency in winter, and increasing farming risks. In this study, liver transcriptome analysis was investigated in T. ovatus under acute low temperature conditions (20 and 15 °C) using RNA sequencing (RNA-Seq) technology. Inter-groups differential expression analysis and trend analysis screened 1219 DEGs and four significant profiles (profiles 0, 3, 4, and 7), respectively. GO enrichment analysis showed that these DEGs were mainly related to metabolic process and cell growth and death process. KEGG enrichment analysis found that DEGs were mainly associated with lipid metabolism, carbohydrate metabolism, and cell growth and death, such as gluconeogenesis, glycolysis, fatty acid oxidation, cholesterol biosynthesis, p53 signaling pathway, cell cycle arrest, and apoptotic cell death. Moreover, protein-protein interaction networks identified two hub genes (FOS and JUNB) and some important genes related to metabolic process and cell growth and death process, that corresponding to enrichment analysis. Overall, gluconeogenesis, lipid mobilization, and fatty acid oxidation in metabolic process and cell cycle arrest and apoptotic cell death in cell growth and death process were enhanced, while glycolysis, liver glycogen synthesis and cholesterol biosynthesis in metabolic process were inhibited. The enhancement or attenuatment of metabolic process and cell growth and death process is conducive to maintain energy balance, normal fluidity of cell membrane, normal physiological functions of liver cell, enhancing the tolerance of T. ovatus to cold stress. These results suggested that metabolic process and cell growth and death process play important roles in response to acute cold stress in the liver of T. ovatus. Gene expreesion level analysis showed that acute cold stress at 15 °C was identified as a critical temperature point for T. ovatus in term of cellular metabolism alteration and apoptosis inducement, and rewarming intervention should be timely implemented above 15 °C. Our study can provide theoretical support for breeding cold-tolerant cultivars of T. ovatus, which is contributed to high-quality productions fish production.

Whole-Genome Sequencing Analyses Reveal the Whip-like Tail Formation, Innate Immune Evolution, and DNA Repair Mechanisms of Eupleurogrammus muticus.

Smallhead hairtail (Eupleurogrammus muticus) is an important marine economic fish distributed along the northern Indian Ocean and the northwest Pacific coast; however, little is known about the mechanism of its genetic evolution. This study generated the first genome assembly of E. muticus at the chromosomal level using a combination of PacBio SMRT, Illumina Nova-Seq, and Hi-C technologies. The final assembled genome size was 709.27 Mb, with a contig N50 of 25.07 Mb, GC content of 40.81%, heterozygosity rate of 1.18%, and repetitive sequence rate of 35.43%. E. muticus genome contained 21,949 protein-coding genes (97.92% of the genes were functionally annotated) and 24 chromosomes. There were 143 expansion gene families, 708 contraction gene families, and 4888 positively selected genes in the genome. Based on the comparative genomic analyses, we screened several candidate genes and pathways related to whip-like tail formation, innate immunity, and DNA repair in E. muticus. These findings preliminarily reveal some molecular evolutionary mechanisms of E. muticus at the genomic level and provide important reference genomic data for the genetic studies of other trichiurids.

Integrated Transcriptomics and Metabolomics Reveal Changes in Cell Homeostasis and Energy Metabolism in Trachinotus ovatus in Response to Acute Hypoxic Stress.

Trachinotus ovatus is an economically important mariculture fish, and hypoxia has become a critical threat to this hypoxia-sensitive species. However, the molecular adaptation mechanism of T. ovatus liver to hypoxia remains unclear. In this study, we investigated the effects of acute hypoxic stress (1.5 ± 0.1 mg·L-1 for 6 h) and re-oxygenation (5.8 ± 0.3 mg·L-1 for 12 h) in T. ovatus liver at both the transcriptomic and metabolic levels to elucidate hypoxia adaptation mechanism. Integrated transcriptomics and metabolomics analyses identified 36 genes and seven metabolites as key molecules that were highly related to signal transduction, cell growth and death, carbohydrate metabolism, amino acid metabolism, and lipid metabolism, and all played key roles in hypoxia adaptation. Of these, the hub genes FOS and JUN were pivotal hypoxia adaptation biomarkers for regulating cell growth and death. During hypoxia, up-regulation of GADD45B and CDKN1A genes induced cell cycle arrest. Enhancing intrinsic and extrinsic pathways in combination with glutathione metabolism triggered apoptosis; meanwhile, anti-apoptosis mechanism was activated after hypoxia. Expression of genes related to glycolysis, gluconeogenesis, amino acid metabolism, fat mobilization, and fatty acid biosynthesis were up-regulated after acute hypoxic stress, promoting energy supply. After re-oxygenation for 12 h, continuous apoptosis favored cellular function and tissue repair. Shifting from anaerobic metabolism (glycolysis) during hypoxia to aerobic metabolism (fatty acid ß-oxidation and TCA cycle) after re-oxygenation was an important energy metabolism adaptation mechanism. Hypoxia 6 h was a critical period for metabolism alteration and cellular homeostasis, and re-oxygenation intervention should be implemented in a timely way. This study thoroughly examined the molecular response mechanism of T. ovatus under acute hypoxic stress, which contributes to the molecular breeding of hypoxia-tolerant cultivars.

Transcriptomic Response of the Liver Tissue in Trachinotus ovatus to Acute Heat Stress.

Trachinotus ovatus is a major economically important cultured marine fish in the South China Sea. However, extreme weather and increased culture density result in uncontrollable problems, such as increases in water temperature and a decline in dissolved oxygen (DO), hindering the high-quality development of aquaculture. In this study, liver transcriptional profiles of T. ovatus were investigated under acute high-temperature stress (31 °C and 34 °C) and normal water temperature (27 °C) using RNA sequencing (RNA-Seq) technology. Differential expression analysis and STEM analysis showed that 1347 differentially expressed genes (DEGs) and four significant profiles (profiles 0, 3, 4, and 7) were screened, respectively. Of these DEGs, some genes involved in heat shock protein (HSPs), hypoxic adaptation, and glycolysis were up-regulated, while some genes involved in the ubiquitin-proteasome system (UPS) and fatty acid metabolism were down-regulated. Our results suggest that protein dynamic balance and function, hypoxia adaptation, and energy metabolism transformation are crucial in response to acute high-temperature stress. Our findings contribute to understanding the molecular response mechanism of T. ovatus under acute heat stress, which may provide some reference for studying the molecular mechanisms of other fish in response to heat stress.

Chromosome-Level Genome Assembly Provides Insights into the Evolution of the Special Morphology and Behaviour of Lepturacanthus savala.

Savalani hairtail Lepturacanthus savala is a widely distributed fish along the Indo-Western Pacific coast, and contributes substantially to trichiurid fishery resources worldwide. In this study, the first chromosome-level genome assembly of L. savala was obtained by PacBio SMRT-Seq, Illumina HiSeq, and Hi-C technologies. The final assembled L. savala genome was 790.02 Mb with contig N50 and scaffold N50 values of 19.01 Mb and 32.77 Mb, respectively. The assembled sequences were anchored to 24 chromosomes by using Hi-C data. Combined with RNA sequencing data, 23,625 protein-coding genes were predicted, of which 96.0% were successfully annotated. In total, 67 gene family expansions and 93 gene family contractions were detected in the L. savala genome. Additionally, 1825 positively selected genes were identified. Based on a comparative genomic analysis, we screened a number of candidate genes associated with the specific morphology, behaviour-related immune system, and DNA repair mechanisms in L. savala. Our results preliminarily revealed mechanisms underlying the special morphological and behavioural characteristics of L. savala from a genomic perspective. Furthermore, this study provides valuable reference data for subsequent molecular ecology studies of L. savala and whole-genome analyses of other trichiurid fishes.

Gene Expression Profile and Co-Expression Network of Pearl Gentian Grouper under Cold Stress by Integrating Illumina and PacBio Sequences.

Pearl gentian grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) is a fish of high commercial value in the aquaculture industry in Asia. However, this hybrid fish is not cold-tolerant, and its molecular regulation mechanism underlying cold stress remains largely elusive. This study thus investigated the liver transcriptomic responses of pearl gentian grouper by comparing the gene expression of cold stress groups (20, 15, 12, and 12 °C for 6 h) with that of control group (25 °C) using PacBio SMRT-Seq and Illumina RNA-Seq technologies. In SMRT-Seq analysis, a total of 11,033 full-length transcripts were generated and used as reference sequences for further RNA-Seq analysis. In RNA-Seq analysis, 3271 differentially expressed genes (DEGs), two low-temperature specific modules (tan and blue modules), and two significantly expressed gene sets (profiles 0 and 19) were screened by differential expression analysis, weighted gene co-expression networks analysis (WGCNA), and short time-series expression miner (STEM), respectively. The intersection of the above analyses further revealed some key genes, such as PCK, ALDOB, FBP, G6pC, CPT1A, PPARα, SOCS3, PPP1CC, CYP2J, HMGCR, CDKN1B, and GADD45Bc. These genes were significantly enriched in carbohydrate metabolism, lipid metabolism, signal transduction, and endocrine system pathways. All these pathways were linked to biological functions relevant to cold adaptation, such as energy metabolism, stress-induced cell membrane changes, and transduction of stress signals. Taken together, our study explores an overall and complex regulation network of the functional genes in the liver of pearl gentian grouper, which could benefit the species in preventing damage caused by cold stress.

Demographic history and population genetic analysis of Decapterus maruadsi from the northern South China Sea based on mitochondrial control region sequence.

Late Pleistocene climate oscillations are believed to have greatly influenced the distribution, population dynamics, and genetic variation of many marine organisms in the western Pacific. However, the impact of the late Pleistocene climate cycles on the demographic history and population genetics of pelagic fish in the northern South China Sea (SCS) remains largely unexplored. In this study, we explored the demographic history, genetic structure, and genetic diversity of Decapterus maruadsi, a typical pelagic fish, over most of its range in the northern SCS. A 828-832 bp fragment of mitochondrial control region were sequenced in 241 individuals from 11 locations. High haplotype diversity (0.905-0.980) and low nucleotide diversity (0.00269-0.00849) was detected, revealing low levels of genetic diversity. Demographic history analysis revealed a pattern of decline and subsequent rapid growth in the effective population size during deglaciation, which showed that D. maruadsi experienced recent demographic expansion after a period of low effective population size. Genetic diversity, genetic structure, and phylogenetic relationship analysis all demonstrated that no significant genetic differentiation existed among the populations, indicating that D. maruadsi was panmictic throughout the northern SCS. Periodic sea-level changes, fluctuation of the East Asian Monsoon, and Kuroshio variability were responsible for the population decline and expansion of D. maruadsi. The demographic history was the primary reason for the low levels of genetic diversity and the lack of significant genetic structure. The life history characteristics and ocean currents also had a strong correlation with the genetic homogeneity of D. maruadsi. However, the genetic structure of the population (genetic homogeneity) is inconsistent with biological characteristics (significant difference), which is an important reminder to identify and manage the D. maruadsi population carefully.