Pelteobagrus vachelli is a well-known commercial species in Asia. However, a sudden lack of oxygen will result in mortality and eventually to pond turnover. Studying the molecular mechanisms of hypoxia adaptation in fishes will not only help us to understand fish speciation and the evolution of the hypoxia-signaling pathway, but will also guide us in the breeding of hypoxia-tolerant fish strains. Despite this, the genetic regulatory network for miRNA-mRNA and the signaling pathways involved in hypoxia responses in fish have remained unexamined. In the present study, we used next-generation sequencing technology to characterise mRNA-seq and miRNA-seq of control- and hypoxia-treated P. vachelli livers to elucidate the molecular mechanisms of hypoxia adaptation. We were able to find miRNA-mRNA pairs using bioinformatics analysis and miRNA prediction algorithms. Furthermore, we compared several key pathways which were identified as involved in the hypoxia response of P. vachelli. Our study is the first report on integrated analysis of mRNA-seq and miRNA-seq in fishes and offers a deeper insight into the molecular mechanisms of hypoxia adaptation. qRT-PCR analysis further confirmed the results of mRNA-Seq and miRNA-Seq analysis. We provide a good case study for analyzing mRNA/miRNA expression and profiling a non-model fish species using next-generation sequencing technology.

T. fasciatus (Takifugu fasciatus) faces the same problem as most warm water fish: the water temperature falls far below the optimal growth temperature in winter, causing a massive death of T. fasciatus and large economic losses. Understanding of the cold-tolerance mechanisms of this species is still limited. Integrated application of multi-omics research can provide a wealth of information to help us improve our understanding of low-temperature tolerance in fish. To gain a comprehensive and unbiased molecular understanding of cold-tolerance in T. fasciatus, we characterized mRNA-seq and metabolomics of T. fasciatus livers using Illumina HiSeq 2500 and UHPLC-Q-TOF MS. We identified 2544 up-regulated and 2622 down-regulated genes in the liver of T. fasciatus. A total of 40 differential metabolites were identified, including 9 down-regulated and 31 up-regulated metabolites. In combination with previous studies on proteomics, we have established an mRNA-protein-metabolite interaction network. There are 17 DEMs (differentially-expressed metabolites) and 14 DEGs-DEPs (differentially co-expressed genes and proteins) in the interaction network that are mainly involved in fatty acids metabolism, membrane transport, signal transduction, and DNA damage and defense. We then validated a number of genes in the interaction network by qRT-PCR. Additionally, a number of SNPs (single nucleotide polymorphisms) were revealed through the transcriptome data. These results provide key information for further understanding of the molecular mechanisms of T. fasciatus under cold stress. The data generated by integrated application of multi-omics can facilitate our understanding of the molecular mechanisms of fish response to low temperature stress. We have not only identified potential genes and SNPs involved in cold tolerance, but also show that some nutrient metabolites may be added to the diet to help the overwintering of T. fasciatus.

Heterosis is a complex biological phenomenon in which hybridization produces offspring that exhibit superior phenotypic characteristics compared with the parents. Heterosis is widely utilized in agriculture, for example in fish farming; however, its underlying molecular basis remains elusive. To gain a comprehensive and unbiased molecular understanding of fish heterosis, we analyzed the mRNA, miRNA, and proteomes of the livers of three catfish species, Pelteobagrus fulvidraco, P. vachelli, and their hybrid, the hybrid yellow catfish “Huangyou-1” (P. fulvidraco ♀ × P. vachelli ♂). Using next-generation sequencing and mass spectrometry, we show that the nonadditive, homoeolog expression bias and expression level dominance pattern were readily identified at the transcriptional, post-transcriptional, or protein levels, providing the evidence for the widespread presence of dominant models during hybridization. A number of predicted miRNA-mRNA-protein pairs were found and validated by qRT-PCR and PRM assays. Furthermore, several diverse key pathways were identified, including immune defense, metabolism, digestion and absorption, and cell proliferation and development, suggesting the vital mechanisms involved in the generation of the heterosis phenotype in progenies. We propose that the high parental expression of genes/proteins (growth, nutrition, feeding, and disease resistance) coupled with low parental miRNAs of the offspring, are inherited from the mother or father, thus indicating that the offspring were enriched with the advantages of the father or mother. We provide new and important information about the molecular mechanisms of heterosis, which represents a significant step toward a more complete elucidation of this phenomenon. Graphical Abstract Highlights mRNA-seq, miRNA-seq, proteomes of P. fulvidraco, P. vachelli, hybrid Huangyou-1. Predicted miRNA-mRNA-protein pairs were found and validated by qRT-PCR and PRM. Immune, metabolism, digestion, absorption, proliferation, development generate heterosis. High parental gene/protein with low parental miRNAs inherit from the mother or father.