Draft Genome of the Korean smelt Hypomesus nipponensis and its transcriptomic responses to heat stress in the liver and muscle

Abstract

Backgrounds: Hypomesus nipponensis is a cold-freshwater fish species as a winter economic resource of aquaculture in South Korea. Due to its high susceptibility to abnormal water temperature from global warming. Aim of the study: Here, we present the first draft genome of H. nipponensis and transcriptomic changes in molecular mechanisms or intracellular responses under heat stress. Methods: We combined Illumina and PacBio sequencing technologies to generate the draft genome of H. nipponensis. Based on the reference genome, we conducted transcriptome analysis of liver and muscle tissues under normal (NT, 5°C) versus warm (HT, 23°C) conditions, to identify heat stress-induced genes and gene categories. Results: We observed a total of 1,987 contigs, with N50 of 0.46 Gbp with a largest contig (3.03 Mbp) in the assembled genome. A total number of 20,644 protein coding genes were predicted, and 19,224 genes were functionally annotated: 15,955 genes for Gene Ontology (GO) terms; and 11,560 genes for KO (KEGG Orthology). In the missing and gained genes analysis, we detected 4,461 (22.16%), 2,825 (10.62%), and 1,499 (3.09%) genes with coverage less than 10% and for gained genes found from alignment to other species that human, zebrafish and salmon, respectively, we observed 1,133 (5.49%), 1,670 (8.09%), and 229 (1.11%) genes with coverage less than 10% compared with above species, respectively. From transcriptome analysis, a total of 297 and 331 differentially expressed genes (DEGs, adjusted p-value < 0.05) were identified in the liver and muscle tissues, respectively. Gene enrichment analysis of DEGs indicates that up-regulated genes were significantly enriched for lipid biosynthetic process (GO:0008610, P < 0.001) and regulation of apoptotic process (GO:0042981, P < 0.01), and down-regulated genes by immune responses such as myeloid cell differentiation (GO:0030099, P < 0.001) in the liver under heat stress. In muscle tissue, up-regulated genes were enriched for hypoxia (GO:0001666, P < 0.05), transcription regulator activity (GO:0140110, P < 0.001) and calcium-release channel activity (GO:0015278, P < 0.01), and down-regulated genes for nicotinamide nucleotide biosynthetic process (GO:0019359, P < 0.01). The results of KEGG pathway analysis were similar to that of gene enrichment analysis. Conclusion: The draft genome and transcriptomic of H. nipponensis will be used as a useful genetic resource for functional and evolutionary studies. Our findings will improve understanding of the molecular mechanisms and heat responses and be useful for better survival of the smelt and its closely related species under global warming.