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

Abstract

Pond smelt (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, a large number of smelt die in hot summer. Here, we present the first draft genome of H. nipponensis and transcriptomic changes in molecular mechanisms or intracellular responses under heat stress. 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. We observed a total of 1,987 contigs, with N₅₀ of 0.46 Mbp 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 KEGG Orthology (KO). We conducted the lost and gained genes analysis compared with three species that human, zebrafish and salmon. In the lost genes analysis, we detected smelt lost 4,461 (22.16%), 2,825 (10.62%), and 1,499 (3.09%) genes compare with above three species, respectively. In the gained genes analysis, we observed smelt gain 1,133 (5.49%), 1,670 (8.09%), and 229 (1.11%) genes compare with above species, respectively. From transcriptome analysis, a total of 297 and 331 differentially expressed genes (DEGs) with False discovery rate (FDR) < 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. 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 will be useful for predicting survival of the smelt and its closely related species under global warming.