A global transcriptional analysis of Megalobrama amblycephala revealing the molecular determinants of diet-induced hepatic steatosis

Blunt snout bream (Megalobrama amblycephala), a prevalent species in China's intensive polyculture systems, is highly susceptible to hepatic steatosis, resulting in considerable losses to the fish farming industry. Due to a lack of genomic resources, the molecular mechanisms of lipid metabolism in M. amblycephala are poorly understood. Here, a hepatic cDNA library was generated from equal amounts of mRNAs isolated from M. amblycephala fed normal-fat and high-fat diets. Sequencing of this library using the Illumina/Solexa platform produced approximately 51.87 million clean reads, which were assembled into 48,439 unigenes with an average length of 596 bp and an N50 value of 800 bp. These unigenes were searched against the nucleotide (NT), non-redundant (NR), Swiss-Prot, Cluster of Orthologous Groups (COG), and Kyoto Encyclopedia of Genes and Genome (KEGG) databases using the BLASTn or BLASTx algorithms (E-value ≤ 10− 5). A total of 8602 unigenes and 22,155 unigenes were functionally classified into 25 COG categories and 259 KEGG pathways, respectively. Furthermore, 22,072 unigenes were grouped into 62 sub-categories belonging to three main Gene Ontology (GO) terms. Using a digital gene expression analysis and the M. amblycephala transcriptome as a reference, 477 genes (134 up-regulated and 343 down-regulated) were identified as differentially expressed in fish fed a high-fat diet versus a normal-fat diet. KEGG and GO functional enrichment analyses of the differentially expressed unigenes were performed and 12 candidate genes related to lipid metabolism were identified. This study provides a global survey of hepatic transcriptome profiles and identifies candidate genes that may be related to lipid metabolism in M. amblycephala. These findings will facilitate further investigations of the mechanisms underlying hepatic steatosis in M. amblycephala.