Transcriptomic and biochemical analyses revealed the improved growth, lipid metabolism, and flesh quality of grass carp (Ctenopharyngodon idellus) by dietary Eucommia ulmoides bark and leaf supplementation

The objective of this study was to investigate the dietary effects of Eucommia ulmoides bark and leaf (EB, EL) supplementation on the growth, lipid metabolism, flesh quality, and transcriptome of grass carp (Ctenopharyngodon idellus). EB and EL were individually added to the basal diet (control) at concentrations of 20 g/kg and 40 g/kg, respectively, and then the three diets were fed to grass carp (59.7 ± 0.3 g) for 60 d. The results showed that the weight gain was improved, and the feed conversion ratio was decreased by supplementation with EB and EL (P < 0.05). Compared to the control, the EB and EL groups showed higher flesh hardness; water-holding capacity; and collagen, docosahexaenoic acid (DHA), and n-3 polyunsaturated fatty acids (n-3PUFAs) contents and lower mesenteric lipid and muscle crude lipid contents (P < 0.05). Moreover, dietary EB and EL supplementation increased the activities of superoxide dismutase and glutathione peroxidase and decreased the contents of malondialdehyde and protein carbonyl in flesh (P < 0.05). In muscle transcriptome profiling, a total of 979, 1980 differentially expressed genes (DEGs) were identified, and 29, 199 Gene Ontology (GO) terms and 13, 39 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were significantly enriched in the EB and EL groups, respectively. Some key pathways and genes involved in promoting growth, lipid metabolism and flesh quality were obtained, including mTOR and PPAR signaling pathways, muscle cytoskeleton- and extracellular matrix-related genes (myosin and collagen), etc. Overall, dietary EB and EL supplementation improved the growth, lipid metabolism, and flesh quality of grass carp, and several potential pathways and genes were identified behind the improvement mechanism of EB and EL supplementation.As a traditional herb, Eucommia ulmoides (E. ulmoides) has been utilized in East Asia for at least 2 000 years. In recent years, E. ulmoides has been applied in the culture of fish for its functions of promoting growth, lipid metabolism, and flesh quality. However, the underlying molecular mechanism of improving growth, lipid metabolism, and flesh quality is not well understood. Our study showed that the improvement of flesh quality is the combined effect of antioxidant capacity, muscle texture, water-holding capacity, and nutritional composition. Additionally, several potential pathways and differentially expressed genes were identified through RNA sequencing to further study the improvement mechanism of dietary E. ulmoides bark and leaf supplementation on growth, lipid metabolism, and flesh quality in fish.