Effects of dietary protein levels on growth performance and liver transcriptome changes in juvenile top-mouth culter Erythroculter ilishaeformis

An eight-week growth experiment was conducted to investigate the growth performance and liver transcriptome response of juvenile top-mouth culter Erythroculter ilishaeformis fed with different dietary protein levels. Six isoenergetic diets supplemented with protein at a level of 25%, 30%, 35%, 40%, 45% and 50% were prepared. The fish (initial mean weight 6.27 ± 0.30 g, 35 fish per tank, n = 3) were fed the six test diets in a recirculating aquaculture system at 30 ± 2 °C. Our results showed that weight gain rate (WGR) increased from 161.88% to 247.20% with the increase of dietary protein level from 25% to 40%, and then kept nearly stabilized with further increases in dietary protein concentration. Feed conversion ratio (FCR) showed an opposite trend to WGR. Using WGR and FCR as the response criterion, the optimal protein requirement for maximum growth of this juvenile fish species was 41.14% and 43.75%, respectively. The liver transcriptome profiles using RNA-seq technology showed that a total of 189.13 million clean data was generated and 202,677 unigenes with 435,540 transcripts were assembled. Unigenes were then searched against four databases including Swiss-Prot, COG, KOG, and KEGG. Differentially expressed genes (DEGs) were considered as a threshold of the fold change (FC) ≥ 2 and a false discovery rate (FDR) ≤ 0.01 in a comparison. Trend analysis revealed that these DEGs were significantly clustered into five clusters. Several DEGs associated with growth (igf1 and ST), lipid metabolism (fasn and accα), glucose metabolism (pepck, pk, gk and g6pd), aspartate biosynthesis (AST), immune (MHC-II), were validated with qPCR analysis. Moreover, function analysis indicated that the DEGs were mainly involved in metabolism, immunity, antioxidation, and a variety of signal pathways. Taken together, our data provide a comprehensive understanding of the feed formulation and molecular mechanism underlying the effects of dietary protein on E. ilishaeformis.