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This study is aimed at evaluating the effects of dietary protein-to-energy ratios on the growth, immunological response, antioxidative capacity, liver and intestinal histology, and growth-related gene expression of hybrid yellow catfish (Pelteobagrus fulvidraco ♀ × Pelteobagrus vachelli ♂). Eight diets were formulated to form different protein/energy ratios of 84, 88, 90, 93, 95, 96, 99, and 103 mg/kcal (P/E84, P/E88, P/E90, P/E93, P/E95, P/E96, P/E99, and P/E103), respectively. These diets contain different levels of gross energy (GE), ranging from 4.13 to 4.76 kcal g-1. Seven hundred and twenty healthy fish (17.15 ± 0.02 g) were randomly dispersed into 24 rectangular fiberglass tanks with 8 treatments in triplicate groups. The fish fed a P/E ratio of 95 mg/kcal demonstrated the best growth and feed utilization. A significant (P < 0.05) increase in percent weight gain (WG%) and specific growth rate (SGR) was seen as the dietary P/E ratio ameliorated from P/E84 to P/E95, followed by a decreased pattern in these parameters. Feed conversion ratio (FCR) and daily feed intake (DFI) were significantly impacted by dietary P/E ratios (P < 0.05). Additionally, an optimum P/E ratio improved intestinal morphology. However, low or high P/E ratio diets can cause oxidative stress, impaired liver function, and significantly reduced nonspecific immunity. The expression of target of rapamycin (TOR) and insulin-like growth factor-1 (IGF1) genes in the liver was considerably influenced by dietary protein-to-energy ratios (P < 0.05). Based on the statistical analysis of WG% against the dietary P/E ratio, the optimal P/E ratio for the studied species was estimated to be 92.92 mg/kcal. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are endogenous small non‐coding RNAs that play important roles in many biological processes. The function of miRNAs in skin colour differentiation and pigmentation has been reported in many animals. However, the researches about their molecular mechanisms of regulating the formation of different skin colour of koi carp (Cyprinus carpio L.) are deficient. In this study, we detected the temporal and spatial expression levels of ccr‐miR‐196a (hereinafter referred to as miR‐196a) and then predicted its target mRNA of microphthalmia‐associated transcription factor a (mitfa), which is a key regulatory transcript factor in the melanogenesis pathway, with bioinformatics and dual‐luciferase reporter methods. Furthermore, we silenced the miR‐196a in vivo with an antagomir to detect whether the miR‐196a would affect the expression levels of mitfa mRNA and its downstream genes including tyrosinase (tyr), tyrosinase‐related protein 1 (tyrp1) and dopachrome tautomerase (dct). The results revealed that mitfa and its downstream genes' mRNA expressions all significantly increased in the treatment group compared with the control group. We presumed that miR‐196a might regulate the pigmentation of koi carp through targeting the mitfa. The finding provided the fundamental information regarding miRNA–mRNA interaction on different skin colour pigmentation in koi carp. [ABSTRACT FROM AUTHOR]