Culture Salinity Alters Growth Performance,Dietary Protein Requirement And Nutrients Metabolism Related Genes Expression In Juveniles Genetically Improved Farmed Tilapia(GIFT)(Oreochromis Niloticus)

Genetically improved farmed tilapia(GIFT)juveniles(Oreochromis niloticus)is one of the main tilapia cultivars in China,which has the advantages of fast growth,low oxygen tolerance,high yield and eurysalinicity.At present,dietary protein requirements of GIFT juveniles under different salinities are still unclear,and the mechanism of dietary protein level on the regulation of nutrient metabolism of juvenile tilapia in freshwater and brakish water is still unknown.Therefore,this paper takes GIFT juveniles as the research object,discussed the effects of dietary protein levels and culture salinity on growth and nutrition metabolism of GIFT juveniles.It mainly included: the growth performance,the whole fish body composition and nutritional metabolism related gene expression.This study mainly included the following two parts:1 Effects of dietary protein level and culture salinity on growth performance and whole body composition of GIFT juveniles(Oreochromis niloticus)In this experiment,fish meal,corn gluten meal and casein were used as protein sources,soybean oil and soybean lecithin were used as fat sources,and corn starch was used as sugar sources.Six diets with different protein levels(18.20%,24.32%,30.60%,36.42%,43.27% and 49.49%)were prepared.GIFT juveniles were taken as the experimental fish.A total of 720 GIFT juveniles with an initial body weight of 3.35 g were randomly divided into 6 groups with 3 replicates per group and 20 fish per replicate at 0‰(freshwater)and 8‰(brakish water)salinity.They were cultured in indoor recirculating aquaculture tanks(180L).The fish were acclimatized to the experimental environment for two weeks before the experiment.The fish were fed to saturation by hand four times daily,and the breeding cycle was 8 weeks.During the experiment,the water temperature was 27±2℃,p H was 7.4±0.4,dissolved oxygen was greater than 6 mg/L,ammonia nitrogen was less than 0.05 mg/L.The results showed that dietary protein level and culture salinity had no significant effect on survival rate(SR)(P>0.05).The average final weight(AFW),weight gain rate(WGR)and specific growth rate(SGR)increased significantly with increasing dietary protein levels up to a certain level(36.42%)in freshwater and brackish water(P<0.05).In addition,the feed conversion ratio(FCR)was significantly affected by the dietary protein levels,salinities and their interaction(P<0.05).The FCR reached the minimum value at the protein level of43.27% at 0‰ salinity and the minimum value at the protein level of 49.49% at 8‰ salinity.With an increase in dietary protein levels,hepatosomatic index(HSI)and viscerosomatic index(VSI)were significantly decreased(P<0.05).Salinity had no significant effect on HSI and VSI(P>0.05).Compared with the 18.2% protein level,crude protein content increased significantly in 36.42% and 49.49%(P<0.05)at freshwater and brackish water(P<0.05).Whole body lipid and ash contents were significantly affected by salinity(P<0.05),but it was not affected significantly by the dietary protein level(P>0.05).8‰salinity significantly decreased whole body lipid content(P<0.05),the trend of whole body ash content was opposite(P<0.05).The dietary protein levels and salinity had no effect on the whole body moisture content(P>0.05).Furthermore,no significant changes were observed in the composition of the whole fish body as a result of the interaction between dietary protein levels and salinity(P>0.05).In terms of the FCR and WGR,the broken-line model analysis suggested that the optimal protein levels for GIFT juveniles were 33.70%,38.21%(0‰)and 37.70%,37.86%(8‰),respectively.2 Effects of dietary protein levels and culture salinity on TOR signaling pathway and nutrient metabolism of GIFT juvenilesIn order to investigate the effects of dietary protein level and culture salinity on TOR signaling pathway and related regulatory mechanisms of nutrient metabolism of GIFT juveniles,The core genes of TOR signaling pathway and glucose metabolism in liver of GIFT juveniles were analyzed.The results showed that the relative m RNA levels of insulin growth factor 1(IGF-1)was increased firstly and then decreased with an increase in DPLs,which reaching a maximum at 36.42% dietary protein levsl.49.49% dietary protein level increased the relative expression of IGF-1 at 0‰ salinity.Culture salinity had no significant effect on IGF-1 m RNA levels(P>0.05).Dietary protein level had no significant effect on the relative expression of hepatic target of rapamycin(TOR),ribosomal protein S6(S6),4E binding protein 1(4E-BP1)(P>0.05),but they increased significantly with increasing salinity(P<0.05).Furthermore,the relative m RNA expression levels of IGF-1,TOR,S6 and 4E-BP1 were not significantly affected by the interaction of dietary protein levels and salinity(P>0.05).The expression levels of GS and GLUT2 decreased with an increase in dietary protein level at 8‰ salinity while they were not affected by dietary protein levels at0‰ salinity.Dietary protein levels significantly decreased the m RNA level of GK(P<0.05).The relative expression levels of FAS and GK decreased while GS and GLUT2 increased with increasing salinity(P<0.05).Phosphoenolpyruvate carboxykinase(PEPCK)m RNA level was not affected by the dietary protein level(P>0.05),but it increased with increasing salinity(P<0.05).As well,the levels of expression for PEPCK genes were not affected by their interaction(P>0.05).Hepatic glycogen decreased significantly with the increasing of dietary protein level but not by salinity(P>0.05).Therefore,appropriate dietary protein level and 8‰ culture salinity could increase the expression of downstream gene 4E-BP1 and promoting protein synthesis via activating the TOR signaling pathway.Meanwhile,the GIFT juveniles contributed to enhanced glycogen synthesis at 8‰ salinity,while fats were deposite at 0 ‰ salinity.