| Grass carp is crucial to Chinese aquaculture industry.The protein source of grass carp compound feed is soybean meal rather than fish meal.However,due to the instability of the international environment,imported soybeans are in short supply in China,and its prices have also risen incessantly.Which bring great pressure and challenge to the grass carp culture.Yellow mealworm(YM)is rich in lipid,protein and other nutritional elements.YM also has the characteristics of fast reproduction,short growth cycle,and utilization of waste for growth and so on.It can be said that it is environmentally renewable resources.So,the YM can be used as a new non-food protein substitute source in aquatic feed.Therefore,this study took grass carp as the research object,and explored the effects of YM replacement of dietary soybean meal(SM)on growth performance and flesh quality of grass carp and its influencing mechanism,aiming to supply reference for rational substitution of SM in grass carp compound feed.1.Effects of YM replacement of dietary SM on growth performance,health state,and flesh quality of grass carpA 90-day feeding trial was conducted to study the influence of dietary soybean meal(SM)replaced by YM on growth,feed utilization,health state,and flesh quality of grass carp.360 fish were fed 6 diets including graded concentrations of YM: YM(control),YM15,YM30,YM45,YM75 and YM100,corresponding to 0%,15%,30%,45%,60%,and 100% dietary SM was replaced by YM.Result indicated that the growth performance and biological parameters of grass carp were not affected by dietary YM.The whole body crude lipid of YM100 was significantly decreased compared to YM group(P < 0.05).The muscle crude protein in YM15 and YM100 groups was significantly lower than that in YM30(P < 0.05).Except that total cholesterol(TCHO)in both YM45 and YM100 was significantly increased compared with the YM(P < 0.05),no serious difference observed on other biochemical indices(P > 0.05).Compared with YM,both skin and muscle color,and muscle water holding capacity were not influenced by YM(P > 0.05).Muscle p H of YM15 was significantly increased when compared to other treatments(P < 0.05);The contents of muscle total trimethylamine and trimethylamine oxide increased with the elevation of dietary YM,and the contents of trimethylamine oxide and total trimethylamine in YM100 group were significantly elevated when compared to other treatments(P < 0.05).In addition,muscle K value in YM100 group was significantly decreased compared to YM group(P < 0.05),indicating that muscle freshness of YM100 group was significantly improved.The muscle hardness,adhesiveness,spring,gumminess,chewiness and hydroxyproline contents were increased with the increase of dietary YM,and the muscle adhesiveness and spring of YM100 were significantly elevated when compared with the YM group(P < 0.05).The hydroxyproline level of YM75 was significantly higher than that of YM group and YM15 group(P < 0.05).Muscle fiber density,the length of sarcomere,bright band and dark band of YM100 were markedly higher than YM group(P < 0.05).According to the results of principal component analysis,texture characteristics in YM100 group was optimal among all groups.Metabolomic results showed that the contents of muscle metabolites EPA in both YM30 and YM100 groups were significantly upregulated with the increase of dietary YM(P < 0.05).In conclusion,the YM can completely replace dietary SM without negative effect on growth performance and health state of grass carp,but the muscle quality was improved.This improvement effect on muscle quality may be related to the increase of EPA content in muscle.2.Effects of addition of selenium yeast to practical diet after replacement of SM with YM on growth performance and flesh quality of grass carp5 experimental diets were designed: practical diet;SM was completely replaced by YM;and 0.3,0.6 and 0.9mg/kg yeast selenium was added after SM was completely replaced by YM,which were named SM,YM,3SE,6SE,9SE,respectively.300 grass carp were fed with five experimental diets for 70 days in pond net cage.Results indicated that fish growth performance in YM and 9SE groups was seriously reduced compared to SM(P< 0.05),but no difference was found among SM,3SE,6SE groups(P > 0.05).Compared with SM,the muscle selenium content in YM group was decreased,while that in other treatment groups was increased.And the selenium of 6SE and 9SE was seriously elevated when compared to SM(P < 0.05).The muscle crude lipid of supplement with selenium yeast group was markedly decreased(P < 0.05).The whole body crude protein in selenium supplementation treatments was seriously higher than SM(P < 0.05).The muscle EAA and AA in 3SE group was markedly higher than those in YM and 9SE treatments(P < 0.05).Muscle SFA and MUFA in YM were obviously higher than those in other treatments(P <0.05),and PUFA contents in YM group were seriously higher than those in other treatments except 3SE(P < 0.05).The EPA level of YM,3SE and 9SE treatments was markedly elevated(P < 0.05)(P > 0.05).The genes of Elo4 and Elo8 were seriously up-regulated in YM group(P < 0.05),while the expression of FAD was significantly up-regulated in YM and 3SE treatments(P < 0.05).Compared with 3SE and 6SE treatments,the muscle lactic acid and drip loss of SM and YM group were markedly elevated(P < 0.05),but p H showed an opposite trend among all groups(P < 0.05).The muscle hydroxyproline of 3SE group was obviously elevated compared to control treatment(P < 0.05).Compared to SM,YM and 9SE,muscle freshness in 3SE was seriously increased(P < 0.05).The results of PCA showed that the muscle texture characteristics in 3SE was the best,while that of SM was the worst.In addition,the muscle fiber density of SM and 9SE was significantly declined(P < 0.05).In terms of muscle ultrastructure,the lengths of sarcomere in 3SE and 6SE groups were markedly longer than SM(P < 0.05).The muscle antioxidant capacity of grass carp was significantly enhanced in 3SE group(P < 0.05).In conclusion,the growth performance of grass carp can be reduced by completely replacing SM with YM in practical diet,but the negative effect on growth can be reversed by adding yeast selenium(0.3mg/kg),and which resulted in improving the muscle quality.3.Effects of YM replacement of dietary SM on the muscle EPA level and genes related to fatty acid synthesis expression of grass carpTo investigate the effects of YM replacement of dietary SM on the EPA content and expression of genes related to fatty acid synthesis of grass carp,two experimental diets were designed: the control diet(CN);The SM of control diet was completely substituted by YM.Grass carp were fed with the two experimental diets for 56 days in a pond net cage,and sampled at the 2nd,4th and 8th week.Results showed that there were no differences in both final body weight and biological parameters between control and YM group at 2,4and 8 weeks(P > 0.05).In terms of muscle chemical composition,no difference was found between control and YM group at week 2,4 and 8(P > 0.05).The muscle EPA level in YM group was higher than that of control group at 2,4 and 8 weeks,but only at 8th week showed a significant difference(P < 0.05).In terms of fatty acid desaturase(FAD),at the4 th and 8th week,the expression level of FDA was significantly increased in YM group(P< 0.05).In terms of fatty acid elongase(Elo),the Elo2 expression was significantly increased in YM group at 2nd week(P < 0.05).The expression of Elo4 and Elo6 was significantly upregulated in YM group at 4th week(P < 0.05).The expressions of Elo5 and Elo8 were significantly increased in YM at 4th and 8th week(P < 0.05).In conclusion,after the complete substitution of SM by YM,the muscle EPA level was significantly increased after the 4th week,which may be caused by the upregulated the genes of fatty acids elongase and desaturase.4.The regulation mechanism of EPA on muscle development of grass carpIn this research,we investigated the effect of EPA on the muscle development of grass carp and potential molecular mechanisms via both in vivo and in vitro.In vitro,grass carp muscle cell treated with 50 μM EPA for 48 h,which markedly upregulated mechanistic target of rapamycin(mTOR)signaling pathway-related genes(mTOR,S6K1 and 4E-BP)(P < 0.05).In vivo,270 grass carp(27.92±0.02 g)were randomly fed 3 diets: control diet(CN),supplement 0.3% EPA diet(EPA),supplement both 0.3% EPA and 30mg/kg rapamycin diet(EPA+Rap)for 56 days.Results showed that dietary supplement EPA significantly promoted grass carp growth performance by increasing FBW,SGR and PE(P< 0.05),compared to CN and EPA+Rap group.There is no difference in the viscerosomaic index(VSI)and body height(BH)among all groups(P > 0.05),whereas the dressed fish weight(DFW)and body length(BL)in EPA group were obvious higher than other groups(P < 0.05),which indicates the increased FBW was due to muscle growth.In addition,the crude protein of muscle,liver and whole body in EPA group was markedly increased compared to CN and EPA+Rap(P < 0.05).Dietary EPA significantly increased the muscle hyperplasia via elevating fiber density(P < 0.05),whereas no difference was found in fiber diameter among all group(P > 0.05).The muscle sarcomere length was also promoted by dietary EPA(P < 0.05).According to the result of PCA,the muscle amino acid and body fatty acid composition was significant influenced by dietary EPA,and the total weight of the muscle amino acid and fatty acid and in EPA group was optimal among all groups.Further study showed that the mTOR,S6K1,4E-BP,fgf6 a,fgf6b,myf5,myhc,myod,mrf4 m RNA expression,and total mTOR,S6K1,myhc,myog,myod protein in EPA group was seriously higher than that of CN and EPA+Rap(P < 0.05).In conclusion,dietary EPA promoted the muscle development via activating the mTOR signaling pathway,and further increased the protein deposition and growth performance of grass carp.To sum up,(1)In the semi-purified diet,yellow mealworm could completely replacement of dietary SM(24%),and which improved flesh quality of grass carp.(2)In the practical diet,the growth performance of grass carp was depressed after the SM was completely replaced by YM,but the negative effect on the growth performance was reversed by adding an 0.3 mg/kg yeast selenium in the diet;And dietary selenium yeast further improved the flesh quality.(3)In both semi-purified diet and practical diet,the replacement of SM by yellow mealworm increased the muscle EPA of grass carp.The increase of muscle EPA may be related to the elevated the genes expressions of fatty acid desaturase and elongase.(4)EPA promoted the development of grass carp muscle by activating mTOR signaling pathway. |
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