| Accurate nutritional regulation of broodstock is an interdisciplinary study area of reproductive biology and nutrition in aquaculture,and the basis for artificial reproduction of fish.Fish oil(FO)rich in long-chain polyunsaturated fatty acids(LCPUFA)can significantly promote the reproductive development of marine teleosts.Spotted scat(Scatophagus argus)is a valuable fish in the Southeast coast of China.Yet,the molecular mechanism of dietary FO to promote ovarian development is unclear.In this study,we first observed the annual changes in ovarian development and fatty acid composition in female broodstock,and then compared the effects of feeding Spotted scat with FO and soybean oil(SO)-enriched diets on ovarian development,and analyzed the fatty acid composition,ovarian and liver transcriptome and metabolome,serum hormone and biochemistry indexes.The study aimed to reveal the molecular mechanism of FO in promoting the development of the ovary in Spotted scat.(1)From May 2019 to March 2020,the female broodstock Spotted scat was continuously collected at two months intervals to observe the annual ovarian development and fatty acid composition of tissues.Fish body weight,total length,body length and condition factor(CF)gradually increased while the fish were growing up.In July 2019,the gonadal somatic index(GSI)increased significantly,all ovaries were at stage IV,and the accumulation of lipid droplets was the most obvious.Ovarian saturated fatty acids(SFA),monounsaturated fatty acids(MUFA),n-3 PUFA and n-6 PUFA were the highest in July 2019.The ovarian n-3 PUFA/n-6 PUFA ratio was the highest in May 2019.The hepatopancreas somatic index(HSI)in November,January and March were significantly higher than that of May,July and September.The viscerasomatic index(VSI)was significantly higher from November to March,and July.The accumulation of liver lipid droplets was the least from May to July.The absolute content of liver SFA,MUFA,n-6 PUFA in January and March was significantly higher than that of May and July.Liver n-3 PUFA in May was significantly higher than that in March,July and November.The liver n-3 PUFA/n-6PUFA ratio in May was the highest.The above results show that the breeding season of Spotted scat is around July.From winter to spring,the abdominal cavity accumulates fat to reserve energy for ovarian development in the next year.The liver is responsible for absorbing n-3 PUFAs before the breeding season and transporting them to the ovaries during the breeding season.(2)Female broodstock Spotted scat(242.83±16.10g)were fed diets containing 8%FO(treatment group)and 8% soybean oil(control group,SO).The fish were sampled at 0,20,40 and 60 days.F and S represent FO and SO,respectively.The numbers represent the days of culturing,and the sampling on day 0 was the C0 group.The CF,GSI,and the VSI in the F60 group were significantly higher than that of the C0 group,and were also numerically higher than those in the S60 group,but the differences were insignificant.The absolute content of ovarian DHA,EPA and n-3PUFA in F60 group was significantly higher than that of S60 group.Compared with the S60 group,68 genes were up-regulated and 193 genes were down-regulated in the F60 group.They are involved in biological processes such as estrogen metabolism(Cyp3a27),steroidogenesis(PPARγ),lipid accumulation and oocyte maturation(Jun,Jund,Plin2),inflammatory factors and oxidative stress(Aldh3a2,Cxcl12).The differential metabolites in the ovarian metabolome were mainly enriched in KEGG signaling pathways such as steroid biosynthesis,unsaturated fatty acid biosynthesis,aromatic compound degradation,and arachidonic acid metabolism.The above results indicate that dietary FO affects the estradiol metabolism in the ovary,regulates lipid accumulation and insulin sensitivity,and reduces inflammatory factors,thus promoting ovarian development in female broodstock Spotted scat.(3)Transcriptome sequencing was used to analyze the effect of higher dietary n-3 LC-PUFA level on gene expression in the liver of female broodstock Spotted scat.The liver transcriptome analysis showed 497 up-regulated and 267 down-regulated genes in the F40 group.Among them,the expression levels of three estrogenregulated genes(Vtga,Vtgb,and Zp4)were significantly higher in the F40 than in the S40 group.This expression patterns of these genes could be related to the upregulation of Hsd17b7(the key gene of liver steroid hormone synthesis)and the down-regulation of the Hsp90(the estrogen receptor chaperone).The expression levels of Foxo1 a and Lep,which are involved in the lipid metabolism,decreased significantly in the F40 group,which may be related to the lower level of HSI in the F40 group.The genes related to liver n-3 LC-PUFA absorption and transport,Fabp2 and Mfsd2ab,were significantly up-regulated in the F40 group,indicating that fish actively adapt to high FO diets.Briefly,high FO diets can influence the expression of genes related to liver n-3 LC-PUFA metabolism and reproduction,inhibit liver fat accumulation,and promote liver health and gonad development.(4)The serum hormone and protein levels were analysed in the female broodstock Spotted scat fed with FO and SO.In the F60 group,the contents of estradiol,VTG,leptin and insulin were significantly higher than that of S60 group,while serum VLDL of F60 group was significantly lower than that of S60.This further proved that FO promotes the biosynthesis of sex steroid hormones and VTG in female broodstock.The results also suggested that dietary FO may affect the ovarian development of Spotted scat by regulating these proteins and hormones related to glucose and lipid metabolism,including VLDL,leptin and insulin.In conclusion,this study used female broodstock Spoted scat as experimental organism,and preliminarily clarified that dietary FO affects ovarian development via regulating E2 metabolism and VTG biosynthesis in fish.Dietary FO could also be able to regulate hormones related to glucose and lipid metabolism. |
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