| Grouper is a general name of Perciformes,Percoidei and Epinephelidae.It is widely distributed in tropical and subtropical waters of Pacific Ocean,Atlantic Ocean and Indian Ocean.It is a kind of fish with delicious meat,rich nutrition,low fat content and important commercial value.Grouper has developed into a kind of high-quality and rare fish cultured in the south and the north in China.In 2021,the output of grouper aquaculture in China reached 204119 t,making it the third largest aquaculture fish.However,with the rapid development and expansion of culture scale,lack of excellent germplasm is still the main restriction factor of grouper culture industry.The jinhu grouper,Epinephelus fuscoguttatus ♀× E.tukula ♂,has been studied in recent years by using the sperm of E.tukula preserved in ultra-low temperature and eggs of E.fuscoguttatus.A hybrid that has the advantages of high fertilization rate,high hatching rate,low malformation rate and rapid growth.At present,jinhu grouper has been large-scale aquaculture in many coastal provinces,has been recognized by the majority of farmers.This topic focuses on the research of hybrid new germplasm jinhu grouper as follows:1.Analysis of digestive characteristics of Epinephelus fuscoguttatus ♀ × E.tukula ♂ and E.fuscoguttatus ♀× E.lanceolatus ♂:Juvenile jinhu grouper and pearl gentian grouper(E.fuscoguttatus ♀ × E.lanceolatus ♂)with similar genetic background and the same breeding conditions were selected,and fed with pellet feed at a water temperature of(24.04 ± 1.87)℃.After ingestion,the wet weight of the stomach contents was determined and the gastric emptying characteristics were analyzed.Then,the linear model,the exponential model and the square root model were compared to fit the gastric emptying curve,and the digestive enzyme activities in the stomach and intestine were analyzed.The results showed that the gastric emptying characteristics of juvenile grouper jinhu and pearl gentian showed a typical pattern of first slow,then fast and then slow after feeding,and the best fitting models were square root models.The regression equations of stomach contents(Y)and postingestion time(t)were Y0.5=1.0586-0.0142 t(R2=0.9018,P<0.0001)and Y0.5=0.9690-0.0187 t(R2=0.8835,P<0.0001),respectively.With the increase of gastric emptyling degree,intestinal amylase activities of juvenile grouper jinhu and those of juvenile grouper pearl gentian had no significant differences(P>0.05),but the other indexes changed significantly(P<0.05).Moreover,the activities of intestinal lipase and intestinal chyme protease of juvenile jinhu grouper were significantly higher than those of juvenile pearl gentian grouper(P<0.05).The results showed that the feeding level and gastric emptyling rate of juvenile jinhu grouper were higher than those of juvenile pearl gentian grouper,and the digestion and absorption ability of protein and lipid in the process of gastric emptyling of jinhu grouper was stronger than that of juvenile pearl gentian grouper.2.Analysis of hypoxia tolerance of Epinephelus fuscoguttatus ♀× E.tukula ♂and tiger grouper:Oxygen consumption rate and asphyxia point of juvenile tiger grouper and jinhu grouper were measured in a closed breathing chamber.After the normal dissolved oxygen[(5.71±0.31)mg/L]and dissolved oxygen decreased to 4.0 mg/L,3.0 mg/L,2.0 mg/L,1.0 mg/L,0.4 mg/L for 1 h and after the normal dissolved oxygen level was restored for 3 h,liver tissues of juvenile tiger grouper and jinhu grouper were taken.Some indexes of antioxidant enzyme activity and energy utilization were determined.The experimental results showed that when the water temperature was at(31.18±0.38)℃,the oxygen consumption rate of jinhu grouper juveniles was 0.16 mg/(g·h),which was significantly higher than that of the female tiger grouper juveniles(P<0.05).The suffocation points of tiger grouper and jinhu grouper juveniles were 0.22 mg/L and 0.24 mg/L,respectively,and the difference between the two species was not significant.During the process of decreasing dissolved oxygen concentration and returning to normal dissolved oxygen,there was a significant difference(P<0.05)between superoxide dismutase(SOD),glutathione peroxidase(GSH-Px),and lactate dehydrogenase(LDH)enzyme activities and lactic acid(LD)content in the liver of tiger grouper juveniles,as well as SOD,catalase(CAT),GSH-Px,total antioxidant capacity(T-AOC),LDH enzyme activities in the liver of jinhu grouper juveniles.However,the activity change of T-AOC and CAT in the liver of tiger grouper juveniles and the content of LD in the liver of jinhu grouper juveniles were not significant.After 3 h of reoxygenation,the activity of SOD in the liver of tiger groupers increased by 16.7%and that of GSH-Px increased by 42.5%,while the content of LD increased by 2.8%.There were significant differences in these indicators between the reoxygnation group and the normoxic group(P<0.05).However,there were no significant differences in the indicators of jinhu grouper juveniles compared with the normoxic group,except for the activity of GSH-Px(P<0.05).During the change in dissolved oxygen level,the activation levels of SOD,GSH-Px,CAT,and T-AOC in the liver of jinhu groupers reached 29.1%,17.9%,42.4%,and 76.0%,respectively,which were higher than the maximum activation levels of corresponding antioxidant enzymes in tiger grouper livers.The experimental results showed that jinhu groupers had a slightly higher oxygen consumption rate than tiger groupers and that their hypoxia tolerance was higher.Thus,jinhu groupers can be cultured on a large scale and in high density.After being subjected to hypoxia stress,compared with tiger groupers,jinhu groupers could efficiently activate the antioxidant defense system to protect the body from oxidative damage and had faster recovery ability after hypoxia.3.Transcriptome characteristics of Epinephelus fuscoguttatus ♀× E.tukula ♂and tiger grouper under temperature stress:To understand the molecular mechanism of jinhu grouper and tiger grouper under temperature stress,we studied the transcriptome of liver,muscle and brain tissue at three temperatures(18℃,24℃ and 30℃).The results showed that 28331 genes and 4380 DEEGs were obtained by Illumina Novaseq 6000 platform,respectively.GO term and KEGG enrichment analysis indicated that binding,catalytic activity,metabolic process,carbon metabolism,complement and coagulation cascade,AMPK signaling pathway,FoxO signaling pathway,and ECM-receptor interaction may be involved in temperature adaptation regulation of tiger grouper and jinhu grouper.We also found different expression patterns of several PPAR signaling pathway related genes and HSPs genes in tiger grouper and jinhu grouper.In addition,WGCNA was used to identify the target modules,EF.greenyellow,EF.sienna3,EF.brown,EFET.salmon,EFET.red and EFET.pink,significantly associated with temperature stress in the liver,muscle and brain tissues of tiger grouper and jinhu grouper respectively,and a series of hub genes were found through the network diagram of these six target modules.These studies can deepen understanding of the complex molecular mechanisms of temperature stress in grouper,and provides a potential strategy for aquatic animals to mitigate environmental stress. |
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