| In the global aquaculture, because of the imbalance of nutrition intake, most of the cultured fish have accumulated a lot of fat in the abdomen, liver and muscle. The accumulation of excess fat in fish would impair fish health, which causes fatty liver disease, reduces stress resistance, reduce meat quality and even result in fish death. Therefore, in order to solve the problem of excessive fat accumulation in fish, the lipid metabolism research in fish has been paid worldwide attention.The knowledge of the anatomic distribution and total mass of fatty tissues is the basic premise to study the mechanism of fat accumulation in economic fishes. However, this issue has not been well documented. Therefore, it is necessary to use a new method to study fish fat distribution and content. This research is to use magnetic resonance imaging (MRI) technique for detection of fat content and distribution in the whole fish, which is applied to detecting fat distribution and content in different kinds of fish. In the first experiment, an optimized T1-weighted MRI technique was developed to scan grass carp, turbot, tilapia, pompano and large yellow croaker. The volume of fatty tissues in these fish was calculated by ImageJ. The fat content of liver and muscle in these fish was detected by Soxhlet extractor method. Three-dimensional (3D) images that quantitatively integrated the total volumes of fatty tissues in these fish were constructed by Visage Imaging Amira 5.4.3 for the first time. From 2D and 3D MRI images, we identified two patterns of fat distribution in fish:visceral-fat dominant (grass carp, tilapia, pompano and large yellow croaker) and subcutaneous-fat dominant (turbot). The volumes of fat tissues assessed by MRI were highly consistent with those obtained by Soxhlet extractor method. In a fasting experiment on tilapia, the MRI signal of mesenteric fat tissues successfully distinguished between fish before and after 28-d starvation. Although MRI technique has limitations for measuring fat in organs not specifically dedicated to fat storage, e.g., liver, muscle and intestine, the method will help researchers to gain insights into the distribution, size, volume and shape of the fatty tissues, especially adipose tissues, in intact fish. This could be a powerful tool in future studies of fish lipid metabolism.There are many factors that affect lipid metabolism. The studies have been shown that gender affects the process of lipid metabolism in mammals. However, this issue in fish has not been well reported. So, it is important to explore gender differences for the impact of mechanism in lipid metabolism. The second experiment was to explore the effects of gender on lipid metabolism in zebrafish (Danio rerio).In this present study, a 5-weeks feeding trial has been conducted to investigate the effects of dietary oil supplementation on the growth performance, triglyceride content in liver and muscle, fatty acid oxidation decomposition rate, lipid metabolism genes and RNA-seq of male (0.1568+0.03g) and female (0.1839+0.05g) zebrafish. Three diets containing 10,70 and 130 mg/kg soybean oil were fed to male and female zebrafish, respectively. Female zebrafish obtained higher weight gain than male zebrafish. Female zebrafish fed 130 mg/kg soybean oil diet gained the most abdominal adipose, triglyceride content in liver, weak ability of fatty acid oxidation. With the increase of the level of soybean oil in dietary, the expression levels of ACC, FAS and PPARa mRNA decreased in female zebrafish liver. In male zebrafish liver, the expression levels of ACC mRNA decreased and the expression levels of CPT1, VTG mRNA increased. The expression levels of CPT1, FAS and PPARa mRNA increased in female and male zebrafish muscle. Transcriptome sequencing results showed that the ability of fatty acid oxidation decreased and the ability of transport triglycerides increased in liver of male zebrafish fed high fat diet. The ability of fatty acid oxidation and triglycerides increased, and the ability of triglycerides synthesis decreased in liver of male zebrafish fed low fat diet. The ability of transport triglycerides and triglycerides decomposition increased, and the ability of fatty acid oxidation decreased. In conclusion, the effects of gender on lipid metabolism in zebrafish are obvious. So, the hypothesis of this experiment is that estrogen has the role of promoting fish fat accumulation.To further explore the influence of sex hormones on lipid metabolism in fish, and the third experiment to investigate the effects of different levels of estrogen on lipid metabolism in male zebrafish. In this study, three concentration of 500ng/L,1000ng/L,2000ng/L 17β-estradiol (17β-E2) treated male zebrafish respectively, wherein the co-solvent acetone as the control group. All male zebrafish(0.2548+0.036lg) were fed 70 mg/kg soybean oil diet, which was a period of five weeks of breeding experiments. This experiment was conducted to investigate the effects of 17(3-E2 on male zebrafish on the growth performance, triglyceride content in liver, abdomen and muscle, lipid metabolism genes and RNA-seq of male zebrafish. The results showed that 17(3-E2 increased body mass in male zebrafish. With the increase of 17β-E2 concentration, triglyceride content in liver and abdominal was affected, but triglyceride content in muscle did not change. Exposed to 17β-E2 environment, the expression level of FAS mRNA decreased. When 17β-E2 was 500ng/L, the expression level of DGAT1 a DGAT1b mRNA was the highest, while the expression level of DGAT2 and VTG mRNA increased with the increase concentration of 17β-E2. The transcriptome sequencing results showed that the capacity of transport triglycerides and fatty acid oxidation tended to decrease in 17β-E2 2000ng/L treatment groups male zebrafish liver. In conclusion,17β-E2 affect lipid metabolism in zebrafish and the effect of 17β-E2 on lipid metabolism in male zebrafish change with the increase of 17β-E2 concentration. Thus, the effect of estrogen on lipid metabolism in fish is the factor of fat accumulation. |
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