Toxic Effect And Mechanism Of Diethylstilbestrol On Water Flea Daphnia Magna

In recent years,trace estrogen has been frequently detected in natural water around the world,and its toxic effect and mechanism on aquatic ecosystem has became frontier research hotspot.Estrogen in the aquatic environment can affect the growth and reproduction process of aquatic organisms and even disturb the homeostasis of the aquatic ecosystem.In China,thousands of tons of synthetic estrogen enter the water body in process of production and utilization every year.Diethylstilbestrol（DES）is a typical synthetic estrogen,but its toxicity to aquatic organisms and mechanism are not very clear.In this study,DES was selected as the most sensitive estrogen to Daphnia magna（D.magna）by acute toxicity test.Subsequently,according to OECD 211 guideline for chemical testing of D.magna,the toxic effect and toxicity mechanism of DES on D.magna were investigated through the combination of chronic reproduction test,transcriptome sequencing technology,and multi-generation test.The toxicity mechanism of DES to D.magna was preliminarily elucidated from physiological changes,transcriptome level and population level.The conclusions are as follows:（1）Compared with OECD 202 chemical test guideline for D.magna,it was founded that the48h-EC₅₀of reference toxicant potassium dichromate was 0.7749 mg·L^-1,which met the requirements of the test guideline.Comparing the acute toxicity of four kinds of estrogens to D.magna,it was found that the 48h-EC₅₀of DES was 1.889 mg·L^-1,which was much lower than other estrogens.Therefore,D.magna is the most sensitive to the toxicity of DES.（2）In the experiment on chronic reproduction of D.magna with DES,it was founded that the first spawning time was significantly delayed at low,medium,and high concentrations(2、200、1000μg·L^-1).At high concentration,the body length was decreased at 7 and 21 days.With low concentration,the total number of offspring was increased,while at medium and high concentrations,the total number of offspring was decreased,but there was no significant difference compared with the solvent control group.It showed that DES exposure affected the growth and reproduction process of D.magna.（3）The effects of DES on gene expression and molecular pathway of D.magna were studied by using new transcriptome sequencing.The results showed that genes involved in energy metabolism and lipid metabolism-related pathways were down-regulated at low concentration.With medium and high concentrations,genes involved in pathways related to the digestive system,transportation,and catabolism were down-regulated,while genes involved in the pathways related to lipid metabolism were up-regulated.Among them,the down-regulation of genes involved in cholesterol metabolism in the digestive system is the main reason for the inhibition of growth and reproduction,which was reflected by the physiological changes such as delayed first spawning time and reduced body length.（4）To further study the toxicity effect of DES on the population level of D.magna,two successive generations of exposure tests（F0,F1）and two successive generations of recovery tests（F2,F3）were carried out.The results showed that the first spawning time and the total number of offspring of F1 generation were changed at three exposure concentrations,and the body length was inhibited at a high concentration.In the F2 generation recovery group,the first spawning time and body length were affected at high concentration.The total number of offspring was still inhibited at medium and high concentrations of F3 generation,meanwhile,the body length was decreased at high concentration.It was founded that D.magna was more sensitive to DES under continuous exposure,and the toxicity was transmitted through reproduction in the population,thus posing a threat to the health of population.This study preliminarily explained the toxic effect and mechanism of DES on D.magna,providing a scientific basis for environmental risk assessment and management of estrogen and warning people to use estrogen properly.