| Stock enhancement have effectively restored and increased fishery resources and improved the ecological environment,which has become a widely adopted fishery measure in the world.In recent years,with the increasing frequency and scale in conduction of stock enhancement whose spieces is dominated by Four Chinese Carps.From 2016 to 2017,large-scale release activities were held in the lower reaches of the Yangtze River.How much the contribution to the amount of fishery resources the stock enhancement had made and whether it had an impact on the genetic resources of the released species which was need to be effectively assessed.In this study,eleven pairs of microsatellite markers were used to estimate the resource contribution rate of stock enhancement of silver carp in Jiangsu section of the Yangtze River from 2016 to2017 and the potential genetic risks possibly caused by stock enhancement in order to provide basic data and reference basis for the works of stock enhancement of silver carp.The main results of this study are as follows:1.Assessment of stock enhancement in resource contribution rate of silver carpFrom 2016 to 2017,Juveniles of silver carp were released into the lower reaches of the Yangtze River in Jiangsu province.A total of 621 brood stocks were collected from 8 hatcheries,and 475 tails of silver carp from four populations of the lower reaches of the Yangtze River were recaptured.Through paternity identification technology,thirty-nine recaptured silver carps were identified as the released offspring of brood stocks of hatcheries under the cumulative exclusion rate of paternity test 99.996%and95%confidence level.From 2016 to 2017,the contribution rate of stock enhancement of silver carp from the Yangtze River to the wild resource was more than 8.21%.2.Assessment of stock enhancement in genetic riskof silver carp(1)Genetic diversityEleven pairs microsatellite markers of silver carp with high polymorphism ranged from 0.34 to 0.938.Hardy-Weinberg disequilibrium(p<0.001)was found in more than5 loci in 8 populations(four populations from hatcheries and four populations from the Yangtze River),indicating that the distribution of alleles in 8 populations was imbalanced or significantly different.The number of alleles(Na)ranged from 3 to 33in all loci.In all populations,the mean number of alleles(Na)ranged from 5.727 to15.818,the mean number of effective alleles(Ne-Allele)ranged from 4.19 to 6.526,the mean observed heterozygosity(Ho)ranged from 0.625 to 0.727,the mean expected heterozygosity(He)ranged from 0.69 to 0.784,and the mean number of allele richness(Ar)ranged from 16.889 to 30.161.The mean inbreeding coefficient(Fis)among populations also varied,ranging from-0.026 to 0.199.The mean unbiased expected heterozygosity(UHe)ranged from 0.718 to 0.791,indicating that the genetic diversity of all populations was relatively high.The descending order of genetic diversity among populations was as follows:(1)In the Yangtze River populations of silver carp,JJY(Jingjiang section),ZJY(Zhenjiang section),CSY(Changshu section),ZGY(Zhangjiagang section).(2)In hatchery populations of brood stocks of silver carp,RGH(e.g.Rugao Hatchery),QXH(e.g.Quxia Hatchery),KSH(Kunshan Hatchery),TXH(Taixing Hatchery),HTH(Huangtang Hatchery),WLH(Weilai Hatchery),HJH(Hangjiang Hatchery),JRH(Jurong Hatchery).Wilcoxon signed-rank results showed that genetic diversity of the Yangtze River population of silver carp was higher than that of hatchery populations of brood stocks of silver carp,and the number of effective alleles was higher than that of hatchery populations of brood stocks of silver carp.Both populations of the Yangtze River population of silver carp and hatchery populations of brood stock of silver carp were possibly facing inbreeding risk,but the Yangtze River population was more serious.There was no significant difference in allele richness(Ar)between two populations.The effective population size ranged from 18.6 to 311.3.The total the effective population size(N_e)of the Yangtze River population(N_e=301.7)was larger than that of hatchery population of brood stocks of silver carp(N_e=200.1).The genetic diversity(UHe)of populations of silver carp in Jiangsu section of the Yangtze River was less affected by tock enhancement of silver carp,but the genetic risks,inbreeding depression of populations in the Yangtze River was found,which impacted the effective population size in a certain extent.(2)Genetic differentiationAmong the 12 populations,genetic differentiation coefficient ranged from 0.003to 0.067.Three pairs of populations showed moderate genetic differentiation(HJH&HTH,0.053;HJH&JRH,0.064;HTH&JRH,0.067),while the others showed low genetic differentiation coefficient.The inter-population gene flow(Nm)ranged from 3.496 to 79.845(Nm>1),indicating that there was strong gene exchange among12 populations.The gene flow among the four populations of the Yangtze River is far greater than 1(Nm>>1).In all populations,molecular variance analysis(AMOVA)showed that 97.03%of genetic differentiation was attributed to intra-population differentiation,while only 2.97%was attributed to inter-population differentiation.Nei’s unbiased distance ranged from 0.002 to 0.364 between paired populations and Nei’s unbiased identity ranged from 0.695 to 0.998 between paired populations.Cluster analysis showed that all populations could be divided into two groups,one group was populations of brood stocks of silver carp(HJH)in Hanjiang hatchery,and the other group was 11 other populations.Without considering hatchery populations,ZJY(Zhenjiang section of the Yangtze River)and CSY(Changshu section of the Yangtze River),ZGY(Zhangjiagang section of the Yangtze River)gathered first,and then JJY(Jingjiang section of the Yangtze River)joined.The results showed that there was no geographical difference in genetic distance among four populations in the lower reaches of the Yangtze River.The genetic differentiation coefficient of populations of silver carp in Jiangsu section of the Yangtze River was reduced by stock enhancement,and the gene flow among populations was increased,accompanying with the reduced genetic distance between populations.(3)Genetic structureThe curves of LnP(D)and K varying with K value show that when LnP(D)and K are maximal,inflection point appears,and the corresponding K value is 3 which is confirmed the real K value.Eight hatchery populations of brood stocks of silver carp and four populations of silver carp in the lower reaches of the Yangtze River were hybridized from three ancestral subpopulations(mixed genetic clusters).There are some differences between hatchery population and the Yangtze River population.Cluster 1 and Cluster 2 account for a larger proportion in the genetic structure of hatchery population,while Cluster 3 account for a smaller proportion.Cluster 1accounts for the largest proportion in the genetic structure of the Yangtze population,Cluster 2 accounts for a smaller proportion and Cluster 3 accounts for a very low proportion.The genetic structure of populations of silver carp in the lower reaches of the Yangtze River is affected by stock enhancement of silver carp,presenting the trendency of genetic structure simplication. |
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