Population Biology And Genetic Diversity Of Coilia Brachygnathus In The Three Gorges Reservoir Area

Shortjaw tapertail anchovy,C.brachygnathus,belongs to the Engraulidae family of Clupeiformes.C.brachygnathus is mainly distributed in the middle and lower reaches of the Yangtze River and lakes,which connectsthe upper and lower trophic levels of the food web in the freshwater ecosystem and has a very important ecological role and economic value.After the impoundment of the Three Gorges Project,a lake ecosystem with serious vacant ecological niche was formed in the reservoir area.In recent years,it was found that C.brachygnathus,which had not been recorded in the upper reaches of the Yangtze River,began to appear in the reservoir area.The number of C.brachygnathus caught in the reservoir area increased year by year,and it has gradually become one of the common species in the Three Gorges Reservoir.In order to understand the resources status of newly found population of C.brachygnathus in the Three Gorges Reservoir Area,this study selected four sampling sites in Fuling（FL）,Fengdu（FD）,Wanzhou（WZ）and Wushan（WS）in the Three Gorges Reservoir area in the upper reaches of the Yangtze River to carry out a catch survey from August 2018 to April 2020.A total of 153 C.brachygnathus samples were collected.The age structure and growth characteristics of C.brachygnathus population in the Three Gorges Reservoir area were obtained by using scales as age identification material and body length and body weight measurement results.The morphological parameters of C.brachygnathus were analyzed by morphological methods.The genetic diversity and genetic structure of C.brachygnathus population were analyzed by genomic microsatellite markers.The main results are as follows.Age and growth of C.brachygnathus population in the Three Gorges Reservoir areaThe age identification and growth characteristics analysis of 153 samples of C.brachygnathus in the Three Gorges Reservoir area showed that the age group of C.brachygnathus in the Three Gorges Reservoir area was 1-5 years old,among which the dominant age group was 2-3 years old,accounting for 82.35%of the total sample,and the number of fish in the 2 years old was the largest,accounting for 51.63%of the total sample.The body length of C.brachygnathus in the Three Gorges Reservoir Area ranged from 102.1 to 301.6 mm,with an average body length of 196.6 mm,among which the dominant body length group ranged from 145.0 to 234.9 mm,accounting for77.12%of the total sample volume.The body weight of C.brachygnathus in the reservoir area ranged from 1.9 to 79.7 g,with an average body weight of 17.7 g.The dominant body weight group ranged from 0 to 29.9 g,accounting for 84.97%of the total sample volume.The body grows in proportion to its weight.The fitting relationship between body length and body weight is as follows:W=2×10^-7L^3.3884（R²=0.8625,n=153）.The results showed that the growth rate of C.brachygnathus in the Three Gorges Reservoir area was fast,and the age composition of the sample population indicated that the population was in a period of rapid growth.Morphological difference analysis of four sampled populations of C.brachygnathusSixteen morphometric indexes of C.brachygnathus were analyzed by one-way analysis of variance,principal component analysis and cluster analysis.One-way analysis of variance showed that there were significant differences among the four C.brachygnathus groups in five proportional traits:snout length/body length,eye diameter/body length,pectoral fin length/body length,body thickness/body length and body weight/body length.Morphological differences were mainly concentrated in the head and the trunk near the head,among which there was a great morphological difference between FLgroup and the other three groups.Principal component analysis showed that the cumulative contribution rate of seven principal components was72.67%.In the first principal component,the trait load coefficient reflecting the body characteristics of C.brachygnathus was higher;in the second principal component,the trait load coefficient reflecting the head characteristics of C.brachygnathus was higher;in the third principal component,the trait load coefficient reflecting the athletic ability of C.brachygnathus was higher.Cluster analysis results show that among the four groups,WZ group and FD group are grouped into one group,while FL group and WS group are grouped into one group.The Euclidian distance between FD group and WZ group is relatively close,while the Euclidian distance between FL group and the other three groups is relatively far,which is consistent with the results of one-way analysis of variance.In general,there were some differences in external morphology among the four C.brachygnathus groups,but the difference degree were less.Analysis of microsatellite genetic diversity in four sample groups of C.brachygnathusTen polymorphic microsatellite loci were selected to analyze the genetic diversity and genetic structure of four wild C.brachygnathus groups.The results showed that all the microsatellite loci could achieve good amplification in 153 C.brachygnathus samples,a total of 82 alleles were detected among which eight loci were highly polymorphic（PIC>0.5）.The PIC of the loci ranged from 0.3711 to 0.8237,the N_a of the loci ranged from 4-16,the N_e of the loci ranged from 1.8024 to 6.8517,the H_oof the loci ranged from 0.4510-0.8497,the H_e of the loci ranged from 0.4467-0.8569,and the I of the loci ranged from 0.7166-2.1361.Four populations of C.brachygnathus in the Three Gorges Reservoir all had high levels of genetic diversity,and the descending order of genetic diversity was WS>WZ>FD>FL,with the average N_a ranged from5.6 to 7.3,the average N_e ranged from 3.5414 to 3.8315,the average H_o ranged from0.6177 to 0.7000,the average H_e ranged from 0.6823 to 0.7029,and the average PIC ranged from 0.6285 to 0.6484.A total of 13 loci deviated significantly from Hardy-Weinberg equilibrium in four populations,all of which were concentrated in FD population,WS population and WZ population.The level of genetic differentiation among the four C.brachygnathus populations was very low,and the F_ST value among the populations was less than 0.05.The results of the analysis of molecular variance showed that only 0.09%of the variation was due to inter-population,and the genetic variation between populations was mainly from within individuals（95.44%）.The FD and FL populations were combined into one sampling population,and the genetic distance（D_s）between populations ranged from 0.0256 to 0.0271,among which the genetic distance between FD and FL combined population and other populations was relatively far.The UPGMA tree based on Nei’s genetic distance showed that the WZ and WS populations clustered first into one branch,and then into one branch with the FD and FL combined population.In conclusion,this study is the first to analyze the age composition,growth characteristics,morphological characteristics and genetic diversity of the newly found population of C.brachygnathus in the Three Gorges Reservoir area.The results showed that the growth rate of C.brachygnathus in the Three Gorges Reservoir area was fast and the proportion of young age was large proportion.Morphological characteristics analysis showed that the external morphological differences among the four sampling sites were less.The four C.brachygnathus groups showed a high level of genomic microsatellite genetic diversity,which gradually decreased from the downstream to the upstream group.the level of genetic differentiation among sample groups was low.The above results suggest that the population of C.brachygnathus in the Three Gorges Reservoir area is probably derived from the the population in the middle and lower reaches of the Yangtze River by diffusion and migration.Currently,the newly found C.brachygnathus population in the reservoir area is in the diffusion stage of rapid population growth,and the gene exchange among different groups in the reservoir area is frequent.This study could provide basic data for the evolution of aquatic ecosystem in the Three Gorges Reservoir area and the stock biological research and conservation of C.brachygnathus in the upper reaches of the Yangtze River.