The Phenotypic,genetic And Epigenetic Differentiation Of Echinochloa Crusgalli Var. Zelayensis Under Extreme Flooding Stress In The Riparian Zone Of The Three Gorges Reservoir

The hydrological regime of the Three Gorges Reservoir（TGR）contributes to the riparian zone whose vertical depth is 30 m on both sides of the reservoir,and the plants growing on both sides will suffer from periodic flooding out of season.In order to adapt to the condition of prolonged and great depth of waterlogging,plant populations at different elevations need to develop appropriate survival strategies.Previous studies have shown that plants at different elevations in the riparian zone have differentiated in some phenotypic traits and have possessed certain evolutionary potential.To explore the adaptation mechanism that plant populations in different elevation of the Three Gorges Reservoir can survive at different environment and whether such adaptation can continue,the seeds of the dominant annual plant Echinochloa crusgalli var.zelayensis from different elevations（153 m,159 m,165 m,170 m and 180 m）of the Three Gorges Reservoir were planted in a homogeneous garden.Phenotypic differentiation pattern among Echinochloa crusgalli var.zelayensis populations at different elevations was analyzed;based on AFLP molecular markers,the genetic diversity and genetic differentiation of Echinochloa crusgalli var.zelayensis populations at different elevations were analyzed;and based on MSAP molecular markers,epigenetic diversity and epigenetic differentiation of Echinochloa crusgalli var.zelayensis populations at different elevations were studied.The main results are as follows:（1）It was found that there were significant differences in germination,vegetative growth and reproductive growth traits of Echinochloa crusgalli var.zelayensis populations from different elevations.Compared with populations from high-elevation,the seeds from low-elevation germinated earlier and germinated explosively within a short time,and the germination rate was highest among different eleations.Echinochloa crusgalli var.zelayensis populations from low-elevation had the smallest height,the lowest total biomass and the fewest number of lateral branches,while the ratio of blade length to sheath length was the highest;Echinochloa crusgalli var.zelayensis populations from low-elevation had the shortest life history and invested more energy in reproductive allocation,and the biomass of reproductive organs occupied the largest proportion in the total biomass,so as to invest as much energy as possible in effective reproductive growth.The phenotypic variation coefficients of different traits in Echinochloa crusgalli var.zelayensis populations at different elevations were analyzed,and the variation extent of traits such as the number of lateral branches,total biomass,thousand-seed weight and start-up days were the maximum.（2）Six pairs of polymorphic AFLP primers were used to analyze the genetic diversity and genetic differentiation of Echinochloa crusgalli var.zelayensis populations from five elevations.The average polymorphism of each pair of primer was83.43%;the average number of alleles（Na）was 1.45;the average number of effective alleles（Ne）was 1.32;the Nei’s gene diversity index（H）was 0.18,and the Shannon information index（I）was 0.26.The genetic diversity of Echinochloa crusgalli var.zelayensis populations from 170 m in the riparian zone was the highest.The genetic diversity of Echinochloa crusgalli var.zelayensis populations was higher than that of the average for the annual plants（0.13）.The average genetic differentiation coefficient(Ф_ST)of Echinochloa crusgalli var.zelayensis population was 0.109,and the genetic differentiation coefficient of Echinochloa crusgalli var.zelayensis populations between153 m and those from 159 m,165 m,170 m and 180 m increased gradually,following a particular differentiation pattern.The variation tendency of genetic distance and genetic differentiation coefficient was the same.In the analysis of genetic structure of the population,the population from the non-fluctuating zone（180 m）clustered into a single branch;in the riparian zone,populations from 159 m and 165 m clustered into a branch first,then the population from 153 m clustered into the branch,and the population from170 m clustered into a branch.（3）Six pairs of polymorphic MSAP primers were used to analyze the epigenetic diversity and epigenetic differentiation of Echinochloa crusgalli var.zelayensis populations from five elevations.Based on the results of methylation-susceptiple loci（MSL）,The average polymorphism of each pair of primer was 67.59%,the average number of alleles（Na）was 1.47;the average number of effective alleles（Ne）was 1.27;Nei’s gene diversity index（H）was 0.16,and Shannon information index（I）was 0.24,which were lower than the genetic diversity.The epigenetic diversity of Echinochloa crusgalli var.zelayensis populations from 153 m was the highest,which was contrary to the trend of genetic diversity.The study on epigenetic differentiation and epigenetic structure of Echinochloa crusgalli var.zelayensis populations from 5 elevations showed that the average epigenetic differentiation coefficient(Ф_ST=0.104)of Echinochloa crusgalli var.zelayensis populations was slightly lower than genetic differentiation coefficient(Ф_ST=0.109).The variation tendency of epigenetic differentiation was similar to that of genetic differentiation in the riparian zone,and the differentiation coefficient increased with the increase of elevation.The epigenetic structure of the population showed that the population from the non-fluctuating zone（180 m）clustered into a single branch;in the riparian zone,populations from 165 m and 170 m clustered into a branch first,then the population from 159 m clustered into the branch,and the population from 153 m clustered into a branch.To sum up,the operation of the Three Gorges Reservoir resulted in different waterlogged environments,and leading to the phenotypic differentiation including germination,vegetative growth and reproductive traits of Echinochloa crusgalli var.zelayensis populations at different elevations.Genetic differentiation and epigenetic differentiation occurred in Echinochloa crusgalli var.zelayensis populations from different elevations,and the degree of epigenetic differentiation was slightly smaller than that of genetic differentiation.Echinochloa crusgalli var.zelayensis populations at low elevation responsed to the waterlogged environment more actively,and these traits could be heritable under the influence of genetic and epigenetic effects,which is conducive to the persistence of Echinochloa crusgalli var.zelayensis populations in the riparian zone,and therefore Echinochloa crusgalli var.zelayensis populations could be chosen for vegetation restoration in the riparian zone.This study is helpful to understand the adaptation mechanism of annual plants to waterlogged environment and the reason for their long-term existence in the riparian zone of reservoirs,providing theoretical basis and development direction for vegetation restoration and reconstruction in riparian zone of large reservoirs.