Ecological Differentiation Of Dominant Perennial Plant Cynodon Dactylon In The Riparian Zone Of The Three Gorges Reservoir

Since the Three Gorges Reservoir(TGR)was put into operation,the long-term flooding stress has become the main limiting factor for the survival of plants in the riparian zone.The original vegetation including trees,shrubs and herbs have been gradually replaced by new vegetation in the riparian zone,and the new vegetation mainly consists of annuals and several kinds of perennial herbs.The formation of new vegetation has an important influence on the quality of ecological environment and the function of the ecosystem in the riparian zone.The time and intensity of waterlogging are different at different elevations in the riparian zone,therefore,the living conditions of plant populations at different elevation are different.Plant populations of different elevations need to adopt different strategies in order to adapt to this long-term and high-intensity flooded environment.Plants at different elevations in the riparian zone could adapt to their environment through intraspecific differentiation and showed some evolutionary potential.In order to explore how plant populations in the riparian zone of the TGR adapt to different elevation environments,and whether this adaptation mechanism can be sustained,in this study,the dominant perennial plant Cynodon dactylon from different elevations(157 m,162 m,168 m,173 m and 180 m)of the TGR were planted in a homogeneous garden.Phenotypic differentiation pattern among different elevations was analyzed;based on AFLP molecular markers,the genetic diversity and structure of Cynodon dactylon populations from five elevations were analyzed,and the genetic differentiation among populations at different elevations was studied;based on the MSAP molecular markers,the epigenetic diversity and structure of Cynodon dactylon populations from five elevations were analyzed,and the epigenetic differentiation among populations at different elevations was studied.The main results are as follows:(1)The phenotypic traits of the Cynodon dactylon populations from five elevations in the TGR were compared and analyzed,and it was found that there were significant differences in germination,growth and dry-matter distribution of Cynodon dactylon populations from different elevations.Compared with the non-fluctuating zone,operation of high water level for years promoted the germination of Cynodon dactylon and helped Cynodon dactylon to produce more lateral branches.With the increase of elevation,the leaf length,leaf width,leaf area,leaf thickness and stem diameter showed an increasing trend,while the internode length showed a decreasing trend.From low-elevation to high-elevation,the total biomass increased first and then decreased.Plant populations at low-elevation allocated more biomass to the lateral branches,and plants populations at high-elevation allocated more biomass to the main stems and leaves.(2)Six pairs of polymorphic AFLP primers were used to analyze the genetic diversity of Cynodon dactylon populations from five elevations.The results showed that the mean values of Nei’s gene diversity and Shannon information index were0.1110 and 0.1593,respectively.There was genetic differentiation among Cynodon dactylon populations at different elevations,and the differentiation coefficient(Фst)was 0.067.The genetic distance and genetic identity of the populations from five elevations were analyzed.It was found that the highest degree of differentiation appeared between the lowest elevation 157 m in the riparian zone and 180 m in the non-fluctuating zone.The genetic variation of Cynodon dactylon was mainly distributed within the population,up to 93%,but only 7% among populations.The five populations were divided into two groups when the genetic similarity coefficient was0.94.The population of Cynodon dactylon from the non-fluctuating zone(180 m)belong to one group,and the populations from 157 m,162 m,168 m,and 173 m elevations in the riparian zone were clustered into another group.(3)Six pairs of polymorphic MSAP primers were used to analyze the epigenetic diversity of Cynodon dactylon populations from five elevations.Based on the methylation-susceptible loci(MSL)and non-methylated loci(NML),the epigenetic diversity of the Cynodon dactylon population was greater than that of the genetic diversity.The results of molecular variance analysis showed that there was epigenetic differentiation among the populations from five elevations,and the differentiation coefficient(Φst)was 0.115 and the degree of epigenetic differentiation was greater than that of genetic differentiation.The epigenetic variation of Cynodon dactylon was mainly distributed within the population,up to 88%,but only 12% among populations.The five populations were divided into three groups when the similarity coefficient was 0.97.Populations from 157 m and 162 m elevations were clustered into one group;populations from 173 m and 180 m were clustered into another group,and the population from 168 m was separated into one group.To sum up,the operation of the TGR has resulted in phenotypic,genetic,and epigenetic differences in Cynodon dactylon populations at different elevations,showing a certain differentiation pattern and certain evolutionary potential.This study revealed the adaptation mechanism of Cynodon dactylon to the environment at different elevations in the TGR from the molecular and morphological levels.This work will help to understand the impact of flooding on plants and the response of plants to flooding,which has important theoretical guiding significance for the establishment of an ecological environment restoration system in the riparian zone.