One population from each of six plant species along both sides of the Juyong-guan Great Wall, together with one population from each of five species along both sides of a path on a mountain top near Juyong-guan, were selected to study the effect of the Great Wall as a barrier on genetic differentiation between two subpopulations using RAPD markers. Significant genetic differentiation was found between the subpopulations on both sides of the Great Wall. A wind-pollinated woody species, Ulmus pumila, showed less genetic differentiation than four insect-pollinated species: Prunus armeniaca, Ziziphus jujuba, Vitex negundo, and Heteropappus hispidus. Cleistogenes caespitosa, a windpollinated perennial herb, displayed more genetic differentiation between subpopulations than the insect-pollinated species because of its propagation strategy. Although AMOVA analysis showed that subpopulations divided by a mountain path had diverged genetically, the variance component between the subpopulations on both sides of the Great Wall was significantly larger than that between the subpopulations at the control site. Therefore, it is reasonable to deduce that the Juyong-guan Great Wall has served as a physical barrier to gene flow between subpopulations separated for more than 600 years.
Although extensive studies have been conducted on the genetic structure of Arabidopsis thaliana (A. thaliana) populations worldwide, the populations from China have never been studied. In this study, we collected 560 individuals from 19 natural populations of A. thaliana distributed in East China along the lower reaches of the Yangtze River, and two populations from northwest China (Xinjiang Province). We adopted two kinds of molecular marker, inter-simple sequence repeats (ISSRs) and random amplified polymorphic DNA (RAPDs) to investigate the genetic diversity within and among populations, and the correlation between the genetic and geographic distances. Thirteen ISSR primers produced 165 polymorphic bands (PPB) (96%) and 11 RAPD primers produced 162 polymorphic bands (98%) in about 560 individuals. The two marker systems generated similar patterns of genetic diversity in these natural populations. The AMOVA analysis indicated about 42-45% of the total genetic variation existed within populations, and found possible geographic structure. The Mantel test revealed a significant correlation between the geographic distance and the genetic distance of these populations in general. A close genetic relationship was found among four populations in the Jiangxi Province, and these always appeared clustered together as a monophyletic group in unweighted pair-group method with arithmetic averages dendrograms based on both ISSR and RAPD data sets. Based on the observation of recolonization and extinction of naturally distributed populations of A. thaliana, and the pattern of their genetic differentiation, the distribution of this species in China might be a result of natural dispersal under the strong influence of human activity.
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