Genetic Diversity and Population Structure in Polygonum cespitosum: Insights to an Ongoing Plant Invasion

Molecular markers can help elucidate how neutral evolutionary forces and introduction history contribute to genetic variation in invaders. We examined genetic diversity, population structure and colonization patterns in the invasive Polygonum cespitosum, a highly selfing, tetraploid Asian annual introduced to North America. We used nine diploidized polymorphic microsatellite markers to study 16 populations in the introduced range (northeastern North America), via the analyses of 516 individuals, and asked the following questions: 1) Do populations have differing levels of within-population genetic diversity? 2) Do populations form distinct genetic clusters? 3) Does population structure reflect either geographic distances or habitat similarities? We found low heterozygosity in all populations, consistent with the selfing mating system of P. cespitosum. Despite the high selfing levels, we found substantial genetic variation within and among P. cespitosum populations, based on the percentage of polymorphic loci, allelic richness, and expected heterozygosity. Inferences from individual assignment tests (Bayesian clustering) and pairwise FST values indicated high among-population differentiation, which indicates that the effects of gene flow are limited relative to those of genetic drift, probably due to the high selfing rates and the limited seed dispersal ability of P. cespitosum. Population structure did not reflect a pattern of isolation by distance nor was it related to habitat similarities. Rather, population structure appears to be the result of the random movement of propagules across the introduced range, possibly associated with human dispersal. Furthermore, the high population differentiation, genetic diversity, and fine-scale genetic structure (populations founded by individuals from different genetic sources) in the introduced range suggest that multiple introductions to this region may have occurred. High genetic diversity may further contribute to the invasive success of P. cespitosum in its introduced range.分子标记（molecular markers）有助于阐明中性进化力量与引入历史如何促成入侵物种的遗传变异。本研究以引入至北美、高度自交的四倍体亚洲一年生入侵植物直立蓼（Polygonum cespitosum）为对象，对其遗传多样性、种群结构及定殖模式展开探究。我们采用9个二倍体化多态微卫星标记（microsatellite markers），通过对516个个体的分析，研究了其引入范围（北美东北部）内的16个种群，并提出以下3个研究问题：1）不同种群的种群内遗传多样性水平是否存在差异？2）种群是否可形成独立的遗传聚类簇？3）种群结构是否反映地理距离或生境相似性？ 研究结果显示，所有种群的杂合度均处于较低水平，这与直立蓼的自交交配系统相符。尽管自交率极高，但基于多态位点（polymorphic loci）百分比、等位基因丰富度（allelic richness）及期望杂合度（expected heterozygosity）的分析表明，直立蓼种群内部及种群间均存在显著的遗传变异。通过个体归属测试（individual assignment tests，采用贝叶斯聚类（Bayesian clustering）法）及两两FST值的推断分析，我们发现种群间分化程度较高，这表明基因流（gene flow）的作用相对有限，而遗传漂变（genetic drift）的影响更为显著，这可能源于直立蓼的高自交率及有限的种子扩散能力（seed dispersal ability）。 种群结构并未呈现距离隔离（isolation by distance）模式，亦与生境相似性无关。相反，种群结构似乎是繁殖体（propagules）在引入范围内随机扩散的结果，这一过程可能与人类介导的扩散（human dispersal）相关。此外，引入范围内较高的种群分化水平、遗传多样性及精细尺度的遗传结构（即种群由不同遗传来源的个体奠基形成）表明，该区域可能存在多次引入事件。较高的遗传多样性可能进一步促进了直立蓼在其引入区域的入侵成功。