Ruckelshaus Mary H
University of Washington, Department of Botany, Seattle, Washington, 98195.
Department of Biological Science, Florida State University, Tallahassee, Florida, 32306.
Evolution. 1998 Apr;52(2):330-343. doi: 10.1111/j.1558-5646.1998.tb01635.x.
In this study, the first investigation of population structure in an aquatic angiosperm, I show that populations of a marine angiosperm (eelgrass, Zostera marina) are genetically differentiated at a number of spatial scales. I find also that there is no correspondence between geographic and genetic distances separating subpopulations, an increasingly common result in spatially stratified studies of genetic structure in marine invertebrates. F-statistics, calculated for two years from electrophoretic variation at five polymorphic allozyme loci, indicate significant genetic differentiation among sampling quadrats within each of two bays (θ = 0.064-0.208), between tide zones within a bay (θ = 0.025-0.157) and between bays (θ = 0.079). Spatial autocorrelation analysis was used to explore genetic differentiation at smaller spatial scales; estimated patch sizes (within which genetic individuals are randomly associated) indicated no appeciable genetic structure at scales less than 20 m × 20 m. Calculated values of F-statistics were a function of the spatial scale from which samples were drawn: increasing the size of the "subpopulation" included in calculation of fixation indices for the same "total" sample resulted in an increase in the magnitude of f (e.g., from 0.092 to 0.181) and a decrease in θ (e.g., from 0.186 to 0.025). On the basis of the best estimate of the spatial scale of subpopulations, the effective number of migrants per generation (N m) ranges from 1.1 to 2.8. Genetic consequences of the disturbance regime in the eelgrass habitat sampled were extreme variation between years in the allele richness and proportion of heterozygotes in a sample and a positive relationship between the extinction probability of patches and the genetic variance among them. The changes in F-statistics as a function of sampling scale and the observation that θ among sampled quadrats was positively associated with the probability of extinction among quadrats indicated that indirect estimates of gene flow (N m) calculated from θ should be cautiously interpreted in populations that may not yet be in drift-migration equilibrium.
在本研究中，这是对一种水生被子植物种群结构的首次调查，我发现一种海洋被子植物（鳗草，大叶藻）的种群在多个空间尺度上存在遗传分化。我还发现，分隔亚种群的地理距离与遗传距离之间不存在对应关系，这在对海洋无脊椎动物遗传结构的空间分层研究中是一个越来越常见的结果。根据在五个多态性等位酶位点的电泳变异计算的两年F统计量表明，在两个海湾中的每一个海湾内的采样样方之间（θ = 0.064 - 0.208）、一个海湾内的潮间带之间（θ = 0.025 - 0.157）以及海湾之间（θ = 0.079）存在显著的遗传分化。空间自相关分析用于探索较小空间尺度上的遗传分化；估计的斑块大小（在其中遗传个体随机关联）表明在小于20米×20米的尺度上不存在明显的遗传结构。计算得到的F统计量值是所抽取样本的空间尺度的函数：对于相同的“总”样本，在计算固定指数时增加所包含的“亚种群”大小会导致f值增大（例如，从0.092增大到0.181）以及θ值减小（例如，从0.186减小到0.025）。根据对亚种群空间尺度的最佳估计，每代的有效迁移个体数（Nm）范围为1.1至2.8。在所采样的鳗草栖息地中，干扰状况的遗传后果是样本中等位基因丰富度和杂合子比例在年份之间存在极大差异，并且斑块的灭绝概率与它们之间的遗传方差呈正相关。F统计量随采样尺度的变化以及所采样样方之间的θ与样方之间灭绝概率呈正相关这一观察结果表明，在可能尚未处于漂变 - 迁移平衡的种群中，从θ计算得到的基因流间接估计值（Nm）应谨慎解释。
