Can physiographic regions substitute for genetically-determined conservation units? A case study with the threatened plant, Silene spaldingii

Protecting genetic diversity throughout the range of a species is important for conservation, as doing so provides for long-term evolutionary potential and persistence under a changing environment. Conservation of diversity at the intraspecific level requires identification of all genetically distinct population segments within species; i.e., conservation units (CUs). Silene spaldingii occurs in grasslands of the Columbia Plateau region of western North America and is listed as threatened under the Federal Endangered Species Act. The recovery plan identified five physiographic regions across the range of the species to use as surrogates for genetic CUs. We collected leaf samples from an average of 26 plants from each of 19 of the largest populations across all five physiographic regions and used variable microsatellite and chloroplast DNA markers to determine how genetic variation is distributed across the range of the species and how well physiographic regions reflect population structure within this species. Results of several multivariate analyses clustered our samples into four genetic groups which did not correspond well with the physiographic regions. We observed little genetic differentiation among populations in the main range of the species which encompasses nearly all of four contiguous physiographic regions. However, three other distinct genetic groups were identified: two in the disjunct northeast corner and one at the southeast edge of the main range. Modification of the CUs to reflect the genetic groups rather than the physiographic regions would result in CUs which better reflect historical patterns of population structure. Moreover, use of the genetic units to inform translocation and genetic rescue efforts could improve our ability to mimic natural patterns of gene flow. Our results suggest that physiographic regions may not always be an accurate reflection of population structure for threatened or endangered species.

在物种的整个分布范围内保护遗传多样性，对于生物保护工作而言至关重要：此举可为物种在持续变化的环境中提供长期的演化潜力与存续能力。种内多样性保护需识别物种内所有具有遗传分化的种群单元，即保护单元（Conservation Units, CUs）。斯帕尔登蝇子草（Silene spaldingii）分布于北美西部哥伦比亚高原地区的草原生境中，被列入《联邦濒危物种法》的受威胁物种名录。该物种的恢复计划将其分布区划分为5个地貌区域，以作为遗传保护单元的替代划分依据。我们从覆盖全部5个地貌区域的19个最大种群中，每个种群平均采集26株植株的叶片样本；并采用多态性微卫星（microsatellite）标记与叶绿体DNA（chloroplast DNA）标记，以解析该物种的遗传变异在其分布范围内的分布格局，以及地貌区域对该物种种群结构的反映程度。多项多元分析结果显示，我们的样本被聚为4个遗传类群，但这些类群与地貌区域的对应性较差。在覆盖近4个毗邻地貌区域的该物种主要分布区内，种群间几乎未出现遗传分化。但研究还识别出另外3个独立的遗传类群：2个位于主要分布区的间断东北部区域，1个位于主要分布区的东南部边缘地带。若调整保护单元的划分标准，使其匹配遗传类群而非地貌区域，则新的保护单元将更能反映种群结构的历史格局。此外，以遗传类群为依据指导种群移植与遗传拯救工作，可提升我们模拟自然基因流模式的能力。本研究结果表明，对于受威胁或濒危物种而言，地貌区域未必总能准确反映其种群结构。