Eckhart Vincent M, Geber Monica A, McGuire Christopher M
Department of Biology, Grinnell College, Grinnell, Iowa 50112-1690, USA.
Evolution. 2004 Jan;58(1):59-70. doi: 10.1111/j.0014-3820.2004.tb01573.x.
Both genetic differentiation and phenotypic plasticity might be expected to affect the location of geographic range limits. Co-gradient variation (CoGV), plasticity that is congruent with genetic differentiation, may enhance performance at range margins, whereas its opposite, counter-gradient variation (CnGV) may hinder performance. Here we report findings of reciprocal transplant experiments intended to tease apart the roles of differentiation and plasticity in producing phenotypic variation across a geographic border between two plant subspecies. Clarkia xantiana ssp. xantiana and C. xantiana ssp. parviflora are California-endemic annuals that replace each other along a west-east gradient of declining precipitation. We analyzed variation in floral traits, phenological traits, and vegetative morphological and developmental traits by sowing seeds of 18 populations (six of ssp. xantiana and 12 of ssp. parviflora) at three sites (one in each subspecies' exclusive range and one in the subspecies' contact zone), in two growing seasons (an exceptionally wet El Niño winter and a much drier La Niña winter). Significant genetic differences between subspecies appeared in 11 of 12 traits, and differences were of the same sign as in nature. These findings are consistent with the hypothesis that selection is responsible for subspecies differences. Geographic variation within subspecies over part of the spatial gradient mirrored between-subspecies differences present at a larger scale. All traits showed significant plasticity in response to spatial and temporal environmental variation. Plasticity patterns ranged from spatial and temporal CoGV (e.g., in node of first flower), to spatial CnGV (e.g., in flowering time), to patterns that were neither CoGV nor CnGV (the majority of traits). Instances of CoGV may reflect adaptive plasticity and may serve to increase performance under year-to-year environmental variation and at sites near the subspecies border. However, the presence of spatial CnGV in some critical traits suggests that subspecies ranges may also be constrained by patterns of plasticity.
遗传分化和表型可塑性都可能影响地理分布范围边界的位置。共梯度变异（CoGV），即与遗传分化相一致的可塑性，可能会增强在分布范围边缘的表现，而与之相反的反梯度变异（CnGV）则可能会阻碍表现。在此，我们报告了相互移栽实验的结果，旨在厘清分化和可塑性在两个植物亚种之间的地理边界上产生表型变异过程中的作用。黄花克拉克（Clarkia xantiana）原亚种和小花亚种是加利福尼亚特有的一年生植物，它们沿着降水递减的东西向梯度相互取代。我们通过在三个地点（每个亚种的专属分布范围内各一个，亚种接触带一个）播种18个种群（黄花克拉克原亚种6个种群，小花亚种12个种群）的种子，在两个生长季节（一个异常湿润的厄尔尼诺冬季和一个干燥得多的拉尼娜冬季），分析了花部性状、物候性状以及营养体形态和发育性状的变异。两个亚种之间在12个性状中的11个出现了显著的遗传差异，且差异方向与自然状态下相同。这些发现与选择导致亚种差异的假说一致。亚种内部分空间梯度上的地理变异反映了更大尺度上存在的亚种间差异。所有性状对空间和时间环境变异均表现出显著的可塑性。可塑性模式从空间和时间共梯度变异（例如，第一朵花的节位），到空间反梯度变异（例如，开花时间），再到既非共梯度变异也非反梯度变异的模式（大多数性状）。共梯度变异的实例可能反映了适应性可塑性，并且可能有助于在年际环境变异以及亚种边界附近的地点提高表现。然而，某些关键性状中存在空间反梯度变异表明，亚种分布范围也可能受到可塑性模式的限制。
