Coverdale Tyler C, Agrawal Anurag A
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14853, USA.
Department of Entomology, Cornell University, Ithaca, NY, 14853, USA.
Am J Bot. 2021 Sep;108(9):1705-1715. doi: 10.1002/ajb2.1732. Epub 2021 Sep 28.
Mismatches between light conditions and light-capture strategy can reduce plant performance and prevent colonization of novel habitats. Although light-capture strategies tend to be highly conserved among closely related species, evolutionary transitions from shaded to unshaded habitats (and vice versa) occur in numerous plant lineages.
We combined phylogenetic approaches with field and greenhouse experiments to investigate evolutionary constraints on light-capture strategy in North American milkweeds (genus Asclepias) and to determine whether colonization of shaded habitats in this heliophilic clade is associated with reduced plasticity and attenuation of the shade avoidance response.
Colonization of shaded habitats has occurred at least 10 times in this genus, including at least once in each major North American clade. Evolutionary transitions between habitats exhibit strong directional bias, with shifts from full-sun to shaded habitats occurring at least three times as often as the opposite transition. In field and greenhouse experiments, sun species responded to shade by increasing internode length, height, and specific leaf area, consistent with the shade avoidance response; paired shade species exhibited reduced plasticity overall, and only one trait (specific leaf area) responded to experimental shade.
Our results suggest that milkweeds colonized shaded environments multiple times using a light-capture strategy distinct from the ancestral (putatively shade avoidant) strategy, including a general attenuation of plasticity in response to variable light conditions. This pattern bolsters the notion that shade avoidance and tolerance represent divergent evolutionary strategies for maximizing performance under qualitatively different types of shade.
光照条件与光捕获策略之间的不匹配会降低植物的性能，并阻碍其在新栖息地的定殖。尽管光捕获策略在亲缘关系密切的物种中往往高度保守，但在众多植物谱系中都发生了从阴生栖息地到阳生栖息地（反之亦然）的进化转变。
我们将系统发育方法与田间和温室实验相结合，以研究北美马利筋属（马利筋属）光捕获策略的进化限制，并确定这个喜阳类群中阴生栖息地的定殖是否与可塑性降低和避荫反应减弱有关。
该属中阴生栖息地的定殖至少发生了10次，包括在北美每个主要类群中至少发生一次。栖息地之间的进化转变表现出强烈的方向偏差，从全日照栖息地向阴生栖息地的转变发生频率至少是相反转变的三倍。在田间和温室实验中，阳生物种通过增加节间长度、高度和比叶面积来响应遮荫，这与避荫反应一致；与之配对的阴生物种总体上可塑性降低，只有一个性状（比叶面积）对实验遮荫有反应。
我们的结果表明，马利筋属多次利用一种不同于祖先（推测为避荫）策略的光捕获策略定殖到阴生环境中，包括在可变光照条件下可塑性普遍减弱。这种模式支持了这样一种观点，即避荫和耐荫代表了在性质不同的遮荫类型下使性能最大化的不同进化策略。
