Institut für Botanik, Technische Universität Dresden, D-01062 Dresden, Germany.
Am J Bot. 2012 Oct;99(10):1609-29. doi: 10.3732/ajb.1200244. Epub 2012 Sep 14.
A large range of growth forms is a notable aspect of angiosperm diversity and arguably a key element of their success. However, few studies within a phylogenetic context have explored how anatomical, developmental, and biomechanical traits are linked with growth form evolution. Aristolochia (∼500 species) consists predominantly of climbers, but a handful of shrub-like species are known from Aristolochia subgenus Isotrema (hereafter, shortened to Isotrema). We test hypotheses proposing that the establishment of functional traits linked to lianescence might limit the ability to evolve structurally diverse growth forms, particularly self-supporting forms. •
We focus on the origin of the shrub habit in Isotrema, from which we sampled representatives from climbing to self-supporting forms. Morphological, anatomical, and biomechanical characters are optimized on a chloroplast- and nuclear-derived phylogeny. •
Character-state reconstructions revealed that the climbing habit is plesiomorphic in Isotrema and shrub-like forms are derived from climbers. However, shrubs do not constitute a monophyletic group. Both shrubs and climbers show large multiseriate rays, but differ in terms of vessel size and proportion of fibers and soft tissues. •
We suggest that while shrub-like species might have partly escaped from the constraints of life as lianas; their height size and stability are not typical of self-supporting shrubs and trees. Shrubs retained lianoid stem characters that are known to promote flexibility such as ray parenchyma. The transitions to a shrub-like form likely involved relatively simple, developmental changes that may be attributed to heterochronic processes.
多种多样的生长形式是被子植物多样性的显著特征，可以说是它们成功的关键因素之一。然而，在系统发育背景下，很少有研究探讨解剖学、发育和生物力学特征与生长形式进化之间的联系。马兜铃科（约 500 种）主要由攀援植物组成，但也有少数灌木状物种来自马兜铃科马兜铃亚属异托马兜铃属（以下简称异托马兜铃属）。我们验证了一些假说，即与藤本植物相关的功能特征的建立可能限制了进化出结构多样化的生长形式的能力，特别是自支撑形式。
我们专注于异托马兜铃属灌木习性的起源，从这里我们采样了从攀援到自支撑形式的代表。形态学、解剖学和生物力学特征在叶绿体和核衍生的系统发育树上进行了优化。
特征状态重建表明，攀援习性在异托马兜铃属中是原始的，而灌木状形态是从攀援植物进化而来的。然而，灌木并不构成一个单系群。灌木和攀援植物都显示出大型多列射线，但在导管大小和纤维及软组织比例方面存在差异。
我们认为，虽然灌木状物种可能部分逃脱了作为藤本植物的限制；但它们的高度和稳定性并不典型于自支撑的灌木和树木。灌木保留了已知促进柔韧性的藤本茎特征，如射线薄壁组织。向灌木状形态的转变可能涉及相对简单的发育变化，这些变化可能归因于异时性过程。
