Data from: Maidenhair ferns, Adiantum, are indeed monophyletic and sister to shoestring ferns, vittarioids (Pteridaceae)

Across the tree of life, molecular phylogenetic studies often reveal surprising relationships between taxa with radically different morphologies that have long obscured their close affiliations. A spectacular botanical example is Rafflesia, a holoparasite that produces the largest flowers in the world, but that evolved from tiny-flowered ancestors within the Euphorbiaceae. Outside of parasitic lineages, such abrupt transformations are rarely seen. One exception involves the “maidenhair ferns” (Adiantum), which are quintessential ferns: beautifully dissected, terrestrial, and shade loving. The closely related “shoestring ferns” (vittarioids), in contrast, have an extremely simplified morphology, are canopy-dwelling epiphytes, and exhibit greatly accelerated rates of molecular evolution. While Adiantum and the vittarioids together have been shown to form a robust monophyletic group (adiantoids), there remain unanswered questions regarding the monophyly of Adiantum and the evolutionary history of the vittarioids. Here we review recent phylogenetic evidence suggesting support for the monophyly of Adiantum, and analyze new plastid data to confirm this result. We find that Adiantum is monophyletic and sister to the vittarioids. With this robust phylogenetic framework established for the broadest relationships in the adiantoid clade, we can now focus on understanding the evolutionary processes associated with the extreme morphological, ecological, and genetic transitions that took place within this lineage.

在整个生命之树中，分子系统发育研究常能揭示形态迥异的类群间存在的紧密亲缘关系——这类关系长期以来因显著的形态差异而被掩盖。一个极具代表性的植物学案例便是大王花（Rafflesia）：它是世界上花朵最大的全寄生植物，但其演化祖先却是大戟科（Euphorbiaceae）下具小型花朵的类群。在非寄生类群中，这类剧变式的形态转变极为罕见，唯一的例外便是“铁线蕨”（Adiantum）。铁线蕨是典型的蕨类植物：叶片精致分裂、陆生且喜阴。而与之亲缘关系紧密的“书带蕨类”（vittarioids）则截然不同：它们的形态极度简化，为林冠附生植物，且分子演化速率大幅加快。尽管已有研究证实铁线蕨与书带蕨类共同构成一个支持度极高的单系群（monophyletic group，adiantoids），但关于铁线蕨本身的单系性以及书带蕨类的演化历史，仍存在诸多未解决的问题。本研究综述了近年支持铁线蕨单系性的系统发育证据，并通过分析全新的质体数据（plastid data）验证了这一结论。研究结果显示，铁线蕨为单系群，且与书带蕨类互为姐妹群。在建立起铁线蕨类演化支核心亲缘关系的高支持度系统发育框架后，我们便可聚焦于解析该谱系内发生的极端形态、生态与遗传转变背后的演化过程。