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，分子系统发育研究往往能揭示长期以来因形态差异悬殊而被掩盖的近缘类群间的意外亲缘关系。一个极具代表性的植物学案例是大花草属(Rafflesia)：这是一种全寄生植物，拥有世界上最大的花朵，但其演化源自大戟科(Euphorbiaceae)下具小型花朵的祖先类群。除寄生谱系外，这类突变式的形态转变极为罕见。
一个例外便是“铁线蕨属(Adiantum)”，即典型的陆生阴生蕨类，其叶片呈优美的细裂状。与之近缘的“书带蕨类(vittarioids)”则截然不同：形态高度简化，为栖息于林冠层的附生植物，且分子演化速率大幅加快。尽管已有研究证实铁线蕨属与书带蕨类共同构成一个支持度极高的单系群（铁线蕨类群adiantoids），但铁线蕨属的单系性以及书带蕨类的演化历史仍存在未解决的学术问题。
本文综述了近期支持铁线蕨属单系性的系统发育证据，并通过分析新的质体(plastid)数据验证了这一结果。研究发现铁线蕨属为单系群，且与书带蕨类互为姐妹群。借助这一针对铁线蕨类群核心亲缘关系的高支持度系统发育框架，我们如今可以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。