Data from: Stasis and convergence characterize morphological evolution in eupolypod II ferns

Background and Aims: Patterns of morphological evolution at levels above family rank remain underexplored in the ferns. The present study seeks to address this gap through analysis of 79 morphological characters for 81 taxa, including representatives of all ten families of eupolypod II ferns. Recent molecular phylogenetic studies demonstrate that the evolution of the large eupolypod II clade (which includes nearly one-third of extant fern species) features unexpected patterns. The traditional ‘athyrioid’ ferns are scattered across the phylogeny despite their apparent morphological cohesiveness, and mixed among these seemingly conservative taxa are morphologically dissimilar groups that lack any obvious features uniting them with their relatives. Maximum-likelihood and maximum-parsimony character optimizations are used to determine characters that unite the seemingly disparate groups, and to test whether the polyphyly of the traditional athyrioid ferns is due to evolutionary stasis (symplesiomorphy) or convergent evolution. The major events in eupolypod II character evolution are reviewed, and character and character state concepts are reappraised, as a basis for further inquiries into fern morphology. Methods: Characters were scored from the literature, live plants and herbarium specimens, and optimized using maximum-parsimony and maximum-likelihood, onto a highly supported topology derived from maximum-likelihood and Bayesian analysis of molecular data. Phylogenetic signal of characters were tested for using randomization methods and fitdiscrete. Key Results: The majority of character state changes within the eupolypod II phylogeny occur at the family level or above. Relative branch lengths for the morphological data resemble those from molecular data and fit an ancient rapid radiation model (long branches subtended by very short backbone internodes), with few characters uniting the morphologically disparate clades. The traditional athyrioid ferns were circumscribed based upon a combination of symplesiomorphic and homoplastic characters. Petiole vasculature consisting of two bundles is ancestral for eupolypods II and a synapomorphy for eupolypods II under deltran optimization. Sori restricted to one side of the vein defines the recently recognized clade comprising Rhachidosoraceae through Aspleniaceae, and sori present on both sides of the vein is a synapomorphy for the Athyriaceae sensu stricto. The results indicate that a chromosome base number of x =41 is synapomorphic for all eupolypods, a clade that includes over two-thirds of extant fern species. Conclusions: The integrated approach synthesizes morphological studies with current phylogenetic hypotheses and provides explicit statements of character evolution in the eupolypod II fern families. Strong character support is found for previously recognized clades, whereas few characters support previously unrecognized clades. Sorus position appears to be less complicated than previously hypothesized, and linear sori restricted to one side of the vein support the clade comprising Aspleniaceae, Diplaziopsidaceae, Hemidictyaceae and Rachidosoraceae – a lineage only recently identified. Despite x =41 being a frequent number among extant species, to our knowledge it has not previously been demonstrated as the ancestral state. This is the first synapomorphy proposed for the eupolypod clade, a lineage comprising 67 % of extant fern species. This study provides some of the first hypotheses of character evolution at the family level and above in light of recent phylogenetic results, and promotes further study in an area that remains open for original observation.

研究背景与目的：蕨类植物科级以上阶元的形态演化模式仍未得到充分探索。本研究旨在填补这一研究空白，对81个类群的79个形态性状展开分析，涵盖真水龙骨类II（eupolypod II）蕨类全部10个科的代表类群。近期分子系统发育研究表明，这个包含近1/3现存蕨类物种的大型真水龙骨类II演化支的演化模式出人意料：传统意义上的蹄盖蕨类（athyrioid）蕨类虽在形态上看似具有一致性，却零散分布于系统发育树中；而在这些看似保守的类群之间，混杂着形态差异显著、缺乏与近缘类群共有的明显鉴别特征的类群。本研究采用最大似然法（maximum-likelihood）与最大简约法（maximum-parsimony）进行性状优化，以确定联结这些看似迥异类群的性状，并检验传统蹄盖蕨类的多系起源究竟是源于演化停滞（共有祖征，symplesiomorphy）还是趋同演化。本研究还梳理了真水龙骨类II性状演化的主要事件，重新审视了性状及性状状态的概念，为后续蕨类形态学研究奠定基础。

研究方法：本研究通过查阅文献、观察活体植株与标本馆（herbarium）标本完成性状编码，并基于分子数据经最大似然法与贝叶斯分析（Bayesian analysis）得到的高支持度拓扑结构，采用最大简约法与最大似然法进行性状优化。本研究通过随机化方法与fitdiscrete工具检验性状的系统发育信号（phylogenetic signal）。

主要结果：真水龙骨类II系统发育树中的多数性状状态改变发生在科级及以上阶元。形态数据的相对分支长度模式与分子数据一致，符合古老快速辐射演化模型（即长分支下方伴随极短的骨干节间），且仅有少数性状可联结形态差异显著的演化支。传统蹄盖蕨类的界定是基于共有祖征与同塑性状（homoplastic character）的组合。在Deltran优化标准下，由2束维管束构成的叶柄维管束（petiole vasculature）是真水龙骨类II的祖征，同时也是该类群的共有衍征（synapomorphy）。孢子囊群（sori）仅着生于叶脉一侧，这一性状界定了新近被认定的、包含从轴子蕨科（Rhachidosoraceae）到铁角蕨科（Aspleniaceae）的演化支；而孢子囊群着生于叶脉两侧则是狭义蹄盖蕨科（Athyriaceae sensu stricto）的共有衍征。研究结果表明，染色体基数（chromosome base number）x=41是所有真水龙骨类演化支的共有衍征，该演化支包含超过2/3的现存蕨类物种。

研究结论：本研究采用整合研究方法，将形态学研究与当前的系统发育假说相结合，明确阐述了真水龙骨类II蕨类各科的性状演化模式。本研究为此前已认定的演化支提供了强有力的性状支持，而针对此前未被识别的演化支，仅有少量性状可提供支持。孢子囊群着生位置的演化模式较此前假设更为简单：仅着生于叶脉一侧的线形孢子囊群，支持了铁角蕨科（Aspleniaceae）、双盖蕨科（Diplaziopsidaceae）、Hemidictyaceae与轴子蕨科（Rhachidosoraceae）组成的演化支——这一演化支仅在近年被发现。尽管x=41在现存物种中较为常见，但据我们所知，此前从未有研究证实其为祖征状态。这是首次为真水龙骨类演化支提出的共有衍征，该演化支包含67%的现存蕨类物种。本研究基于最新的系统发育研究结果，首次提出了科级及以上阶元的性状演化假说，同时推动了这一仍有待更多原创性观察的研究领域的进一步发展。