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Leaves are lateral determinate structures formed in a predictable sequence (phyllotaxy) on the flanks of an indeterminate shoot apical meristem. The origin and evolution of leaves in vascular plants has been widely debated. Being the main conspicuous organ of nearly all vascular plants and often easy to recognize as such, it seems surprising that leaves have had multiple origins. For decades, morphologists, anatomists, paleobotanists, and systematists have contributed data to this debate. More recently, molecular genetic studies have provided insight into leaf evolution and development mainly within angiosperms and, to a lesser extent, lycophytes. There has been recent interest in extending leaf evolutionary developmental studies to other species and lineages, particularly in lycophytes and ferns. Therefore, a review of fern leaf morphology, evolution and development is timely. Here we discuss the theories of leaf evolution in ferns, morphology, and diversity of fern leaves, and experimental results of fern leaf development. We summarize what is known about the molecular genetics of fern leaf development and what future studies might tell us about the evolution of fern leaf development.

Despite the extraordinary significance leaves have for life on Earth, their origin and development remain vigorously debated. More than a century of paleobotanical, morphological, and phylogenetic research has still not resolved fundamental questions about leaves. Developmental genetic data are sparse in ferns, and comparative studies of lycophytes and seed plants have reached opposing conclusions on the conservation of a leaf developmental program. We performed phylogenetic and expression analyses of a leaf developmental regulator (Class III HD-Zip genes; C3HDZs) spanning lycophytes and ferns. We show that a duplication and neofunctionalization of C3HDZs probably occurred in the ancestor of euphyllophytes, and that there is a common leaf developmental mechanism conserved between ferns and seed plants. We show C3HDZ expression in lycophyte and fern sporangia and show that C3HDZs have conserved expression patterns during initiation of lateral primordia (leaves or sporangia). This expression is maintained throughout sporangium development in lycophytes and ferns and indicates an ancestral role of C3HDZs in sporangium development. We hypothesize that there is a deep homology of all leaves and that a sporangium-specific developmental program was coopted independently for the development of lycophyte and euphyllophyte leaves. This provides molecular genetic support for a paradigm shift in theories of lycophyte leaf evolution.

• Premise of the study: As currently circumscribed, Lastreopsis has about 45 species and occurs in Australia, southern Asia, Africa, Madagascar, and the neotropics. Previous molecular phylogenetic studies suggested that Lastreopsis is paraphyletic. Our study focuses on resolving relationships among the lastreopsid ferns (Lastreopsis, Megalastrum, and Rumohra), the evolution of morphological characters, and an understanding of the temporal and spatial patterns that have led to the current diversity and geographical distribution of its extant species.• Methods: Phylogenetic relationships were recovered under Bayesian, maximum likelihood, and maximum parsimony methods, using a data set of four plastid markers. Divergence time estimates were made using BEAST, and the biogeographic hypotheses were tested under the DEC model and the RASP/S-DIVA methods.• Key results: Lastreopsis was recovered as paraphyletic, and at least one of its clades should be recognized as a distinct genus, Parapolystichum. Coveniella poecilophlebia and Oenotrichia tripinnata were nested within Lastreopsis s.s., Megalastrum and Rumohra as sister to the Lastreopsis s.s., and the Lastreopsis amplissima clades. The initial diversification of the lastreopsids took place at about 56.55 Ma, from a neotropical ancestor.• Conclusions: Taxonomic recognition of Parapolystichum is warranted to preserve the monophyly of Lastreopsis. Diversification among the main clades of the lastreopsid ferns was influenced by climatic and geological changes in the southern hemisphere. The biogeographic history of the group is intimately related to the trans-Antarctic corridor between Australia and South America, with evidence for multiple lineage interchanges between Australia and South America during the Oligocene and the Eocene epochs.

Premise of research. The polybotryoid fern clade is completely Neotropical and consists ofCyclodium,Maxonia,Olfersia,Polybotrya, andPolystichopsis. It has never received a detailed phylogenetic analysis. We performed such an analysis to examine the relationships among species and genera and to map the evolution of their morphological and anatomical characters. Methodology. Our study included 46 (77%) of the 60 species in the clade. It also included 37 outgroup species from 19 genera. We sequenced four plastid DNA markers (rbcL,rps4-trnS,trnG-trnR, andtrnL-trnF) and analyzed the data with maximum likelihood and Bayesian inference. One anatomical and 11 morphological characters were mapped on the resulting phylogenetic trees using the criterion of maximum parsimony. Pivotal results. The polybotryoid clade was strongly supported as monophyletic, as were its component genera. Nearly all its species have long-creeping rhizomes.Polystichopsiswas resolved sister to the other polybotryoid genera. Its monophyly is supported by the morphological synapomorphies of distichous phyllotaxy, long straightish white hairs on the leaves, and tuberculate perines. Two species currently classified inArachniodes(Arachniodes macrostegiaandArachniodes ochropteroides) form a clade withOlfersia. No known morphological characters support this clade.Olfersia, however, is highly distinct from all other polybotryoids by the combination of its imparipinnate laminae, submarginal connecting vein, strong sterile-fertile leaf dimorphy, loss of indusia, and evolution of acrostichoid sori.Maxoniais defined by its terrestrial root-climbing habit and dimorphic sterile and fertile leaves.CyclodiumandPolybotryawere resolved as sister. The presence of peltate indusia is synapomorphic forCyclodium.Polybotryais defined by several morphological synapomorphies: a rhizome anatomy unique among dryopteroid ferns (each individual meristele is surrounded by a dark sclerenchymatous sheath), strong sterile-fertile dimorphy, and loss of indusia. A possible synapomorphy forPolybotryais the terrestrial root-climbing habit. WithinPolybotrya, anastomosing veins and round discrete sori have evolved more than once. Conclusions. This is the first phylogenetic analysis of the polybotryoid ferns. The clade was resolved as monophyletic, as were its genera. An unexpected result was that two species currently classified inArachniodes(A. macrostegiaandA. ochropteroides) were resolved sister toOlfersia. Most of the main clades of polybotryoids were supported by morphological and/or anatomical characters.

We performed a phylogenetic analysis of the traditionally recognized genera of bolbitidoid ferns (i.e.,Arthrobotrya,Bolbitis,Elaphoglossum,Lomagramma, andTeratophyllum) using two noncoding chloroplast spacers:trnL‐trnFandrps4‐trnS. The sampling included 57 species, of which 55 had not been sequenced previously. The results supported the monophyly of bolbitidoid ferns and ofArthrobotrya,Elaphoglossum,Lomagramma, andTeratophyllum; however,Bolbitiswas resolved as polyphyletic. A clade of eight Neotropical species currently placed inBolbitisis sister toElaphoglossum, not the other species ofBolbitis. We refer to this group of species as theBolbitis nicotianifoliaclade.Lomagramma(orBolbitis)guianensis, whose generic placement has been uncertain, was found to belong to theB. nicotianifoliaclade.Bolbitiss.s. was resolved sister to the rest of the bolbitidoid ferns, which are in turn divided into two clades, one consisting ofElaphoglossumand theB. nicotianifoliaclade and the other ofLomagramma,Teratophyllum, andArthrobotrya. We optimized 34 morphological characters on the resulting phylogenetic tree. The characters found to be synapomorphic for bolbitidoid ferns were ventral root insertion, elongated ventral meristeles, sterile‐fertile leaf dimorphism, acrostichoid sori, and the absence of hairs on the leaves. Other characters, such as articulate pinnae, venation patterns, laminar buds, paraphyses, and growth habit, are discussed in relation to the clades they support at different nodes on the tree. The bolbitidoid ferns show a transition series from terrestrial (Bolbitis) to hemiepiphytic (theB. nicotianifoliaclade,Arthrobotrya,Lomagramma, andTeratophyllum) to epiphytic (Elaphoglossum). A sister‐species relationship between the NeotropicalBolbitis serratifoliaand the AfricanBolbitis acrostichoideswas recovered, supporting their relationship as previously postulated on the basis of morphology.

PREMISE OF THE STUDY: Hymenophyllaceae (“filmy ferns”) are a widely distributed group of predominantly tropical, epiphytic ferns that also include some temperate and terrestrial species. Hymenophyllaceae are one of the earliest‐diverging lineages within leptosporangiate ferns, but their fossil record is sparse, most likely because of their low fossilization potential and commonly poor preservation of their delicate, membranaceous fronds. A new species of unequivocal fossil Hymenophyllaceae, Hymenophyllum iwatsukii sp. nov., is described from the Early Cretaceous of Mongolia based on abundant and exceptionally well‐preserved material. METHODS: Bulk lignite samples collected from Tevshiin Govi and Tugrug localities in Mongolia, were disaggregated in water, cleaned with hydrochloric and hydrofluoric acids, washed, and dried in air. Fossils were examined and compared to material of extant Hymenophyllaceae using LM and SEM. KEY RESULTS: The fossil fern specimens are assigned to the Hymenophyllaceae based on their membranaceous laminae with marginal sori that have sessile to short‐stalked sporangia with oblique, complete annuli, and trilete, tetrahedral‐globose spores. Within the family, the fossil material is assigned to the extant genus Hymenophyllum on the basis of bivalvate indusia and short, included receptacles. CONCLUSIONS: Hymenophyllum iwatsukii was likely an epiphyte based on the sedimentary environment in which the fossils are preserved, the associated fossil flora, and the growth habit of extant species of Hymenophyllum. The new fossil species underlines the extent to which morphological characters in Hymenophyllum have been conserved despite significant tectonic, climatic, ecological, and floristic changes since the Early Cretaceous.

• Premise of the study: Closing gaps in the fossil record and elucidating phylogenetic relationships of mostly incomplete fossils are major challenges in the reconstruction of the diversification of fern lineages through time. The cosmopolitan family Dryopteridaceae represents one of the most species-rich families of leptosporangiate ferns, yet its fossil record is sparse and poorly understood. Here, we describe a fern inclusion in Miocene Dominican amber and investigate its relationships to extant Dryopteridaceae.• Methods: The morphology of the fossil was compared with descriptions of extant ferns, resulting in it being tentatively assigned to the bolbitidoid fern genus Elaphoglossum. This assignment was confirmed by reconstructing the evolution of the morphological characters preserved in the inclusion on a molecular phylogeny of 158 extant bolbitidoid ferns. To assess the morphology-based assignment of the fossil to Elaphoglossum, we examined DNA-calibrated divergence time estimates against the age of the amber deposits from which it came.• Key results: The fossil belongs to Elaphoglossum and is the first of a bolbitidoid fern. Its assignment to a particular section of Elaphoglossum could not be determined; however, sects. Lepidoglossa, Polytrichia, and Setosa can be discounted because the fossil lacks subulate scales or scales with acicular marginal hairs. Thus, the fossil might belong to either sects. Amygdalifolia, Wrightiana, Elaphoglossum, or Squamipedia or to an extinct lineage.• Conclusions: The discovery of a Miocene Elaphoglossum fossil provides remarkable support to current molecular clock-based estimates of the diversification of these ferns.

Tectaria brauniana and T. nicotianifolia are unusual in their genus by having creeping rhizomes and two-ranked leaves. Tectaria brauniana is further unusual by having free veins. These rhizome characters of both species, and the free veins of T. brauniana, are typical of Triplophyllum and suggest a relationship to that genus instead of Tectaria. To determine the phylogenetic relationships of T. brauniana and T. nicotianifolia, we used molecular evidence from four plastid DNA markers (rbcL, rps4-trnS, trnG-trnR, and trnL-trnF) to generate a phylogenetic hypothesis for the Tectariaceae. The analysis included the tectarioid genera Arthropteris, Hypoderris, Psammiosorus, Pteridrys, Tectaria, and Triplophyllum. Tectaria brauniana and T. nicotianifolia were recovered as sister to Hypoderris brownii, and these three species were sister to Triplophyllum. These two clades were sister to the rest of Tectaria. Thus, to preserve the monophyly of Tectaria, T. brauniana and T. nicotianifolia are here classified in Hypoderris, a genus previously considered monotypic. We make the following new combination: H. nicotianifolia. In this expanded sense, Hypoderris is characterized by creeping rhizomes, two-ranked leaves, and spiny perispores. The genus occurs in the Caribbean region, Central America, and the Andes from northern Venezuela to Bolivia. A key and illustrations are given for the three species now recognized in Hypoderris.

We did a phylogenetic analysis on 21 of the 23 species of Stigmatopteris, a neotropical montane genus of wet forest floors. The analysis was based on four plastid markers: rbcL, rps4-trnS, trnG-trnR, and trnL-trnF. Two of the most closely related families to the Dryopteridaceae (Didymochlaenaceae and Hypodematiaceae) and 12 dryopteroid genera were used as outgroups. Eight morphological characters were mapped on the resulting tree. Stigmatopteris was recovered as monophyletic and sister to a clade formed by polystichoid ferns such as Arachniodes, Dryopteris, and Polystichum. Synapomorphies that distinguish Stigmatopteris are 1-pinnate-pinnatifid laminae, visible punctate glands in the mesophyll, the presence of hydathodes, uniseriate cilia on the scale margins, and loss of indusia. Within the genus are two main clades. The first consists of four species characterized by pinnae long-decurrent in at least the distal third of the lamina. The second consists of the remaining 17 species in the analysis and has no known morphological synapomorphies. Nested within this second clade is a subclade of six species endemic to southeastern Brazil. Anastomosing veins, a character often used in keys to distinguish the species, evolved three times within the genus.

Keys, full synonymy, descriptions, specimens examined, and illustrations are provided for the fern genus Polystichopsis. The genus occurs in the West Indies, with disjunct populations in Nicaragua and Isla Margarita, Venezuela. Seven species and one hybrid are recognized. The genus can be distinguished from other dryopteroid genera by the combination of creeping rhizomes, distichous leaves, lack of scales on the mature leaves, and straight, whitish multicellular hairs on the leaves. Although the genus has creeping rhizomes, it does not climb as do many other closely related dryopteroid genera. A new species, P. puberula, and new hybrid, P. × sanchezii (=P. chaerophylloides × P. pubescens), are described. Lectotypes are designated for Nephrodium pubescens var. breviculum, P. leucochaete, and P. muscosa. A list is provided of 28 names excluded from Polystichopsis, and another list is provided for accepted names and their synonyms that apply in the genus. Claves, sinonimia completa, descripciones, ejemplares examinados e ilustraciones se suministra para el género de helechos Polystichopsis. El mismo existe en las Antillas con poblaciones disyuntas en Nicaragua y la Isla Margarita, Venezuela. Se reconocen siete especies y un híbrido. El género se distingue de otros géneros dryopteroides por la combinación de rizomas reptantes, hojas dísticas, ausencia de escamas en las hojas maduras y pelos blanquecinos en las hojas. Aunque posee rizomas rastreros, no es trepador como muchos otros géneros driopteroides relacionados. Se describe una especie nueva, P. puberula, y un híbrido nuevo, P. × sanchezii (=P. chaerophylloides × P. pubescens). Se designan lectotipos para Nephrodium pubescens var. breviculum, P. leucochaete y P. muscosa. Se brinda una lista de 28 nombres excluidos de Polystichopsis, y otra lista para los nombres y sus sinónimos que se aplican en el género.