David S. Barrington

A community-derived classification for extant lycophytes and ferns

Phylogeny has long informed pteridophyte classification. As our ability to infer evolutionary trees has improved, classifications aimed at recognizing natural groups have become increasingly predictive and stable. Here, we provide a modern, comprehensive classification for lycophytes and ferns, down to the genus level, utilizing a community‐based approach. We use monophyly as the primary criterion for the recognition of taxa, but also aim to preserve existing taxa and circumscriptions that are both widely accepted and consistent with our understanding of pteridophyte phylogeny. In total, this classification treats an estimated 11 916 species in 337 genera, 51 families, 14 orders, and two classes. This classification is not intended as the final word on lycophyte and fern taxonomy, but rather a summary statement of current hypotheses, derived from the best available data and shaped by those most familiar with the plants in question. We hope that it will serve as a resource for those wanting references to the recent literature on pteridophyte phylogeny and classification, a framework for guiding future investigations, and a stimulus to further discourse.

Ecological and Historical Factors in Fern Biogeography

Remarkable diversity in recognized endemic centres as well as noteworthy disjunct distributions have led students of the pteridophytes to seek historical explanations for these phenomena. A suite of distinctive attributes limit the array of possible explanations: most biogeographically significant are ( I ) the smaller number of species-range determinants, and (2) the substantial capacity for long-distance dispersal. These attributes constrain the train of events triggered by global events and culminating in the particular phylogenetic history of an evolutionary lineage. Fern biogeographers most often infer historical changes in habitat distribution predicated by climatic change from distribution

Major evolutionary events in the origin and diversification of the fern genus Polystichum (Dryopteridaceae)

Recent advances in molecular systematics of the ferns make it possible to address long-standing questions about classification of the major fern genera, such as the worldwide genus Polystichum (Dryopteridaceae), comprising at least 200 species. In this study we examined rbcL sequences and morphological characters from 55 fern taxa: 34 were from Polystichum and 21 were from other genera in the Dryopteridaceae. We found that Phanerophlebia, possibly including Polystichopsis, is the sister group to Polystichum sensu lato (s.l.), including Cyrtomium. Polystichum as commonly recognized is paraphyletic. Our results lead us to suggest recognizing the clade of earliest diverging Polystichum species as a distinct genus (Cyrtomidictyum) and to continue to recognize Cyrtomium as a separate genus, leaving a monophyletic Polystichum sensu stricto (s.s.). We resolved a tropical American clade and an African clade within Polystichum s.s. However, the resemblance between the once-pinnate, bulb-bearing calciphilic species found in Asia and the West Indies appears to be the result of convergent evolution. Optimizing our morphological character transformations onto the combined phylogeny suggests that the common ancestor of Polystichum s.l. and Phanerophlebia had evolved the common features of the alliance, including ciliate petiole-base scales, once-pinnate fronds, ultimate segments with scarious tips, peltate indusia, and microscales.

Systematic inferences from spore and stomate size in the ferns

Size of equivalent cells has traditionally been assumed to be constant within species and variable between species. Systematists have commonly measured cells with constant form, such as spores and stomates, as a means of distinguishing species and hybrids in polyploid complexes, since the best known factor determining cell size is ploidy level (Stebbins, 1950). Preliminary hypotheses about polyploid evolution can be generated directly from herbarium specimens based on spore and stomate measurement data. The first evidence of a relation between cell size and polyploidy in the ferns was Lawtons 1932 demonstration that prothallial cells, lower epidermal cells, and stomates in induced apogamous races of Dryopteris marginalis and Woodwardia virginica vary directly with ploidy level. Butters and Tryons 1948 work on Woodsia x abbeae included epidermal cell and annulus cell measurements that corroborated data from spore germination experiments to document an instance of somatic autopolyploidization. More recently a considerable body of evidence has been assembled supporting the contention that spore size is related to ploidy level in the ferns. Manton (1950) noted that Vancouver Island Cystopteris fragilis (n = 84) has smaller spores than Swiss Cystopteris alpina (n = 126). Hagenah (1961) postulated a polyploid series in species of Cystopteris based on spore-size measurements. In a monograph of Cystopteris, Blasdell (1963) concluded that it was possible to infer ploidy level from spore size in that genus. Blasdell assigned ploidy levels to cytologically unknown components of polyploid complexes based only on spore size (see Lovis, 1977 for a critique). Wagner (1966) demonstrated that there were two varieties of Gymnocarpium dryopteris: a larger-spored typical variety, which is tetraploid, and a smaller-spored variety disjunctum, which is diploid. Schneller (1974) recently provided an analysis of ploidy level and spore size in the Dryopteris filix-mas group of Europe. He concluded that ploidy and spore size are positively correlated for diploid to pentaploid cytotypes in a closely knit phylogenetic group. In an exhaustive analysis of spore features of northeastern North American Isoetes, Kott and Britton (1983) provided new insight into spore-size variability. Whereas megaspore variation within sporangia of a single plant, between sporangia of a single plant, and between plants of a population was found to be more or less equivalent and negligible, variation between populations of a species was demonstrated to be twice that between plants of a population. Microspores were found to be slightly less variable. Kott and Britton (1983) suggested that

Hybridization, Reticulation, and Species Concepts in the Ferns

Hybrids and hybrid species are common among ferns, and they account for many of the problems in species definition in the group. Most systematic inquiry into the evolutionary process in ferns has addressed hybrid species, because meaningful explanations of their origins are feasible (Manton, 1950). As a result, complexes of hybrids, hybrid species, and their progenitor species have been popular subjects for experimental work. Here, we address the definition and changing perception of these hybrid species in the light of improvements in the data available to systematists. Once we have established basic definitions, we demonstrate the utility of recent advances in defining hybrid species of ferns. With this orientation, we investigate the status of hybrid species in the context of reigning species concepts. Renewed reproductive interaction between populations or species following a period of isolation characterizes all hybrids; hence hybrids are often spoken of as the products of secondary contact. Hybrids are unique in that they arise when isolating mechanisms fail; thus they are evolutionarily a consequence of the disruption of the divergence process that leads to ordinary (primary) species. Consequently, the hybrid is at once a novelty and a rehash: it is a novel combination of genetic and morphological features already present in its progenitors. These features need not be intermediate: see Grant (1975) on transgressive segregation and Barrington, 1986a. Fern hybrids are predominantly sterile (Knobloch, 1976), though there is a small, disparate set of variously fertile hybrids (in Pteris, Walker, 1958; in Dryopteris, Whittier & Wagner, 1961; in the Cyatheaceae, Conant & Cooper-Driver, 1980). The origin and evolutionary significance of sterile hybrids have been the subject of most

Phylogeny of Chinese Polystichum (Dryopteridaceae) based on chloroplast DNA sequence data (trnL-F and rps4-trnS)

Polystichum is one of the largest and most taxonomically complex fern genera in China. The evolutionary relationships of Chinese Polystichum and related genera, and the relationship between our Polystichum phylogeny and ecogeographic distribution, were tested by the use of DNA sequence data. Fifty-one species of Polystichum and 21 species in allied genera were sequenced for the plastid intergenic spacers rps4-trnS and trnL-F. Maximum parsimony and Bayesian phylogenetic analyses of both individual and combined data sets showed that Chinese Polystichum as commonly recognized was paraphyletic: one clade (the CCPC clade) included Cyrtomidictyum lepidocaulon, two Cyrtogonellum species, three Cyrtomium species, and a small number of Polystichum species usually occurring on limestone. A second clade, Polystichum sensu stricto, included the remainder of the Polystichum species; these often occur on non-limestone substrates. The remaining Cyrtomium species formed the third clade. Three subclades resolved within Polystichum sensu stricto (s.s.) clade do not correspond with recent sectional classifications, and we outline the issues relevant to a new classification for the genus.

Origin of Hawaiian Polystichum (Dryopteridaceae) in the context of a world phylogeny

A genus-wide molecular phylogeny for Polystichum and allied genera (Dryopteridaceae) was reconstructed to address the biogeographic origin and evolution of the three Hawaiian Polystichum species, all endemic there. The analysis was based on the cpDNA sequences rbcL and the trnL-F spacer from a taxonomically and geographically diverse sample. Parsimony and Bayesian phylogenetic analyses of the combined data support a monophyletic Polystichum and corroborate recent hypotheses as to membership and sequence of origin of the major groups within the genus. The Hawaiian Polystichum species are polyphyletic; two separate lineages appear to have arrived independently from the Old World. The provenance of the diploid Polystichum hillebrandii is continental eastern Asia, while the source of the polyploid lineage comprising tetraploid P. haleakalense and octoploid P. bonseyi is likely continental Asia. From our results, the origin of the Hawaiian species of Polystichum, like many Hawaiian fern genera with several species, is the result of multiple migrations to the islands, rather than single migrations yielding nearly all the local diversity as in the angiosperms. This emerging pattern provides a modern test of the premise that propagule vagility has a central role in determining pattern of evolution.

Systematics of the Carex aquatilis and C. lenticularis lineages: geographically and ecologically divergent sister clades of Carex section Phacocystis (Cyperaceae).

Carex aquatilis is a highly diverse and geographically widespread member of one of the largest genera of flowering plants, Carex, and is ideally suited for the study of the role of hybridization and niche partitioning in ecological speciation. Phylogenetic analyses of nuclear ITS and ETS 1f and chloroplast psbA-trnH DNA sequences support the monophyly of a broadly defined Carex aquatilis-Carex lenticularis lineage, which includes C. aquatilis and C. lenticularis and their allies within section Phacocystis. However, neither taxon is monophyletic as currently circumscribed. The C. aquatilis lineage includes C. aquatilis and four morphologically and molecularly distinct salt-tolerant maritime taxa with which C. aquatilis s.s. is reported to form stabilized homoploid hybrids. The C. lenticularis lineage includes a paraphyletic C. lenticularis and seven allied species from both the New and Old World. The data provided here allow recognition of four species within the North American endemic C. lenticularis and suggest a neotropical origin for the C. lenticularis lineage with subsequent radiation and divergence through northwestern North America to Asia and via northeastern North America to Europe and southern South America. Evolutionary rate analyses indicate an origin for the C. aquatilis-C. lenticularis group around 1.89 million years ago during the early Pleistocene.

Phylogeny and biogeography of exindusiate Andean Polystichum (Dryopteridaceae)

PREMISE OF THE STUDY Uplift of the tropical Andes had a significant impact on the diversification of South American flora and fauna. Recent biogeographic inquiries have established patterns of Andean divergence, but investigations on ferns are scant. The fern genus Polystichum Roth (Dryopteridaceae) combines widespread geographic and elevational distribution with a large number of species to form an ideal system for investigation of the origin and diversification patterns of a fern lineage in the tropical Andes. METHODS The relationships among 42 Polystichum species, including taxa from all major biogeographic regions, were analyzed with 2591 aligned nucleotides from four plastid markers using maximum parsimony and Bayesian inference. The resulting phylogeny was then used to estimate divergence times and reconstruct both ancestral areas and ancestral elevations. KEY RESULTS Tropical Andean South American polystichums that lack an indusium (sori exindusiate) were confirmed to form a monophyletic group. This exindusiate Andean Polystichum clade diverged from a middle-elevation forest lineage now rich in species endemic to Mexico during the middle Miocene (13.12 million years ago). The majority of diversification that followed took place in the montane regions of the central Andes with radiations to the northern Andes, southeastern Brazil, and alpine regions. CONCLUSIONS The monophyletic exindusiate Andean Polystichum lineage diverged from a Mexican lineage in the middle Miocene and diversified in the central Andes before dispersing northward. This south-to-north dispersal pattern, documented for many other Andean lineages, corresponds with episodes of uplift in the tropical Andes.

Special Report The Present Evolutionary and Taxonomic Status of the Fern Genus Polystichum: The 1984 Botanical Society of America Pteridophyte Section Symposium

The fern genus Polystichum has presented major problems in definition and circumscription of species since its description by Roth in 1799. Part of the problem is the vast diversity within the genus: the number of species is reported as more than 175 by Copeland (1947). In addition, hybridization is extremely common (Knobloch, 1976), and agamospory has been reported among experimental plants from Europe (Vida & Reichstein, 1975). There is also substantial evidence of phenotypic and ontogenetic variability within species of the genus. Good progress has been made in solving evolutionary and taxonomic problems in the north-temperate and boreal regions (Manton, 1950; Manton & Reichstein, 1961; Kurata, 1964; Sleep & Reichstein, 1967; Daigobo, 1972; W. Wagner, 1973; D. Wagner, 1979); however, little taxonomic and virtually no evolutionary work has been done on the genus in tropical or austral regions. Hence this large genus of dryopteroid ferns is in need of substantial attention from systematic and evolutionary biologists, especially in tropical regions. Christopher Haufler at the University of Kansas organized a symposium on Polystichum for the American Institute of Biological Sciences meetings in Fort Collins, Colorado, during August of 1984. As Haufler noted in his opening remarks, the purpose of the symposium was to draw attention to a complex and poorly understood genus of ferns, rather than to solve problems by consensus of the speakers. Participants in the symposium focused on the evolutionary problems encountered in New World Polystichum, primarily from a taxonomic and floristic viewpoint. This paper summarizes the Fort Collins symposium.