Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants

Rate heterogeneity among lineages of tracheophytes: integration of molecular and fossil data and evidence for molecular living fossils

Many efforts to date evolutionary divergences by using a molecular clock have yielded age estimates that are grossly inconsistent with the paleontological evidence. Such discrepancies often are attributed to the inadequacy of the fossil record, but many potential sources of error can affect molecular-based estimates. In this study, we minimize the potential error caused by inaccurate topology and uncertain calibration times by using a well-supported tree, multiple genes, and multiple well-substantiated dates to explore the correspondence between the fossil record and molecular-based age estimates for major clades of tracheophytes. Age estimates varied because of gene effects, codon position, lineage effects, method of inferring branch lengths, and whether or not rate constancy was assumed. However, even methods designed to ameliorate the effects of rate heterogeneity among lineages could not accommodate the substantially slower rates observed in Marattia + Angiopteris and in the tree ferns. Both of these clades of ferns have undergone dramatic decelerations in their rates of molecular evolution and are "molecular living fossils," consistent with their relative morphological stasis for the past 165-200 million years. Similar discrepancies between the fossil record and molecular-based age estimates noted in other studies may also be explained in part by violations of rate constancy among lineages.

Selection pressures on stomatal evolution

Fossil evidence shows that stomata have occurred in sporophytes and (briefly) gametophytes of embryophytes during the last 400 m yr. Cladistic analyses with hornworts basal are consistent with a unique origin of stomata, although cladograms with hornworts as the deepest branching embryophytes require loss of stomata early in the evolution of liverworts. Functional considerations suggest that stomata evolved from pores in the epidermis of plant organs which were at least three cell layers thick and had intercellular gas spaces and a cuticle; an endohydric conducting system would not have been necessary for low-growing rhizophytes, especially in early Palaeozoic CO₂ -rich atmospheres. The 'prestomatal state' (pores) would have permitted higher photosynthetic rates per unit ground area. Functional stomata, and endohydry, permit the evolution of homoiohydry and the loss of vegetative desiccation tolerance and plants > 1 m tall. Stomatal functioning would then have involved maintenance of hydration, and restricting the occurrence of xylem embolism, under relatively desiccating conditions at the expense of limiting carbon acquisition. The time scale of environmental fluctuations over which stomatal responses can maximize carbon gain per unit water loss varies among taxa and life forms. Contents Summary 371 I. Introduction 371 II. Monophyly of stomata? 372 III. Roles of stomata in extant plants 373 IV. Ecophysiology of ancestrally astomatous terrestrial plants 375 V. Evolution of stomata 379 VI. Ecophysiological implications of losses of stomata 382 VII. Conclusions 384 Acknowledgements 384 References 384.

Occurrence of polygodial and 1-(2,4,6-trimethoxyphenyl)-but-2-en-1-one from some ferns and liverworts: role of pungent components in bryophytes and pteridophytes evolution

The New Zealand fern Blechnum fluviatile and liverwort, Hymenophyton flabellatum produce the characteristic pungent compounds, (-)-polygodial, a sesquiterpene dialdehyde, and 1-(2,4,6-trimethoxyphenyl)-but-2-en-1-one, respectively. The former compound has been isolated from the Japanese liverwort, Porella vernicosa complex and the latter one from the Japanese fern, Arachinoides standishii. The occurrence of both compounds in both pteridophytes and bryophytes provides another important link between bryophytes and ferns.

Plant evolution and development in a post-genomic context

Large-scale gene-sequencing projects that have been undertaken in animals have involved organisms from contrasting taxonomic groups, such as worm, fly and mammal. By contrast, similar botanical projects have focused exclusively on flowering plants. This has made it difficult to carry out fundamental research on how plants have evolved from simple to complex forms - a task that has been very successful in animals. However, in the flowering plants, the many completely or partially sequenced genomes now becoming available will provide a powerful tool to investigate the details of evolution in one group of related organisms.

Architecture of the sperm cell of Psilotum

In this correlated SEM (scanning electron microscope) and TEM (transmission electron microscope) investigation, we describe architectural details of the multiflagellated sperm cell of Psilotum nudum. Comparisons with other pteridophytes are made to (1) assess the placement of Psilotum among pteridophyte taxa and (2) evaluate structural modifications of sperm cells during land plant evolution. The released spermatozoid of Psilotum coils 2.0 revolutions and is outlined by a parallel band of up to 190 microtubules. The elongated nucleus is highly compacted and parallels the cellular coils with numerous mitochondria and starch-laden plastids distributed along its length. Along the anterior coil is an elaborate locomotory apparatus that includes ∼36 flagella that are inserted into the cell by basal bodies. Subtending the basal bodies is the multilayered structure, which consists of a long narrow lamellar strip and an overlying band of microtubules. An elongated anterior mitochondrion underlies the multilayered structure. Additional amyloplasts and mitochondria are aggregated along the anterior coil in association with the locomotory apparatus, while a fibrous band encircles the leading edge of the cell. Salient features of this cell, including details of the locomotory apparatus, structure and position of organelles, and arrangement of the spline, are shared by spermatozoids of Equisetum and ferns (including eusporangiate and leptosporangiate taxa). Thus, this study provides morphological support for the hypothesis that Psilotum nudum is a member of an assemblage that includes ferns and Equisetum. However, the less streamlined architecture of Psilotum gametes and the lack of architectural features shared with any specific taxon examined to date suggest that Psilotum is an early divergent fern, with relatively remote affinities to Ophioglossaceae and Equisetaceae.

Goodbye to 'one by one' genetics

The completion of the Arabidopsis thaliana (mustard weed) genome sequence constitutes a major breakthrough in plant biology. It will revolutionize how we answer questions about the biology and evolution of plants as well as how we confront and resolve world-wide agricultural problems.

Ancient flowering plants: DNA sequences and angiosperm classification

Phylogenetic analyses of gene sequences provide a clear pattern of which extant flowering plant genera diversified earliest. Combined with complete genomic sequences, these data will vastly improve understanding of the genetic basis of plant diversity.