Dianne Edwards

Bio: Dianne Edwards is an academic researcher from Cardiff University. The author has contributed to research in topics: Devonian & Cooksonia. The author has an hindex of 49, co-authored 198 publications receiving 7401 citations. Previous affiliations of Dianne Edwards include University of Wales & Chinese Academy of Sciences.

The timescale of early land plant evolution

Abstract: Establishing the timescale of early land plant evolution is essential for testing hypotheses on the coevolution of land plants and Earth's System. The sparseness of early land plant megafossils and stratigraphic controls on their distribution make the fossil record an unreliable guide, leaving only the molecular clock. However, the application of molecular clock methodology is challenged by the current impasse in attempts to resolve the evolutionary relationships among the living bryophytes and tracheophytes. Here, we establish a timescale for early land plant evolution that integrates over topological uncertainty by exploring the impact of competing hypotheses on bryophyte-tracheophyte relationships, among other variables, on divergence time estimation. We codify 37 fossil calibrations for Viridiplantae following best practice. We apply these calibrations in a Bayesian relaxed molecular clock analysis of a phylogenomic dataset encompassing the diversity of Embryophyta and their relatives within Viridiplantae. Topology and dataset sizes have little impact on age estimates, with greater differences among alternative clock models and calibration strategies. For all analyses, a Cambrian origin of Embryophyta is recovered with highest probability. The estimated ages for crown tracheophytes range from Late Ordovician to late Silurian. This timescale implies an early establishment of terrestrial ecosystems by land plants that is in close accord with recent estimates for the origin of terrestrial animal lineages. Biogeochemical models that are constrained by the fossil record of early land plants, or attempt to explain their impact, must consider the implications of a much earlier, middle Cambrian-Early Ordovician, origin.

Roots: evolutionary origins and biogeochemical significance

Abstract: Roots, as organs distinguishable developmentally and anatomically from shoots (other than by occurrence of stomata and sporangia on above‐ground organs), evolved in the sporophytes of at least two distinct lineages of early vascular plants during their initial major radiation on land in Early Devonian times (c. 410–395 million years ago). This was some 15 million years after the appearance of tracheophytes and c. 50 million years after the earliest embryophytes of presumed bryophyte affinity. Both groups are known initially only from spores, but from comparative anatomy of extant bryophytes and later Lower Devonian fossils it is assumed that, during these times, below‐ground structures (if any) other than true roots fulfilled the functions of anchorage and of water and nutrient acquisition, despite lacking an endodermis (as do the roots of extant Lycopodium spp.). By 375 million years ago root‐like structures penetrated almost a metre into the substratum, greatly increasing the volume of mineral matter subject to weathering by the higher than atmospheric CO2 levels generated by plant and microbial respiration in material with restricted diffusive contact with the atmosphere. Chemical weathering consumes CO2 in converting silicates into bicarbonate and Si(OH)4. The CO2 consumed in weathering ultimately came from atmospheric CO2 via photosynthesis and respiration; this use of CO2 probably accounts for most of the postulated 10‐fold decrease in atmospheric CO2 from 400–350 million years ago, with significant effects on shoot evolution. Subsequent evolution of roots has yielded much‐branched axes down to 40 μm diameter, a lower limit set by long‐distance transport constraints. Finer structures involved in the uptake of nutrients of low diffusivity in soil evolved at least 400 million years ago as arbuscular mycorrhizas or as evaginations of ‘roots’ (‘root hairs’).

Stomata in early land plants: an anatomical and ecophysiological approach

Abstract: direct comparisons of functions and functioning with extant forms and the conclusion that they are among Descriptions of Silurian and Lower Devonian stomata the most conservative of embryophyte vegetative characbased on cuticles, coalified compressions and permin- ters. Their study in the fossil record has received a major eralizations reveal similarities with those in mosses boost from the current preoccupation with recording and certain ferns, and facilitate understanding about their frequencies (McElwain and Chaloner, 1995; the mechanism of guard cell movements. A detailed Beerling and Woodward, 1997). However, in considering survey of stomatal complexes, substomatal chambers the broader physiological and ecological issues of and cortical tissues in Rhynie Chert plants suggest stomata, they should not be considered in isolation from adaptations to reduce water loss in peristomatal the surrounding and, particularly, the underlying tissues. regions and these, together with a specialized paren- Earlier observers, for example, Whitehouse (1952) and chymatous tissue with an extensive intercellular space Corner (1964) had, in discussing the major anatomical system at the base of the substomatal chamber, point innovations associated with fitness to the terrestrial to high water use efficiency. Lower stomatal frequen- environment, focused on the indispensible role of an cies are discussed in relation to water stress and intercellular space system in association with the postulated high atmospheric carbon dioxide concen- stomatal apparatus. This aspect has received comparattrations. Stomatal numbers and distribution in axial ively little attention in studies based on cuticles or Silurian and basal Devonian fossils form the basis for compression fossils in which ‘soft’ tissues are now speculation on the selective pressures (e.g. the gen- represented by a film of coal. Here, in addition to an eration of a transpiration stream, H 2 O and nutrient update on these conventional approaches, new data are acquisition, temperature control) that led to the evolupresented based on the three-dimensionally preserved tion of stomata, although the fossil record provides no tissues of the Pragian silicified Rhynie Chert plants, direct evidence for evolutionary pathways. which provide a unique insight into the ecophysiology of early land plants.

Charcoal in the Silurian as evidence for the earliest wildfire

Abstract: Rare basal Pridoli plant fossils, which resemble the rhyniophytoid Hollandophyton colliculum and have exceptional three-dimensional cellular anatomy, are preserved as charcoal. As such, these fossils from Ludford Lane in the Welsh Borders are evidence of the earliest recorded wildfire and are the first documented from before the Devonian. The fossils represent the charred remains of a low-growing, low-diversity, in situ rhyniophytoid or cryptospore-bearing plant vegetation charred during a low-temperature fire.

Some genetic consequences of ice ages, and their role in divergence and speciation

Abstract: The genetic effects of pleistocene ice ages are approached by deduction from paleoenvironmental information, by induction from the genetic structure of populations and species, and by their combination to infer likely consequences. (1) Recent palaeoclimatic information indicate rapid global reversals and changes in ranges of species which would involve elimination with spreading from the edge. Leading edge colonization during a rapid expansion would be leptokurtic and lead to homozygosity and spatial assortment of genomes. In Europe and North America, ice age contractions were into southern refugia, which would promote genome reorganization. (2) The present day genetic structure of species shows frequent geographic subdivision, with parapatric genomes, hybrid zones and suture zones. A survey of recent DNA phylogeographic information supports and extends earlier work. (3) The grasshopperChorthippus parallelusis used to illustrate such data and processes. Its range in Europe is divided on DNA sequences into five parapatric races, with southern genomes showing greater haplotype diversity — probably due to southern mountain blocks acting as refugia and northern expansion reducing diversity. (4) Comparison with other recent studies shows a concordance of such phylogeographic data over pleistocene time scales. (5) The role that ice age range changes may have played in changing adaptations is explored, including the limits of range, rapid change in new invasions and refugial differentiation in a variety of organisms. (6) The effects of these events in causing divergence and speciation are explored usingChorthippusas a paradigm. Repeated contraction and expansion would accumulate genome differences and adaptations, protected from mixing by hybrid zones, and such a composite mode of speciation could apply to many organisms.

Genetic consequences of climatic oscillations in the Quaternary.

Abstract: An appreciation of the scale and frequency of climatic oscillations in the past few million years is modifying our views on how evolution proceeds. Such major events caused extinction and repeated changes in the ranges of those taxa that survived. Their spatial effects depend on latitude and topography, with extensive extinction and recolonization in higher latitudes and altitudinal shifts and complex refugia nearer the tropics. The associated population dynamics varied with life history and geography, and the present genetic constitution of the populations and species carry attenuated signals of these past dynamics. Phylogeographic studies with DNA have burgeoned recently and studies are reviewed from the arctic, temperate and tropical regions, seeking commonalities of cause in the resulting genetic patterns. Arctic species show distinct shallow genetic clades with common geographical boundaries. Thus Beringia is distinct phylogeographically, but its role as a refugial source is complex. Arctic taxa do not show the common genetic pattern of southern richness and northern purity in north-temperate species. Temperate refugial regions in Europe and North America show relatively deep DNA divergence for many taxa, indicating their presence over several Ice Ages, and suggesting a mode of speciation by repeated allopatry. DNA evidence indicates temperate species in Europe had different patterns of postglacial colonization across the same area and different ones in previous oscillations, whereas the northwest region of North America was colonized from the north, east and south. Tropical montane regions contain deeply diverged lineages, often in a relatively small geographical area, suggesting their survival there from the Pliocene. Our poor understanding of refugial biodiversity would benefit from further combined fossil and genetic studies.

Post-glacial re-colonization of European biota

Abstract: Population structure is the result of both present processes and past history. Molecular markers are proving of great value in describing the former, and it is important to similarly determine the latter in order to understand their respective contributions. The study of palaeo-climates has also advanced significantly, and in particular that of the Pleistocene ice ages, which modified species ranges considerably. The last ice age and rapid post-glacial colonization of Europe is summarized. Possible population genetic consequences of expansion northward from southern refugia, and those of remaining in these mountainous regions are discussed. A series of recent case studies are detailed where DNA sequence information has been used to describe species genetic variation and subdivision across Europe. These include a grasshopper, the hedgehog, oak trees, the common beech, the black alder, the brown bear, newts, shrews, water vole, silver fir and house mice. These molecular data confirm southern peninsulas of Europe as major ice age refugia, and in most cases demonstrate that genetically distinct taxa emerged from them. They can thus define genomic differences and so greatly augment previous fossil data. The refugial genomes contributed differently in various species to the re-colonization of Europe, with three broad patterns described as paradigms—«grasshopper», «hedgehog» and «bear». These different expansion patterns produced clusters of hybrid zones where they made contact, and it is argued that many species genomes may be further cryptically subdivided. A reduction in diversity from southern to northern Europe in the extent of allelic variation and species subdivision is seen; this is attributed to rapid expansion northward and the varied topography of southern refugia allowing populations to diverge through several ice ages. The differences in DNA sequence indicate that some species have been diverging in refugial regions for a few ice ages at most, whilst distinct lineages in other species suggest much more ancient separation.

Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource

Abstract: Contents I. Introduction 424 II. The phosphorus conundrum 424 III. Adaptations to low P 424 IV. Uptake of P 424 V. P deficiency alters root development and function 426 VI. P deficiency modifies carbon metabolism 431 VII. Acid phosphatase 436 VIII. Genetic regulation of P responsive genes 437 IX. Improving P acquisition 439 X. Synopsis 440 Summary Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants.