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Biology and evolution of ferns and lycophytes.
TA Ranker, CH Haufler. eds. 2008.
Cambridge, UK: Cambridge University Press. £35 (paperback). 480 pp.
About once every decade for the last 80 years, a volume has been produced that reports comprehensively on recent developments in pteridology. Some have an emphasis on particular aspects, such as phylogeny, cytogenetics or gametophytes, while others have a broader coverage. Together with several substantial proceedings of conferences, these books have helped to maintain the momentum of interest in ferns and lycophytes. With the current changes in fashion in botanical curricula and the reduction in research support for pteridology, the stimulus of another major publication of this kind was needed. Ranker and Haufler have followed in the tradition of Verdoorn's multi-authored compendium Manual of Pteridology (1938) and assembled contributions that include a broad selection of topics across the range currently being investigated. They cover all the plant groups traditionally included in the ‘Pteridophytes’, but avoid the use of the name in the title in recognition of the current view that the groups do not share a common ancestor and therefore a collective noun is no longer valid (this has robbed us also of the term ‘pteridologist’, for which there is no convenient alternative.) The 16 chapters are grouped into four parts: ‘Development and Morphogenesis’ (103 pages), ‘Genetics and Reproduction’ (94 pages), ‘Ecology’ (102 pages) and ‘Systematics and Evolutionary Biology’ (167 pages). The list of 28 authors includes several of the elder statesmen of today's ‘pteridology’, along with some of the rising stars. Twenty authors are based in the USA, the remainder with addresses in Canada, Mexico, Japan, Sweden, Switzerland and the UK. This may reflect the fact that both editors are from the USA as much as it mirrors the geographical distribution of current pteridological activity, but it is true that in the UK at least there are now few active research pteridologists who can contribute to such a volume.
The intended readership is stated to be advanced undergraduates, graduates and academic researchers. This means that each chapter should provide sufficient background for those new to the field, a survey of recent developments to bring the reader up to date, and a synthesis of ideas to stimulate further investigation. This in turn means that those who have read earlier accounts might be disappointed to find that some of the content is familiar, particularly in the areas where research progress is slow. However, this is balanced by the fact that this volume provides a one-stop source for beginners.
In Chapter 1, M. Wada discusses ‘Photoresponses in fern gametophytes’. He describes the investigation of most of the growth responses and intracellular reactions (from chloroplast movement to nuclear division) to monochromatic or polarized light, and the current understanding of the photoreceptors (more than 10 already identified in Adiantum) and the mechanisms involved. This account would of course be useful to students new to the topic, but should also be read by fern ecologists interested in the gametophyte in the natural environment and by plant physiologists researching photoresponses in angiosperms.
In Chapter 2, ‘Alternation of generations’, E. Sheffield presents a brief review of the familiar and fundamental issue of the alternation of gametophyte and sporophyte generations. This is nowhere more conspicuous than in the ferns and lycophytes, the only organisms in which both phases are capable of independent, free-living existence. There is little that is new to report about the origin and adaptive success of this two-phase life cycle, but the availability of new techniques promises progress in our understanding of the controlling mechanisms. A review of the events and modifications of the homosporous life cycle is illustrated by some excellent SEM photographs; heterosporous life cycles are not considered. The brevity of the treatment of obligate apomixis, despite its adaptive importance, is perhaps an indication that there have been few recent investigations of its mechanism or ecological success.
In Chapter 3, ‘Meristem organisation and organ diversity’, R. Imaichi presents a concise, clear and well-ordered review of apical development of stems and stem branches, leaves and roots. Also covered are the specialized rhizomes of the whisk ferns, the rhizophores unique to the Selaginellaceae, and the rhizomorphs found only in the Isoetaceae. Progress is slow in this field (only 10 of the 91 papers quoted were written in the last 10 years) and Imaichi covers the major developments of the last 40 years. Most pteridologists, whatever their specialism, would find this account interesting because the events at the meristems, and particularly the apical meristems, determine the conspicuous characteristics that define the ferns, lycophytes and their component groups.
Chapter 4, ‘Population genetics’ by T. Ranker and J. M. O. Geiger, provides a useful critical review of 40 years of investigation by a variety of techniques into population genetics and the reproductive biology fundamental to it. Most observations relate to homosporous ferns and increasingly in recent years depend on molecular techniques. On the basis of the small proportion of the world flora that has been studied, a coherent picture is emerging. Most diploid fern species form mainly unisexual gametophytes and are to a greater or lesser extent outcrossing, with levels of genetic diversity no lower than seed plants. However, some pioneer species that colonize new habitats, many polyploid species, and those with subterranean gametophytes are able to inbreed by intra-gametophytic selfing of bisexual gametophytes. Further clarification will require the use of a broader range of species and the wider application of molecular techniques.
J. J. Schneller, who has been a source of interesting observations on fern reproductive biology for 30 years, has contributed Chapter 5, ‘Antheridiogens’. This provides a useful summary of the history of investigations, mostly in the laboratory, into sex determination by antheridiogen pheromones in gametophytes of homosporous ferns. The account highlights the need for more research, especially into the role of antheridiogens in nature, the occurrence of antheridiogens in other (especially tropical) species, the interaction of antheridiogens with the phytochrome system, and the molecular aspects of genetic regulation of sex determination.
In Chapter 6, ‘Structure and evolution of fern plastid genomes’, P. G. Wolf and J. M. Roper describe the progress in mapping the chloroplast genome. It is nearly 20 years since restriction mapping revealed that lycophytes lack an inverted repeat (IR) present in all other extant vascular plants. This approach has been succeeded by complete genome mapping (first achieved for Adiantum capillus-veneris in 2003 by Wolf et al.) and PCR mapping of the gene order of the IR sequence. Recent results have shown that the rare changes in the structure of the evolutionarily conservative IR provide phylogenetic data. Wolf and Roper describe the methodological approach and present previously unpublished results. New molecular techniques promise significant developments in the near future. The implications of the results will interest all pteridologists, even if the methods can be fully appreciated only by other molecular phylogeneticists.
In a parallel chapter (Chapter 7), T. Nakazato, M. S. Barker, L. H. Rieseberg and G. J. Gastony consider the ‘Evolution of the nuclear genome of ferns and lycophytes’. Genomic studies in these seedless plants lag behind those in flowering plants; the first linkage map of a fern was not published until 2006, more than 20 years after the first for angiosperms. The results from new molecular technologies, including linkage mapping and targeted gene silencing, are fine-tuning the long-established theory that homosporous fern and lycophyte genomes with large numbers of chromosomes are the consequence of repeated genome doubling events followed by gene silencing or even gene elimination. Full resolution of the issues relating to the unique features of genome evolution in ferns and lycophytes requires further research into various aspects, from meiotic pairing, C values, gene expression and gene mapping to whole-genome sequencing. Much of the recent work has focussed on Ceratopteris richardii, a model fern with a conveniently short life cycle; in future it will be necessary to widen the range of plants analysed to include the heterosporous ferns and lycophytes as well as other homosporous species, both palaeopolyploids and neopolyploids, representative of all the main phylogenetic lineages.
Little has been written about tropical fern ecology over the last 100 years but in Chapter 8, ‘Phenology and habitat specificity of tropical ferns’, K. Mehltreter, one of those who has helped to stimulate interest in the last few years, has reviewed recent investigations of habitat preferences and periodicity in biological processes. The facts presented in this chapter reveal not how much but how little we know about tropical fern ecology. There is a need for more quantitative field data obtained over long periods and across a wide range of species and habitats, and in conjunction with the investigations of physiology, biochemistry, morphology and genetics, without which fern ecology is unlikely to be fully understood. This draws our attention to the fact that there is no chapter in this book on the physiology and biochemistry of ferns, reflecting the current lack of interest. The absence of any mention of the gametophytes' role in determining phenology or habitat specificity is no doubt in deference to the chapter that follows.
Chapter 9, ‘Gametophyte Ecology’ by D. R. Farrar, C. Dassler, J. E. Watkins, Jr. and C. Skelton, is one of the longest chapters in the book and goes some way towards reversing the past neglect in print of the gametophyte in natural habitats. No one is better qualified than Farrar to review this topic, although in one or two places this chapter appears to have been written in separate sections by different authors and then ‘bolted’ together. With a focus on leptosporangiate species, the account summarizes what little has been published and adds some new unpublished observations. Contrary to popular belief, the familiar short-lived cordiform prothallus is not the archetypical fern gametophyte; it is an evolutionarily advanced form that is adapted to disturbed terrestrial microhabitats and is largely restricted to modern leptosporangiate species. Most older fern groups, and those species of the more recent Polypodiales that occupy the more stable epiphytic habitats, together present a diverse array of long-lived, elongate and proliferating gametophytes, some with vegetative propagules. The chapter finishes with some interesting original observations on differences between laboratory-raised and field-grown gametophytes, which in turn point towards some of the factors influencing development in the wild. Because of the central role of the gametophyte generation in establishment and reproduction, this chapter should be essential reading for everyone with an interest in fern ecology.
Chapter 10, ‘Conservation biology’ by N. N. Arcand and T. A. Ranker, presents a concise outline of the issues relating to the conservation of ferns and lycophytes, in particular endemic species of high conservation priority and species in identified diversity ‘hot-spots’. It covers the case for conservation, the range of threats to diversity and their causes, and the different practical approaches to in situ and ex situ conservation. Although habitat restoration is highlighted, there is no discussion of the contentious issues relating to re-introduction (planting at sites where a species once grew but has become extinct) and augmentation (planting in populations endangered because of their very small number of individuals) using ex situ-raised plants. The need for more evaluation of conservation requirements, and for more research into fern and lycophyte ecology and in situ development, is made clear. The comprehensive list of relevant literature, mostly from the last 10 years, will be useful.
In Chapter 11, ‘Ex situ conservation of ferns and lycophytes – approaches and techniques’, V. C. Pence enlarges on one aspect of practical conservation, touched upon in the previous chapter and likely to be of growing interest as the various available techniques are developed. In most cases, the various types of ex situ gene banks are derived from initial samples taken as spores from surviving wild plants or natural soil spore banks, but where spores are not available or not viable, techniques known as ‘in vitro collecting’ (IVC) are being developed for establishing tissue cultures from excised explants obtained from plants in the wild without harming them. It is unlikely that there is one method of ex situ conservation that is optimal for all species, so more research into the effectiveness of alternative techniques and the requirements of individual species, as well as the physiology of spores and plant tissues in storage, is needed in order to facilitate the establishment of more long-term conservation germplasm banks.
Chapter 12, ‘Species and speciation’ by C. H. Haufler, will be largely familiar to anyone who has read earlier reviews by this author, but it is convenient to have this well-written account along with all the other topics. There was a time when speciation in ferns and lycophytes was considered to be a less dynamic process than in angiosperms, but this is not so. Primary speciation (diploid divergence after geographic or ecological isolation despite the capacity for wide spore dispersal), secondary speciation (involving genetically isolated autopolyploids, ecological isolation in rare fertile hybrids, and allopolyploids derived from sterile hybrids), and tertiary speciation (divergence in polyploids, sometimes involving gene silencing and ‘diploidization’) have all been detected in one or other group of ferns and lycophytes. Haufler issues a plea for more research into the role of the gametophyte in defining habitat specificity because ecological specialization in peripheral populations can be an important step in speciation, particularly in tropical species. We must now view ferns not as a relatively few widely dispersed species but as many, more narrowly defined, often cryptic species.
In Chapter 13, ‘Phylogeny and evolution of ferns; a paleontological perspective’, G. R. Rothwell and R. A. Stockey consider the early stages of that phylogeny. They state that their account is intended for the non-paleontologist, but the reader will benefit considerably from being familiar with the morphological and phylogenetic terminology as well as the (American) names of the geological periods and their dates. The authors' interpretation of the relationships among euphyllophytes (i.e. excluding lycophytes) is not the only one and the concentrated text is not an easy read, but it is a good reference source. Throughout, only the sporophyte is considered, implying that no gametophytes have been found in the fossil record. The chapter is illustrated by more than 70 photographs but much of their potential value is lost because they are so small, with up to 20, together with captions, on a single page. The authors finish with a plea for including fossil evidence in any phylogenetic analysis; basing it entirely on studies of living representatives often leads to a different interpretation.
After more than 170 years of publication on the subject, the literature on biogeography of ferns and lycophytes is now overwhelming, so in Chapter 14, ‘Diversity, biogeography and floristics’, R. Moran has limited himself to three interrelated topics: diversity, long-distance dispersal, and vicariance, and to 214 references. Diversity increases from the temperate latitudes to the equator and is greatest in the middle altitudes of mountains. The whole of Britain and Ireland has just over 70 native species whereas a single tree in Costa Rica can contain 50 species. While mountains can form barriers to migration of species, there is considerable circumstantial evidence for long-distance dispersal. Molecular phylogeny also indicates a role for continental drift and climate change in explaining some instances of disjunct distribution. Moran finishes by highlighting the need for more plant collecting and publication of floras and more research into, for example, speciation rates and the influence of ploidy, morphology and life cycle characteristics on ecological and geographical distribution. Progress will be accelerated by greater dissemination of information via the web and by the questions raised by new phylogenetic trees derived from improved DNA sequencing.
In Chapter 15, ‘Fern phylogeny’, E. Scheuttpelz and K. M. Pryer present a series of diagrams of the fern ‘tree of life’ with accompanying commentary and supporting references. This clearly summarizes the current hypotheses concerning fern relationships based on molecular phylogenetic analysis in a way that those not routinely immersed in the intricacies can easily refer to and follow. The account starts with the early vascular plant divergences that explain the abandonment of the concept of ‘pteridophytes’ (a term previously used to include all the spore-bearing and ‘seed-free’ plants) and that justify the inclusion of horsetails and whisk ferns within a broadly defined group of ‘ferns’. It then progresses through the early separation of leptosporangiates from other ferns and the divergence of the monophyletic older families like Osmundaceae and Hymenophyllaceae from the remainder, to the relationships between the leptosporangiate families, including the ‘tree ferns’ (not all have trunks), heterosporous water ferns, and the ‘Polypods’, a group united by having sporangia with a vertical annulus interrupted by the stalk, and which contains the majority of fern species. Within these groupings, relationships between genera and selected species are suggested. At this level there are many uncertainties, as well as inconsistencies between conclusions based on molecular analyses and those based on morphology. However, this framework derived from DNA sequence analysis provides a focus for further resolution of evolutionary patterns.
The final Chapter 16, ‘Fern classification’, is essentially an update of the 2006 paper (in Taxon) by the same authors: A. R. Smith, K. M. Pryer, E. Schuettpelz, P. Korall, H. Schneider and P. Wolf. The classification is based on the consensus relationships that are presented in Chapter 15 by two of these authors; the common authorship results in close compatability, but also a little repetition, between the two chapters. Ferns are described as a monophyletic group of about 9000 species (compare with the estimate of over 12 000 given in Chapter 14) with several shared characteristics. Accounts are presented of 37 fern families in 11 orders within four classes: Psilotopsida, Equisetopsida, Marattiopsida and Polypodiopsida. For each family there is a description of characters, the numbers and names of genera, chromosome numbers, synonyms, references and outstanding classification problems. Although recent molecular analyses have produced some surprises, and despite the need to resolve some circumscriptions, many of the families and monotypic genera that had been recognized in past major classifications, mostly morphologically based, still have strong support, with evidence of monophyly. The chapter finishes with appendices listing familial and generic names, both those accepted by the authors and others. The family summaries and genus placements, although no doubt already familiar to those who work in the field of fern classification, provide a useful reference source for those who do not.
Throughout the volume, which finishes with a comprehensive index, the editors' beneficial influence is apparent in the consistent approach of many of the chapters (introduction and plan, historical survey, review of important recent advances, future directions, and a comprehensive list of relevant references, more than 200 in some chapters) and in the commendable rarity of typographical errors. I found only three that were potentially misleading. First, on page 368, line 2, ‘13 600’ should be ‘1 360’; second, ‘Smith et al, 2006a’ in Table 16·1, page 418, should be ‘Smith et al, 2006b’; third, the editors will not be happy that ‘homoeologous’ was changed to ‘homologous’ in several places on pages 112, 178, and 179 after they handed over the final draft.
This book will be indispensable for complementing any fern biology or systematics courses still taught at university. It will provide essential background information for those beginning research in any of the areas covered. Botanists working in similar areas of seed-plant biology will benefit from reading about the parallel processes in a different vascular plant group. However, for all but the most dedicated of amateur fern enthusiasts, many chapters will probably appear to be too detailed and technical, despite the fact that contained within them are statements that might well interest them.