26 results in Biology and Evolution of Ferns and Lycophytes
11 - Ex situ conservation of ferns and lycophytes – approaches and techniques
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- By Valerie C. Pence, Plant Conservation Division, Center for Research of Endangered Wildlife, Cincinnati Zoo and Botanic Garden, Cincinnati, OH 45220, USA
- Edited by Tom A. Ranker, University of Colorado, Boulder, Christopher H. Haufler, University of Kansas
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Summary
Introduction
Although generally less conspicuous than seed plants, ferns and lycophytes have been the object of human interest since at least the time of the ancient Greeks. As with other plant and animal species, habitat loss is the major threat, but harvesting for medicine, food, or as ornamental plants has also depleted some species in the wild. Of the approximately 13000 named species, approximately 800 are of conservation concern (Walter and Gillett, 1998). However, some areas of the world are still poorly explored for ferns and lycophytes, and although the exact number of endangered species is uncertain, many species are in habitats that are under pressure. While conserving species in situ is the ideal, it cannot always be ensured. As a result, ex situ conservation methods can play a complementary role in ensuring the survival of these species, by providing a back-up of genetic diversity for the populations in the wild.
Growing ferns and lycophytes ex situ as horticultural or botanically interesting specimens has a long history, but other ex situ conservation methods, such as spore banking and cryopreservation are more recent. Whereas spore banking follows protocols similar to those of seed banking, the independent alternation of generations in ferns and lycophytes provides opportunities for cryopreservation that are not available in seed plants. Previous reviews have described some of these possibilities (Page et al., 1992; Pattison et al., 1992; Pence, 2002).
16 - Fern classification
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- By Alan R. Smith, University Herbarium, University of California, Berkeley, CA 94720, USA, Kathleen M. Pryer, Department of Biology, Duke University, Durham, NC 27708, USA, Eric Schuettpelz, Department of Biology, Duke University, Durham, NC 27708, USA, Petra Korall, Department of Phanerogamic Botany, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden, Harald Schneider, Natural History Museum, Cromwell Road, London SW7 5BD, UK, Paul G. Wolf, Department of Biology, Utah State University, Logan, UT 84322, USA
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Summary
Introduction and historical summary
Over the past 70 years, many fern classifications, nearly all based on morphology, most explicitly or implicitly phylogenetic, have been proposed. The most complete and commonly used classifications, some intended primarily as herbarium (filing) schemes, are summarized in Table 16.1, and include: Christensen (1938), Copeland (1947), Holttum (1947, 1949), Nayar (1970), Bierhorst (1971), Crabbe et al. (1975), Pichi Sermolli (1977), Ching (1978), Tryon and Tryon (1982), Kramer (in Kubitzki, 1990), Hennipman (1996), and Stevenson and Loconte (1996). Other classifications or trees implying relationships, some with a regional focus, include Bower (1926), Ching (1940), Dickason (1946), Wagner (1969), Tagawa and Iwatsuki (1972), Holttum (1973), and Mickel (1974). Tryon (1952) and Pichi Sermolli (1973) reviewed and reproduced many of these and still earlier classifications, and Pichi Sermolli (1970, 1981, 1982, 1986) also summarized information on family names of ferns. Smith (1996) provided a summary and discussion of recent classifications.
With the advent of cladistic methods and molecular sequencing techniques, there has been an increased interest in classifications reflecting evolutionary relationships. Phylogenetic studies robustly support a basal dichotomy within vascular plants, separating the lycophytes (less than 1% of extant vascular plants) from the euphyllophytes (Figure 16.1; Raubeson and Jansen, 1992, Kenrick and Crane, 1997; Pryer et al., 2001a, 2004a, 2004b; Qiu et al., 2006). Living euphyllophytes, in turn, comprise two major clades: spermatophytes (seed plants), which are in excess of 260000 species (Thorne, 2002; Scotland and Wortley, 2003), and ferns (sensu Pryer et al. 2004b), with about 9000 species, including horsetails, whisk ferns, and all eusporangiate and leptosporangiate ferns.
Part III - Ecology
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- By Tom A. Ranker, University of Hawai'i at Manoa, Honolulu, HI, USA, Christopher H. Haufler, University of Kansas Lawrence, KS, USA
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4 - Population genetics
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- By Tom A. Ranker, Botany Department, University of Hawaii at Manoa, Honolulu, HI 96822, USA, Jennifer M. O. Geiger, Department of Natural Sciences, Carroll College, Helena, MT 59625, USA
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Summary
Introduction
William Henry Lang (1923) and Irma Andersson (later Andersson-Kottö; e.g., Andersson, 1923, 1927; Andersson-Kottö, 1929, 1930, 1931) were pioneers in the field of fern genetics. Lang (1923) was the first to demonstrate simple Mendelian inheritance in a fern with his experimental study of the inheritance of entire versus incised leaf margins in Scolopendrium vulgare. Andersson studied inheritance in ferns and was the first to introduce the use of an agar-based growth medium for the experimental study of fern gametophytes (Andersson, 1923). These pioneers paved the way for future explorations of fern and lycophyte population genetics.
In considering how ferns and lycophytes develop and maintain genetic variation, contemporary investigators have used an array of techniques to explore several primary, intertwining topics such as the population genetic implications of reproductive biology (including genetic load), genetic diversity and structure of populations, gene flow and divergence, and the genetics of dispersal and colonization. The goal of this chapter is to review the fern and lycophyte population genetic literature across these broad categories, to provide a synthesis of current knowledge, and to suggest possible future directions of study. We will focus primarily on homosporous taxa because little population genetic research has been conducted on heterosporous taxa.
Population genetics and reproductive biology
An understanding of the reproductive biology of individuals and populations is fundamental for discussing the genetics of populations.
3 - Meristem organization and organ diversity
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- By Ryoko Imaichi, Department of Chemical and Biological Sciences, Faculty of Science, Japan Women's University, 8-1, Mejirodai 2-chome, Tokyo 112-8681, Japan
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Summary
Introduction
Vascular plants are classified into two groups, microphyllous lycophytes and megaphyllous euphyllophytes (ferns including whisk ferns and horsetails, and seed plants). This classification is based on comparative morphology (Kenrick and Crane, 1997), and it is consistent with recent molecular phylogenetic analyses (Qiu and Palmer, 1999; Pryer et al., 2001; 2004, Qiu et al., 2006; see Chapter 15). Based on this classification, it seems likely that the stem, the leaf, and the root evolved independently in both plant groups. In addition, the root-producing organs called the rhizophore and rhizomorph have evolved only in lycophytes (Kato and Imaichi, 1997). The evolutionary origins of these organs have been proposed mainly based on comparative morphology and anatomy of extant as well as fossil plants (Gifford and Foster, 1989; Stewart and Rothwell, 1993).
Each organ develops through a series of individual morphogenetic events (ontogeny), so attention should be and has been focused on the role of developmental changes during evolutionary diversification. If the ontogeny of a given organ is modified by addition or deletion of specific morphogenetic events to or from the original morphogenetic series, or is modified by alteration of the timing of morphogenetic events, such as retardation or acceleration (heterochrony), the final organ shape could change, leading to evolution of specialized and novel organs (e.g., Gould, 1977; Kluge, 1988; Imaichi and Kato, 1992). Therefore, comparison of morphogenetic events (development) among organs in the context of phylogeny should offer clues for improving the evolutionary scenarios of certain organs.
10 - Conservation biology
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- By Naomi N. Arcand, Department of Geography, University of Colorado, Boulder, CO 80309, USA, Tom A. Ranker, Botany Department, University of Hawaii at Manoa, Honolulu, HI 96822, USA
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Summary
Introduction
Within the USA, Connecticut was the first state to pass a plant protection law in 1869, and it was for a fern: Lygodium palmatum populations were declining due to over-collection for horticultural uses (Yatskievych and Spellenberg, 1993). Over a century later, the same fern was the source of a short article in The New York Times on June 16, 1985, which described road relocation negotiations to avoid two patches of the rare Lygodium palmatum in Burlington County, New Jersey (Haitch, 1985). The article reads, “The issue was at a stalemate in December. Score one for the ferns. Burlington will move the road 200 ft. east of the originally planned route to bypass the plants …” (Haitch, 1985). The beginnings of the American Fern Society in 1893, followed quickly by the publication of the Fern Bulletin (for 5¢ each), and later the American Fern Journal in 1910, attest to the early importance of ferns and lycophytes to US aficionados (Benedict, 1941).
There are approximately 13600 named species of ferns and lycophytes globally (Hassler and Swale, 2001; Chapter 14). Because new species continue to be described and because of persistent regional gaps in floristic treatments, the real number of fern and lycophyte species is not yet known. Due to declining abundance or local extirpation, fern and lycophyte species of conservation concern have been identified for certain areas and have become the focus of international conservation efforts.
9 - Gametophyte ecology
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- By Donald R. Farrar, Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, USA, Cynthia Dassler, Museum of Biological Diversity, Ohio State University, Columbus, OH 43210, USA, James E. Watkins, 16 Divinity Avenue, Harvard University, Cambridge, MA 02138, USA, Chanda Skelton, Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, USA
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Summary
Introduction
Seed plant ecologists would find incredulous a proposal to study the ecology of a species that did not include critical examination of all aspects of recruitment in the field, relying instead on laboratory studies of seed germination, seedling growth and mortality, etc. Yet students of fern and lycophyte ecology are limited to data on gametophyte growth and reproduction collected almost exclusively from laboratory studies. They are expected to assume that these studies accurately reflect growth and reproduction in nature. Field investigations of gametophyte biology are minimal on such critical topics as: the role of morphological and physiological diversity among gametophyte taxa in habitat selection; the time frame and method of gametophyte development, maturation, sexual differentiation, and sporophyte production; the breeding systems and habitats effectively contributing new recruits to sporophyte populations; or the number and frequency of recruits. In addition to producing a decidedly unbalanced view of fern and lycophyte ecology, the absence of ecological data on the gametophytic phase of fern biology has left science with important misconceptions regarding this critical phase of the life cycle. The gametophyte not only provides the opportunity for sexual reproduction (and thus controls genetic diversity) but also determines (along with vegetative reproduction) recruitment, species habitat selection, species migration, and, ultimately, fern and lycophyte evolution.
Reasons for the dearth of field studies on gametophyte ecology are both perceived and real. It is much more difficult to find and to identify gametophyte plants than to do the same with sporophytes.
8 - Phenology and habitat specificity of tropical ferns
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- By Klaus Mehltreter, Departamento de Sistemática Vegetal, Instituto de Ecología, A. C., Xalapa, Veracruz 91000, México
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Summary
Introduction
The focus of this chapter is two aspects of fern sporophyte ecology: phenology and habitat specificity. I define phenology as the study of the periodicity of biological processes caused by intrinsic factors (hormones, circadian clock) or triggered by extrinsic, environmental factors, mainly rainfall, temperature, and photoperiod, or some combination of those elements. Habitat specificity is defined as the biotic and abiotic conditions that favor the development and, consequently, the presence and abundance of fern species on a spatial scale.
Historical summary
Descriptive treatments considering ecological aspects of ferns and lycophytes have been organized geographically (Christ, 1910) and by vegetation types and/or growth forms (Holttum, 1938; Tryon, 1964; Page, 1979a). The latter organization is followed for the two ecological issues treated within this chapter, starting with terrestrial species, followed by rheophytes (fluvial plants), lithophytes (rock plants), epiphytes, and climbers. All other growth forms (e.g., hemi-epiphytes, mangrove ferns) are either treated marginally within the nearest group (e.g., tree ferns within terrestrial ferns, mangrove ferns within rheophytes) or omitted because of lack of information.
Holttum (1938) observed that ferns and lycophytes are rarely dominant in any plant community. His statement that most vegetation types would not be greatly modified if all ferns were removed reflects the low importance he accorded ferns in a functional context within tropical forest ecosystems. In fact, we simply do not understand the ecological importance of ferns, because few studies have addressed this issue. Page (1979a) presented an opposite point of view.
Part IV - Systematics and evolutionary biology
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- By Tom A. Ranker, University of Hawai'i at Manoa, Honolulu, HI, USA, Christopher H. Haufler, University of Kansas Lawrence, KS, USA
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7 - Evolution of the nuclear genome of ferns and lycophytes
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- By Takuya Nakazato, Department of Biology, Indiana University, Bloomington, IN 47405, USA, Michael S. Barker, Department of Biology, Indiana University, Bloomington, IN 47405, USA, Loren H. Rieseberg, Department of Botany, University of British Columbia, Vancouver V6T 1Z4, Canada and Department of Biology, Indiana University, Bloomington, IN 47405, USA, Gerald J. Gastony, Department of Biology, Indiana University, Bloomington, IN 47405, USA
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Summary
Introduction
Analyses of gene expression and function, genetic networks, population polymorphisms, and genome organization at the whole genome level have enabled research on previously intractable questions (reviewed in Wolfe and Li, 2003). Among plant lineages, genomic approaches have been most widely applied in the angiosperms, where significant resources have been developed. Angiosperm studies utilizing genome scale analyses have made several important advances, including the identification of an extensive history of genome duplications (Blanc et al., 2003; Schlueter et al., 2004; Cui et al., 2006), progress in understanding flower development and evolution (Doust et al., 2005; Whibley et al., 2006), characterization of the genetics underlying speciation and adaptation (Bradshaw and Schemske, 2003; Rieseberg et al., 2003; Lai et al., 2005; Eyre-Walker, 2006), the identification and mapping of recombination hot spots (Drouaud et al., 2006), and the discovery and role of microRNAs (Bartel and Bartel, 2003; Bartel, 2004). Genomic analyses will undoubtedly continue to provide tests of longstanding questions and offer novel perspectives in biology. For example, modern genomic analyses are capable of explaining the origin of the exceptionally high chromosome numbers of homosporous ferns and lycophytes, a result that will shed light on eukaryotic genome organization and evolution.
Although there are rich biological and taxonomic resources for ferns and lycophytes, the genomics of these seed-free plants is still in its infancy, and the tools necessary for genomic studies lag behind those available for seed plants. The first homosporous fern linkage map was published only recently (Nakazato et al., 2006), whereas a large number of linkage maps for seed plants have accumulated since the 1980s
Contents
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14 - Diversity, biogeography, and floristics
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- By Robbin C. Moran, The New York Botanical Garden, Bronx, NY 10458, USA
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Summary
Introduction
The biogeography of ferns and lycophytes can be studied from several points of view and with various methods. It might, for instance, examine the distribution of species on a single tree (Krömer and Kessler, 2006; Schuettpelz and Trapnell, 2006), or the frequency and abundance of species over large regions (Ruokolainen et al., 1997; Lwanga et al., 1998; Tuomisto et al., 2003; Jones et al., 2005; Tuomisto and Ruokolainen, 2005), or the relationships of species on different continents (e.g., Moran and Smith, 2001; Parris, 2001). Methods can be as varied as producing lists of plants growing on different soil types (e.g., Young and León, 1989; van der Werff, 1992), calculating the percentage of floristic similarity between different regions (Dzwonko and Kornás, 1978, 1994; Pichi Sermolli, 1979), or analyzing the phylogeny of a clade in relation to its geography and geological history (e.g., Geiger and Ranker, 2005; Hoot et al., 2006). These and other approaches have contributed to what is now an overwhelming amount of literature on the subject. To limit the subject for this chapter, three themes have been chosen: diversity, long-distance dispersal, and vicariance. After discussing these, a summary of the current state of floristics is given because biogeography is ultimately based on that subject.
Historical review
The earliest works on fern and lycophyte biogeography were mostly tabular summaries of the percentages and/or occurrences of species in different regions of the world (D'Urville, 1835; Baker, 1868; Lyell, 1879).
Part I - Development and morphogenesis
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- By Tom A. Ranker, University of Hawai'i at Manoa, Honolulu, HI, USA, Christopher H. Haufler, University of Kansas Lawrence, KS, USA
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1 - Photoresponses in fern gametophytes
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- By Masamitsu Wada, Department of Biology, Tokyo Metropolitan University, Minami Osawa 1-1, Hachioji-shi, Tokyo 192-0397, Japan
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Summary
Introduction
Fern gametophytes are ideal model systems for study of the mechanisms of photomorphogenesis from the standpoint of physiology, photobiology, and cell biology (Wada, 2003, 2007; Kanegae and Wada, 2006). Positive aspects of the fern system include the following. (1) Spores can be preserved at room temperature and they germinate under appropriate conditions within about a week in many species, becoming gametophytes that grow rapidly, at least in their critical early stages. (2) Gametophytes are nutritionally autonomous, facilitating ease of cultivation. (3) Gametophytes are not enclosed by other tissue, so that observation, light irradiation, and experimental manipulation are readily performed. (4) Each developmental step can be controlled synchronously because gametophytes are highly sensitive to light. Each step in development is completely dependent on light; indeed, without light, development does not proceed.
Since the nineteenth century, especially in Germany, fern gametophytes have been used (see Dyer, 1979a) to study photo-physiological phenomena, such as light dependent spore germination (Mohr, 1956a), differentiation from one-dimensional protonemata to two-dimensional prothalli (Mohr, 1956b), and intracellular dichroic orientation of phytochrome (Etzold, 1965). Even though fern gametophytes are very good materials for the study of both photobiology and cell biology, only a few laboratories use them presently, probably for the following reasons. (1) Although mutants can be obtained easily by phenomenological screening (gametophytes are haplophase), making crosses for genetic studies is difficult and time consuming. (2) The biochemistry is also challenging because collecting enough gametophyte tissue for biochemical analyses is difficult.
5 - Antheridiogens
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- By Jakob J. Schneller, Institut für Systematische Botanik, Universität Zürich, CH-8008 Zürich, Switzerland
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Introduction
In homosporous ferns individual gametophytes are generally able to form both antheridia and archegonia. No genetic regulation that determines the sex of the haploid generation has been demonstrated. Growth, temperature, light conditions, environmental characteristics, soil conditions, and, in many cases, antheridia-inducing substances can influence the development of antheridia and archegonia (Voeller, 1964; Miller, 1968; Voeller and Weinberg, 1969). We can therefore describe homosporous ferns as having labile sex expression (Korpelainen, 1998).
The antheridia-inducing substances are called antheridiogens, and are products (hormone-like substances) of the metabolism of prothalli. The term antheridiogen characterizes the function but not the chemical composition. In the literature there is a variety of different terms for antheridiogen, for instance, A-substance (Döpp, 1950, 1959, 1962), antheridogen (Pringle, 1961), pheromone (e.g., Näf et al., 1975; Scott and Hickok, 1987), and hormone (e.g., Näf, 1962; Näf et al., 1975; Raghavan, 1989). Schraudolf (1985) distinguished between the pheromonal (effective on neighboring individuals) and the hormonal (effective within an individual plant) phase of antheridiogens. Here, we will use antheridiogen, the term that is favored in the literature.
History of discovery
Döpp (1950) was the first to discover a naturally produced substance that induces antheridia formation in young prothalli. He showed that substrate from maturing prothallial cultures of bracken (Pteridium aquilinum) induced antheridia formation in young prothalli of its own species and those of Dryopteris filix-mas. The same was true also when using aquatic extractions.
Preface
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- By Tom A. Ranker, University of Hawai'i at Manoa, Honolulu, HI, USA, Christopher H. Haufler, University of Kansas Lawrence, KS, USA
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Summary
Over the past century, books on basic research into ferns and lycophytes have largely focused on particular topics, floras, or methods of study. Setting the stage for understanding fern structure and evolution was a three-volume masterpiece by Frederick O. Bower, published between 1923 and 1928 by Cambridge University Press, and titled simply, The Ferns. In 1950, Cambridge also published Irene Manton's magnum opus, Problems of Cytology and Evolution in the Pteridophyta, establishing a new era of exploring the genetics and evolution of ferns and lycophytes. Books concentrating on laboratory studies have included Adrian Dyer's multi-authored The Experimental Biology of Ferns and Valayamghat Raghavan's Developmental Biology of Fern Gametophytes. Others, such as the detailed and well illustrated Ferns and Allied Plants published in 1982 by Rolla and Alice Tryon, were more systematically focused. Several books have captured the exchange of information at international conferences such as The Phylogeny and Classification of Ferns edited by A. C. Jermy, J. A. Crabbe, and B. A. Thomas in 1973, the Biology of Pteridophytes edited by A. Dyer and C. Page in 1985, a 1989 volume Systematic Pteridology edited by K. H. Shing and K. U. Kramer and based on a Beijing conference, and Pteridology in Perspective edited by J. M. Camus, M. Gibby, and R. J. Johns in 1996. These and others have synthesized ideas on particular areas of basic research, and helped to maintain excitement and communication about fern and lycophyte biology.
6 - Structure and evolution of fern plastid genomes
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- By Paul G. Wolf, Department of Biology, Utah State University, Logan, UT 84322, USA, Jessie M. Roper, Department of Biology, Utah State University, Logan, UT 84322, USA
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Summary
Introduction
The concept of the genome, as the haploid complement of genes of an organism, is far from recent. The term genome is usually attributed to Hans Winkler in 1920 (Ledergerg and McCray, 2001). However, fine scale maps and understanding of the function of genes in the context of the genome did not begin until the 1970s after DNA sequencing techniques were developed. The term genome (and its corresponding genomics) can mean different things to different people (Ledergerg and McCray, 2001) but here we will focus on structural and evolutionary aspects of genomes in ferns. Although genomics is generally reserved for the main (nuclear) component of an organism, that topic is covered in Chapter 7. Instead we narrow the focus here to the chloroplast (i.e., plastid) genome. This small, well-defined genome is found in all green plants. Among land plants the plastid genome is highly conserved in structure and gene content (Palmer, 1985b). Compared to most nuclear genomes studied, plastid genomes contain a high proportion of DNA that codes for proteins and for RNA (ribosomal and transfer). Much of the non-coding regions (between protein-encoding genes) is transcribed and may well have important regulatory functions.
Because the plastid genome contains a high density of genes of well-studied processes, the genome is an excellent model for investigations into the relationship between genome structure and function. This field represents an ideal starting point leading into the much more complex field of the study of nuclear genomes.
Part II - Genetics and reproduction
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- By Tom A. Ranker, University of Hawai'i at Manoa, Honolulu, HI, USA, Christopher H. Haufler, University of Kansas Lawrence, KS, USA
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2 - Alternation of generations
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- By Elizabeth Sheffield, School of Biological Sciences, University of Manchester, Manchester M13 9PT, UK
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Introduction
What is meant by the term “alternation of generations”? There is no consensus on this, but a plethora of definitions and interpretations. For example: “The alternation of a sexual phase and an asexual phase in the life cycle of an organism. The two phases, or generations, are often morphologically, and sometimes chromosomally, distinct.” This is the current Encyclopedia Britannica version, one of the broadest, and one of the most defensible. One alternative is: “The succession of multicellular haploid and diploid phases in some sexually reproducing organisms …” (Purves et al., 2004). The latter is typical of the definitions found in biological textbooks, and as we shall see, restricts the process too much to be useful to fern biologists. The essential feature of the process upon which most authors agree is the presence of distinct multicellular forms. This distinguishes a set of organisms from those with only a single multicellular phase (such as humans, which reproduce, at least at present, via single-celled gametes that, on fusion, generate a multicellular phase morphologically comparable with the parent form that generated the gametes). Organisms with a single multicellular phase include those like ourselves, where the conspicuous phase is diploid (“diplonts”), and those in which the haploid phase is the only one with more than single cells (“haplonts”).
The possession of two different free-living forms allows each to exploit different environments. The tiny spores of the ferns allow genes to travel far beyond the immediate vicinity of the parent.
12 - Species and speciation
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- By Christopher H. Haufler, Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA
- Edited by Tom A. Ranker, University of Colorado, Boulder, Christopher H. Haufler, University of Kansas
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- Book:
- Biology and Evolution of Ferns and Lycophytes
- Published online:
- 11 August 2009
- Print publication:
- 19 June 2008, pp 303-331
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- Chapter
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Summary
Introduction
Two of the most basic elements of evolutionary biology, species and speciation, are also among the most enigmatic and consistently debated. Systematists seem to have a love/hate relationship with both of these topics, and have devoted literally thousands of pages over the past century and a half to exploring what species are and how they originate. In this chapter, general aspects and contemporary perspectives on species and speciation in ferns and lycophytes will be discussed and interpreted.
Are species real or imagined?
When studying biodiversity, a fundamental question that emerges is, “Do species exist?” Why is the variety of life on earth subdivided into a set of discontinuous and distinct groups rather than existing as a seamless series of intergrading populations? Although this appears to be a central question for biologists to answer, prominent authorities consider it to be “one of the most intriguing unsolved problems of evolutionary biology” (Coyne and Orr, 2004). How do the clearly observable distinctions between the groups we label species arise, and what maintains separate ancestor–descendant lineages through time and space? According to some scientists (including Charles Darwin), species may be arbitrary human constructs erected for our convenience (see also Raven, 1976; Mishler and Donoghue, 1982). On the other hand, we can all detect and give names to non-overlapping distinctions among natural populations of organisms.