bluuebunny/crossref_metadata_2025_split huggingface dataset:
THE ORGANIZATION AND EVOLUTION OF MICROTUBULAR ORGANELLES IN CILIATED PROTOZOA
Authors: LYNN DENIS H. 
Published: January 2008

Summary(1) Ciliated protozoa are viewed as unicellular organisms structured in a hierarchy of organizational levels that include the macromolecular, suborganellar, unit organellar, organellar complex, and organellar system.(2) The ciliate cortex is divided into two major functional regions, the somatic region and the oral region. The fundamental component of the cortex is an organellar complex, the kinetid, whose organizing centre is the kinetosome with which are associated three fibrillar associates diagnostic of ciliates. These three fibrillar associates are the periodically striated kinetodesmal fibril and two microtubular ribbons, the transverse and postciliary ribbons.(3) Somatic and oral kinetids are found to be of three major types: monokinetids are composed of one kinetosome and its fibrillar associates; dikinetids are composed of two kinetosomes and their fibrillar associates; polykinetids are composed of more than two kinetosomes and their fibrillar associptes.(4) The mechanisms underlying kinetid function and development remain largely unexplored. Research into the molecular biology and ultrastructure, especially of mutant forms, should provide basic insights in the near future.(5) The conservation of kinetid structure across major phyla of organisms suggests that this subcellular structure should be useful in phylogenetic analysis despite the concepts of ‘chemical identity’ and ‘organic design’.(6) The evolutionary rate of change of oral features is greater than that of somatic features, probably due to developmental and ecological factors. Nevertheless both cortical regions are constrained by the phenomenon of structural conservatism; that is, the conservation of structure through time is inversely related to the level of biological organization.(7) Eight major groupings of ciliate species are recognized, based on ultrastruc‐tural features of the cortex. Several examples of differences between these eight groups and the groups presently recognized are discussed.
SPATIAL ORDER IN MICROBIAL ECOSYSTEMS
Authors: WIMPENNY JULIAN W. T. 
Published: January 2008

Summary(1) It is suggested that microbiologists are concentrating too much at present on homogeneous laboratory models such as the chemostat to simulate natural microbial ecosystems, which are not usually homogeneous but structured in time and space.(2) Natural microbial ecosystems that are spatially heterogeneous are reviewed and discussed.(3) Microbial interactions are briefly discussed in so far as they are relevant to spatially organized ecosystems.(4) Concepts such as ‘niche’ and ‘habitat’ are defined and discussed in terms of their applicability to microorganisms growing at specific points in solute gradient systems. The definitions currently accepted lack precision because they do not take into account the importance of spatial and temporal coordinates. Use of the words ‘compartment’ and ‘domain’ is advocated. The former corresponds to a cell, the latter to a region around it where the compartment has some influence, acting either as a source or as a sink for particular solutes. The terms ‘niche’ and ‘habitat’ are redefined for the activity domains of a compartment and the habitat domains of the ecosystem. The importance of the vectorial flow of solutes is stressed.(5) Current methodology in microbial ecology is briefly reviewed.(6) Methods specifically designed for investigating heterogeneous ecosystems are described, These include: (i) The gradostat, a system of interlinked culture vessels allowing the establishment of steady‐state bidirectional solute gradients. (ii) Percolating columns. (iii) Capillary methods. (iv.) The thin film fermenter. (v) The use of two‐dimensional diffusion plates or stopped time‐dependent gradient plates. (vi) Gel‐stabilized diffusion models established vertically in glass containers such as beakers. (vii) The use of one‐dimensional gradient‐plate techniques to determine the habitat characteristics of different microbial species. (viii) Investigations into naturally structured systems such as the bacterial colony.
ADAPTIVE STRATEGIES OF COLONIZING ANIMAL SPECIES
Authors: Parsons P. A. 
Published: January 2008

Summary1. An understanding of the adaptive strategies of colonizing animal species depends upon an integration of population genetics and ecology, but behavioural components should not be ignored especially higher in the phylogenetic series than insects.2. An ecologically marginal habitat from which colonists are derived can be regarded as one in which physical stresses of climatic origin tend to be variable and extreme, so that resources are unpredictable and short lived. Prerequisites for genetic analysis are therefore phenotypes relatable to the r K continuum of adaptive strategies. These can be called ‘ecological phenotypes’.3. Ecological phenotypes include tolerances to environmental stresses, development time, and resource utilization variability. Such phenotypes enable distinctions to be made between colonizing species and non‐colonists. For example, colonizing species have ecological phenotypes incorporating high resistance to physical stress, rapid development time, and the exploitation of an array of food resources. They are ecologically versatile generalists. This includes the use of ethanol as a resource in Drosophila.4. There is a substantial literature on variation in central and marginal populations based upon gene and chromosome polymorphisms. Most data show a reduction of chromosome polymorphisms and of lethals and semilethals towards the margins, but no equivalent reduction in enzyme polymorphisms. Widespread species tend to have low levels of chromosome polymorphisms as in marginal populations, but enzyme polymorphism levels vary too much among species for meaningful interpretations. Since these are genotypic assessments not directly relatable to the field situation, the somewhat unsatisfactory nature of these data from the interpretative point of view is understandable. In addition, the fundamental issue is not the variability of the genome, but the nature and role of loci controlling ecological phenotypes.5. Ecological phenotypes can be analyzed at the population level with isofemale strains as the starting material. In theory, genetic activity can then be localized to the chromosomal and even genic levels in a species such as D. melanogaster. Isofemale strain studies in D. melanogaster are interpretable in terms of the r K continuum, and so reflect adaptive strategies in nature.6. It is highly likely that the genetic architecture of ecological phenotypes of marginal populations mainly comprises a few additive genes of relatively large effect. This is an architecture permissive of rapid adaptation to new habitats, provided that the appropriate genes are present. Discussions of speciation via the founder principle, a colonization event in itself, have invoked a similar explanation.
THE TRANSPORT AND FUNCTION OF SILICON IN PLANTS
Authors: RAVEN JOHN A. 
Published: January 2008

SummaryA number of lines of evidence (Mr, number of ‐OH groups, measured fluxes at inner mitochondrial membranes) suggest the intrinsic PSi(OH)4of about 10‐10m s‐1in the plant cell plasmalemma. While relatively low, such a PSi(OH)4could maintain the intracellular concentration of Si(OH)4equal to that in the medium for a phytoplankton cell of 5 μm radius growing with a generation time of 24 h. Such passive entry could not account for SiO, precipitation such as is required for scale (Chrysophyceae) or wall (Bacillariophyceae) production in terms of either the generation of a super‐saturated solution or the quantity of SiO2required; active transport occurs at the plasmalemma (and possibly at an internal membrane) of such cells. The energy required for silicification, even in a diatom with an Si/C ratio of 0.25, is only some 2% of the total energy (as NADPH and ATP) needed for growth; the energy cost of leakage of Si(OH)4due to the intrinsic permeability of lipid bilayers to Si(OH)4is never more than 10% of the cost of silicification.In vascular land plants the entry of Si from the soil into the xylem can involve a flux ratio (mol Si/m3water) that is less than (e.g. Leguminoseae) equal to (e.g. many Gramineae) or greater than (e.g.Oryza, Equisetum) the concentration (mol m‐3) in the bathing solution. Even the low influx of the Leguminoseae cannot be accounted for by the ‘lipid solution’ value of PS(OH)4, but requires entry coupled (phenomenologically) to water influx with a reflexion coefficient of about 0.9. The situation in most Gramineae is described by such a coupling with a reflexion coefficient near O, while the accumulation of Si (relative to water) inOryzaandEquisetuminvolves an apparent reflexion coefficient which is negative, i.e. an active transport system stoichiometrically related to water flux. Even in Leguminoseae with a transpiration‐stream concentration of Si(OH)4of only 20 mmol m‐3(cf. the soil solution at 200 mmol m‐3), the fact that only I % of the water in the xylem is retained in the plant means that Si(OH)4at transpirational termini approaches saturation; super‐saturation, and precipitation of SiO, occurs in Gramineae andEquisetum.SiO2precipitation occurs mainly near transpirational termini but can also occur in the xylem vessels and endodermis of roots, for example. Si(OH)2mobility in the phloem seems to be very restricted.The energy costs of SiO2relative to organic compounds as structural and defensive materials are in the ratio of 1:10‐1:20 (on the basis of weight of material). The relative rarity of SiO2as a structural material is discussed in the context of the evolution of Si(OH)4‐transport mechanisms.
ADHESION IN BYSSALLY ATTACHED BIVALVES
Authors: WAITE J. HERBERT 
Published: January 2008

SummaryThe byssus is a structure produced by marine bivalve molluscs to adhere, usually permanently, to substrata under water. As the adhesion of synthetic polymers to surfaces is predictably compromised by the presence of water, particularly bulk water, it is of particular interest to discover the mechanism of byssal adhesion. In most species, the byssus consists of at least four essential components: acid mucopolysaccharides, adhesive protein, fibrous proteins, and an oxidative enzyme, polyphenoloxidase. The function of the mucopolysaccharide component is still uncertain, but it can conceivably be used by the animal as a temporary adhesive, a surface modifying agent, and/or a stabilizing filler for the permanent adhesive. The adhesive protein known as the polyphenolic protein in Mytilus is but a thin plaque applied to the substrate surface by the foot of the animal. The molecular and physical properties of this adhesive protein conform remarkably well to what one expects of an ideal synthetic polymer, i.e. high molecular weight, abundance of large and polar side chains, near‐zero surface contact angle, and total water‐insolubility after setting. The fibrous proteins constitute the major portion of the thread or ribbon‐like material connecting the animal to the adhesive plaque on the substrate surface. These proteins are packed in ordered crystalline arrays, e.g. β‐pleated sheet and collagen helix (in mytilids) as is to be expected from structural tensile elements of Nature. The enzyme polyphenoloxidase is presumed to induce intermolecular cross‐linking of proteins in the fibrous and adhesive portions of the byssus. In Mytilus the natural substrates of the enzymc may be the dopa‐containing polyphenolic protein and accessory gland protein.
FUNCTIONAL ASPECTS OF DROSOPHILA COURTSHIP
Authors: EWING ARTHUR W. 
Published: January 2008

Summary1. The elements that make up the courtship behaviour of males and of females are briefly described. It is pointed out that some of the terms used, such as female ‘repelling’ behaviour, are misleading as they do not reflect the known functions of the behaviours.2. Evidence has been presented for a number of distinct pheromones with different functions during courtship. These claims are critically examined as the evidence is incomplete and at times conflicting. It seems unlikely that any pheromones other than those acting over a very short distance are involved in courtship. There is sound evidence for an aphrodisiac pheromone produced by all females which stimulates male courtship. A pheromone, which may be the same one, is produced by males less than 12 h old, which also stimulates male courtship. No function is ascribed to this pheromone. Fertilized females either produce less aphrodisiac pheromone or they may, in addition, produce one that inhibits male courtship. Mature males may also produce an inhibitory pheromone. Females produce a contact pheromone which is species‐specific and involved in sexual isolation. It is not at present clear whether this is different from the aphrodisiac pheromone.3. There is considerable variability in the importance of vision in courtship. Many species will mate satisfactorily in the dark, suggesting that visual stimuli are not critical. Most species use vision to orient towards one another and for males to track and follow females. Even in light‐independent species such as D. melanogaster, specific visual signals may be used in courtship although they are not obligatory. Thus the red eye of the male is a sexual signal for females. Conversely, some light‐dependent species do not appear to make use of visual signals as a major factor in courtship. Some, however, do perform behaviours that are clearly visual and which may act to emphasize markings on wings, head or body.4. The majority of Drosophila species perform courtship songs by vibrating one or both wings. The songs produced by males sexually stimulate the females. They are species specific and there is considerable indirect and some direct evidence that the songs are involved in sexual isolation. Males of many species produce two different songs during courtship and it is probable that one is concerned mainly with sexual stimulation and the other with species recognition. Females of certain species of Drosophila and Zaprionus also sing during courtship and these songs may aid species recognition by males. In addition males and unreceptive females perform ‘aggressive’ songs.5. Almost all studies of Drosophila courtship have been made in very confined conditions in the laboratory. Interpretation of some of the results obtained in this way may require modification in the light of ecological research and observation of courtships under more natural conditions.
THE NATURAL CONTROL OF POPULATIONS OF BUTTERFLIES AND MOTHS
Authors: DEMPSTER J. P. 
Published: January 2008

Summary1. Life‐table data for 14 species of Lepidoptera are analysed by the k‐factor technique of Varley & Gradwell (1960). Two factors are shown to be of particular importance in determining fluctuations in abundance from one generation to the next. These key factors are predators and the failure of females to lay their full complement of eggs.2. Data from 24 studies are reviewed to identify any density‐dependent factors that would be capable of regulating the populations about an equilibrium density. In eight studies no density‐dependent relationships could be identified, and in a further 13 the only density dependence demonstrated was due to intraspecific competition for resources. It is argued that competition is incapable of regulating populations at low density. In the other three studies, natural enemies are thought to be acting in a density‐dependent manner, but their ability to regulate the populations is questioned.3. The frequency of over‐population and of extinction is reviewed and both appear to occur frequently in Lepidoptera. This, coupled with the failure of most studies to demonstrate the existence of density‐dependent processes capable of regulating populations, leads the author to reject the model of regulation about an equilibrium density in favour of a model of population limitation by a ceiling set by resources.4. Fluctuations in resource availability may be important in determining variations in the abundance of many Lepidoptera, but at present too few ecologists have quantified the carrying capacity of habitats occupied by the species they study to generalize about this.
FACTORS AFFECTING THE SECRETION OF LUTEINIZING HORMONE IN THE EWE
Authors: MARTIN GRAEME B. 
Published: January 2008

Summary(1) Luteinizing hormone (LH) is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre‐pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle. Secretion is probably also pulsatile during the preovulatory surge of LH.(2) The secretion of LH is affected by the ovarian steroids, oestradiol and progesterone, both of which act principally to reduce the frequency of the pulses. During the luteal phase the two steroids act synergistically to exert this effect, and during anoestrus oestradiol acts independently of progesterone. Androstenedione secreted by the ovary apparently has no role in the control of LH secretion.(3) The amplitude of the pulses may also be affected by the steroids but there are conflicting reports on these effects, some showing that amplitude is lowered by the presence of oestrogen and others showing increases in amplitude in the presence of oestrogen and progesterone.(4) The secretion of LH pulses is affected by photoperiod, social environment and nutrition. Under the influence of decreasing day‐length, oestradiol alone cannot reduce the frequency of pulses and the ewe experiences oestrous cycles. When day‐length is increasing, the hypothalamus becomes more responsive to oestradiol which reduces the frequency of the pulses.(5) A hypothetical pheromone secreted by rams can increase the frequency of the LH pulses in anoestrous ewes and thereby induce ovulation, possibly by inhibiting the negative feedback exerted by oestradiol.(6) The relationships between nutrition and reproduction are poorly understood, but it seems likely that the effects of nutrition are mediated partly through the hypothalamus and its control of the secretion of LH pulses.(7) The pulses of LH secreted by the anterior pituitary gland are evoked by pulses of GnRH secreted by the hypothalamus. The location of the centre controlling the GnRH pulses and the neurotransmitter involved are not known.
DEVELOPMENTAL MECHANISMS UNDERLYING THE FORMATION OF ATAVISMS
Authors: HALL BRIAN K. 
Published: January 2008

Summary1. Atavisms emerge as evidence of localized modifications in development of an organ or of one of its parts. Different developmental processes can be triggered within the same organ rudiment, presumably in response to the same stimulus. We saw that that stimulus can have a genetic basis in a mutational event, which can be selected for. We also saw that atavism can be produced by experimental manipulation within developing systems ‐increased growth of the chick fibula, enamel production from avian ectoderm, and balancer formation in amphibians. Such atavisms are not based on heritable genetic changes. They indicate the developmental plasticity that exists within embryos and the relative ease with which development can be switched from one programme to another.2. Examination of mutants (wingless chicks), limbless vertebrates and experimental manipulation of embryos, shows that cell death, inductive tissue interactions and altered patterns of growth are developmental mechanisms used in the formation of atavisms.3. Differential development mechanisms can be triggered within the same organ at the same time to produce atavisms. In the guinea pig, formation of atavistic digit V involves prolongation of growth of metatarsal V whereas formation of atavistic digit I involves development of a new metatarsal I.4. Secondary functional modifications ensure that the atavism is integrated with the other components of the functional unit, as illustrated by extra digits in horses or guinea pigs and fibulae in birds. Atavistic 2nd and 4th digits in horses arise by continued growth of their primordia. A consequent reduction in the growth rate of digit 3, the normal single functional digit, enables all three digits to attain approximately equal lengths and so potentially to function. The altered functional load transmitted to the limbs results in secondary but correlated alterations in muscles and skeletal elements in other portions of the limbs. The fact that embryonic digit 2 normally develops to a more advanced state than digit 4 explains why digit 2 more often develops atavistically, for if variation in growth rate is the basis for the atavistic digit, digit 2 has an advantage over digit 4.5. Atavisms should not be an embarrassment to the evolutionary biologist. They are the outward and visible sign of a hidden potential for morphology change possessed by all organisms. Neither basic capacity to form the organ nor patterning information is lost. Modification of components of inductive tissue interactions helps to explain how organs are lost during evolution (also see Regal, 1977); retention of the basic mechanism explains how structures can be revived as atavisms (also see Rachootin & Thomson, 1981). Frequency of atavisms thus provides an indication of the degree of modification or loss of the underlying developmental programme.
PARASITISM AND THE UNIONACEA (BIVALVIA)
Authors: KAT PIETER W. 
Published: January 2008

SummaryIt is proposed that the incorporation of a unique parasitic stage in the life‐cycle of unionaceans which involves an obligate relationship between a vertebrate host, usually a fish, and a highly modified larval stage, the glochidium, has had far‐reaching consequences with respect to overall morphology, extent of species' geographic ranges, and rate of speciation in the group.Glochidia are separable into three main types with respect to overall shape and attachment features, and are retained in variously modified brood pouches. When mature, glochidia are released in several different ways which reflect various adaptations involved in either attracting the fish host and/or increasing the probability of attachment. Glochidia do not seem capable of host selection, and the reaction of the host to the parasite seems to be the main factor in determining specificity. Release of glochidia is synchronized to correspond to periods of predictable host availability, such as during spawing migrations and nesting behaviour. Other adaptations include modifications of glochidial conglutinates to mimic host food items, and modifications of the unionacean mantle edges to attract hosts. In all cases, a good correlation exists between the type of lure used and host food preferences, but, despite these adaptations, host specificity among unionaceans seems low.Parasitism among unionaceans is postulated to be mainly advantageous in terms of predictability of dispersal by habitat‐specific hosts, but parasitism is hypothesized to entail constraints in terms of the degree to which shell shape and life‐habit can be diversified among unionaceans. The type of host parasitized is considered to affect the rate of diversification among populations and speciation among unionaceans: those that parasitize strictly freshwater hosts are more likely to exhibit highly individualistic populations in different drainages with respect to molecular genetic and soft‐part characters, while those that parasitize anadromous or saltwater‐tolerant hosts show little differentiation among widely distributed populations.