Showing papers in "Philosophical Transactions of the Royal Society B in 1987"
TL;DR: Bat wing morphology is considered in relation to flight performance and flight behaviour to clarify the functional basis for eco-morphological correlations in flying animals, and adaptive trends in wing adaptations are predictably and closely paralleled by echolocation call structure.
Abstract: Bat wing morphology is considered in relation to flight performance and flight behaviour to clarify the functional basis for eco-morphological correlations in flying animals. Bivariate correlations are presented between wing dimensions and body mass for a range of bat families and feeding classes, and principal-components analysis is used to measure overall size, wing size and wing shape. The principal components representing wing size and wing shape (as opposed to overall size) are interpreted as being equivalent to wing loading and to aspect ratio. Relative length and area of the hand-wing or wingtip are determined independently of wing size, and are used to derive a wingtip shape index, which measures the degree of roundedness or pointedness of the wingtip. The optimal wing form for bats adapted for different modes of flight is predicted by means of mechanical and aerodynamic models. We identify and model aspects of performance likely to influence flight adaptation significantly; these include selective pressures for economic forward flight (low energy per unit time or per unit distance (equal to cost of transport)), for flight at high or low speeds, for hovering, and for turning. Turning performance is measured by two quantities: manoeuvrability, referring to the minimum space required for a turn at a given speed; and agility, relating to the rate at which a turn can be initiated. High flight speed correlates with high wing loading, good manoeuvrability is favoured by low wing loading, and turning agility should be associated with fast flight and with high wing loading. Other factors influencing wing adaptations, such as migration, flying with a foetus or young or carrying loads in flight (all of which favour large wing area), flight in cluttered environments (short wings) and modes of landing, are identified. The mechanical predictions are cast into a size-independent principal-components form, and are related to the morphology and the observed flight behaviour of different species and families of bats. In this way we provide a broadly based functional interpretation of the selective forces that influence wing morphology in bats. Measured flight speeds in bats permit testing of these predictions. Comparison of open-field free-flight speeds with morphology confirms that speed correlates with mass, wing loading and wingtip proportions as expected; there is no direct relation between speed and aspect ratio. Some adaptive trends in bat wing morphology are clear from this analysis. Insectivores hunt in a range of different ways, which are reflected in their morphology. Bats hawking high-flying insects have small, pointed wings which give good agility, high flight speeds and low cost of transport. Bats hunting for insects among vegetation, and perhaps gleaning, have very short and rounded wingtips, and often relatively short, broad wings, giving good manoeuvrability at low flight speeds. Many insectivorous species forage by `flycatching' (perching while seeking prey) and have somewhat similar morphology to gleaners. Insectivorous species foraging in more open habitats usually have slightly longer wings, and hence lower cost of transport. Piscivores forage over open stretches of water, and have very long wings giving low flight power and cost of transport, and unusually long, rounded tips for control and stability in flight. Carnivores must carry heavy loads, and thus have relatively large wing areas; their foraging strategies consist of perching, hunting and gleaning, and wing structure is similar to that of insectivorous species with similar behaviour. Perching and hovering nectarivores both have a relatively small wing area: this surprising result may result from environmental pressure for a short wingspan or from the advantage of high speed during commuting flights; the large wingtips of these bats are valuable for lift generation in slow flight. The relation between flight morphology (as an indicator of flight behaviour) and echolocation is considered. It is demonstrated that adaptive trends in wing adaptations are predictably and closely paralleled by echolocation call structure, owing to the joint constraints of flying and locating food in different ways. Pressures on flight morphology depend also on size, with most aspects of performance favouring smaller animals. Power rises rapidly as mass increases; in smaller bats the available energy margin is greater than in larger species, and they may have a more generalized repertoire of flight behaviour. Trophic pressures related to feeding strategy and behaviour are also important, and may restrict the size ranges of different feeding classes: insectivores and primary nectarivores must be relatively small, carnivores and frugivores somewhat larger. The relation of these results to bat community ecology is considered, as our predictions may be tested through comparisons between comparable, sympatric species. Our mechanical predictions apply to all bats and to all kinds of bat communities, but other factors (for example echolocation) may also contribute to specialization in feeding or behaviour, and species separation may not be determined solely by wing morphology or flight behaviour. None the less, we believe that our approach, of identifying functional correlates of bat flight behaviour and identifying these with morphological adaptations, clarifies the eco-morphological relationships of bats.
1,641 citations
TL;DR: The 'rules' that are emerging for DNA bendability and, from the results of other workers, on intrinsically bent DNA, are likely to be useful in considering looping and bending of DNA in other processes in which it is thought to be wrapped around a protein core.
Abstract: The DNA of a nucleosome core particle is wrapped tightly around a histone octamer with approximately 80 base pairs per superhelical turn. Studies of both naturally occurring and reconstituted systems have shown that DNA sequences very often adopt well-defined locations with respect to the octamer. Recent work in this laboratory has provided a structural explanation for this sequence-dependent positioning in terms of the differential flexibility of different sequences and of departures from smooth bending. The 'rules' that are emerging for DNA bendability and, from the results of other workers, on intrinsically bent DNA, are likely to be useful in considering looping and bending of DNA in other processes in which it is thought to be wrapped around a protein core.
164 citations
TL;DR: Three different types of amperometric enzyme electrode are described and the mechanism of the enzyme oxidation is discussed and the reaction is shown to take place by heterogeneous redox catalysis and not by homogeneous mediation.
Abstract: Three different types of amperometric enzyme electrode are described. The first type uses a conducting organic-salt electrode to oxidize NADH. Results for sensors for ethanol and for bile acids are presented. In the second type of sensor, flavoenzymes are directly oxidized on the surface of the conducting organic-salt electrode. Results for five different enzymes are described. The mechanism of the enzyme oxidation is discussed and the reaction is shown to take place by heterogeneous redox catalysis and not by homogeneous mediation. The enzymes are strongly adsorbed on the electrode; microelectrodes for in vivo studies can be constructed without a membrane. Results for in vivo studies of changing glucose levels in the brain of a freely moving rat are presented. The third type of sensor is designed to measure low levels of toxic gases such as H2S and HCN. This is done by monitoring the inhibition by the toxic gas of the activity of the respiratory enzyme cytochrome oxidase.
157 citations
TL;DR: In this article, the authors describe and name these linked geomorphic and lithostratigraphic features, and outline on a reconnaissance basis their distribution within the Severn Estuary.
Abstract: The Severn Estuary on the west coast of Britain is large, macrotidal and well mixed, receiving fine sediment from many sources. Within the last few thousand years, at least four discrete lithostratigraphic units, predominantly of sandy to silty clay, have accumulated along the shores of the estuary in the upper intertidal zone. The three youngest are continuing to be deposited, each beneath a distinctive geomorphic surface reached by a proportion of the tides. These surfaces form a stair-like succession on the salt marshes and high mud flats, the most elevated and outermost of the surfaces overlying the formation that, of the three, began to accumulate earliest. We here describe and name these linked geomorphic and lithostratigraphic features, and outline on a reconnaissance basis their distribution within the estuary. The (upper) Wentlooge Formation consists of pale green estuarine silty clays that began to accumulate 2500-3000 years ago and ceased to form in the Roman period or soon after. Reclamation during the Roman period isolated the Wentlooge Surface on large areas of tidal wetland in the lower estuary. The soil that developed on this surface is recognized as the Wentlooge palaeosol in those places where post-Roman breaching of the Roman sea defences led to a resumption of tidal sedimentation. The Rumney Surface is the most elevated of the geomorphic surfaces on the salt marshes of the estuary. It overlies thick largely pink sandy to silty clays, termed the Rumney Formation, that began to form at times ranging from the early mediaeval to the early modern periods. Mediaeval reclamation of wetlands led to the isolation of the Oldbury Surface during the early stages in the accumulation of the Rumney beds. Locally, the breaching of the mediaeval seabanks led to renewed tidal sedimentation on the Oldbury Surface. Wherever the Wentlooge and Rumney formations are seen in contact, the base of the latter so far proves to be sharp and erosional. Locally, the Rumney Formation is found to abut against and smother a low embayed cliff cut into the upper Wentlooge beds. The Awre Surface forms the intermediate level on the salt marshes and overlies pink to grey sandy to silty clays (Awre Formation) that bank against a low cliff and gently shelving platform cut into older deposits. This unit began to accumulate probably in the 19th century. The lowest step on the salt marshes is formed by the Northwick Surface, underlain by the grey sandy to silty clays of the Northwick Formation. Like the Rumney and Awre beds, the Northwick Formation abuts against a cliff and shelving platform eroded into older sediments, chiefly the Rumney and Awre formations. The erosion surface at the base of the Northwick Formation dates from the 19th century and the beds themselves from the early 20th century. The reconnaissance shows that these linked formations and geomorphic surfaces are represented in most parts of the Severn Estuary. The evidence available for dating - archaeological, historical, radiocarbon, and geochemical - points to the broad synchroneity of each formation throughout this extensive area. The geochemistry of the tidal sediments, reflecting the pollution history of the estuary, is particularly useful in the recognition and correlation of the Northwick Formation. These associated geomorphic and lithostratigraphic features point to the instability of the shores of the estuary and to important horizontal movements of the strand over the last 2000 years. Intrinsic as well as external factors may have controlled these oscillations, but which factor, or factors, was responsible for the movements recorded cannot as yet be decided.
125 citations
TL;DR: The evidence suggests that the eogyrinids, like the archeriids, were long-bodied with a presacral count of 40 and forelimb function in Pholiderpeton was more flexible than that proposed for the related Proterogyrinus.
Abstract: Pholiderpeton scutigerum is an eogyrinid amphibian from the Coal Measures of Yorkshire (Westphalian A). The holotype has been prepared by airbrasive and dental-mallet techniques to reveal the most completely preserved of British embolomeres. The skull and braincase morphology of Pholiderpeton is closely similar to that of Eogyrinus but has provided new information about many of the skull roofing bones and palate, in particular the pterygoid-epipterygoid complex. The braincase has augmented information gained from other embolomere specimens and a new reconstruction has been attempted. A supraoccipital was not present but the otic capsule was roofed dorsally by the opisthotics, as in loxommatids. Study of jaw function in Pholiderpeton suggests that it did not conform to a pure ‘kinetic inertial’ system but that it was essentially unspecialized. The so-called ‘kinetic line’ at the junction of the eogyrinid skull table and cheek was not mobile but was probably a butt joint to resist vertical compression generated during jaw closure. Present on the specimen, and previously undescribed in eogyrinids, were elements of the cervical region, including the atlas neural arch and pleurocentrum, the axis neural arch and rib as well as the pectoral girdle and forelimb, allowing reconstruction of these regions to be made. The eogyrinids resemble Archeria in the structure of pectoral limb and girdle. Forelimb function in Pholiderpeton was more flexible than that proposed for the related Proterogyrinus . The evidence suggests that the eogyrinids, like the archeriids, were long-bodied with a presacral count of 40. The cervical region of embolomeres shows some similarities with that of Proterogyrinus but with a more fully ossified atlas pleurocentrum. The atlas vertebra is also compared with that of Archeria , previously undescribed. A study of the skull table in the genus Archeria is included to assist in taxonomic studies of eogyrinids. A number of consistent characters were found to exist in this region within one species of embolomere. The genus Pholiderpeton Huxley (1869) is shown to be synonymous with Eogyrinus Watson (1926) and therefore takes nomenclatural priority, but it may be distinguished from the related Palaeoherpeton on a number of characters. The isolated skull table pertaining to Pteroplax is not that of an eogyrinid, and is removed from the family. It shows resemblances to Archeria and Proterogyrinus . The North American eogyrinids are retained within the family Eogyrinidae. The family Eogyrinidae is most closely related to the monogeneric family Anthracosauridae, and together these two form the sister-group of the Archeriidae. The Proterogyrinidae is the sister-group of all three, and together these families form the group Embolomeri. A cladogram to express these relationships is presented.
93 citations
TL;DR: Responses to oil-derived hydrocarbons at the molecular, subcellular, cellular and whole animal levels of organization are considered, with particular emphasis on the use of marine molluscs as sentinel organisms for assessing pollutant effects.
Abstract: The impact of pollutants on an organism is realized as perturbations at different levels of functional complexity. This presentation considers responses to oil-derived hydrocarbons at the molecular, subcellular, cellular and whole animal levels of organization, with particular emphasis on the use of marine molluscs as sentinel organisms for assessing pollutant effects. A number of biological effects measurements are described which have been used in the development of early-warning systems based upon reactions to hydrocarboninduced damage. These include those of the microsomal cytochrome P-450 dependent monooxygenase system involved in metabolism of organic xenobiotics, functional and structural responses of lysosomes to hydrocarbons, quantitative structural alterations in the cells of the digestive and reproductive systems and effects on physiological scope for growth and parallel correlations of this latter parameter with ecological parameters such as species diversity. Aspects of recovery processes are also considered. Examples of both laboratory and field studies are cited to illustrate both the application of these approaches and the functional integration of the responses at the various levels of biological organization. This ability to link the various parameters in a functional manner is believed to strengthen the rationale for their use in impact assessment.
93 citations
TL;DR: The electrochemistry of redox proteins is now well established, and derivatives of the ferricinium ion act as excellent electron-transfer reagents from the enzyme glucose oxidase, capable of detecting glucose in blood.
Abstract: The electrochemistry of redox proteins is now well established. Conditions exist which allow electron-transfer reactions of all simple proteins to proceed rapidly and reversibly at electrodes. Coupling of the electrode reaction to enzymes, for which the redox proteins act as cofactors, allows exploitation of this good electrochemistry. This is well illustrated by the enzyme-catalysed electrochemical oxidation of p -cresol to p -hydroxybenzaldehyde, which has been shown to proceed along with coupling to the electrode via the copper protein, azurin, or the organometallic compound ferroceneboronic acid. Ferrocene derivatives, in general, show a degree of versatility, coupling the electron-transfer reactions of many enzymes. Thus derivatives of the ferricinium ion act as excellent electron-transfer reagents from the enzyme glucose oxidase. The system is capable of detecting glucose in blood. Similar procedures, in conjunction with the appropriate enzyme, have yielded assays for, among others, H 2 O 2 and cholesterol.
93 citations
TL;DR: The operational characteristics of enzyme electrodes are adaptable to any given analytical problem, and enzymic anti-interference layers are a tool for improving the sensor specificity.
Abstract: Starting from the state of the art, principles for improving the analytical characteristics of enzyme electrodes are discussed. Coupling of appropriate amperometric electrode processes with enzyme systems, e.g. urease or aminopeptidases, results in a simplification of operation. Optimal sample frequencies are realized on the basis of enzyme membranes, with both a small characteristic diffusion time and a high enzyme activity, applied in a well-designed sample-processing system. Coupled enzyme reactions of the sequence or competition type are successfully used for extension to new analytes, e.g. inhibitors, cofactors or alternative substrates. Cyclization of the analyte enhances the sensitivity of enzyme electrodes to the nanomolar concentration range. Enzymic anti-interference layers are a tool for improving the sensor specificity. The operational characteristics of enzyme electrodes are thus adaptable to any given analytical problem.
92 citations
TL;DR: Details of a novel type of optical biosensor for immunoassays, the fluorescence capillary-fill device (FCFD), based on a straightforward adaptation of the technology used to mass manufacture liquid-crystal display cells to give cheap disposable immunosensors are reported.
Abstract: This paper reports, for the first time, details of a novel type of optical biosensor for immunoassays, the fluorescence capillary-fill device (FCFD). This is based on a straightforward adaptation of the technology used to mass manufacture liquid-crystal display (LCD) cells to give cheap disposable immunosensors. These merely require contact by the sample to give a result in about a minute, and use certain principles of optical fibres and waveguides to avoid the need for operator attention, for physical separation methods or for washing steps. After a very brief introductory review and classification of optical biosensors, the main features of the FCFD and its associated instrumentation are described. The optical characteristics of the FCFD are then described, followed by accounts of the immunoassay method, the measurement system used in the experiments, the fabrication of FCFD sensors and a detailed description of the design of a competitive immunoassay for human immunoglobulin G (hIgG). The experimental details and the results of a first attempt at such an assay are then presented and discussed. It is concluded that the demonstration of this assay is a significant achievement, because the format of the FCFD, its manufacturing process and its instrumentation are completely novel. Certain problem areas have been identified and quantified; intended further work on these is outlined.
86 citations
TL;DR: Central projections of neurons innervating sensory structures on the head of larval Manduca sexta were traced by using methods of anterograde cobalt-diffusion to find mechanosensory neurons in all parts of the head and its sensory appendages.
Abstract: Central projections of neurons innervating sensory structures on the head of larval Manduca sexta were traced by using methods of anterograde cobalt-diffusion. Regions of the deutocerebrum and tritocerebrum in the brain receive input from the antenna, labrum, maxilla, labial palps, hypopharynx and other unidentified regions of the buccal cavity. Antennal, maxillary and labial inputs project to the larval antennal centre (LAC) of the deutocerebrum. Stemmatal neurons and a few antennal neurons project into the protocerebrum. The suboesophageal ganglion (SEG) receives input from mechanosensory neurons in all parts of the head and its sensory appendages. Some mechanosensory neurons project further to the first thoracic ganglion. In addition to receiving input from chemosensory neurons of the maxilla, the SEG may also receive chemosensory input from epipharyngeal sensilla of the labrum.
86 citations
TL;DR: The three different prokaryotic replication systems that have been most extensively studied use the same basic components for moving a DNA replication fork, even though the individual proteins are different and lack extensive amino acid sequence homology.
Abstract: The three different prokaryotic replication systems that have been most extensively studied use the same basic components for moving a DNA replication fork, even though the individual proteins are different and lack extensive amino acid sequence homology. In the T4 bacteriophage system, the components of the DNA replication complex can be grouped into functional classes as follows: DNA polymerase (gene 43 protein), helix-destabilizing protein (gene 32 protein), polymerase accessory proteins (gene 44/62 and 45 proteins), and primosome proteins (gene 41 DNA helicase and gene 61 RNA primase). DNA synthesis in the in vitro system starts by covalent addition onto the 3'OH end at a random nick on a double-stranded DNA template and proceeds to generate a replication fork that moves at about the in vivo rate, and with approximately the in vivo base-pairing fidelity. DNA is synthesized at the fork in a continuous fashion on the leading strand and in a discontinuous fashion on the lagging strand (generating short Okazaki fragments with 5'-linked pppApCpXpYpZ pentaribonucleotide primers). Kinetic studies reveal that the DNA polymerase molecule on the lagging strand stays associated with the fork as it moves. Therefore the DNA template on the lagging strand must be folded so that the stop site for the synthesis of one Okazaki fragment is adjacent to the start site for the next such fragment, allowing the polymerase and other replication proteins on the lagging strand to recycle.
TL;DR: To enter into symbiosis with nitrogen-fixing microorganisms may be the most controllable method for eukaryotes to obtain fixed nitrogen and to be better adapted to a free-living than to a symbiotic existence.
Abstract: Eukaryotic organisms do not fix nitrogen. Animals generally have no need to do so because of their complex food-acquisition and waste-disposal systems. Plants, by using carbon polymers for structural purposes, minimize their need for nitrogen. When very nitrogen-limited, to enter into symbiosis with nitrogen-fixing microorganisms may be the most controllable method for eukaryotes to obtain fixed nitrogen. Filamentous, heterocystous nitrogen-fixing cyanobacteria may be better adapted to a free-living than to a symbiotic existence, because of their complexity. In symbioses, their photosynthetic machinery becomes redundant and the need to differentiate heterocysts as well as derepress nif genes may be a disadvantage. This could in part account for the greater success of symbioses involving the structurally simpler genera Frankia , Rhizobium and Bradyrhizobium . Nitrogen fixation by legume nodules can be controlled by varying the oxygen supply. This control may be effected by a variable diffusion resistance, enabling oxygen required for ATP synthesis to be matched to available photosynthate. Such a resistance, which is probably located in the nodule cortex, may also be used to reduce nitrogen fixation in the presence of combined nitrogen and could also facilitate rapid responses to other forms of stress. Alternative resistances to gaseous diffusion may operate when water supplies are restricted. Rhizobium and Bradyrhizobium follow different patterns of differentiation into nitrogen-fixing bacteroids. These patterns are coupled with retention or loss of viability and with significant or no natural enrichment of the bacteroids with 15 N respectively. The basic patterns of each type are subject to host-modification. Recent studies on structures of primitive legume nodules show some parallels both with actinorhizas and with nodules on Parasponia induced by Bradyrhizobium . In particular, distribution of rhizobia in nodule tissues is intercellular and infection threads are formed only when bacteria ‘enter’ host cells; there is no intracellular ‘bacteroid’ stage. These threads are retained in the active nitrogen-fixing cells. Many legumes and some actinorhizas are not infected via root hairs. Therefore two of the stages often considered typical of the development of effective legume nodules, i.e. ‘release’ of bacteria into vesicles bounded by peribacteroid membrane and infection through root hairs, can be omitted; these omissions may be of use in attempts to transfer nodulating ability to new genera.
TL;DR: The large-T antigen is a complex multifunctional protein, and to delineate its activity more precisely in viral DNA replication and cellular transformation, small functional domains of the protein have been expressed in Escherichia coli and analysed by using a very extensive library of anti-T monoclonal antibodies.
Abstract: The small eukaryotic DNA tumour virus, SV40, has long provided a very useful model for the study of eukaryotic DNA replication and cellular transformation The viral gene product, large-tumour (large-T) antigen, is essential for the initiation of viral DNA replication and the initiation and maintenance of SV40-virus-mediated cellular transformation The large-T antigen is a complex multifunctional protein, and to delineate its activity more precisely in viral DNA replication and cellular transformation, small functional domains of the protein have been expressed in Escherichia coli and analysed by using a very extensive library of anti-T monoclonal antibodies
TL;DR: The intimate association of the SAR with the upstream/enhancer elements, the presence of clustered sequences highly homologous to the topoisomerase II cleavage consensus, and the localization of topoisomersase II in the scaffold, suggest a structure-function relation between chromosome organization and gene expression.
Abstract: We have been able to map specific DNA fragments at the bases of chromatin loops with the help of a novel extraction procedure by using lithium-3',5'-diiodosalicylate. One such scaffold-attached region (SAR) is found in the non-transcribed spacer in each repeat of the histone gene cluster, on a 657 base pair (b.p.) restriction fragment. Exonuclease III digestion has localized two protein-binding domains on the SAR of the histone cluster. Each covers approximately 200 b.p. and they are separated by a nuclease-accessible region of about 100 b.p. These domains are rich in sequences closely related to the topoisomerase II cleavage consensus. We have studied the scaffold association of three developmentally regulated genes of Drosophila melanogaster: alcohol dehydrogenase (Adh), the homoeotic gene fushi tarazu (ftz) and Sgs-4, a gene encoding one of the glue proteins secreted by third-instar larvae. We find regions attached to the nuclear scaffold (SARS) both 5' and 3' of all three genes, defining small domains ranging from 4.5 to 13 kilobases. In the case of Adh, a gene with two promoters, we find two upstream and two downstream SARS. Those 5' of the gene co-map with regulatory regions for the adult and the larval transcripts, respectively. For Sgs-4, the 5' SAR covers 866 b.p. immediately upstream of the transcript, and encompasses the 200 b.p. regulatory region defined by two deletion mutants that produce little or no Sgs-4 protein. In ftz the 5' SAR is found 4.8 kilobases upstream of the start of transcription within a 2.5 kilobase element required for a high level of ftz expression in the early embryo. Sequence analysis of five upstream SARS reveals clusters of sequences closely related to the cleavage consensus of topoisomerase II. In addition, they contain multiple copies of two sequence motifs: a specific 10 b.p. A-rich sequence, and another 10 b.p. T-rich stretch. In conclusion, the intimate association of the SAR with the upstream/enhancer elements, the presence of clustered sequences highly homologous to the topoisomerase II cleavage consensus, and the localization of topoisomerase II in the scaffold, suggest a structure-function relation between chromosome organization and gene expression.
TL;DR: The Wentlooge Level of the former tidal wetland northeast of Cardiff, reclaimed during the Roman period, exposes a series of late Flandrian lithostratigraphic units dominated by estuarine clays.
Abstract: The shore of the former tidal wetland northeast of Cardiff, called the Wentlooge Level and reclaimed during the Roman period, exposes a series of late Flandrian lithostratigraphic units dominated by estuarine clays. Chief among these are the (upper) Wentlooge Formation (up to 2.5 m thick) and the Rumney Formation (up to 2.7 m thick). The (upper) Wentlooge Formation consists of greenish grey slightly silty clays and is capped by an immature palaeosol yielding signs of clay illuviation. Extending down from a level within the palaeosol is a reticulate system of deep drainage ditches, some of which are locally filled with Romano-British cultural debris, regarded as dating their excavation. Such fills are sealed by the soil. Marine erosion after the Roman period but before late mediaeval times destroyed much of the upper Wentlooge Formation together with a presumed Roman sea defence. The then unprotected drainage ditches were invaded during this episode of coastal retreat and widened out to form a series of large embayments that defined flat-topped headlands on which the palaeosol formed a capping. The overlying Rumney Formation is a series of pink estuarine silty clays with some sands and gravels. It infills the embayments of the coastline cut in the Wentlooge beds and smothers its headlands to a thickness of about 1.4 m. Deposition in the embayments began in the 15th century, but on the headlands was delayed for a further 100-200 years. The outward movement of the shore recorded by the mudflat and marsh deposits of the earlier part of the Rumney Formation was eventually reversed with the formation of a bold mud cliff that has continued to retreat inland to its present position. On the upper part of this cliff are exposed sand sheets which record sand bodies formed during the retreat, at the heads of inlets, as pocket beaches, and as landward-facing bars cast up on to the salt marsh during high tides and storms. Two younger lithostratigraphic units of recent date, the Awre Formation and the Northwick Formation, have stratigraphical relations similar to the Rumney Formation and record further movements of the strand. The coastal oscillations recorded by the lithostratigraphy are attributable to fluctuations in the role and strength of waves at the shore, as governed by either medium-term weather changes or the positions of offshore shoals, or by the morphology of the shore itself.
TL;DR: It is considered that there was no `post-pygidial abdomen', as claimed by Cisne (1975, 1981), the structure so inter-preted being the most posterior, six or seven tiny coxae and leg branches, preserved crowded together and overlapping in particular regions of the body were concerned with capture and ingestion of food.
Abstract: Sixteen specimens from the classic locality quarried by C. E. Beecher, in the Frankfort Formation near Rome, New York State, U.S.A., have been examined. Pyrite has replaced the exoskeleton and lines or infills confined spaces such as within the doublure and appendages. Appendages are compressed beneath the flattened exoskeleton and retain their original relationship to one another; biramous appendages have in most cases been rotated, commonly showing the posterior face beneath the cephalon, the anterior face beneath thorax and pygidium. The specimens were collected and prepared by C. D. Walcott, most having been exposed from the ventral side and lie approximately parallel to the bedding, three are oblique-lateral in relation to bedding. Seven have been additionally prepared using a gas pressure abrasion machine, photographs taken in reflected light, the specimens submerged in alcohol, and explanatory camera-lucida drawings made. The antennae may not have had an originally lyriform configuration. Additional evidence supports the view that the cephalon bore three pairs of biramous appendages as J. L. Cisne claimed (Science, Wash. 186, 13-18 (1974); Fossils Strata 4, 45-63 (1975); Palaeontogr. Am. 9, 99-142 (1981)), but there is no indication that a metastome was present. Coxae are poorly preserved, the last-formed posterior coxa small, triangular, coxae becoming progressively elongate forward along the series, and deeper beneath the cephalon. Neither the ventral membrane nor the coxa-body junction has been observed. The interpretation of the form of the coxa, and attachment of leg branches, by Cisne (1975, 1981), is considered to have been based on misinterpretation of an X-ray stereograph; the leg branch is inserted into the full depth of the abaxial coxal margin. Endites of the podomeres of the leg branch are deep, acutely triangular in shape, on podomeres 1-4 of the posterior branches; podomeres become progressively more elongate forward along the series, endites less acutely triangular, and are present only on the proximal podomere anteriorly, as C. E. Beecher (Am. J. Sci. 1, 251-256 (1896)) showed. The tips of the endites were spinose. We consider that there was no `post-pygidial abdomen', as claimed by Cisne (1975, 1981), the structure so inter-preted being the most posterior, six or seven tiny coxae and leg branches, preserved crowded together and overlapping, backwardly directed across and behind the pygidial doublure. The shaft of the exite was rigidly attached to the upper, posterior side of the coxa, was broad proximally, tapering, and obliquely subdivided. It bore about 50 filaments and a small, setose terminal lobe. The filaments are preserved as imbricated, flattened strips, closely spaced and parallel, always dorsal to the leg branches. The most posterior limb pair was the last-formed, differing growth rates between portions of the earlier-formed limbs led to the graded differentiation shown by the biramous limb series. The differences between coxae and leg branches in particular regions of the body were concerned with capture and ingestion of food. A new reconstruction is made, the exoskeletal convexity based on uncrushed, enrolled specimens of the similar species Triarthrus beckii. Exoskeletal structures show that T. eatoni could have enrolled to form an enclosed capsule, within which all the appendages must have been accommodated. The biramous limbs are restored in a hanging stance, the inner ends of the coxae close together. Both branches of the limbs projected well below the margins of the exoskeleton, but only the tips were visible in dorsal view. T. eatoni was a benthic animal that walked on the substrate, and could have launched itself off the bottom and drifted above it. It was probably a generalized deposit feeder, scavenger and predator, exploring the surface of the mud and digging into it for small organic fragments, and capable of catching and squeezing small prey in the formidable array of posterior endites and spines. Food was passed forward to the mouth by the interaction of the spinose mesial edges of pairs of coxae, larger cephalic coxae helped push food toward the mouth. T. eatoni is preserved predominantly in dark shales, that were formed in muddy environments of the outer continental shelf and upper slope.
TL;DR: An overview of the natural systems of the North Sea is given in this paper, where the effects of pollution and other human activities are discussed as well as the difficulties in assessing them where they are obscured by natural changes.
Abstract: An overview is given of the natural systems of the North Sea: water-circulation, topography and geology of the sea floor, sediment transport, influx of trace constituents (nutrients, trace metals, organic compounds), biological systems and their interrelations. The effects of pollution and other human activities are discussed as well as the difficulties in assessing them where they are obscured by natural changes.
TL;DR: Four new species of parasitic crustaceans belonging to the class Tantulocarida are described, two of which are placed in a new genus, Onceroxenus, and three of them parasitize deep-sea tanaids, one of them a deep sea asellote.
Abstract: Four new species of parasitic crustaceans belonging to the class Tantulocarida are described, two of which are placed in a new genus, Onceroxenus. Three of them parasitize deep-sea tanaids, the other, a deep sea asellote. Microdajus langi, originally classified as an epicaridean isopod, is recognized as a tantulocaridan. It is reported from Scottish waters for the first time and from new host species. These records include the shallowest depth, 22 m, known for a tantulocaridan. Cumoniscus kruppi, a parasite of cumaceans, is also recognized as a tantulocaridan. The Tantulocarida now comprises eleven species and five genera, here assigned to the Basipodellidae and two new families, the Deoterthridae and Microdajidae. Several life cycle stages are described and arranged in two developmental sequences. Evidence for a possible third sequence was found. Male development involves a unique type of metamorphosis in which the free-living adult differentiates from a dedifferentiated mass of tissue contained within the expanded trunk of the tantulus larva. Throughout this metamorphosis the male is supplied with nutrients from the host via a tissue connection, the umbilical cord, and the permanently attached larval head. The non-feeding adult male lacks cephalic appendages but possesses two clusters of aesthetascs on its anterior margin. It is free swimming and has six pairs of large thoracopods without endites. The first two thoracic somites are incorporated into the cephalothorax. The abdomen bears a posteriorly directed, median stylet, interpreted as the intromittent organ. It originates on the first abdominal somite. The adult female has a large sac-like trunk attached by the larval head. The larval trunk is sloughed leaving a scar but no complete moult occurs. Eggs develop within the trunk sac and hatch directly at the infective tantulus larval stage. This extreme condensation of early ontogeny is compared with that of other crustaceans and is interpreted as an adaptation to parasitism in situations where a high dispersal ability is not advantageous. In some females the trunk sac forms behind the head but the larval trunk is retained. Small and large females of this type are described, the largest being 737 $\mu m$ in length. These probably represent females in which sloughing of the larval trunk has failed but it is possible that each may have contained a free-living adult female of comparable size to the adult male. The tantulus larva is described in detail. Scanning electron microscopy reveals that the thoracopodal endites have a complex apical armature, including coupling spines which serve to link the members of a leg pair. Tantulocaridans are permanently attached to their host by the oral disc, presumably by means of an adhesive. In the centre of the disc they make a minute puncture (between 0.5 and 2.0 $\mu m$ in diameter) through the host integument, probably with the aid of their cephalic stylet. This constitutes their only access to the body fluids of the host. The phylogenetic relationships of the Tantulocarida are discussed. They appear to be related to the barnacles (Thecostraca), both groups possessing a median penis derived from the seventh trunk limb. Their possession of a thorax of six somites and the location of the male gonopores on trunk somite seven suggests an affinity with a larger group containing the Thecostraca and the Copepoda.
TL;DR: The angular acceleration receptor system (crista/cupula system) of the statocyst of Octopus vulgaris has been thoroughly reinvestigated, and detailed information is presented regarding its morphometry, ultrastructure, and neuronal and synaptic organization.
Abstract: The angular acceleration receptor system (crista/cupula system) of the statocyst of Octopus vulgaris has been thoroughly reinvestigated, and detailed information is presented regarding its morphometry, ultrastructure, and neuronal and synaptic organization. In each of the nine crista sections, some receptor hair cells are primary sensory cells with an axon extending from their base. Also, there are large and small secondary sensory hair cells without axons, which make afferent synapses with large and small first-order afferent neurons. The afferent synapses are of two morphologically distinct types, having either a finger-like or a flat postsynaptic process; both can be seen in the same hair cell. In addition to the afferents, there is a dense plexus of efferent fibres in each crista section, and efferent synapses can be seen at the level of the hair cells and of the neurons. The morphometric analysis of the nine crista sections shows obvious differences between the odd-numbered (C1, C3, C5, C7, C9) and the even-numbered (C2, C4, C6, C8) crista sections: they differ in length, in the number of the small primary sensory cells and in the number of the small first-order afferent neurons. Centrifugal cobalt filling of the three crista nerves revealed a disproportionate innervation of the nine crista sections: the anterior crista nerve innervates section C1 and the first half of section C2, the medial crista nerve innervates the second half of section C2, sections C3, C4, C5, and the first half of section C6, and the posterior crista nerve innervates the second half of section C6, and sections C7, C8 and C9. In each of the three crista nerves, only 25% of the total number of axons are afferent fibres, the remaining 75% are efferent. To each of the nine crista sections a cupula is attached. In the form and size of the cupulae there is again a conspicuous difference between the odd and the even crista sections: a small wide-based cupula is attached to each of the odd crista sections, whereas the even crista sections each have a large narrow-based cupula with a small area of attachment. The results are discussed with reference to their functional consequences.
TL;DR: Virgin externae of the parasitic barnacle Sacculina carcini Thompson were exposed to settlement of male cyprids and the formation of the trichogon and the kentrogon are regarded as homologous instars.
Abstract: Virgin externae of the parasitic barnacle Sacculina carcini Thompson were exposed to settlement of male cyprids. The events from settlement around the mantle aperture to arrival of the male cypris cells into the receptacle of the externa were studied by transmission and scanning electron microscopy. A small, hitherto unknown larva, the trichogon, escapes from the cyprid within ca. 20 min after settlement. A thin cuticle armed with long spines is preformed beneath the carapace of the male cyprid, and after metamorphosis this cuticle encloses the free trichogon. The trichogon is up to 220 $\mu$ m long, unsegmented, has a variable amoeboid shape and a very simple structure. It includes parts of the cypris epidermis and two other types of cypris cells, but it has no appendages, muscles, sense organs or nervous tissue. The trichogon migrates through the mantle cavity of the externa and arrives at the entrance to the receptacle duct within 2 h after settlement. During the ensuing migration through the receptacle duct, the trichogon loses its spine-armed cuticle. Once inside the receptacle, the trichogon cells and the female cells of the receptacle are in direct contact, with no intervening cuticle. The implanted trichogon is regarded as a very specialized dwarf male. The formation of the trichogon from male cyprids, and of the kentrogon from female cyprids has many similarities, and the trichogon and the kentrogon are regarded as homologous instars. A trichogon is present in the Sacculinidae, the Lernaeodiscidae and most probably also in the Peltogastridae; i.e. in the same families where a kentrogon has been demonstrated to accomplish invasion of the decapod host.
TL;DR: In this article, the effects of the discharge of cuttings on the benthic environment has been shown to be very severe, but only in a very localized area around the installations.
Abstract: Although there were originally no statutory obligations for North Sea oilfield developers to monitor the environmental impact of their activities, many companies undertook such studies voluntarily. The central and northern North Sea is principally a level-bottom habitat and can be broadly considered as being dominated by variations of the classical Amphiura benthic community. Early approaches to monitoring studies involved the use of grids of sampling stations extending several kilometres in every direction from the proposed site of the installation. More recently it has been found that the major impact on the environment is from the discharge of oil-based drilling cuttings at the platforms and drilling rigs. Efforts are now concentrated closer to the installations, using transects starting as near to the source of the discharge as possible. By using community parameters such as diversity and equitability, it has been shown that the fauna responds with a dramatic drop in values of these measures close to the platform. However, in most surveys, background values are regained between 500 and 1000 m from the installation. This seems to be the case regardless of whether diesel or low toxicity oil-based drilling fluids are used. Numbers of individuals and biomass responded in a similar way at some installations using `low toxicity' oil-based drilling muds but increased at others using diesel oil-base. The latter response is similar to that of areas of great organic enrichment while the drop in numbers is more indicative of disturbed or toxic conditions. The markedly patchy distribution of drilling cuttings around the production platforms calls into question the sampling strategies that have been adopted for offshore surveys in the past. The extreme variation of figures, particularly oil levels in sediments, makes it almost impossible to establish firm connections between cause and effect. The effects of the discharge of cuttings on the benthic environment has been shown to be very severe, but only in a very localized area around the installations. It is suggested that attention is now focused on the persistence of the oil in the cuttings and that future monitoring strategies should include this in their scope.
TL;DR: Temperate-zone hypertidal desiccation cracks appear to be distinct from the associations developed in other marginal and continental environments, taking into account their stratigraphical variation.
Abstract: Field studies, including the close temporal monitoring of selected sites allow desiccation cracks in the hypertidal subzone to be classified on the basis of (i) degree of desiccation, (ii) overall shape of individual cracks, (iii) shape of crack margins, (iv) the angle at which cracks join and the number of sides to a desiccation pillar, and (v) the degree and kind of preferred orientation of cracks. In the Severn Estuary and The Wash, and on the north Norfolk coast, these properties are controlled by the thickness, lithology and degree of stratification of the exposed muds, and by the history of drying as determined by positional, tidal and climatic factors. The cracks are initiated at such defects as bird’s footprints, plant stems, and pebbles and shells, and grow slowly along mainly orthogonal paths under the influence of principal stresses that change in orientation with the spread of the fractures they themselves have caused. Hypertidal desiccation cracks commonly open and fill more than once, filling may be accomplished by either a repetition of the same material or a succession of different ones (for example, mud, sand, mud clasts, bivalve shells). The closely monitored sites reveal that hypertidal cracks develop chiefly during the late spring and summer on time scales of hours to months, but that significant fracturing may also occur during the winter, when periods of dry windy weather coincide with relatively weak tides. Because of the nature of the controls, the degree of desiccation and the type and pattern of the cracks varies stratigraphically within the hypertidal zone in a systematic manner which is similar in all the areas studied. Late- and terminal-stage cracks are most prevalent in the upper hypertidal subzone, whereas early-stage forms predominate in the lower subzone. Non-orthogonal patterns predominate in the upper subzone, where various factors promote the destratification of the sediment. Orthogonal fractures are best developed in the middle subzone, where thick muds can be accumulated but where exposure can be lengthy. As an assemblage, and taking into account their stratigraphical variation, temperate-zone hypertidal desiccation cracks appear to be distinct from the associations developed in other marginal and continental environments.
TL;DR: The dental plates of the Devonian lungfish Chirodipterus australis Miles (Osteichthyes; Dipnoi) are shown to have achieved their characteristic morphology by a growth process different from that assumed for the plates of genera such as Dipterus.
Abstract: The dental plates of the Devonian lungfish Chirodipterus australis Miles (Osteichthyes; Dipnoi) are shown to have achieved their characteristic morphology by a growth process different from that assumed for the plates of genera such as Dipterus. Each plate was thickened by the addition of layers of bone that also extended the plate labially, thus providing the base on which and into which dentine grew. Distinctive features of the dentition are: (a) labial increase of the dentine mass by the addition of blister-like denticles of simple enamel-covered dentine, which is initially ingrown by pleromic dentine and subsequently resorbed and replaced by petrodentine; (b) increase in the midline by a similar process that results in the addition of one (or possibly two) new ridges; (c) resorption of the posterior edge of the pterygoid plates and the posterior and posteromedial edges of the prearticular plates, with subsequent development over the resorbed surfaces of several generations of simple regenerative dentine; (d) resorption and redeposition of pleromic dentine and bone in a triangular region posteromedially on the pterygoid plates; (e) the formation of tuberosities that simulate teeth at a short distance in from the labial edge, by four processes: formation of an undulating plate margin, differential growth of petrodentine (hard compact dentine) within the pulp cavity, differential wear of the petrodentine and the adjacent bone plus pleromic dentine, and slightly greater growth of the petrodentine towards the occlusal surface relative to the adjacent bone and dentine; (f) expansion of the large flat surfaces of the plates by gradual replacement of the bone and dentine at the proximal ends of the furrows and also by the development of linkages of petrodentine across the furrows; (g) development of isolated tuberosities on the flat posterolateral parts of the plates. The petrodentine of the ridges, tuberosities and plateaus of the plates is indistinguishable structurally and in its mode of growth from the petrodentine in extant species of dipnoans. Plates similar to those of C. australis have been observed in Stomiahykus, Archaeonectes, Conchodus, Palaedaphus and Sunwapta, as well as several species usually referred to as Dipterus. Sunwapta may be congeneric with C. australis We propose that the term \`dental plate' be used as a general term to cover the crushing plates of dipnoans; that the term \`tooth plate' be restricted to those types that grew by the addition of true teeth to the labial margins of the plates, to form radiate rows; and that the term `dentine plate' be used for those that added dentine to their margins without the formation of teeth. The oldest known genera with tooth plates and dentine plates are Speonesydrion and Dipnorhynchus respectively.
TL;DR: It is suggested that the osphradium is primarily used in sexual biology (coordination of spawning, search for a mate), a role altered within the Gastropoda (search for food, osmoreceptor, p ${\_O\_2}$ -receptor).
Abstract: The fine morphology of the osphradia of six placophorans and eight bivalves, representing all major subgroups of both classes, is described. In addition, the branchial and lateral sense organs of Lepidopleurus cajetanus (Placophora) have been investigated ultrastructurally. Whereas osphradial fine structure is very uniform within the Bivalvia, there are differences between Ischnochitonina and Acanthochitonina, supporting the separation of both groups. Major differences in the conditions of the mantle cavity divide Recent Placophora into the orders Lepidopleurida and Chitonida. The homology of the molluscan osphradium throughout the phylum is discussed in detail. It is concluded that the terminal sense organ (Caudofoveata, Solenogastres), the adanal sensory stripes (Placophora-Chitonida), the interbranchial and post-anal papillae of Nautilus (Cephalopoda), and the organ of Lacaze (Gastropoda-Basommatophora) are homologous with the organs of Spengel (Prosobranchia, Opisthobranchia, Bivalvia), all to be called osphradial sense organs (or osphradia). After discussion it is concluded that the osphradium is a chemoreceptor and not a mechanoreceptor as suggested by many authors. This is shown by the physiological evidence so far reported but mainly by the existence of paddle cilia in the osphradial epithelia throughout the Mollusca, which are typical of molluscan chemoreceptors. It is suggested that the osphradium is primarily used in sexual biology (coordination of spawning, search for a mate), a role altered within the Gastropoda (search for food, osmoreceptor, p ${\_O\_2}$ -receptor).
TL;DR: The Bivalvia are shown to comprise six clusters which have been arranged in the following classificatory system, in general agreement with that adopted in the Treatise on invertebrate palaeontology.
Abstract: Thirty-seven superfamilies of Lamellibranchia (predominantly suspension-feeding) were graded according to their overall more primitive or more advanced structural condition, by tabulation based on two organ systems exhibiting progressive series of character-states (ctenidia, marginal pallial fusion). Commencing with the more primitive superfamilies, these were investigated in groups, recording the occurrence of those character-states that were especially relevant to the group in question. By this means the superfamilies were arranged in five clusters, and the structural characteristics of each cluster were clearly reported. By these methods, and together with three superfamilies of Protobranchia, the Bivalvia are shown to comprise six clusters which have been arranged in the following classificatory system. Subclass 1 Protobranchia Order 1 Nuculoida Order 2 Solemyoida Subclass 2 Lamellibranchia Order 3 Pteriomorpha Order 4 Mesosyntheta Suborder Trigonioida Suborder Unionoida Order 5 Anomalodesmata Suborder Pholadomyoida Suborder Septibranchia Order 6 Gastropempta Suborder Veneroida Suborder Myoida This classification is in general agreement with that adopted in the Treatise on invertebrate palaeontology (ed. R. C. Moore, University of Kansas Press, 1969-71) save for the following points. (i) Four subclasses are suppressed, reasons given. (ii) The superfamilies Crassatellacea, Carditacea and Leptonacea are transferred out of the Veneroida to join the Unionacea in an expanded suborder Unionoida. This improved classification differs in a few respects from the taxonomic arrangement previously obtained by computer analysis (R. D. Purchon, Phil. Trans. R. Soc. Lond. B 284, 425-436 (1978)) owing to additional information, and to more sensitive methods of analysis. The methods used provide clear reasons for the classification adopted, and permit easy adjustments to the classification if these are required after incorporation of any new information that may become available. The principal attributes of the nuculoid protobranchs and of the subclass Lamellibranchia are compared and contrasted in tabular form. A diagram, incorporating four hypothetical ancestral stages, illustrates the probable course of evolution in the class Bivalvia. An appendix supplies five further tabulations demonstrating the relevance to classification of the structural variation within certain of the organ systems. Throughout these six tabulations a majority of the superfamilies exhibit uniformity in their associations.
TL;DR: An account is given of a laboratory investigation designed to evaluate the extent to which the freshwater pulmonate snail Biomphalaria glabrata (Say) can utilize various species of aquatic plants when presented in the following forms over different time scales.
Abstract: An account is given of a laboratory investigation designed to evaluate the extent to which the freshwater pulmonate snail Biomphalaria glabrata (Say) can utilize various species of aquatic plants, mainly macrophytes, when presented in the following forms over different time scales: normal plants; dried plant material; homogenized plant material in calcium alginate matrices; water-soluble filtrates of plant homogenates in the medium. The following propositions, derived from the theory of phased coevolution of components of the module consisting of the epiphytic bacteria, algae, snails and macrophytes, are evaluated on the basis of the present results and others including those obtained in this laboratory. That as the snails had become specialized to exploit surface communities of epiphytic algae, decaying plant material and dissolved organic matter (DOM) early in their evolutionary history they would continue to exploit these resources when they later become associated with aquatic macrophytes. That pulmonate snails would tend to be feeding generalists capable of adapting to food of varying chemical composition, given sufficient time, provided it was sufficiently small or flaccid. That although macrophytes and snails show a strong positive relationship, the living macrophyte tissue would be little used by the snails. That the hard outer envelope, inherited from their terrestrial ancestors, would remain as the major defence mechanism of aquatic macrophytes against attack by snails and other aquatic invertebrates. That aquatic macrophytes would invest little in the nutrient deficiency strategy to reduce attack by invertebrates such as snails. That truly aquatic submerged macrophytes would not possess secondary plant compounds (SPC) that would be molluscicidal. Emergent parts of subaquatic or aquatic plants might be expected to be better sources of SPC with molluscicidal factors than submerged aquatic plants. Species of epiphytic or planktonic algae might be better sources of SPC with molluscicidal effects than aquatic macrophytes. That the strategies developed by pulmonate snails for obtaining their energy supplies would not be conducive to rapid speciation. The analysis of the present and other related results supports these propositions. Predictions based on the theory of mutualism involving the snails, macrophytes and other components of the module also receive some support from an analysis of the present results. The additional empirical work that could be undertaken to test this theory is briefly discussed.(ABSTRACT TRUNCATED AT 400 WORDS)
TL;DR: Biosensor research is likely to play an increasingly important role in generating analytical information in all sectors of human endeavour, from medicine to the military, and will form the basis of cheap, simple devices for acquiring chemical information.
Abstract: A biosensor is an analytical device that responds to an analyte in an appropriate sample and interprets its concentration as an electrical signal via a suitable combination of a biological recognition system and an electrochemical transducer. As a result of recent scientific and technological progress, such devices are likely to play an increasingly important role in generating analytical information in all sectors of human endeavour, from medicine to the military. In particular, biosensors will form the basis of cheap, simple devices for acquiring chemical information, bringing sophisticated analytical capabilities to the non-specialist and general public alike. The market opportunities for the rapid exploitation of novel developments in this sector are substantial. Biosensor research is also likely to have a significant impact on the development of modern electronics.
TL;DR: Cell-free systems from eggs of the frog Xenopus laevis are able to perform most of the acts of eukaryotic chromosome replication in vitro, which now includes the crucial regulatory step of initiation, which had only been achieved for viral systems previously.
Abstract: Cell-free systems from eggs of the frog Xenopus laevis are able to perform most of the acts of eukaryotic chromosome replication in vitro. This now includes the crucial regulatory step of initiation, which had only been achieved for viral systems previously. Purified DNA or nuclei are able to initiate and complete semiconservation replication in egg extracts in vitro (Blow & Laskey, Cell 47, 557-587 (1986)). Replication does not require specialized DNA sequences either in vitro or in microinjected eggs, but in both systems large templates replicate more efficiently than small templates. In some cases replication can re-initiate, excluding the possibility that replication is primed by preexisting primers in the template preparations. When nuclei are replicated in vitro, only one round of replication is observed in a single incubation resembling the single round of replication observed for purified DNA after micro-injection. The mechanism that prevents re-initiation of replication within a single cell cycle is discussed and certain models are eliminated. Nucleosome assembly from histones and DNA has also been studied in cell-free systems from Xenopus eggs. Fractionation has led to the identification of two acidic proteins called nucleoplasmin and N1, which bind histones and transfer them to DNA. The sequences of both proteins have been determined by cDNA cloning and sequencing. Both proteins are found as complexes with histones in eggs.
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TL;DR: This paper is concerned with looking into the future and trying to discern the shape of the directions nitrogen fixation research will take, and how to use techniques to forge new nitrogen-fixing systems or associations or to improve existing ones, or to pursue some combinations of the two.
Abstract: This paper is concerned with looking into the future and trying to discern the shape of the directions nitrogen fixation research will take. Accordingly, much of it may be proved incorrect or impracticable; this is the danger for anyone who makes forecasts. It seems clear that, although rapid progress is being made in our theoretical understanding of the nitrogen fixation process, little of that progress has yet been applied in a practical sense to improve crop production. Our future directions need to encompass this phase of application. One of the dilemmas is to decide how to use our techniques: to forge new nitrogen-fixing systems or associations, or to improve existing ones, or to pursue some combinations of the two. In the legume systems, there is still much slack in technology to be taken up across the world. Simple problems in production, such as widespread boron deficiency in Thailand, remain to be corrected. Some questions to be considered include the following: (i) The ability to manipulate expression of sym and nif genes exists; what are we going to do with it? (ii) Acid tolerance in legume bacteria remains a major problem. What conditions such tolerance, and how can it be recognized and exploited? (iii) Nitrogen fixation in legume nodules depends on dicarboxylate supplies from the plant, apparently because the legume controls what the nodule bacteroids receive. Would a greater supply of dicarboxylates improve nitrogen fixation? Would making other classes of substrates available to bacteroids in larger amounts have beneficial effects? (iv) ‘Alternative’ nitrogenases are now known; can they be used beneficially in existing or new systems?