Showing papers in "Philosophical Transactions of the Royal Society B in 1981"

The Population Dynamics of Microparasites and Their Invertebrate Hosts

Abstract: We show how directly transmitted microparasites, broadly defined to include viruses, bacteria, protozoans and fungi, may regulate natural populations of invertebrate hosts. The study combines elements of conventional epidemiology (where the host population is assumed constant) with elements of prey-predator studies (which conventionally emphasize how prey and predator populations may be regulated by their interaction). To this end, we construct simple models embodying the essentials of the dynamical interaction between invertebrate hosts and their directly transmitted microparasites. In successive refinements, these models include the effects of recovery and disease-induced mortality, castration or diminished reproduction of infected hosts, vertical transmission, latent periods of infection, stress-related pathogenicity, the interplay between disease and other density-dependent constraints on host population growth, and free-living infective stages. In analysing the dynamical behaviour of these models, we focus on: the possible regulation of the host population by the parasite; the basic reproductive rate of the parasite, and the way in which it affects the dynamics and the evolution of the host-parasite association; and the threshold host density and its implications for endemic or epidemic maintenance of the infection. These are examined in the light of synoptic compilations of field and laboratory data on: birth rates (and disease-induced reduction thereof), natural death rates and disease-induced death rates of hosts; latent periods and efficiencies of vertical transmission of pathogens; the rate of production and lifetime of free-living infective stages; and some characteristics of long-term cycles and of epidemic outbreaks of disease in forest insects. In particular, our models suggest that the baculovirus and microsporidian infections of many temperate forest insects will tend to produce stable cycles in host abundance and in prevalence of infection, with periods in the range 5-12 years. Enough is known about the European larch budmoth and an associated granulosis virus for us to undertake a detailed comparison between theory and data that strongly suggests that the observed 9-10 year cycles are driven by the host-parasite interaction. We also discuss the possible control of invertebrate pest species by pathogens, showing how our models could guide laboratory or field studies, to help estimate whether a given pathogen is capable of regulating the target pest population, and, if so, roughly what quantity is needed to effect a specific level of (local) control. Throughout, the emphasis is on the biological ingredients of the models, and on the biological conclusions to be drawn; mathematical details are given in appendixes.

The stimulation of inositol lipid metabolism that accompanies calcium mobilization in stimulated cells: defined characteristics and unanswered questions.

Abstract: It now appears to be generally agreed that the 'phosphatidylinositol response', discovered in 1953 by Hokin & Hokin, occurs universally when cells are stimulated by ligands that cause an elevation of the ionized calcium concentration of the cytosol. The initiating reaction is almost certainly hydrolysis of an inositol lipid by a phosphodiesterase. Phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate all break down rapidly under such circumstances. However, we do not yet know which of these individual reactions is most closely coupled to receptor stimulation, nor do we know where in the cell it occurs. With many stimuli, inositol phospholipid breakdown is closely coupled to occupation of receptors and appears not to be a response to changes in cytosol [Ca2+]: this provoked the suggestion that it may be a reaction essential to the coupling between activation of receptors and the mobilization of Ca2+ within the cell. In a few situations, however, it appears probable that inositol lipid breakdown can occur as a result of the rise in cytosol [Ca2+] that follows receptor activation: such observations gave rise to the alternative opinion that inositol lipid breakdown cannot be related to stimulus-response coupling at calcium-mobilizing receptors. It now seems likely that these two views are too rigidly polarized and that some cells probably display both receptor-linked and Ca2+-controlled breakdown of inositol lipids. Both may sometimes occur simultaneously or sequentially in the same cell.

On the Annual Cycle of the Blue-Green Alga Microcystis Aeruginosa Kutz. Emend. Elenkin

Abstract: A 4 year series of field, light-microscope and ultrastructural observations is presented to illustrate biological aspects of the annual cycle of natural Microcystis populations enclosed in Lund tubes. Nine morphological stati, all referable to M. aeruginosa f. aeruginosa , feature at various stages of the cycle. Summer bloom-forming populations originate from vegetative colonial stock that overwinters on the bottom sediment each year, but there is no mass transfer of these colonies to the water column: intensive growth from individual cells in the old colonies leads to the formation of new infective colonies, being stimulated when the bottom water approaches anoxia and light penetrates to the bottom sediments. Growth is slow but the developing populations sustain only minor losses through grazing and settling out, eventually becoming dominant over other species. Allelopathy possibly contributes to this effect. In postmaximal populations, several mechanisms can contribute to net buoyancy loss and a (usually) rapid recruitment of vegetative colonies to the sediments is observed. Hypotheses are advanced to account for the observed behaviour, and some of these have been tested in the laboratory. The apparent physiological flexibility of Microcystis seems well suited to growth and survival in the microenvironments encountered in eutrophic lakes.

A Computer Implementation of a Theory of Human Stereo Vision

Abstract: Recently, Marr & Poggio (1979) presented a theory of human stereo vision. An implementation of that theory is presented, and consists of five steps. (i) The left and right images are each filtered with masks of four sizes that increase with eccentricity; the shape of these masks is given by delta 2G, the Laplacian of a Gaussian function. (ii) Zero crossings in the filtered images are found along horizontal scan lines. (iii) For each mask size, matching takes place between zero crossings of the same sign and roughly the same orientation in the two images, for a range of disparities up to about the width of the mask's central region. Within this disparity range, it can be shown that false targets pose only a simple problem. (iv) The output of the wide masks can control vergence movements, thus causing small masks to come into correspondence. In this way, the matching process gradually moves from dealing with large disparities at a low resolution to dealing with small disparities at a high resolution. (v) When a correspondence is achieved, it is stored in a dynamic buffer, called the 2 1/2-dimensional sketch. To support the adequacy of the Marr-Poggio model of human stereo vision, the implementation was tested on a wide range of stereograms from the human stereopsis literature. The performance of the implementation is illustrated and compared with human perception. Also statistical assumptions made by Marr & Poggio are supported by comparison with statistics found in practice. Finally, the process of implementing the theory has led to the clarification and refinement of a number of details within the theory; these are discussed in detail.

Dietary Hypotheses and Human Evolution

Abstract: Hypotheses concerning the diet of early hominids have played an important role in discussions on human evolution. Three investigations have helped define the extent to which dietary hypotheses may be taken and still be testable. Comparative anatomy is a fairly coarse approach, which despite convergences allows only the most specialized diets to be ruled out. A biomechanical analysis makes it clear that the changes in jaw and tooth form are subtle and outside the resolution given by present understanding of cranial function. Analysis of the microscopic tooth wear of extant species has been carried out. Major dietary types can be distinguished by their microwear. The microwear on fossil hominids appears to rule out certain diets that have been proposed for them.

Calcium control of exocytosis and endocytosis in bovine adrenal medullary cells.

Abstract: Experimental analysis of the mechanisms of exocytosis and endocytosis has hitherto been hampered by the inaccessibility of the intracellular sites at which they are controlled. We have recently developed a technique that overcomes this problem. Cells are subjected to intense electric fields of brief duration; this renders the plasma membrane permeable without impairing its ability to participate in exocytosis and endocytosis. Working with `leaky' bovine adrenal medullary cells, catecholamine release has a rather specific requirement for Mg-ATP, is activated by micromolar concentrations of ionized Ca and can be inhibited by Mg, detergents, trifluoperazine, high osmotic pressure and various anions. The mechanism of activation by Ca is discussed in some detail.

The localization of motoneurons supplying the hindlimb muscles of the mouse

Abstract: The method of retrograde axonal transport of horseradish peroxidase (HRP) has been used to localize the motoneurons that innervate the mouse hindlimb musculature. Motoneurons were labelled following either intramuscular injection of an HRP solution or application of HRP to the cut end of a muscle nerve. When intramuscular injection was used the nerves to adjacent muscles were cut and deflected from the injection site to prevent motoneurons projecting to these muscles being labelled with HRP. For some muscles this procedure was inadequate since the nerves to adjacent muscles were too short to enable adequate deflexion. The motoneurons projecting to these muscles were labelled by the method of cut nerve exposure. The motoneurons that project to a single muscle or a group of muscles were organized as longitudinal columns. The positions of such motonuclei within the lateral motor column were similar in different animals for any given muscle or muscle group. Motoneurons innervating the anterior and medial femoral muscles were located in spinal segments L1 and L2. Motoneurons innervating the remaining hindlimb muscles were found in segments L3-L5. Topographic relationships between muscle motonuclei were in general found to be similar to those described for the cat. The principal differences to be noted between the two species were that the adductors motonucleus did not overlap with the hamstrings motonucleus in the mouse. Also the motonuclei supplying the deep flexors of the crural musculature and intrinsic musculature of the foot were located more ventrally relative to the posterior crural motonucleus in the mouse as compared to the cat. Consideration of muscle homologies between vertebrate classes enabled comparisons of the localization of motonuclei between the mouse and the other species studied. It was found that topographical relations between motonuclei were similar in all the species so far studied. There was no absolute correlation between the rostrocaudal position of a motonucleus and the position in the hindlimb of the muscle that it innervated. In general, motonuclei innervating muscles derived from the dorsal muscle mass were located lateral to motonuclei innervating muscles derived from the ventral muscle mass. Furthermore, within each muscle mass there is a relationship between rostrocaudal position of a motonucleus and the anteroposterior position of the muscle it supplies. Thus there is a relation between position of a motonucleus within the spinal cord and the derivation from the embryonic muscle mass of the muscle that it supplies.

Components of the Mental Lexicon

Abstract: This paper sets out to identify, in information-processing terms, the elementary functional components of the mental lexicon and their interrelations. In particular it is concerned with the independent status of lexical codes for written and spoken language, and their relations to each other and to a language-free cognitive representation. Our evidence is based on the performance of language transcoding tasks (such as reading aloud or writing to dictation) in brain-damaged adult subjects. We review evidence for the functional independence of non-linguistic, cognitive representations, and for word-specific, lexical codes in both phonological and orthographic form. The data rule out the hypothesis of a modality-free or abstract lexicon mediating communication between lexical and cognitive representations. The data also reject the dominance of phonological over orthographic codes in access to and from word meanings. We can find no satisfactory evidence for independent lexicons used in language reception and language production. Different syntactic word-classes are known to cause greater or less difficulty for various categories of aphasic patients. We show, however, that the relative difficulties of this or that word-class change according to the input and output modalities involved. We consider two theoretical accounts of these differences. Our discussion raises questions about the boundary between psychological mechanisms that are, properly, mechanisms of language, and those that, while necessary for the intelligent use of language, are themselves language-independent.

On the three-dimensional structure and catalytic mechanism of triose phosphate isomerase

Abstract: The essence of evolution is change in the linear sequence of nucleotides of the DNA, giving, rise to altered or new genes and their corresponding products. It is these changes that determine the complex three-dimensional structure of proteins and their conglomerates. Thus, to seek an understanding of protein evolution we must first seek to understand evolution at the DNA level and the constraints that this may place on the evolution of protein structure. In this paper I shall review briefly some of the implications of recent advances in our knowledge of gene structure at the DNA level for the understanding of gene organization and its evolution (see also Bodmer 1981).

On pattern formation mechanisms for lepidopteran wing patterns and mammalian coat markings.

Abstract: The patterns on wings of Lepidoptera can be generated with a few pattern elements, but no mechanism has been suggested for producing them. I consider two of the basic patterns, namely, central symmetry and dependent patterns. A biochemically plausible model mechanism is proposed for generating major aspects of these patterns, based on a diffusing morphogen that activates a gene or colour-specific enzyme in a threshold manner to generate a stable heterogeneous spatial pattern. The model is applied to the determination stream hypothesis of Kuhn & von Engelhardt (Wilhelm Roux Arch. Entw Mech. Org. 130, 660 (1933)), and results from the model compared with their microcautery experiments on the pupal wing of Ephestia kuhniella. In the case of dependent patterns, results are compared with patterns on specific Papilionidae. For the same mechanism and a fixed set of parameters I demonstrate the important roles of geometry and scale on the spatial patterns obtained. The results and evidence presented here suggest the existence of diffusion fields of the order of several millimetres, which are very much larger than most embryonic fields. The existence of zones of polarizing activity is also indicated. Colour patterns on animals are considered to be genetically determined, but the mechanism is not known. I have previously suggested that a single mechanism that can exhibit an infinite variety of patterns is a candidate for that mechanism, and proposed that a reaction-diffusion system that can be diffusively driven unstable could be responsible for the laying down of the spacing patterns that generates the prepattern for animal coat markings. For illustrative purposes I consider a practical reaction mechanism, which exhibits substrate inhibition, and show that the geometry and scale of the domain (part of the epidermis) play a crucial role in the structural patterns that result. Patterns are obtained for a selection of geometries, and general features are related to the coat colour distribution in the spotted Felidae, giraffe, zebra and other animals. The patterns depend on the initial conditions, but for a given geometry and scale are qualitatively similar, a positive feature of the model and a necessary model attribute in view of the pattern individuality on animals of the same species.

The Emergence of Man in Africa and Beyond

Abstract: Transformations of the nervous, masticatory, locomotor and manipulatory systems, with accompanying functional changes, marked the emergence of the Hominidae and of the genus Homo . Various systems evolved in a mosaic fashion. The manipulatory and locomotor systems hominized early, probably with the emergence of the hominid family. Major changes of brain form and size occurred later, with the emergence of Homo . The functional counterpart of brain change is often thought of as cultural behaviour (material and non-material). However, the evolution of a propensity for culture would not alone have ensured the perpetuation of culture. Only an advanced mechanism for social transmission could have handed on the culture itself: evolving speech was such an advanced mechanism. Direct and indirect evidence suggests that emergent Homo (though not Australopithecus ) possessed at least the rudiments of a speech faculty about 2 Ma ago. Thereafter, biological and cultural evolution were in a positive reciprocal feedback relationship. In this autocatalytic system, speech was a crucial component: by making possible spoken teaching and learning, it enabled culture to evolve beyond what could be conveyed by grunts, snorts or nudges.

The polar transport of auxin and vein patterns in plants

Abstract: The hormone auxin is transported through many plant tissues with a definite velocity. It is thought that certain channels, or pumps, located at the basal ends of cells, are responsible for the hormone's transport. It is also known that auxin will induce veins when applied to suitable tissues. T. Sachs has suggested that it is the flow of the hormone that induces vessels. He suggests that discrete strands form because the transport capacity of a pathway increases with the flux that that pathway carries, leading to a canalization of flow. I cast this in the form of a more specific hypothesis: I suppose the permeability for the transport of auxin through the basal plasmalemma of a cell (by means of whatever kind of pump or channel) to increase with flux. I then show that discrete veins will form provided that the transport permeability increases rapidly enough with flux, and provided that the movement of auxin is not too polar, in the sense that there is a substantial amount of diffusive movement of auxin in addition to polar transport. The same hypothesis offers an explanation for the loops of veins found under certain conditions.

The role of ionic currents in establishing developmental pattern

Abstract: This paper reviews a theory of pattern establishment and pattern restoration by endogenous ionic currents. These currents are supposed to be generated by a certain separation of ion leaks and ion pumps in cell membranes. In so far as these currents act back to further this separation, they would be part of a regenerative process that initially establishes positional values. Later in development, particularly in epimorphic regeneration, when positional values are restored or extended, these currents are supposed to leak through sites of discontinuity in such values and thus trigger growth. This paper also reviews the factual evidence for this view: evidence that developmental currents are, indeed, very widespread; evidence in a few cases, particularly in Cecropia follicles and in wounded cavy skin, that they can generate substantial voltage differences or gradients; evidence that comparable artificial fields can move charged macromolecules along cell membranes and polarize cell growth; and direct evidence in a few case, particularly fucoid eggs, Cecropia follicles and regenerating amphibian limbs, that ion currents do, in fact, act back to direct or further development. The paper also presents a particular theory, based upon ionic currents, of the reversal of thyroid cell polarity by serum.

Embryonic growth and innervation of rat skeletal muscles. I. Neural regulation of muscle fibre numbers.

Abstract: Individual rat embryos were chronically paralysed during development in utero by insertion of small slow-release capsules containing tetrodotoxin (TTX). In others, motoneurons were destroyed by individual injections of $\beta$ -bungarotoxin ( $\beta$ -BTX). These embryos were compared with normal controls so that the effects of innervation and of use on embryonic development of skeletal muscles could be defined. The use of Sirius Red to stain muscle basal lamina made it possible to count muscle units (one or more myotubes contained within a single basal lamina sheath) with light microscopy. (i) In left hemidiaphragm muscles from control embryos about 20% of the adult number of muscle units was present on day 17 of gestation, the time at which formation of secondary myotubes was first observed, and about 70% at birth. New units continued to form during the first postnatal week. (ii) Skeletal muscles still formed following destruction of muscle innervation at day 14 of gestation, although at day 21 of gestation (birth) they were smaller and thinner than controls. The number of units in these muscles remained constant from day 18 through day 21 of gestation, and was similar to that in day 17 controls. No secondary myotubes were seen histologically. (iii) Following destruction of motoneurons at day 16, 17 1/2 or 18 of gestation, new muscle units continued to form for about 1 1/2 d, indicating that the temporary presence of innervation affected later development of muscle tissue. The denervated muscles contracted spontaneously during this time. (iv) TTX-induced paralysis from day 16 onwards was at least as effective as denervation in inhibiting the generation of new muscle units. There was a dose-response relation in the ability of TTX to reduce the generation of new muscle units beyond the amount necessary to paralyse nerves, and it is suggested that the higher concentrations also blocked myogenic contractions. High doses of TTX stopped generation of new units within half a day. (v) Even the slightest degree of recovery from paralysis was accompanied by restoration of the proper number of muscle units. This shows that there is no `critical period' for muscle fibre development within the last five prenatal days in the rat. (vi) It is concluded that development of primary myotubes is autonomous, but that development of secondary myotubes, which form the majority of muscle cells in the adult, is dependent on innervation. Even when nerves are present, new muscle fibres cannot be generated in the absence of muscle contraction.

Evolution of hominid bipedalism and prehensile capabilities

Abstract: In this paper, I present an updated version of the hylobatian model for the proximate ancestors of the Hominidae. The hylobatians are hypothesized to have been relatively small creatures that were especially adapted for vertical climbing on tree trunks and vines and for bipedalism on horizontal boughs. They were no more disposed toward suspensory behaviours than are modern chimpanzees and bonobos. According to this evolutionary scenario, bipedalism preceded the emergence of the Hominidae. The earliest hominids would be recognized as diurnally terrestrial bipeds that stood with full extension of the knee joints and walked with greater extension of the lower limbs than is common in non-human primates that are induced to walk bipedally on the ground. The wealth of hominid fossils from the Hadar Formation, Ethiopia, and the Laetolil Formation, Tanzania, are generally compatible with the hylobatian model. They show that by ca. 4 Ma B.P. habitually terrestrial, bipedal hominids had evolved from arboreal ancestors. The Hadar hominids had curved fingers and toes, strong great toes and thumbs, and other features that suggest that they were rather recently derived from arboreal hominids and that they probably continued to enter trees, perhaps for night rest and some foraging. The hominid hand bones from Hadar evince no features that are distinctly related to knuckle-walking. They relate neatly to counterparts in the hand of O.H. 7, a specimen that was found with stone tools. However, there is no evidence that the Hadar hominids of 3 Ma ago engaged in tool behaviour.

Glyceraldehyde-3-phosphate dehydrogenase

Abstract: Conflicting experimental evidence of the pathway of catalysis for the enzyme from rabbit, pig and lobster muscle tissues is reviewed. Transient kinetic studies with the enzyme from rabbit muscle are presented. The results are shown to be consistent with the double-displacement mechanism of catalysis originally proposed by Segal & Boyer (1953). The rate constant for combination of the aldehyde form of the substrate with the NAD+ complex of the enzyme is about 3 X 10(7) M-1 S-1, and for all four subunits of the molecule the rate constant for hydride transfer in the ternary complex formed is greater than 10(3) S-1, consistent with their simultaneous participation in catalysis. Recent steady-state kinetic studies with the rabbit muscle enzyme, in contrast to earlier studies, also provide evidence to support the Segal-Boyer pathway if the kinetic effects of the negative cooperativity of NAD+ binding are taken into account. Experimental data for the binding of NAD+ to the enzyme from muscles and from Bacillus stearothermophilus, and their interpretations, are also briefly reviewed. The information currently available from X-ray crystallography regarding the structures of holoenzyme and apoenzyme from B. stearothermophilus and lobster muscle is outlined.

Zinc and Gene Expression

Abstract: During the last two decades, zinc has been shown to be a functionally essential component of more than 120 enzymes and concurrently has been recognized as indispensable to normal cellular growth, development and differentiation of all species. However, the manner in which it exerts its effects are still unknown. Studies of the phytid E. gracilis have demonstrated that zinc is critical for transcription and translation, affecting the metabolism of RNA polymerases, mRNA and proteins, among which are histones and arginine-containing peptides. These and other data lead to the suggestion that this element has a fundamental role in gene activation and/or repression, accounting for its pervasive effects on cell metabolism.

The speaker’s linearization problem

Abstract: The process of speaking is traditionally regarded as a mapping of thoughts (intentions, feelings, etc.) onto language. One requirement that this mapping has to meet is that the units of information to be expressed be strictly ordered. The channel of speech largely prohibits the simultaneous expression of multiple propositions: the speaker has a linearization problem - that is, a linear order has to be determined over any knowledge structure to be formulated. This may be relatively simple if the informational structure has itself an intrinsic linear arrangement, as often occurs with event structures, but it requires special procedures if the structure is more complex, as is often the case in two- or three-dimensional spatial patterns. How, for instance, does a speaker proceed in describing his home, or the layout of his town? Two powerful constraints on linearization derive, on the one hand, from ‘mutual knowledge’ and, on the other, from working memory limitations. Mutual knowledge may play a role in that the listener can be expected to derive different implicatures from different orderings (compare ‘she married and became pregnant’ with ‘she became pregnant and married’). Mutual knowledge determinants of linearization are essentially pragmatic and cultural, and dependent on the content of discourse. Working memory limitations affect linearization in that a speaker’s linearization strategy will minimize memory load during the process of formulating. A multidimensional structure is broken up in such a way that the number of ‘return addresses’ to be kept in memory will be minimized. This is attained by maximizing the connectivity of the discourse, and by backtracking to stored addresses in a firstin-last-out fashion. These memory determinants of linearization are presumably biological, and independent of the domain of discourse. An important question is whether the linearization requirement is enforced by the oral modality of speech or whether it is a deeper modality-independent property of language use.

The Arthropod Sidneyia Inexpectans, Middle Cambrian, Burgess Shale, British Columbia

Abstract: Old and new specimens of Sidneyia inexpectans have been studied and are accompanied by explanatory drawings and photographs. New reconstructions of the animal are given together with a three-dimensional model. The body consisted of a cephalon with a long backwardly directed doublure, a thorax of nine articulating somites, abdomen with cylindrical exoskeleton of two or three somites and a telson. A caudal fan was formed by a pair of uropods articulating at the posterior margin of the last abdominal somite. The cephalon had stalked eyes and preoral antennae but no walking or grasping appendages. The first four somites of the thorax had paired uniramous, prehensile walking legs attached to the body by broad coxae with spiny gnathobases. The coxae were smaller on the five posterior thoracic somites and the paired appendages were biramous, each bearing a gill supported on a flap attached at its proximal end to the first podomere of the leg. The coxa-body attachment resembles that of modern merostomes and is in advance of trilobites. Evidence suggests that Sidneyia was a bottom-living, carnivorous animal eating larger and harder food than trilobites. Gut contents include ostracodes, hyolithids, small trilobites and phosphatic debris. Sidneyia is the earliest known form which could be an ancestor to merostomes, but its body plan and absence of chelicera distinguishes Sidneyia from this group. The holotype of Amiella ornata Walcott, 1911 is reinterpreted and its synonomy with S. inexpectans is confirmed.

The pathological basis and microanatomy of occlusive thrombus formation in human coronary arteries

Abstract: Myocardial necrosis, usually called infarction, occurs in different patterns. A common form is necrosis of one segment of the left ventricle, i.e., anterior, septal, lateral or posterior. This regional infarction is consistently associated with an acute occlusive thrombosis of the artery supplying that region. Diffuse necrosis involving the whole circumference, usually the subendocardial zone, of the ventricle is not consistently associated with thrombi. Occlusive thrombi identified in post-mortem coronary arteriograms have been reconstructed in their entirety from serial sections at 150 μm intervals. Most occlusive thrombi were found to be associated with a dissection track into the intima at an atheromatous plaque. The break into the plaque usually extended over several millimetres, often in spirals, so that a mass of thrombus within the plaque compressed the original lumen. Previous accounts of plaque rupture or cracking greatly underestimated the magnitude of the dissection of blood into the intima.

Design of Glycolysis

Abstract: The design of the glycolytic pathway resulting from the continuous refinement of evolution is discussed with regard to three aspects. 1. Functional and structural properties of individual enzymes. The catalytic constants of the glycolytic enzymes are remarkably optimized; the turnover numbers are within one order of magnitude. The same is true for the molarities of catalytic centres in the cytosol, as is noted for yeast. Functional properties of the enzymes are reflected in their tertiary and quaternary structures. 2. Regulatory mechanisms of single enzymes. A classification of the various types of enzymic control mechanisms operating in the glycolytic pathway is given. In addition to the usual Michaelis-Menten saturation kinetics and the various types of inhibition there is control by positive and negative effectors based on oligomeric structures (fast acting, fine control) as well as regulation by chemical interconversion structures (fast acting, fine control) as well as regulation by chemical based on enzymes cascades (slow acting, very effective). 3. Functional and regulatory mechanisms of the whole glycolytic reaction pathway. A prominent feature is the high enzyme:substrate ratio, which guarantees fast response times. However, a quantitative treatment of the overall kinetics is limited by an incomplete knowledge of the enzymes' dynamic and chemical compartmentation as well as some of their control properties. From an analysis of the oscillatory state, certain control points in the glycolytic chain can be located that coincide with major branching points to other metabolic pathways. These points are controlled by fast-acting cooperative enzymes that operate in a flip-flop mechanism together with the respective antagonistic enzymes, preventing futile cycles. The gating enzymes leading to the glycogen store and the citric acid cycle are of the slow-acting but very effective interconvertible type. The combination of all the complex and intricate features of design yields a glycolytic network that enables the cell to respond to its various metabolic needs quickly, effectively and economically.

Plesiosaur Ancestors from the Upper Permian of Madagascar

Abstract: Numerus well preserved fossils from the Upper Permian of Madagascar are structurally intermediate between primitive diapsid reptiles and nothosaurs and plesiosaurs. Claudiosaurus germaini (gen.nov., sp.nov.) is similar in its basic anatomy to eosuchian reptiles such as Thadeosaurus colcanapi (gen.nov., sp.nov.), but the absence of a lower temporal bar and the closure of the palate are characteristics of sauropterygian reptiles. Claudiosaurus shows the initiation of aquatic adaptations in the proportions and reduced ossification of the carpus and manus. A third pair of sacral ribs is partially incorporated. The small size of the skull, the nature of the palate and marginal dentition and the long neck are suggestive of aquatic feeding habits. Claudiosaurus does not, however, show the specific adaptations for aquatic locomotion seen in either nothosaurs or plesiosaurs. Even the most primitive known species of nothosaurs and plesiosaurs are too specialized in the postcranial skeleton for direct comparison with Claudiosaurs , although the similarities to the skull roof of primitive nothosaurs are very close. The configuration of the cheek in nothosaurs almost certainly resulted from the loss of the lower temporal bar from a pattern like that of Youngina , rather than from the ventral emargination of the cheek. The nature of the Upper Permian sediments in Madagascar and the tectonic environment of their deposition indicate accumulation in deep rift valleys, some parts of which were open to the sea. The presence of oolites replaced with collophane suggests a rich phosphate source such as deep marine upwellings. Similar upwellings of phosphate have also been associated with the evolution of the marine iguanas on the Pacific coast of South America. The concept of the derivation of nothosaurs from protorosaurs or araeosceloids may be traced to misunderstandings of the nature of the cheek in both Nothosaurus and Protorosaurus . Araeoscelis , despite the possession of a solid cheek, is closely related to Petrolacosaurus , an ancestral diapsid.

Allometry of Bat Wings and Legs and Comparison with Bird Wings

Abstract: Allometric equations on wing dimensions versus body mass are given for eight species of megabats and 76 species of microbats, on forearm length versus mass for 14 species of megabats and 90 species of microbats, and on lower leg length versus mass for 11 species of megabats and 45 species of microbats. Megabats have, on average, shorter wing span, small wing area, higher wing loading and lower aspect ratio than have frugivorous microbats and the insectivorous vespertilionids of similar mass. Vespertilionids have the longest span, largest wing area and lowest wing loading in relation to body mass of the bat groups for which regression lines were calculated (megabats, frugivorous microbats, vespertilionids, molossids), characteristics that are important for slow flight and manoeuvrability for insect capture. Molossids have the highest wing loading of the groups. There is a weak tendency towards higher aspect ratio for larger bats than for smaller ones (positive slope). The slopes for most characters fit geometric similarity or have confidence intervals including the value for geometric similarity. Only in three cases does the slope lie nearer that for elastic similarity: for the forearm in nycterids and emballonurids and the lower leg length in molossids. Also in these cases the confidence intervals are wide and include the value for elastic similarity and that for geometric similarity as well. In megabats the slope for the lower leg length is much steeper than for geometric similarity. The slope for the forearm length is rather similar to that for wing span in the various groups. Megabats and frugivorous microbats have rather similar slopes for all the characters measured, but differ from the other groups only in wing area, wing loading and aspect ratio. The two frugivorous bat groups also have about the same elevation of the regression lines for aspect ratio and forearm length. Megabats and frugivorous microbats thus show a close convergence in wing area, wing loading, aspect ratio and forearm length. The regression equations provide `norms' for the respective bat groups. Those species that deviate 10% or more from the mean trends for wing measurements are divided into different groups, based on the wing's aspect ratio and loading. Bats with low aspect ratio wings usually have large pinnae, which improve the ability to discover small objects such as insects on leaves. Families or species of bats with wings of low aspect ratio are, for instance, Megadermatidae, Nycteridae, Rhinolophus ferrumequinum (Rhinolophidae), Chrotopterus auritus (Phyllostomidae) and Plecotus (Vespertilionidae). The group with average aspect ratio wings contains bats with different kinds of flight style and foraging behaviour, for instance many pteropodids, phyllostomids and vespertilionids. Bats with high aspect ratio wings are, for instance, Molossidae, Rhynchonycteris naso (Emballonuridae) and Nyctalus leisleri (Vespertilionidae). The regression lines for wing span, area and loading in megabats lie almost in the region of the lines for Greenewalt's (1975) passeriform group, whereas the span and area for vespertilionid bats are larger and the wing loading much smaller than for most birds of similar mass. Molossid bats have a larger relative wing span and aspect ratio than have most birds, and a wing area and loading similar to those of small birds of the passeriform group. Vespertilionid bats have about the same aspect ratio as birds of the passeriform group, whereas megabats have somewhat lower ratios. Molossid bats show strong convergence with swifts and swallows in foraging behaviour and in wing form. Similar convergences can be found between various vespertilionid bats, flycatchers and swallows.

Positional information and pattern formation.

Abstract: Spatial patterns of cellular differentiation may arise from cells first being assigned a position, as in a coordinate system, and then interpreting the positional value that they have acquired. This interpretation will depend on their genetic constitution and developmental history. Different patterns may thus arise from similar positional fields. The specification of positional value may involve a positional signal, such as the concentration of a diffusible morphogen, but can also depend on how long the cells remain in a particular region, such as a progress zone. Positional values may also be acquired by direct transfer from one cell layer to another, as in directed embryonic induction. Positional value, unlike a positional signal, involves long-term memory, and can be regarded as a type of cell determination. Cells of the same differentiation class may have different positional values and may thus be non-equivalent. Evidence is presented for a signal providing positional information along the antero-posterior axis during chick limb development. This signal has properties similar to those of a diffusible morphogen.

Cell Patterning in Dictyostelium

Abstract: We summarize studies on stalk and spore cell formation in D. discoideum cell monolayers, aimed at revealing factors involved in controlling the prestalk:prespore pattern in this organism. We propose that there are no cell interactions dependent on cell contact per se. Formation of mature stalk cells from isolated amoebae incubated in a buffered salts medium requires only cyclic AMP and a lipid-like factor (DIF) released by cells developing at high density. In addition, a variety of sporogenous mutants can form spores rapidly and efficiently when incubated at low density in tissue culture dishes containing a similar cyclic AMP and salts medium. In some cases spore formation is improved by the addition of one or other of a variety of protective agents such as bovine serum albumin. Wild-type amoebae at low density form prespore cells under the same conditions. We present some evidence that DIF is the activator of prestalk cell formation in a two-component patterning mechanism of the kind proposed by Wolpert et al. (Symp. Soc. exp. Biol. 25, 391-415 (1971)) and Gierer & Meinhardt (Kybernetik 12, 30-39 (1972)). We also provide data indicating that the role of inhibitor is played by ammonia, an idea first mooted by Sussman & Schindler (Differentiation 10, 1-5 (1978)).

Language and the Mind

Abstract: ‘We may usefully think of the language faculty, the number faculty, and others, as “mental organs”, analogous to the heart or the visual system or the system of motor coordination and planning. There appears to be no clear demarcation line between physical organs, perceptual and motor systems, and cognitive faculties in the respects in question.’ (N. Chomsky, Rules and representations , p. 39.) I shall argue that the view thus expressed is open to philosophical criticism that is not adequately rebutted by the lengthy philosophical discussion in Rules and representations . The notion of a mental structure appears to involve a philosophical confusion, one that is sometimes nicknamed the ‘hardware-software fallacy’; but this nickname is unfortunate since the postulated structures appear to be intended to be too ghostly to be hardware and too concrete to be software. Philosophical criticism of the notion of a mental structure does not imply that there cannot be illuminating structural descriptions assignable by grammar to linguistic expressions. But it concerns the characterization of the appropriate vehicle to embody the capacities that are expressed in linguistic performance, and thus the relation between mind and body. If the role of linguistics, like the role of psychology, is described in a way that is free of philosophically inappropriate analogy, then the discipline of linguistics can indeed be regarded as a branch of psychology; for language is the paradigm expression of mind.

The role of syllables in speech processing: infant and adult data

Abstract: An empirical account is offered of some of the constants that infants and adults appear to use in processing speech-like stimuli. From investigations carried out in recent years, it seems that syllable-like sequences act as minimal accessing devices in speech processing. Ss are aware in real time of syllabic structure in words and respond differently to words with the same initial three phonemes if the segmental one is CV/... and the other CVC/.... Likewise, infants seem to be aware that a 'good' syllable must have at least one alternation if it is composed of more than one phoneme. When the segment is only one phoneme long, its status is necessarily somewhere between that of the phoneme and the syllable. An important problem that arises with the syllable is that it is an unlikely device for speech acquisition. Indeed, there are a few thousand syllables and the attribution of a given token to a type is far from obvious. Even if physical invariants for syllables in contexts were to be found, the task facing the child still remains one of sorting thousands of types from many more tokens. Issues concerning acquisition versus stable performance will be addressed to further constrain possible models. In addition, I try to show that even though information processing models are useful tools for describing synchronic sections of organisms, the elements that can account for development will have to be uncovered in neighbouring branches.

The Functional Anatomy of a Permian Dicynodont

Abstract: A specimen of Dicynodon trigonocephalus from the Madumabisa mudstones of Zambia is described. The jaw adductor musculature is reconstructed. It is concluded that two slips of the adductor externus medialis were present. A posterior adductor ran from the quadrate to the medial surface of the lower jaw and a forwardly running muscle may have inserted on the reflected lamina of the angular. The masticatory cycle is much as Crompton & Hotton (1967) proposed, but new interpretations are developed for the movable quadrate and the W-shaped jaw hinge. It is proposed that the horny jaw covering inserted into the longitudinal dentary sulcus. The reflected lamina of the angular is discussed. As well as being a possible site for adductor muscle insertion, it is also suggested that hyoid musculature attached to it. The nasal capsule and cranial nerves and blood vessels are described. The forelimb and pectoral girdle are described and the main muscles reconstructed. It is concluded that the principle movements of the forelimb were protraction and retraction with only a small degree of long axis rotation. Rotation of the glenoid to lengthen the stride may have occurred. Flexion and extension of the lower limb was a particularly powerful action, but in contrast protraction and retraction were not powerful. The pectoral girdle and forelimb are seen as providing a stable and strong base for the massive anterior part of the body, but not generating much locomotory thrust. In contrast the hindlimb is supplied with musculature (the ilio-femoralis) that generates a powerful locomotory thrust by retraction and long axis rotation. The caudi-femoralis is much reduced but the ischio-trochantericus muscle has assumed importance in pulling the femur postero-ventrally. The pubo-ischio-femoralis externus is in a less advantageous position to do this and has assumed a role largely of a ventral adductor. In the axial skeleton, rotation of the head may occur, to a limited extent, between the atlas neural arch and the odontoid. The neck is a flexible region but the neck vertebrae bear strong transverse processes for origin of the extensive musculature that supports the head. However, the trend more posteriorly is to limit flexibility of the vertebral column by more vertically oriented zygapophyses and by ligamentous and muscular ties between vertebrae. The insertions of the longissimus dorsi have been modified from the presumed primitive condition and occupy only the dorsal surface of the transverse process. There are five sacral vertebrae. The centra of the first four are fused. The fifth is free. The anterior and posterior zygapophyses are reduced. The neural spines become longer antero-posteriorly, but very narrow transversely. The sacral ribs are all expanded but not fused either to the ilium or to the centra. The caudal region consists of possibly thirteen vertebrae and is probably complete. Ribs are present up to the fifth caudal vertebra. From vertebra 6 backwards haemal arch facets are seen on the centra. It is concluded that Dicynodon trigonocephalus was a slow-moving herbivore, nevertheless capable of a sustained locomotory effort. It may have avoided predators by crepuscular or cryptic behaviour. The powerful masticatory mechanism of D. trigonocephalus would have allowed exploitation of tough and bulky food sources. The relationships of D. trigonocephalus are discussed. It is possible that it is a close relative of the Permian stock that gave rise to Lystrosaurus.

Structure and activity of phosphoglycerate mutase.

Abstract: The structure of yeast phosphoglycerate mutase determined by X-ray crystallographic and amino acid sequence studies has been interpreted in terms of the chemical, kinetic and mechanistic observations made on this enzyme. There are two histidine residues at the active site, with imidazole groups almost parallel to each other and approximately 0.4 nm apart, positioned close to the 2 and 3 positions of the substrate. The simplest interpretation of the available information suggests that a ping-pong type mechanism operates in which at least one of these histidine residues participates in the phosphoryl transfer reaction. The flexible C-terminal region also plays an important role in the enzymic reaction.

Neuropsychological studies of verbal semantic systems

Abstract: The selective impairment of verbal comprehension in patients with cerebral lesions provides a powerful and direct source of evidence regarding the properties of verbal semantic systems. Word frequency is shown to be an important determinant of performance for both spoken and written word comprehension, indicating that the aphasic patient may have a quantitative reduction in capacity according to task difficulty. But this cannot be an exhaustive account. Evidence for the vulnerability of subordinate as compared with superordinate information is reported, which it is argued indicates that the semantic representations of single words are hierarchical or ordered in their degree of specificity. Finally, evidence is presented for selective impairments of the comprehension of words from particular semantic categories. Double dissociations of deficits of the concrete-abstract and of objects-non-object concepts are reported. It is argued that the verbal semantic meaning systems are categorically organized.