Permeation of membranes by the neutral form of amino acids and peptides: Relevance to the origin of peptide translocation

The flux of amino acids and other nutrient solutes such as phosphate across lipid bilayers (liposomes) is 10⁵ slower than facilitated inward transport across biological membranes. This suggests that primitive cells lacking highly evolved transport systems would have difficulty transporting sufficient nutrients for cell growth to occur. There are two possible ways by which early life may have overcome this difficulty: (1) The membranes of the earliest cellular life-forms may have been intrinsically more permeable to solutes; or (2) some transport mechanism may have been available to facilitate transbilayer movement of solutes essential for cell survival and growth prior to the evolution of membrane transport proteins. Translocation of neutral species represents one such mechanism. The neutral forms of amino acids modified by methylation (creating protonated weak bases) permeate membranes up to 10¹⁰ times faster than charged forms. This increased permeability when coupled to a transmembrane pH gradient can result in significantly increased rates of net unidirectional transport. Such pH gradients can be generated in vesicles used to model protocells that preceded and were presumably ancestral to early forms of life. This transport mechanism may still play a role in some protein translocation processes (e.g., for certain signal sequences, toxins and thylakoid proteins) in vivo.Abbreviations LUV large unilamellar vesicle - pH transmembrane pH gradient - PAH polyaromatic hydrocarbon Correspondence to: A.C. Chakrabarti     

Arterial pulsation-driven cerebrospinal fluid flow in the perivascular space: a computational model

This study was conducted to determine whether local arterial pulsations are sufficient to cause cerebrospinal fluid (CSF) flow along perivascular spaces (PVS) within the spinal cord. A theoretical model of the perivascular space surrounding a "typical" small artery was analysed using computational fluid dynamics. Systolic pulsations were modelled as travelling waves on the arterial wall. The effects of wave geometry and variable pressure conditions on fluid flow were investigated. Arterial pulsations induce fluid movement in the PVS in the direction of arterial wave travel. Perivascular flow continues even in the presence of adverse pressure gradients of a few kilopascals. Flow rates are greater with increasing pulse wave velocities and arterial deformation, as both an absolute amplitude and as a proportion of the PVS. The model suggests that arterial pulsations are sufficient to cause fluid flow in the perivascular space even against modest adverse pressure gradients. Local increases in flow in this perivascular pumping mechanism or reduction in outflow may be important in the etiology of syringomyelia.     

Arterial Pulsation-driven Cerebrospinal Fluid Flow in the Perivascular Space: A Computational Model

This study was conducted to determine whether local arterial pulsations are sufficient to cause cerebrospinal fluid (CSF) flow along perivascular spaces (PVS) within the spinal cord. A theoretical model of the perivascular space surrounding a "typical" small artery was analysed using computational fluid dynamics. Systolic pulsations were modelled as travelling waves on the arterial wall. The effects of wave geometry and variable pressure conditions on fluid flow were investigated. Arterial pulsations induce fluid movement in the PVS in the direction of arterial wave travel. Perivascular flow continues even in the presence of adverse pressure gradients of a few kilopascals. Flow rates are greater with increasing pulse wave velocities and arterial deformation, as both an absolute amplitude and as a proportion of the PVS. The model suggests that arterial pulsations are sufficient to cause fluid flow in the perivascular space even against modest adverse pressure gradients. Local increases in flow in this perivascular pumping mechanism or reduction in outflow may be important in the etiology of syringomyelia.     

Predation intensity in a rocky intertidal community

Summary The predation intensity exerted by populations of the gastropod Thais lapillus at different study areas in the rocky intertidal community of New England is unrelated to predator density. Specifically, very similar intensities are exerted by populations differing in density by at least an order of magnitude. Predation intensity is, in part, a joint function of individual rates of prey consumption and various environmental characteristics. Major factors potentially affecting the individual feeding rates of Thais are (1) prey abundance and productivity, (2) other predators, (3) canopy-forming algae, (4) wave shock, (5) desiccation and (6) snail phenotype and/or history. The effects of the first two of these factors seem unimportant. The effects of the latter 4 on prey consumption rates were studied by estimating field feeding rates of snails held in cages with prey in microhabitats which were characterized by one of two alternative states of each factor. For example, microhabitats could be exposed or protected, at higher or lower levels in the mid intertidal, or under a canopy or not. In addition, exposed-phenotype or protected-phenotype snails were used in each experiment.All of factors (3) to (6) had statistically significant effects except wave shock. The latter would probably also have had a significant effect if the experiments had been performed in the stormier part of the year as well as late summer. The results indicate that sparse populations of Thais can exert intense predation pressure on their prey if they are in protected sites covered with a dense canopy (i.e. in cool, moist habitats in calm waters). Areas with sparser canopy (i.e. greater desiccation stress) and more severe wave shock or both apparently reduce average feeding rates of snails. This appears to explain the paradoxical lack of correlation between predation intensity and snail density.An unexpected result with potentially major implications is the nonlinear response of Thais feeding rates to combinations of factors (3) to (6). Four-way analyses of variance on experiments at exposed and protected sites indicate that 7 of 14 1st-order interactions, 2 of 8 2nd-order interactions, and even 1 of 2 3rd-order interactions are statistically significant. These results suggest that individual predators cannot be assumed to be identical, and that socalled higher order interactions cannot be safely ignored in models of interacting multi-species systems. Hence, it appears that to obtain a thorough understanding of the organization of natural communities, both field and theoretical ecologists alike should begin to grapple with such complexities of nature rather than ignore them.     

Behavioral rhythms in the Nilgiri langur,Presbytis johnii

Eighteen complete daily profiles of 16 behaviors were compiled for three troops of Nilgiri langurs from Periyar Sanctuary in South India. Daily behavioral peaks from these profiles were tested for their associations with each other. Daily rhythms of feeding and resting showed four to eight peaks per day. When all observation days were lumped, however, a bimodal curve resulted as noted by most authors who have studied activity rhythms. The major daily behavioral associations were as follows: (1) feeding occurred during high rates of movement although not during major movement periods; (2) measures of troop movement were interrelated and occurred when troop members were dispersed; (3) movement was related to such active behaviors as whooping displays, coughing, juvenile whining, urinating, and scratching; (4) the above active behaviors showed some positive associations with each other; (5) play, grooming, and scratching showed positive associations with each other; and (6) rest periods were primarily periods of close social contact when grooming occurred and juveniles were with their mothers.     

Detecting surface‐feeding behavior by rorqual whales in accelerometer data

The movement of marine animals feeding at the sea surface is restricted by wave drag and a reduction in propulsive efficiency. Many rorqual whale species lunge feed at the surface, yet existing methodologies for detecting lunges in accelerometer data have not been applied to surface‐feeding behavior. Our study aimed to develop a method to detect surface‐feeding behavior in accelerometer data and in doing so, determine whether wave drag influences the detection of surface‐feeding behavior. A new acceleration parameter is described that considers the forward acceleration of the animal relative to its pitch. The new parameter, along with information on the deceleration and pitch angle, was then used in an automatic lunge detecting algorithm followed by a visual classification method that detected approximately 70% of the lunges observed during focal follow sampling. The forward acceleration of lunges decreased significantly with increasing proximity to the surface. This lower acceleration at the surface may influence the ability to detect lunge feeding behavior close to the surface. Future research should attempt to determine the cause of this relationship, which may be the influence of changes in the forces acting on the whale or behavioral flexibility by the whale.     

A mechanism for the evolution of altruism among nonkin: positive assortment through environmental feedback

The evolution of altruism often requires genetic similarity among interactors. For structured populations in which a social trait affects all group members, this entails positive assortment, meaning that cooperators and noncooperators tend to be segregated into different groups. Several authors have claimed that mechanisms other than common descent can produce positive assortment, but this claim has not been generally accepted. Here, we describe one such mechanism. The process of "environmental feedback" requires only that the cooperative trait affects the quality of the local environment and that individuals are more likely to leave low-quality than high-quality environments. We illustrate this dynamic using an agent-based spatial model of feeding restraint. Depending on parameter settings, results included both positive assortment (required for the evolution of altruism) and negative assortment (required for the evolution of spite). The mechanism of environmental feedback appears to be a general one that could play a role in the evolution of many forms of cooperation.     

Periodic activity of the cortical cytoskeleton of the lymphoblast: modelling by a reaction-diffusion system

The cytoplasm of cultured human lymphoblasts has a particular organization. A ring formed by actin and myosin delimits a region of the cell where an active membrane veil forms. Depolymerization of the microtubular cytoskeleton releases this ring, which then oscillates between the two poles of the cell. This periodic movement has been studied and described with great precision. We have used these experimental results to model the oscillation of the ring. We have constructed a system of reaction—diffusion equations designed to simulate the behaviour of the actin and have considered three variables representing, respectively, the proteins involved in actin nucléation, F-actin bound to the plasma membrane and free F-actin, pan of which is assumed to constitute the ring, together with myosin. There are two types of results. First, from a theoretical point of view, we have succeeded in simulating a perfect back-and-forth movement of a wave. Second, this model suggests that actin alone, by virtue of its intrinsic properties, can generate an oscillatory movement within the cell, without the need for other oscillators.     

Evolutionary changes in the timing of gut morphogenesis in larvae of the marine annelid Streblospio benedicti

The planktonic larvae of marine invertebrates are diverse in their nutritional modes, suggesting that evolutionary transitions in larval nutritional mode have been frequent. One approach to identifying the developmental changes that play important roles in such transitions is to compare "intermediate" larval forms to closely related larvae representative of their common ancestor. Here we make such a comparison between obligately planktotrophic and facultatively feeding larvae of the poecilogonous polychaete annelid Streblospio benedicti. We used feeding experiments to show that the derived, facultatively feeding larvae of this species develop the ability to feed at a later developmental stage (five muscle bands) than planktotrophic larvae (two to three muscle bands). This delay in the onset of feeding ability does not appear to be caused by delay in the formation of particle capture structures, but instead by delay in the development of a continuous, functional gut. These observations are consistent with the hypothesis that evolutionary increases in egg size in annelids lead predictably to heterochronic delays in gut development, and hence to transitions in larval nutritional mode.     

Some aspects of feeding in thecosomatous pteropod molluscs

Various aspects of the feeding behavior of thecosomatous pteropods of the families Cavoliniidae and Cymbuliidae have been examined both in the field with SCUBA, and in the laboratory. It was found that the family Cavoliniidae employs a ciliary-mucus feeding mechanism in which the mantle cavity water current and pallial gland are of primary importance in food collection. The feeding mechanism is discussed, as well as related aspects including buoyancy control, particle size range of natural diets, oxygen consumption, and water filtration rates. It is concluded that Cavolinia must have a high efficiency of retention and assimilation of available carbon.     

纺锤水蚤摄食生态学研究进展

纺锤水蚤(Acartia)是温带、亚热带近岸水域优势的小型桡足类,在区域生态系统的物质循环与能量流通中起着重要作用。综述了国内外对纺锤水蚤食性、摄食机制、摄食影响因子及摄食转化效率的研究工作:纺锤水蚤为杂食性,偏爱高营养的动物性饵料,摄食浮游植物和微型浮游动物时分别采取滤食策略和伏击策略,摄食过程受到自身生理状态和环境因子的共同调节。其对食物的利用效率约为60%,其他40%通过Sloppy feeding和排泄等方式以溶解形式释放到海水中。目前摄食研究多以实验模拟为主,自然现场研究手段有限,分子生物学技术有望促进认识其在自然生态系统中的地位与作用。     

The use of feeding noises to determine the algal foods being consumed by individual intertidal molluscs

Summary To study the diets of individual animals in the context of intraspecific resource partitioning, it is desirable to detect what individuals are eating without disturbing them. Animals such as slow-moving molluscs on two-dimensional algal foods would be convenient to study, but the mouth is usually difficult to see, especially with limpets. However, one can often hear how an herbivorous mollusc is feeding. Even when the mouth region can be checked for feeding movement, feeding noises can indicate to what degree a mollusc is licking microscopic material off the surface of a plant versus biting into the plant, though licking microscopic material off the plants seems to be rare. Noises also indicate the food's texture, identifying the food species when several different algae are near the mollusc's mouth. Comparing various molluscan taxa, differences in radular structure and movement are associated with different feeding noises, even while different molluscs are eating the same alga. Sound thus aids in specifying which species are feeding where molluscs are close together.Feeding is most common on wet surfaces at night. While the molluscs are above water or less than 5 cm deep in calm water, several listening methods are useful after some practice. Even the unaided ear can hear emerged molluscs rasping resonant kelps. One can detect rasping by molluscs greater than 1 cm in length by gently contacting the alga closest to the mouth with a stethoscope or with a gum rubber tube sealed against one's ear. A cassette tape recorder with a contact microphone and headphones is useful for both emerged and submerged animals. Representative feeding noises have been documented using oscillograms from tape recordings. Analogous sounds in both terrestrial and marine environments can be useful in numerous behavioral studies.     

Feeding Mechanisms in Sharks

Although many sharks have a rather general vertebrate body plan,they display a number of specializations for feeding that beliethe notion that they are "primitive." These specializationsinclude a battery of highly developed exteroceptive systemssuch as vision, olfaction, acoustico-lateralis sense and electroreception;and a cranial morphology that has been molded into a numberof functionally adaptive forms. These forms result in grasping,sucking, crushing, gouging, cutting and filtering systems offeeding. With relatively few exceptions elasmobranch feedingmechanisms share such features as subterminal or inferior mouths,a dynamic tooth replacement system, hyostylic jaw suspensionand a kinetic, protractile upper jaw. The importance of eachof these components is discussed. The evolution of the highdiversity of mechanical feeding systems in such a small groupof vertebrates has probably been facilitated by the morphologicalsimplicity of the basic feeding mechanism. This radiation wasaccomplished by modifications in jaw length, the length andsupporting angle of the hyomandibula, the size of the gape,dentition and changes in the relative size of the cranial musculature.The evolutionary pattern of shark feeding mechanisms is complex,there being several examples of both parallelism and convergence.A long-jawed, grasping form (similar to, but not identical withChlamydoselachus) is here considered primitive. From a subsequentbenthic sucking and grasping ancestor, similar in many respectsto some living batoids,radiated crushing, ray-like forms; cutting,squaloid forms; and gouging, lamniform and carcharhiniform types.From the latter developed sucking and grasping, or crushingforms such as modern orectolobiforms, triakids and heterodontiformsharks. From several levels (primary crushing, secondary crushingand gouging) there emerged filter-feeding forms representedtoday by mobulids, rhiniodontids and cetorhin.     

Graptoloid feeding efficiency, rotation and astogeny

Two methods are used to examine feeding strategies in graploloids; the first profiles different sets of zooids on the colony, the second treats the colony as a whole. Both of these techniques have advantages. The choice between them brings into question our concepts of the degree of coloniality shown by graptoloids. Using a whole colony model. graptoloids can be shown to have sampled the water with variable efficiency. as defined in this paper. Planar forms were relatively inefficient, generally sampling less than 10% of the available water. Inclined forms frequently approached 75% efficiency. Biserial forms and strdight monograptids roulinely exceeded 100%. sampling each unit of water more than once. Rotation of the rhabdosome during movement increased the efficiency of horizontal and inclined forms. It reduced the efficiency of scandent biserials and straight monograptids. These were both advantageous effects. Astogenetic changes in colony size and form would have had a profound effect on feeding efficiency.□ Graptoloid, ecology, astogeny     

Muscle energy transfer during a given wave-like movement of human body segments

The paper deals with a movement of two voluntary segments fixed in a joint and connected by a muscle in a multi-segment biomechanical system of human body. The muscle model is a four-element mechanical system. The mechanical movement energy brought into the "segments-muscle" system from the segments preceding the next ones is studied. The movement in which the total multi-segment system of the human body participates is described by the wave equation. Conditions concerning applying active muscle efforts and correlating velocities of muscle ends movement which provide the maximal value of transferred energy have been found. It is shown that the use of "artificial muscles" type devices promotes activization of energy transfer processes between segments.     

ParA ATPases can move and position DNA and subcellular structures

Prokaryotic chromosomes and plasmids can be actively segregated by partitioning (par) loci. The common ParA-encoding par loci segregate plasmids by arranging them in regular arrays over the nucleoid by an unknown mechanism. Recent observations indicate that ParA moves plasmids and chromosomes by a pulling mechanism. Even though ParAs form filaments in vitro it is not known whether similar structures are present in vivo. ParA of P1 forms filaments in vitro at very high concentrations only and filament-like structures have not been observed in vivo. Consequently, a 'diffusion-ratchet' mechanism was suggested to explain plasmid movement by ParA of P1. We compare this mechanism with our previously proposed filament model for plasmid movement by ParA. Remarkably, ParA homologues have been discovered to arrange subcellular structures such as carboxysomes and chemotaxis sensory receptors in a regular manner very similar to those of the plasmid arrays.     

An Intermediate in the evolution of superfast sonic muscles

Background

Intermediate forms in the evolution of new adaptations such as transitions from water to land and the evolution of flight are often poorly understood. Similarly, the evolution of superfast sonic muscles in fishes, often considered the fastest muscles in vertebrates, has been a mystery because slow bladder movement does not generate sound. Slow muscles that stretch the swimbladder and then produce sound during recoil have recently been discovered in ophidiiform fishes. Here we describe the disturbance call (produced when fish are held) and sonic mechanism in an unrelated perciform pearl perch (Glaucosomatidae) that represents an intermediate condition in the evolution of super-fast sonic muscles.

Results

The pearl perch disturbance call is a two-part sound produced by a fast sonic muscle that rapidly stretches the bladder and an antagonistic tendon-smooth muscle combination (part 1) causing the tendon and bladder to snap back (part 2) generating a higher-frequency and greater-amplitude pulse. The smooth muscle is confirmed by electron microscopy and protein analysis. To our knowledge smooth muscle attachment to a tendon is unknown in animals.

Conclusion

The pearl perch, an advanced perciform teleost unrelated to ophidiiform fishes, uses a slow type mechanism to produce the major portion of the sound pulse during recoil, but the swimbladder is stretched by a fast muscle. Similarities between the two unrelated lineages, suggest independent and convergent evolution of sonic muscles and indicate intermediate forms in the evolution of superfast muscles.     

A MOVING IMAGE OF FLAGELLA: NEWS AND VIEWS ON THE MECHANISMS INVOLVED IN AXONEMAL BEATING

Beating of cilia and flagellae allows movement of the fluid surrounding isolated cells (for example: protists) or epithelia (bronchial tissue) but is also responsible for the movement of unicellular organisms in this medium (such as spermatozoa or protists). This paper aims to describe: (1) the biochemical and structural elements of the ‘9 +2’ structure called the axoneme; (2) the mechanisms of wave generation and propagation along the axoneme of cilia and flagellae are then described, stating that in most models of wave propagation, a clear distinction is made between the dynein-dependent microtubule sliding which represents the oscillatory motor and the bending mechanism which regulates wave propagation. In current models, the bending propagation is supported by a bind /relax cyclic mechanism which propagates in register, but frame-shifted, with the powering action of the dynein motor along the axoneme. While a large amount of knowledge was accumulated about the motor, little is known about the resisting elements regulating the bending. (3) The present study also puts forward ideas as to how these organelles have been highly conserved throughout eucaryotic evolution, and concludes with suggestions for further fields of investigation into this unique mechanical device used for cell movement.