Random Encounters Research Articles

The electrostatic character of the ribosomal surface enables extraordinarily rapid target location by ribotoxins

Alpha-sarcin ribotoxins comprise a unique family of ribonucleases that cripple the ribosome by catalyzing endoribonucleolytic cleavage of ribosomal RNA at a specific location in the sarcin/ricin loop (SRL). The SRL structure alone is cleaved site-specifically by the ribotoxin, but the ribosomal context enhances the reaction rate by several orders of magnitude. We show that, for the alpha-sarcin-like ribotoxin restrictocin, this catalytic advantage arises from favorable electrostatic interactions with the ribosome. Restrictocin binds at many sites on the ribosomal surface and under certain conditions cleaves the SRL with a second-order rate constant of 1.7 x 10(10) M(-1) s(-1), a value that matches the predicted frequency of random restrictocin-ribosome encounters. The results suggest a mechanism of target location whereby restrictocin encounters ribosomes randomly and diffuses within the ribosomal electrostatic field to the SRL. These studies show a role for electrostatics in protein-ribosome recognition.

The Teaching Effectiveness of Standardized Patients

Teaching nursing students therapeutic communication skills begins in the classroom and extends to the clinical environment. The usual method of instruction consists of random patient encounters observed by faculty and measures of competence that rely on paper-and-pencil tests. Using standardized patients (SPs) offers an alternative approach to the traditional method of teaching. Standardized patients are individuals who have been carefully trained to present an illness or scenario in a standardized, unvarying manner. This pilot study compared use of SPs with the usual method of instruction in a class of undergraduate nursing students. Results indicated that students who participated in the SP method overwhelmingly described the experience as positive, creative, and meaningful. No significant differences were found between the two groups on measures of interpersonal skills, therapeutic communication skills, and knowledge of depression

The Conservative Physiology of the Immune System. A Non‐Metaphoric Approach to Immunological Activity

Historically, immunology emerged as a biomedical science, concerned with host defense and production of anti-infectious vaccines. In the late 50s, selective theories were proposed and from then on, immunology has been based in a close association with the neo-Darwinian principles, such as random generation of variants (lymphocyte clones), selection by extrinsic factors (antigens)—and, more generally, on genetic determinism and functionalism. This association has had major consequences: (1) immunological jargon is full of “cognitive” metaphors, founded in the idea of “foreignness”; (2) the immune system is described with a random clonal origin, coupled to selection by random encounters; and (3) physiological events are virtually absent from immunological descriptions. In the present manuscript, we apply systemic notions to bring forth an explanation including systemic mechanisms able to generate immunological phenomena. We replace “randomness plus selection” and the notion of foreignness by a history of structural changes which are determined by the coherences of the system internal architecture at any given moment. The importance of this systemic way of seeing is that it explicitly attends to the organization that defines the immune system, within which it is possible to describe the conservative physiology of the immune system. Understanding immune physiology in a systemic way of seeing also suggests mechanisms underlying the origin of immunopathogeny and therefore suggests new insights to therapeutic approaches. However, if seriously acknowledged, this systemic/historic approach to immunology goes along with a global conceptual change which modifies virtually everything in the domain of biology, as suggested by Maturana.

Contemporary art and the role of museums as situational media

In this paper I analyse some examples from contemporary art and describe the role of museums as a situational media, through which a special kind of sociality is produced. In this context, I want to draw attention to the so-called ‘new institutionalism’, which seems to be the current denominator among different fields within the art world (including both art institutions and contemporary artists). New institutionalism refers to a recent resurgence of interest in the influence of art institutions on the production and reception of art. It generally understands art in terms of the ideas of openness, networking and process.Consequently, the reception of art should not be perceived only as a private contemplation by an individual, but must be understood also as a form of social, collective experience. According to French sociologist Michel Maffesoli, art is more and more experienced and interpreted collectively. It is experienced in a temporal and social process, which is similar to urban everyday experience. It is based on random events and occasional encounters, which in their own logic reinforce a sense of togetherness and social collectivity.This creates an economy which is, in a specific sense, essentially un-productive. Neither in the example of the open discussion in the Stockholm gallery about an incident implicating Israel's ambassador to Sweden, nor in the case of the events of the Art Project by the Italian artist Michelangelo Pistoletto does art aim for a specific end product (exhibition, artwork, cultural programme, correct interpretation, etc.). The implication is that art practice should be more about creating situations, encounters and interactions, which are in a constant state of process, than about creating art objects. This makes everything in and around the art equally relevant and important. In this way, art practice offers to society a plurality of values and promotes the enjoyment of this plurality, i.e. the mobility and choice between different, parallel truths.

Associations between adult spectral tarsiers

While anecdotal observations of gregarious behavior in nocturnal prosimian primates are common, most anthropologists continue to refer to them as solitary, perhaps based on the assumption that the occasional social interactions observed via ad libitum methods represent random chance encounters and not patterned social interactions. In this paper, I test the null hypothesis that nocturnal encounters between spectral tarsier (Tarsius spectrum) group members, outside of the sleeping tree, are the result of chance. Three male-female pairs were radio-collared and observed over a 4-month period, using continuous focal animal sampling at the Tangkoko Nature Reserve (Sulawesi, Indonesia). Using Waser's random gas model, I found that spectral tarsiers spent more time in proximity to other group members than expected by chance, given the size of their home range and nightly path length. Adult group members spent 11% of the night in physical contact and an additional 17% of the night within a 10-m radius of one another. Spectral tarsiers were also observed to significantly increase the amount of time spent foraging when located less than 10 m from another group member. Individuals foraging in proximity to another adult group member had lower insect capture rates compared to individuals who were not foraging in proximity to another adult group member. If living in a group is costly to these tarsiers' foraging efficiency, then why don't they actively avoid one another when foraging? One situation in which it might benefit tarsiers to be gregarious is high predation pressure. Preliminary results suggest that predation pressure by snakes may be the most likely factor selecting for the tarsiers to forage in proximity.

Subunit Assembly of N-Methyl-d-aspartate Receptors Analyzed by Fluorescence Resonance Energy Transfer

N-methyl-d-aspartate (NMDA) receptors play major roles in synaptic transmission and plasticity, as well as excitotoxicity. NMDA receptors are thought to be tetrameric complexes mainly composed of NMDA receptor (NR)1 and NR2 subunits. The NR1 subunits are required for the formation of functional NMDA receptor channels, whereas the NR2 subunits modify channel properties. Biochemical and functional studies indicate that subunits making up NMDA receptors are organized into a dimer of dimers, and the N termini of the subunits are major determinants for receptor assembling. Here we used a biophysical approach, fluorescence resonance energy transfer, to analyze the assembly of intact, functional NMDA receptors in living cells. The results showed that NR1, NR2A, and NR2B subunits could form homodimers when they were expressed alone in HEK293 cells. Subunit homodimers were also found existing in heteromeric NMDA receptors formed between NR1 and NR2 subunits. These findings are consistent with functional NMDA receptors being arranged as a dimer of dimers. In addition, our data indicated that the conformation of NR1 subunit homodimers was affected by the partner NR2 subunits during the formation of heteromeric receptor complexes, which might underlie the mechanism by which NR2 subunits modify NMDA receptor function.

Effects of density and spatial heterogeneity on foraging behaviour and fitness correlates of Armadillidium vulgare (Isopoda Oniscidea)

The encounter rate of focal individuals of Armadillidium vulgare in experimental arenas increased with increasing density of other isopods. This increase was only significantly more than would be predicted from a random encounter model when the additional animals were of a different species, Porcellio scaber. Increasing density of both species resulted in focal individuals spending a higher proportion of time walking while in high quality food patches, and a smaller proportion of their time feeding on the high quality food. This was only significant when the high quality food was most heterogeneously distributed and was more significant following encounters with P. scaber than with other A. vulgare. At higher densities focal individuals alternated turns less while searching in the low quality food matrix Relative growth rates and survivorship were both higher at higher population densities (278 m–2) than in the low density treatments of 28 m–2. We suggest that at low densities, which are below those observed for three field populations over twenty years, the advantages of having additional individuals with which to aggregate while sheltering may outweigh the costs of increased interference competition during foraging. The effects of spatial heterogeneity of food resources on interference competition between species differing in aggressiveness are discussed. The possible advantages of aggregating with another species, that has different morphological, physiological and behavioural adaptations to the terrestrial environment, while sheltering are examined in relation to the Allee effect of the positive fitness consequences of increasing population densities from very low levels.

Dual Nature of the Genome: Genes for the Individual Life and Genes for the Evolutionary Progress of the Population

Biological evolution is here postulated to be driven coordinately by the products of specific evolution genes and by non-genetic elements such as the intrinsic properties of matter and random encounter with environmental factors. Evolution genes are supposed to have their own evolutionary history in which second-order selection was exerted at the population level. The products of evolution genes can act as generators of genetic variations and/or as modulators of the frequency of genetic variation. Three major natural strategies, each with a number of specific mechanisms contribute to the overall spontaneous production of genetic variants. Each of these three strategies contributes its own specific quality to genetic variation. The difficulties of experimentally investigating these strategies and a wider discussion of some of the postulates within the scientific community are outlined. Finally, the general relevance of the postulated duality of the genome for our world view is briefly mentioned.

Persuasion dynamics

We here discuss a model of continuous opinion dynamics in which agents adjust continuous opinions as a result of random binary encounters whenever their difference in opinion is below a given threshold. We concentrate on the version of the model in the presence of few extremists which might drive the dynamics to generalized extremism. A network version of the dynamics is presented here, and its results are compared to those previously obtained for the full-mixing case. The same dynamical regimes are observed, but in rather different parameter regions. We here show that the combination of meso-scale features resulting from the first interaction steps determines the asymptotic state of the dynamics.

Interactions between wolves Canis lupus and dogs C. familiaris in Finland

We examined wolf Canis lupus attacks on domestic dogs C. familiaris in six Finnish wolf territories occupied by mated pairs and packs. Most incidents (76%, N = 21 confirmed cases) took place inside one territory. The wolves mostly (70%) attacked dogs in house yards. It appeared that wolves in the territory were actively seeking for dogs rather than killing them as a result of random encounters. A strong tendency to attack dogs seemed to be adopted by pups born to the wolf pack. We did not find evidence that the density of primary prey or resident dogs were associated with the risk of wolf attacks.

Minorities in a model for opinion formation

AbstractWe study a model for social influence in which the agents' opinion is a continuous variable [G. Weisbuch et al., Complexity 2002, 7(2), 55]. The convergent opinion adjustment process takes place as a result of random binary encounters whenever the difference between agents' opinions is below a given threshold. The inhomogeneity in the dynamics gives rise to a complex steady state structure, which is also highly dependent on the threshold and on the convergence parameter of the model. © 2004 Wiley Periodicals, Inc. Complexity 9: 24–30, 2004

Video Game Internal Turfs and Turfs of Play

Video Game Internal Turfs and Turfs of Play

Evolution of sexual asymmetry

BackgroundThe clear dominance of two-gender sex in recent species is a notorious puzzle of evolutionary theory. It has at least two layers: besides the most fundamental and challenging question why sex exists at all, the other part of the problem is equally perplexing but much less studied. Why do most sexual organisms use a binary mating system? Even if sex confers an evolutionary advantage (through whatever genetic mechanism), why does it manifest that advantage in two, and exactly two, genders (or mating types)? Why not just one, and why not more than two?ResultsAssuming that sex carries an inherent fitness advantage over pure clonal multiplication, we attempt to give a feasible solution to the problem of the evolution of dimorphic sexual asymmetry as opposed to monomorphic symmetry by using a spatial (cellular automaton) model and its non-spatial (mean-field) approximation. Based on a comparison of the spatial model to the mean-field approximation we suggest that spatial population structure must have played a significant role in the evolution of mating types, due to the largely clonal (self-aggregated) spatial distribution of gamete types, which is plausible in aquatic habitats for physical reasons, and appears to facilitate the evolution of a binary mating system.ConclusionsUnder broad ecological and genetic conditions the cellular automaton predicts selective removal from the population of supposedly primitive gametes that are able to mate with their own type, whereas the non-spatial model admits coexistence of the primitive type and the mating types. Thus we offer a basically ecological solution to a theoretical problem that earlier models based on random gamete encounters had failed to resolve.

Energy Requirements for Inverted CIDEP in Reactions between eaq- or Radical Anions and Phenoxyl Radicals

Inverted (A/E) chemically induced dynamic electron polarization (CIDEP) is observed for random encounters of radical pairs such as the phenoxyl radical together with either e a q - or the radical anion of benzoate. A number of experiments have been carried out to clarify the relationship of the energetics of the reaction of oxidizing and reducing radicals to CIDEP behavior. Steady-state and time-resolved ESR experiments were carried out to determine the sense of F-pair CIDEP for a number of different radical pairs consisting of a phenoxyl radical together with one of a series of radical anions of para-substituted benzoates. Inverted polarization (A/E) was found for e a q - or the radical anions of benzoate and 4-methyl, 4-methoxy, and 4-carboxybenzoates in the presence of phenoxyl or 4-methylphenoxyl radicals. Other radical anions such as those of 4-formylbenzoate showed normal polarization. The pair consisting of e a q - with the p-benzosemiquinone radical anion also showed inverted polarization, but normal polarization was seen for all radical anions paired with the semiquinone radical anion. The corresponding excited-state energies of the reaction product (phenolate anion or hydroquinone dianion) were determined from the emission spectra, and the energies of the radical pairs were estimated from electrochemical experiments. The inverted CIDEP is clearly the result of a change in the normal energy ordering of the radical-pair states so that the triplet state is lower. The observations are discussed in terms of the electron-transfer reaction within the radical pairs using the approach presented by Kobori et al. (Kobori, Y.; Sekiguchi, S.; Akiyama, K.; Tero-Kubota, S. J. Phys. Chem. A 1999, 103, 5416. Kobori, Y.; Akiyama, K.; Tero-Kubota, S. J. Chem. Phys 2000, 113, 465).

T cell-dendritic cell interaction in vivo: random encounters favor development of long-lasting ties.

Understanding the complexity of the functional communication between cells composing the immune system is central to improving our capacity to manipulate it and conceive better strategies to combat microbial pathogens. So far, these studies have been based on immunohistochemistry of fixed tissues and in vitro attempts to reproduce functional connections between cells. The application of two-photon laser microscopy to the observation of viable immune cells in their natural environment where foreign antigens are carried to trigger an immune response opens a new era for these studies. They reveal exceptional properties of the locomotion of T cells that facilitate encounters with dendritic cells and the receipt of information that promotes T cell survival, death, or initiation of immune responses. These studies also complement in vitro observations addressing the importance of time of stimulation in determining T cell fates.

Vector opinion dynamics in a model for social influence

We present numerical simulations of a model of social influence, where the opinion of each agent is represented by a binary vector. Agents adjust their opinions as a result of random encounters, whenever the difference between opinions is below a given threshold. Evolution leads to a steady state, which highly depends on the threshold and a convergence parameter of the model. We analyze the transition between clustered and homogeneous steady states. Results of the cases of complete mixing and small-world networks are compared.

A Neutral Model of Stone Raw Material Procurement

Stone tool assemblage variability is considered a reliable proxy measure of adaptive variability. Raw material richness, transport distances, and the character of transported technologies are thought to signal (1) variation in raw material selectivity based on material quality and abundance, (2) optimization of time and energy costs associated with procurement of stone from spatially dispersed sources, (3) planning depth that weaves raw material procurement forays into foraging activities, and (4) risk minimization that sees materials transported in quantities and forms that are energetically economical and least likely to fail. This paper dispenses with assumptions that raw material type and abundance play any role in the organization of mobility and raw material procurement strategies. Rather, a behaviorally neutral agent-based model is developed involving a forager engaged in a random walk within a uniform environment. Raw material procurement in the model is dependent only upon random encounters with stone sources and the amount of available space in the mobile toolkit. Simulated richness-sample size relationships, frequencies of raw material transfers as a function of distance from source, and both quantity-distance and reduction intensity-distance relationships are qualitatively similar to commonly observed archaeological patterns. In some archaeological cases it may be difficult to reject the neutral model. At best, failure to reject the neutral model may mean that intervening processes (e.g., depositional time-averaging) have erased high-frequency adaptive signals in the data. At worst, we may have to admit the possibility that Paleolithic behavioral adaptations were sometimes not responsive to differences between stone raw material types in the ways implied by current archaeological theory.

Dispersion movements in ants: spatial structuring and density-dependent effects.

This paper examines whether the characteristics of individual dispersion movements in ants are changed when workers are moving solitarily or in a group. We analyzed the trajectories of workers of the species Messor sancta moving solitarily or in groups of different size (5, 10, 15 individuals), tested for density-dependent effects on their trajectory characteristics and investigated through resampling techniques whether ants are able to spatially structure their movements through direct (e.g. contact) or indirect (pheromone deposited on the ground) interactions. In addition to group size, the effects of the nutritional state of the colony and of the state of the area on which ants were dispersing were also examined. Solitary ants moved faster and had more sinuous trajectories than ants moving in a group. We found however no significant differences in trajectory characteristics between groups of different size. Whatever the group size, ants from starved colonies moved more slowly and had more direct trajectories than their counterpart coming from fed colonies. On the other hand, the state of the area on which ants were moving had no direct significant effect on dispersion movement. Ants dispersing in a group moved independently and did not coordinate their movements through direct or indirect interactions. However, the geometry of their path was changed not only through the effect of random encounters with other workers but also through an active modification of their movement when they perceived directly or indirectly the presence of nearby workers.

Tidal Breakup of Comets

In the context of the survival of periodic comets of different origins, rotational breakup and tidal disruption could be important, especially of the short period comets injected from the Kuiper belt. This is because long-period comets from the distant Oort cloud tend to be subject to thermal stress and volatile 'explosion' far more severely. A simple calculation using the Opik method of random planetary close encounters was performed to estimate the probability of tidal disruption of comets and scattered Kuiper belt objects (SKBOs) during their orbital migration. It was found that a large fraction of the short period comets and SKBOs might have been internally fragmented by single or multiple close encounters with the outer planets.

Frequency, composition and variation in external food stores constructed by orb-web spiders: Nephila edulis and Nephila plumipes (Araneae : Araneoidea)

Few examples of food hoarding have been documented in spiders, yet two Australian species of orb-web spiders, Nephila edulis and N. plumipes, typically incorporate previously captured prey into the web. The effect of prey density and prey-encounter rate on the storage behaviour of adult female N. edulis was tested in the laboratory. Prey density had a significant effect on the propensity to construct external caches: when more food was available, food caches were larger than when the supply of prey was limited. Caching behaviour also differed with the rate of prey encounter, even though the total amount of food supplied was the same. When prey were encountered at constant rates, spiders allocated more food to external storage compared with random encounter rates. Finally, we tested the quality of different prey types for external or internal storage. N. edulis were fed with blow-flies or crickets, and these prey were stored in the web, discarded or totally consumed. Crickets were typically consumed or stored, while flies were more frequently discarded. Field observations of the storage behaviour in N. edulis and N. plumipes found surprising differences in the composition of the cache. While N. plumipes incorporated only animal material, N. edulis also utilised plant material, suggesting that the storage band in N. edulis has other, non-food-storing functions. Field experiments indicated that the presence or absence of external stores in the web of N. plumipes had no influence on mortality, weight gain, or the presence of Argyrodes kleptoparasites.