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PubMed1988

Frequency dependence and competition.

Intraspecific competition implies interaction among the individuals of a population, so natural selection on genotypic variation in characters related to the competition will necessarily be frequency dependent. Intraspecific antagonistic competition exhibits properties similar to other behavioural interactions between individuals. In exploitative intraspecific competition the interactions among individuals are less direct. Exploitation modifies the abundance of the various limiting resources according to the use of these resources by the individual members of the population. The amount of resource available to an individual is therefore a function of the phenotypes present in the population, through their density and frequency.

Animals↗
PubMed2000

Evolutionarily Stable Strategies for Consuming a Structured Resource.

A general consumer-resource model assuming discrete consumers and a continuously structured resource is examined. We study two foraging behaviors, which lead to fixed and flexible patch residence times, in conjunction with a simple consumer energetics model linking resource consumption, foraging behavior, and metabolic costs. Results indicate a single, evolutionarily stable foraging strategy for fixed and flexible foraging in a nonspatial environment, but flexible foraging in a spatial environment leads to consumer grouping, which affects the resource distribution such that no single foraging strategy can exclude all other strategies. This evolutionarily stable coexistence of multiple foraging strategies may help explain a dichotomous pattern observed in a wide variety of natural systems.

competitive coexistence↗
PubMed2005

The plant's capacity in regulating resource demand.

Regulation of resource allocation in plants is the key to integrate understanding of metabolism and resource flux across the whole plant. The challenge is to understand trade-offs as plants balance allocation between different and conflicting demands, e.g., for staying competitive with neighbours and ensuring defence against parasites. Related hypothesis evaluation can, however, produce equivocal results. Overcoming deficits in understanding underlying mechanisms is achieved through integrated experimentation and modelling the various spatio-temporal scaling levels, from genetic control and cell metabolism towards resource flux at the stand level. An integrated, interdisciplinary research concept on herbaceous and woody plants and its outcome to date are used, while drawing attention to currently available knowledge. This assessment is based on resource allocation as driven through plant-pathogen and plant-mycorrhizosphere interaction, as well as competition with neighbouring plants in stands, conceiving such biotic interactions as a "unity" in the control of allocation. Biotic interaction may diminish or foster effects of abiotic stress on allocation, as changes in allocation do not necessarily result from metabolic re-adjustment but may obey allometric rules during ontogeny. Focus is required on host-pathogen interaction under variable resource supply and disturbance, including effects of competition and mycorrhization. Cost/benefit relationships in balancing resource investments versus gains turned out to be fundamental in quantifying competitiveness when related to the space, which is subject to competitive resource exploitation. A space-related view of defence as a form of prevention of decline in competitiveness may promote conversion of resource turnover across the different kinds of biotic interaction, given their capacity in jointly controlling whole plant resource allocation.

Energy Metabolism↗
PubMed2000

Erythrocytes possess an intrinsic barrier to nitric oxide consumption.

It has been reported that free hemoglobin (Hb) reacts with NO at an extremely high rate (K(Hb) approximately 10(7) M(-1) s(-1)) and that the red blood cell (RBC) membrane is highly permeable to NO. RBCs, however, react with NO 500-1000 times slower. This reduction of NO reaction rate by RBCs has been attributed to the extracellular diffusion limitation. To test whether additional limitations are also important, we designed a competition test, which allows the extracellular diffusion limitation to be distinguished from transmembrane or intracellular resistance. This test exploited the competition between free Hb and RBCs for NO generated in a homogenous phase by an NO donor. If the extracellular diffusion resistance is negligible, then the results would follow a kinetic model that assumes homogenous reaction without extracellular diffusion limitation. In this case, the measured effective reaction rate constant, K(RBC), would remain invariant of the hematocrit, extracellular-free Hb concentration, and NO donor concentration. Results show that the K(RBC) approaches a constant only when the hematocrit is greater than 10%, suggesting that at higher hematocrit, the extracellular diffusion resistance is negligible. Under such a condition, the NO consumption by RBCs is still 500-1000 times slower than that by free Hb. This result suggests that intrinsic RBC factors, such as transmembrane diffusion limitation or intracellular mechanisms, exist to reduce the NO consumption by RBCs.

Animals↗
PubMed2002

Interference competition and species coexistence.

Interference competition is ubiquitous in nature. Yet its effects on resource exploitation remain largely unexplored for species that compete for dynamic resources. Here, I present a model of exploitative and interference competition with explicit resource dynamics. The model incorporates both biotic and abiotic resources. It considers interference competition both in the classical sense (i.e. each species suffers a net reduction in per capita growth rate via interference from, and interference on, the other species) and in the broad sense (i.e. each species suffers a net reduction in per capita growth rate via interference from, but can experience an increase in growth rate via interference on, the other species). Coexistence cannot occur under classical interference competition even when the species inferior at resource exploitation is superior at interference. Such a trade-off can, however, change the mechanism of competitive exclusion from dominance by the superior resource exploiter to a priority effect. Now the inferior resource exploiter can exclude the superior resource exploiter provided it has a higher initial abundance. By contrast, when interference is beneficial to the interacting species, coexistence is possible via a trade-off between exploitation and interference. These results hold regardless of whether the resource is biotic or abiotic, indicating that the outcome of exploitative and interference competition does not depend on the exact nature of resource dynamics. The model makes two key predictions. First, species that engage in costly interference mechanisms (e.g. territoriality, overgrowth or undercutting, allelopathy and other forms of chemical competition) should not be able to coexist unless they also engage in beneficial interference mechanisms (e.g. predation or parasitism). Second, exotic invasive species that displace native biota should be superior resource exploiters that have strong interference effects on native species with little or negative cost. The first prediction provides a potential explanation for patterns observed in several natural systems, including plants, aquatic invertebrates and insects. The second prediction is supported by data on invasive plants and vertebrates.

Animals↗
PubMed2003

Competitive binding of fatty acids and the fluorescent probe 1-8-anilinonaphthalene sulfonate to bovine beta-lactoglobulin.

The use of spectroscopy in the study of fatty acids binding to bovine beta-lactoglobulin (BLG) appears to be a difficult task, as these acid compounds, assumed as the protein natural ligands, do not exhibit favorable optical response such as, for example, absorption or fluorescence. Therefore, the BLG fatty-acid equilibrium has been tackled by exploiting the competition between fatty acids and ANS, a widely used fluorescent hydrophobic probe, whose binding sites on the protein have been characterized recently. Two lifetime decays of the ANS-BLG complex have been found; the longer one has been attributed to the internal binding site and the shorter one to the external site. At increasing fatty acids concentration, the fractional weight associated with ANS bound to the internal site drops, in agreement with a model describing the competition of the dye with fatty acids, whereas the external site occupancy appears to be unaffected by the fatty acids binding to BLG. This model is supported by docking studies. An estimate of the acid-binding affinities for BLG has been obtained by implementing the fitting of the bound ANS intensities with a competitive binding model. A relevant dependence has been found upon the solution pH, in the range from 6 to 8, which correlates with the calyx accessibility modulated by the conformation of the EF loop. Fatty acids with longer aliphatic chains (palmitate and laurate) are found to display larger affinities for the protein and the interaction free energy nicely correlates with the number of contacts inside the protein calyx, in agreement with docking simulations.

Anilino Naphthalenesulfonates↗
PubMed2004

The effect of spatial heterogeneity and parasites on the evolution of host diversity.

Both spatial heterogeneity and exploiters (parasites and predators) have been implicated as key ecological factors driving population diversification. However, it is unclear how these factors interact. We addressed this question using the common plant-colonizing bacterium Pseudomonas fluorescens, which has been shown to diversify rapidly into spatial niche-specialist genotypes when propagated in laboratory microcosms. Replicate populations were evolved in spatially homogeneous and heterogeneous environments (shaken and static microcosms, respectively) with and without viral parasites (bacteriophage) for approximately 60 bacterial generations. Consistent with previous findings, exploiters reduced diversity in heterogeneous environments by relaxing the intensity of resource competition. By contrast, exploiters increased diversity in homogeneous environments where there was little diversification through resource competition. Competition experiments revealed this increase in diversity to be the result of fitness trade-offs between exploiter resistance and competitive ability. In both environments, exploiters increased allopatric diversity, presumably as a result of divergent selection for resistance between populations. Phage increased total diversity in homogeneous environments, but had no net effect in heterogeneous environments. Such interactions between key ecological variables need to be considered when addressing diversification and coexistence in future studies.

Animals↗
PubMed1996

A repair competition assay to assess recognition by human nucleotide excision repair.

We developed a competition assay to compare, in a quantitative manner, the ability of human nucleotide excision repair (NER) to recognise structurally different forms of DNA damage. This assay uses a NER substrate consisting of M13 double-stranded DNA with a single and uniquely located acetylaminofluorene (AAF) adduct, and measures the efficiency by which multiply damaged plasmid DNA competes for excision repair of the site-directed modification. To validate this assay, we tested competitor DNA containing defined numbers of either AAF adducts or UV radiation products. In both cases, repair of the site-directed NER substrate was inhibited in a damage-specific and dose-dependent manner. We then exploited this competition assay to determine the susceptibility of bulky adozelesin-DNA adducts to human NER.

Base Sequence↗
PubMed1993

Biosensor for direct determination of glucose and lactate in undiluted biological fluids.

This paper describes the implementation of a bienzyme sensor for the direct determination of glucose and lactate in undiluted biological samples. The biosensor exploits the competitive action versus the substrate itself by two different enzymes immobilized into a sandwich of two different membranes. In this way the quantity of substrate reaching the indicating electrode is reduced and this determines an extension of the linearity range.

Biosensing Techniques↗
PubMed2004

How should public policy respond to the challenges of modern biotechnology?

Modern biotechnology has been a topic of public and political debate around the world for over 30 years; continuing scientific breakthroughs keep it high on the agenda. Policy responses have been diverse, fragmented and incoherent, at levels ranging from local regions to agencies of the United Nations, but particularly in national administrations and the European Union. Reactions have been ambivalent, combining fears about conjectural risks with concern to maintain competitiveness and exploit beneficial applications. Adverse public perceptions have become a significant influence on policy, in combination with more cynical and self-interested motives of some of the players in policy debates. Future options, in Europe and elsewhere, are constrained by past and continuing mistakes and over-reactions.

Biotechnology↗
PubMed2003

Beef cattle growing and backgrounding programs.

The stocker industry is one of many diverse production and marketing activities that make up the United States beef industry. The stocker industry is probably the least understood industry sector and yet it plays a vital role in helping the industry exploit its competitive advantage of using forage resources and providing an economical means of adjusting the timing and volume of cattle and meat in a complex market environment.

Animal Husbandry↗
PubMed1996

Kinetics of association and dissociation of two enantiomers, NSC 613863 (R)-(+) and NSC 613862 (S)-(-) (CI 980), to tubulin.

The kinetics of binding of R- and S-enantiomers were studied by the fluorescence stopped-flow technique. For the R-enantiomer, the time course of the increase in fluorescence is best fitted by a sum of two exponentials. In pseudo-first-order conditions, the first observed rate constant showed a linear concentration dependence whereas the second showed a hyperbolic one. The dissociation rate constants were determined independently by displacement experiments with 2-methoxy-5-(2,3,4-trimethoxyphenyl)-2,4,6-cycloheptatrien-1-one (MTC). The two exponential phases were assumed to be due to a two-step binding mechanism: an initial binding followed by a conformational change. This is different from colchicine and MTC binding, where the two phases show a hyperbolic concentration dependence and are attributed to the parallel binding to different isoforms of tubulin [Banerjee, A., & Luduena, R. F. (1992) J. Biol. Chem. 267, 13335-13339]. R-isomer binding did not discriminate between the tubulin isoforms. The temperature dependence of all the rate constants were measured, and the entire thermodynamic reaction path was constructed. For the S-isomer, the direct fluorescence stopped-flow study showed that the signals were largely imputable to the fluorescence of the binding at low-affinity sites [Leynadier, D., Peyrot, V., Sarrazin, M., Briand, C., Andreu, J. M., Rener, G. A., & Temple, C., Jr. (1993) Biochemistry 32, 10674-10682]. Therefore, we exploited the competition between R- and S-isomers to determine the binding kinetics of the S-isomer to the R-site. The observed rate constants for competitive binding showed a linear concentration dependence, thus allowing us to calculate the association rate constant of the S-isomer to the R-site. The kinetics of displacement of the S-isomer by MTC allowed the dissociation rate constant for the S-isomer to be determined. The binding of both enantiomers to tubulin in presence of tropolone methyl ether (analog of the colchicine C ring) was decreased, indicating the involvement of the C subsite.

Animals↗
PubMed2006

Directed assembly and separation of self-assembled monolayers via electrochemical processing.

Separated domains of 1-dodecanethiolate were fabricated via solution displacement of preformed 1-adamantanethiolate self-assembled monolayers on Au{111}. Subsequently, the 1-adamantanethiolate domains were desorbed selectively, and the substrate was exposed to a 1-octanethiol solution, creating artificially separated self-assembled monolayers of 1-dodecanethiolate and 1-octanethiolate. The molecular order of each lattice type and the apparent height differences imaged with scanning tunneling microscopy and the two distinct cathodic peaks observed with cyclic voltammetry indicated distinct separated domains of each lattice type in the separated self-assembled monolayers. By manipulating the intermolecular interaction strengths of the patterned molecules, we are able to control the structure and properties of the separated self-assembled monolayers via the exploitation of competitive adsorption and the utilization of electrochemical processing, which can be extended to other self-assembly patterning techniques such as microdisplacement printing.

Adamantane↗
PubMed2001

Evolutionary origins of stigmatization: the functions of social exclusion.

A reconceptualization of stigma is presented that changes the emphasis from the devaluation of an individual's identity to the process by which individuals who satisfy certain criteria come to be excluded from various kinds of social interactions. The authors propose that phenomena currently placed under the general rubric of stigma involve a set of distinct psychological systems designed by natural selection to solve specific problems associated with sociality. In particular, the authors suggest that human beings possess cognitive adaptations designed to cause them to avoid poor social exchange partners, join cooperative groups (for purposes of between-group competition and exploitation), and avoid contact with those who are differentially likely to carry communicable pathogens. The evolutionary view contributes to the current conceptualization of stigma by providing an account of the ultimate function of stigmatization and helping to explain its consensual nature.

Adaptation, Psychological↗
PubMed1995

Thrombin-thrombomodulin interaction: energetics and potential role of water as an allosteric effector.

The interaction of rabbit lung thrombomodulin (TM) and C-terminal hirudin 54-65 fragment (Hir54-65) with human alpha-thrombin were investigated by exploiting their competitive inhibition of thrombin-fibrinogen interaction. Measurements of Ki values for TM and Hir54-65 interactions with human alpha-thrombin performed over a temperature range spanning from 10 to 40 degrees C showed a constant enthalpy for both ligands. The enthalpic and entropic contributions to the free energy of binding, however, are different for TM and the hirudin peptide. The calculated values of delta H and delta S, in fact, were -47.3 +/- 2.51 kJ (-11.3 +/- 0.6 kcal)/mol and -42.7 +/- 7.9 J (-10.2 +/- 1.9 cal)/mol.K for the hirudin peptide, while being -22.9 +/- 2.09 kJ (-5.47 +/- 0.5 kcal)/mol and 102.50 +/- 6.69 J (24.5 +/- 1.6 cal)/mol.K respectively for TM binding. These findings indicate that the interaction between thrombin and Hir54-65 is largely driven by the enthalpic contribution, whereas the positive entropy change is the driving force for the formation of the thrombin-TM complex. In other experiments performed in the presence of various concentrations of either sorbitol or sucrose it could be demonstrated that the value of the equilibrium association constant for thrombin-TM interaction increases as a function of the osmotic pressure, while the thrombin-Hir54-65 interaction was not affected by the same conditions. Moreover, control experiments showed that no major conformational changes are produced on TM by osmotic pressures used in the present study. From these experiments it was calculated that roughly 35 water molecules are released into the bulk water upon TM binding. Such a phenomenon, which is likely to be responsible for the entropic change described above, indicates the relevance of hydration processes for the formation of the thrombin-TM adduct.

Allosteric Regulation↗
PubMed2001

Changes in thymic function in HIV-positive patients treated with highly active antiretroviral therapy and interleukin-2.

Despite its potent antiviral activity, highly active antiretroviral therapy (HAART) only exerts a marginal effect on CD4+ T-cell regeneration in HIV-infected subjects. Combination therapies aimed at boosting T-cell activity and maturation may provide an important contribution to the restoration of immune function. Here, we report the results obtained by a two-year follow-up of a cohort of HIV-infected patients treated with a combination of HAART and interleukin-2 (IL-2). In these patients, in addition to a series of quantitative virological and immunological parameters, we investigated T-cell regeneration by an immunophenotypic assay monitoring CD4+ naïve T cells, and by analysis of thymic function, through the quantification of the excision DNA products of T-cell receptor rearrangement (TRECs) in lymphocytes. Compared with HAART alone, we found that the IL-2 combination therapy was equally effective in reducing the levels of viremia and marginally more effective in decreasing proviral DNA load. Strikingly, the IL-2 combination produced a marked increase in the number of CD4+ T cells bearing a naïve phenotype (CD45RA+, CD62L+), which was apparent for over 96 weeks after therapy. To assess whether these cells were the product of improved T-cell generation, we exploited a competitive quantitative molecular assay to quantify TRECs in peripheral blood lymphocytes. Surprisingly, we found that the levels of these molecules were unchanged in these patients. These findings indicate that improved thymic function does not account for the early rise of CD4 naïve cells in HIV-positive patients treated with IL-2, and suggest that alternative mechanisms of T-cell maturation and differentiation are responsible for this event.

Adult↗
PubMed2006

Oxidation of evaporated porous silicon rugate filters.

Rugate filters are thin-film optical interference coatings with sinusoidal variation of the refractive index. Several of these filters were fabricated with glancing angle deposition, which exploits atomic competition during growth to create nanoporous materials with controllable effective refractive index. This method enables the fabrication of devices with almost arbitrary refractive index profiles varying between the ambient, 1.0, and the index of the film material, in this case silicon with an index of 4.0 (at 600 nm). As these filters are inherently porous, oxidation of the silicon can occur throughout the device layer, and here we study the intentional oxidation of silicon filters by high-temperature reaction with gaseous oxygen. We find that a significant portion of the silicon filter oxidizes in approximately 10 min when heated to 600 degrees C-650 degrees C in an oxygen environment; oxidation then continues slowly over several hours. The presence of water vapor has little apparent effect on the oxidation reaction, and attempts to oxidize with ozone at room temperature were unsuccessful. As silicon filters oxidize to become silica, spectral blueshifts and increased short-wavelength transmittance are observed. Measured and calculated transmittance spectra generally agree, although the lack of absorption and dispersion in the theoretical model limits detailed comparison.

Journal Article↗
PubMed2004

Aphid specific predators in potato in Belgium.

In summer 2003, aphid specific predators populations were sampled in four potato fields located in Wallonia. Ladybirds, hoverflies and lacewings were the most abundant predators collected. A total of 6 ladybird, 8 syrphid and 2 lacewing species was found. Predator species composition and abundance greatly differed from field to field, while aphids populations were more or less similar with a peak of 3 to 5 aphid/potato leaf in mid-July. All chrysopidae sampled belong to the Chrysoperla kolthoffi (NAVAS) species except 2 specimens of Chrysopa perla L. Episyrphus balteatus DEGEER was the dominant syrphid species. Sphaerophoria scripta L. and Syrphus vitripiennis MEIGEN were subdominant species and 1 or 2 specimens of Melanostoma scalare F., Melanostoma mellinum (L.), Melliscaeva cinctella (ZETTERSTED), Metasyrphus corollae (F.) and Platycheirus albimanus F. were also collected. Coccinella septempunctata (L.) and Propylea quatuordecimpunctata (L.) were the dominant ladybird species. P. 14-punctata was proportionally more abundant in beating samples than with visual inspection samples, indicating that this ladybird is probably more discrete than C. septempunctata and that their populations can be underestimated with only visual inspection. Adalia bipunctata (L.) was also present in nearly all samples, but with lower levels of populations. Coccinella quinquepunctata (L.) and Adalia tenpunctata (L.) were found in small numbers at one occasion. Larvae and adults of Harmonia axyridis PALLAS, the multicoloured Asian beetle introduced in glasshouse for aphid control, were found in 3 fields out of 4, with 1.5 to 6% of ladybirds sampled. This is the first record of H. axyridis in open field in Belgium. As this beetle has been found in numbers at different locations and with different growth stages including larvae and pupae, it is supposed that H. axyridis is adapted to ware potato aphid and able to complete a full life cycle in this crop. As this species is very competitive for exploitation of food resources and aggressive for other predators, they are several questions about the evolution in the future of aphid specific predator biodiversity in ware potato.

Animals↗