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PubMed1999

Undertaking specialization in the desert leaf-cutter ant Acromyrmex versicolor.

We investigated undertaking behaviour in the desert leaf-cutter ant to determine whether colonies show undertaking specialization, how task specialization is regulated and the consequences of specialization on colony performance. Task specialization has been hypothesized to be a result of internal physiological or genetic factors that govern worker behaviour. On the other hand, it has been suggested that task specialization could be a result of spatial structuring of workers allowing only a subset to have the opportunity to perform certain tasks. To test between these two hypotheses, we experimentally introduced dead ants into laboratory colonies with individually marked workers, and recorded all encounters and undertaking behaviours. Workers demonstrated individual specialization, in that some workers removed the dead ant more frequently than expected from encounter rate. Although individual workers differed in their opportunity to perform undertaking, this did not account for the specialization. These results suggest that undertaking specialization is governed by internal differences among workers. In addition, undertaking specialists removed corpses more successfully, and in the largest colony, more quickly, than nonspecialist workers, indicating that internally governed specialization is an important part of overall colony efficiency. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1999

Identification, sequence, and mapping of the mouse multiple PDZ domain protein gene, Mpdz.

The PDZ domain gained its name from the three proteins that were first seen to have homology by virtue of these domains, the mammalian postsynaptic density protein, PSD-95, the Drosophila discs-large septate junction protein, DLG, and the mammalian epithelial tight-junction protein zona occludens, ZO-1. Over 50 PDZ domain-containing genes have been recognized so far from almost any organism subjected to sequencing, including mammals, nematodes, yeast, plants, and bacteria. The domain consists of an approximately 90-amino-acid-residue unit, which is often repeated in the protein. The majority of residues form a conserved spatial structure while a few amino acids in critical positions confer protein binding specificity. A subgroup of PDZ domains have been shown to recognize a short carboxy-terminal amino acid motif, T/SXV (Ser/Thr-X-Val-COO-), where X is any amino acid. We have identified and completely sequenced a gene, Mpdz, that encodes a mouse protein containing 13 such domains. We have also mapped the gene to a series of overlapping deletions on mouse chromosome 4 and can therefore determine that its function is not essential for embryonic development or neonatal survival.

Animals↗
PubMed2002

Development and implementation of the Norwegian monitoring programme for agricultural landscapes.

This paper describes the development and implementation of the Norwegian monitoring programme for agricultural landscapes--the '3Q programme'. The main objective of the scheme is to indicate development trends in the agricultural landscape, and their consequences for spatial structure, biodiversity, cultural heritage and accessibility. The monitoring programme aims to give policy feedback and provide data to fulfill international reporting requirements. This paper describes the background to the programme and reasons behind the choice of methods. Results are presented to show the accuracy of the methods employed and the range of indicator values recorded in the programme. Strengths and limitations of the monitoring programme are discussed, and potential future improvements and developments are outlined. Although there remains a potential for methodological improvement, we stress the importance of establishing a baseline to enable the detection of development trends in a rapidly changing environment.

Agriculture↗
PubMed2000

Evolution of seed dormancy due to sib competition: effect of dispersal and inbreeding.

The effect of dispersal and inbreeding on the evolution of seed dormancy to avoid sib competition is theoretically investigated, using a model which assumes a plant population with patchy spatial structure in a constant environment. Applying the inclusive fitness method, the evolutionarily stable dormancy rates are analytically derived for three cases: (a) an asexual haploid population, (b) a diploid-hermaphrodite population in which the dormancy rate is controlled by seeds, and (c) a diploid-hermaphrodite population in which the dormancy rate is controlled by mother plants. The evolutionarily stable dormancy rates decrease in the order of case (c), case (a), and case (b). In all the cases, the evolutionarily stable dormancy rates increase with decreasing the dispersal rate. Although inbreeding generally increases the evolutionarily stable dormancy rates, inbreeding due to selfing reduces the rate exceptionally in case (c).

Biological Evolution↗
PubMed2000

Lessons on pattern formation from planet WATOR.

It is well known that if reacting species experience unequal diffusion rates, then dynamics that lead to a constant steady state in a "well-mixed" environment can in a spatial setting lead to interesting patterns. In this paper, we focus on complementary pattern formation mechanisms that operate even when the diffusion rates are equal. In particular, we can say that when the mean-field ODE has an attracting periodic orbit then the stochastic spatial model will have large-scale spatial structures in equilibrium. We explore this mechanism in depth through the dynamics of the simulator WATOR.

Animals↗
PubMed2000

Motional instabilities in prey-predator systems.

Differential fluxes can destabilize the locally stable stationary density distributions in interaction systems with diffusion, advection, and/or locomotion of chemical or biological species. By this method they can cause the formation of stationary or travelling spatial structures. Different scenarios of this general mechanism of spatio-temporal pattern formation in reaction-diffusion-advection systems will be demonstrated, using a simple two-species predator-prey system as an example model.

Animals↗
PubMed2002

The stability of cooperation involving variable investment.

In this paper, we attempt to reconcile the results of several studies which have investigated the evolution of cooperation between non-relatives in systems where investment in partners can vary. In contrast to previous proposals, we show for the first time that variable-investment cooperation can be readily maintained in inter-species mutualistic relationships even in the absence of spatial structure, but that the stability of this interaction is dependent on the particular investment-response rule that is employed. By allowing the evolution of investment-response parameters in both inter- and intra-specific versions of the continuous variable-investment Prisoners' Dilemma we show that, in the absence of further factors, the raise-the stakes (RTS) strategy is likely to evolve into a simpler, variable investment form of Give-as-Good-as-you-Get that initially offers a high fixed amount and subsequently matches its partner's investment. Nevertheless, we demonstrate that genuine RTS-like strategies will still be selected if two intuitively reasonable conditions hold: if individuals are limited in terms of the total they can invest in cooperative actions over their lifetimes, and if there are always some individuals in a population that cannot cooperate.

Animals↗
PubMed1998

An evaluation of linear model analysis techniques for processing images of microcirculation activity.

Sequences of images of the cortical surface can be processed to reveal information about the cortical microcirculation, regional cerebral blood flow (rCBF), and changes induced by neuronal activity. This study examined the use of different analysis methodologies on intrinsic optical images taken from rat sensory motor cortex and testes. Generalized linear model (GLM) analysis was used and compared with standard signal processing methods including principal component analysis. The GLM method has been used by Friston et al. (1994, Hum. Brain Map., 1: 214-220) in the analysis of functional magnetic resonance imagery to identify regions of focal activity. We investigated the use of this method to analyze video image data of the modulation of rCBF from rat cortex. The results revealed spatiotemporal variations in rCBF in response to stimulation within local regions of cortex. The advantage of the GLM method is that it augments ordinary signal processing methods with an estimate of statistical reliability. The use of different wavelengths of illumination reveals spatial structures with different temporal relationships. In image time series data collected under green and red illumination a phase difference was found in the low frequency approximately 0.1 Hz vasomotion oscillation. This phase difference occurred in data from both cortex and testes. A possible explanation of these differences is that the spectral absorption characteristics of the tissue reflect changes in the volume proportions of the different hemoglobin derivatives in interacting with the modulation of the volume of blood. It is suggested that the combination of these effects produces the phase differences we detect.

Animals↗
PubMed1996

Lotka's game in predator-prey theory: linking populations to individuals.

This paper further examines an individual-based model of a spatially distributed predator-prey population that demonstrates strong spatial structuring in contrast with predictions from its representative analytic formulation. Examination of a small, localized population reveals that extinctions due to demographic stochasticity dominate the dynamics. Local extinction dynamics produce wave pulses and the interactions of these wave pulses constitute global dynamics. The results motivate a population-level cell-based model with each cell representing a local population and parameterized by local extinction probabilities, rather than individual-based interaction rates. A detailed comparison of spatiotemporal plots from the two modelling frameworks shows that the population-level model captures the broad range of dynamics exhibited by the individual-based model. The agreement between these two complementary theoretical frameworks, one formulated at the level of individuals, the other at the level of populations, provides a mechanistic understanding of the dynamics.

Crowding↗
PubMed1998

Spatial aspects of interspecific competition.

Using several variants of a stochastic spatial model introduced by Silvertown et al., we investigate the effect of spatial distribution of individuals on the outcome of competition. First, we prove rigorously that if one species has a competitive advantage over each of the others, then eventually it takes over all the sites in the system. Second, we examine tradeoffs between competition and dispersal distance in a two-species system. Third, we consider a cyclic competitive relationship between three types. In this case, a nonspatial treatment leads to densities that follow neutrally stable cycles or even unstable spiral solutions, while a spatial model yields a stationary distribution with an interesting spatial structure.

Competitive Behavior↗
PubMed1999

The effects of habitat fragmentation on persistence of source-sink metapopulations in systems with predators and prey or apparent competitors.

We consider systems with one predator and one prey, or a common predator and two prey species (apparent competitors) in source and sink habitats. In both models, the predator species is vulnerable to extinction, if productivity in the source is insufficient to rescue demographically deficient sink populations. Conversely, in the model with two prey species, if the source is too rich, one of the prey species may be driven extinct by apparent competition, since the predator can maintain a large population because of the alternative prey. Increasing the rate of predator movement from the source population has opposite effects on prey and predator persistence. High emigration rate exposes the predator population to danger of extinction, reducing the number of individuals that breed and produce offspring in the source habitat. This may promote coexistence of prey by relaxing predation pressure and apparent competition between the two prey species. The number of sinks and spatial arrangement of patches, or connectivity between patches, also influence persistence of the species. More sinks favor the prey and fewer sinks are advantageous to the predator. A linear pattern with the source at one end is profitable for the predator, and a centrifugal pattern in which the source is surrounded by sinks is advantageous to the prey. When the dispersal rate is low, effects of the spatial structure may exceed those of the number of sinks. In brief, productivity in patches and patterns of connectivity between patches differentially influence persistence of populations in different trophic levels.

Animals↗
PubMed1991

Atomic and molecular physics in the gas phase.

The spatial and temporal distributions of energy deposition by high-linear-energy-transfer radiation play an important role in the subsequent chemical and biological processes leading to radiation damage. Because the spatial structures of energy deposition events are of the same dimensions as molecular structures in the mammalian cell, direct measurements of energy deposition distributions appropriate to radiation biology are infeasible. This circumstance has led to the development of models of energy transport based on a knowledge of atomic and molecular interactions that enable one to simulate energy transfer on an atomic scale. Such models require a detailed understanding of the interactions of ions and electrons with biologically relevant material. During the past 20 years, there has been a great deal of progress in our understanding of these interactions, much of it coming from studies in the gas phase. These studies provide information on the systematics of interaction cross sections, and lead to knowledge of the regions of energy deposition where molecular and phase effects are important-knowledge that guides development in appropriate theory. In this report, studies of the doubly differential cross sections, which are crucial to the development of stochastic energy deposition calculations and track structure simulation, are reviewed. We discuss areas of understanding and address directions for future work. Particular attention is given to experimental and theoretical findings that have changed the traditional view of secondary electron production for charged-particle interactions with atomic and molecular targets.

Animals↗
PubMed1994

Asymptotic behaviour of reaction-diffusion systems in population and epidemic models. The role of cross diffusion.

Cross diffusion has been widely considered in the mathematical modelling of spatially structured ecological and epidemic systems, either in the mechanical description of diffusion or in the stochastic point process description of interacting populations. In this paper mathematical results recently obtained by the authors about the asymptotic behaviour of reaction-diffusion systems with full matrices of diffusion are applied to classes of biological systems.

Animals↗
PubMed1986

Multivariate analysis of Kenya's urban system.

"The study analyzes the structure, spatial interaction, and dimension of the post-colonial urban systems of Kenya using component factor analysis and based on selected socio-cultural, economic, and demographic variables. The study identified seven major dimensions of variation accounting for 72.0%. The analysis of the urban spatial variation is compared and intertwined with studies done in other countries to find out the regional variation of the loadings. Finally, a seven-order characteristics of Kenya urban systems is suggested, based on the study. The accurate knowledge of these salient characteristics of Kenya urban sub-systems is important before any viable spatial planning should be implemented."

Africa↗
PubMed1990

Spatio-temporal interactions and the spatial phase preferences of visual neurons.

We recorded single neuron responses in the cat's lateral geniculate nucleus (LGN) and visual cortex to compound stimuli composed of two sinusoidal gratings in a 2:1 frequency ratio. To probe visual receptive field symmetry, we varied the relative spatial phase of the two components and measured the effect on neuronal responses. We expected that on-center LGN neurons would respond best to gratings combined in positive cosine (bright bar) phase, while off-center LGN neurons would respond best to gratings combined in negative cosine (dark bar) phase. When drifting stimuli were used, cells' phase preferences were roughly 90 deg away from the expected values; when stationary, contrast-modulated stimuli were used, phase preferences were as originally predicted. Computer simulations showed that this discrepancy could be explained by taking into account the cells' temporal properties. Thus, tests using drifting stimuli confound the spatial structure of visual neural receptive fields with their temporal response characteristics. A small sample of data from cortical neurons reveals the same confound.

Action Potentials↗
PubMed1983

Aspects of modelling and simulating tumor growth and treatment.

The proliferation of malignant cells and tumor growth can be studied at various levels and from different viewpoints in the field of tumor biology and oncology. The aim of this paper is to outline how control theory and computer science can pave the way to new approaches to interpreting tumor growth and treatment. The present development is based on the hypothesis that the proliferation of malignant cells may be simulated by an unstable closed-loop control circuit. This type of model only describes the number of cells as a function of time. Therefore, an extended model permitting the study of the spatial structure of tumor growth is chosen. This approach leads to three-dimensional models simulating tumor growth in a vascularized tissue segment and opens the possibility of determining optimized chemotherapeutic tumor-treatment schedules. In the future it may become possible to perform computer simulations of different kinds of tumor treatment prior to clinical therapy.

Animals↗
PubMed1986

Two-dimensional spectrophotometry and pseudo-color representation of chemical reaction patterns.

The formation of spatio-temporal patterns in layers of chemical and biochemical media is observed and quantified by a two-dimensional spectrophotometer based on a computerized video equipment with high spatial, temporal, and intensity resolution. Images are shown, to some extent, in pseudo-colors. The use of pseudo-colors and three-dimensional perspective representations enhances the perception of the information contained in an image and the inherent order of the spatial structures.

Chemical Phenomena↗
PubMed1987

Genetic variability in mitochondrial DNA in a regional population of the great tit (Parus major).

Genetic variation of mitochondrial DNA (mtDNA) in 18 great tits (Parus major) from three neighboring localities in Sweden was investigated with eight tetranucleotide restriction endonucleases. The 18 individuals could be separated into 13 different maternal lineages. The high number of female lineages present in this regional population contrasts with a low level of sequence divergence between the different mtDNA clones, with a mean of 0.19% sequence divergence between all individuals. There was no obvious spatial structuring of mtDNA clones among the three localities. The presence of a high number of different clones with a low degree of sequence divergence could be explained by the effects of a large long-term effective population size, with the mtDNA clones having diverged about 25,000-200,000 years ago.

Animals↗