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At least 73 records · Page 4Linked to original sources
PubMed2002

Spatial structure of shell polychromatism in populations of Cepaea nemoralis: new techniques for an old debate.

A conspicuous shell polychromatism is observed in colonies of Cepaea nemoralis from western France (Brittany). The present study is intended to search for a spatial structure of shell features at this scale and to infer evolutionary processes from the observed patterns. We used a database of morph frequencies (six composite phenotypes were retained) measured on 213 samples regularly distributed on the whole studied area. Data analysis was based on two distinct multivariate methods leading to the following steps: (i) to search for a structure without reference to environmental conditions with a method (global principal components analysis: GPCA) which takes into account the spatial information by means of a neighbouring relationship between sampling points (Delaunay triangulation); (ii) to test the structuring power of environmental conditions by means of two explanatory factors (distance from the sea, altitude) involved in a redundancy analysis (RDA); (iii) to search for a spatial structure using residuals of the previous analysis, ie, after removing effects of environmental conditions. Global covariance accounted for 26.4% of the total variance, leading to a highly significant autocorrelation for each phenotype (step 1). Geographical mapping of factorial scores resulting from global analysis showed a well structured littoral zone and a strong southern-northern inland differentiation. Sixteen percent of the total variance was expressed in RDA but all morphs were not equally concerned. After removing environmental effects, a significant spatial structure still remains but was essentially caused by random processes. We argue for the importance of these last phenomena.

Animals↗
PubMed2004

Spatial structure of dye-doped polymer nanoparticle laser media.

The spatial structure of dye-doped polymer-nanoparticle gain media, reported to generate spatially homogeneous single-transverse-mode laser beams, has been examined by electron microscopy in the nanometer scale. It is found that the distribution of the silica nanoparticles in the laser dye-doped polymer is fairly uniform. There is some aggregation of silica particles into loose nanoclusters. However, the nanocluster dimensions are smaller than those necessary to satisfy the conditions for internal interference in the visible spectrum. This explains the absence of macroscopic spatial inhomogeneities in the emission laser beams.

Journal Article↗
PubMed1995

Spatial structure of two-locus genotypes under isolation by distance.

Extensive Monte Carlo simulations are conducted of spatial distributions of two-locus genotypes in large, continuous populations under isolation by distance models. The results show that substantial patches of double homozygotes are present in the spatial structures, even when loci are unlinked. The stochastic spread of identical two-locus genotypes largely outpowers the tendency for recombination to decouple patterns for separate loci. A spatial patch is a large area containing mostly one double homozygous genotype in a highly contiguous constellation. This patch structure is reflected in high positive spatial autocorrelations and large excesses of pairs, or joins, of identical double homozygotes at short-to-intermediate distances of spatial separation. Although spatial patches of double homozygotes are the dominant spatial feature, and the major contributors to overall high levels of autocorrelations among two-locus genotypes, other substantial features include areas of concentrations of identical genotypes heterozygous at only one locus. One implication of the patch structure is the presence of high levels of linkage disequilibrium, caused by isolation by distance even for unlinked loci, at some spatial scales; yet the disequilibrium in the large total populations is near 0. Thus linkage disequilibrium produced by isolation by distance is highly dependent on spatial scale. Another implication is that high degrees of spatial structuring and autocorrelations are produced for genetic variation controlling quantitative traits, at least when the number of loci is relatively small, under a wide range of situations, even if the trait is selectively neutral. The significance of the results to field studies is also examined.

Diploidy↗
PubMed1988

Two-dimensional spatial structure of receptive fields in monkey striate cortex.

Measurements of the spatial contrast sensitivity function and orientation selectivity of visual neurons in the foveal striate cortex (V1) of primates were interpreted within the context of a model of the two-dimensional spatial structure of their receptive fields. Estimates of the spatial dimensions of the receptive fields along the axis of preferred orientation were derived from the application of the model and were compared with estimates of the smallest spatial subunit in the dimension orthogonal to the preferred orientation. Some measure of agreement was found with corresponding estimates of parameters for psychophysical channels in human foveal vision.

Animals↗
PubMed2000

Does evolution of iteroparous and semelparous reproduction call for spatially structured systems?

A persistent question in the evolution of life histories is the fitness trade-off between reproducing only once (semelparity) in a lifetime or reproducing repeated times in different seasons (iteroparity). The problem can be formulated into a research agenda by assuming that one reproductive strategy is resident (has already evolved) and by asking whether invasion (evolution) of an alternative reproductive strategy is possible. For a spatially nonstructured system, Bulmer (1994) derived the relationship v + PA < 1 (PA is adult survival; vbS and bS are offspring numbers for iteroparous and semelparous breeding strategies, respectively) at which semelparous population cannot be invaded by an iteroparous mutant. When the inequality is changed to v + PA > 1, invasion of a semelparous mutant is not possible. From the inequalities, it is easy to see that possibilities for evolutionary establishment of a novel reproductive strategy are rather narrow. We extended the evolutionary scenario into a spatially structured system with dispersal linkage among the subunits. In this domain, a rare reproductive strategy can easily invade a population dominated by a resident reproductive strategy. The parameter space enabling invasion is far more generous with spatially structured evolutionary scenarios than in a spatially nonstructured system.

Animals↗
PubMedDate not supplied

Spatial structure and richness of ectomycorrhizal fungi colonizing bioassay seedlings from resistant propagules in a Sierra Nevada forest: comparisons using two hosts that exhibit different seedling establishment patterns.

In this study we analyzed the spatial structure of ectomycorrhizal fungi present in the soils as resistant propagules (e.g. spores or sclerotia) in a mixed-conifer forest in the Sierra Nevada, California. Soils were collected under old-growth Abies spp. stands across approximately 1 km and bioassayed with seedlings of hosts that establish best in stronger light (Pinus jeffreyi) or that are shade-tolerant (Abies concolor). Ectomycorrhizal fungi colonizing the roots were characterized with molecular techniques (ITSRFLP and DNA sequence analysis). Wilcoxina, five Rhizopogon species and Cenococcum were the most frequent of 17 detected species. No spatial structure was detected in the resistant propagule community as a whole, but P. jeffreyi seedlings had higher species richness and associated with seven Rhizopogon species that were not detected on A. concolor seedlings. We drew two conclusions from comparisons between this study and a prior study of the ectomycorrhizal community on mature trees in the same forest: (i) the resistant propagule community was considerably simpler and more homogeneous than the active resident community across the forest and (ii) Cenococcum and Wilcoxina species are abundant in both communities.

Abies↗
PubMed2004

A Malthusian curb on spatial structure in microorganism populations.

That all organisms are born in the company of a parent but die alone is a fundamental biological asymmetry. It has been suggested that this provides a deep-rooted source of spatial pattern formation for microorganisms even at the scale of the population. Such a theory, however, neglects the strong influence in nature of the limited and spatially variable availability of food. The tendency, first recognized by Thomas Malthus in the 18th century, of a population to out-strip its food resources will eventually lead, through local starvation, to the suppression of a heterogeneity growing within a population. Using a generic model it is demonstrated that including local food limitation of breeding strongly dampens spatial structure otherwise resulting from birth and death. The extent of this damping is shown to be a function of the strength of the coupling between organisms and their food and of the total abundance of organic material. Moreover, this work provides an example of a density-dependent process acting to diminish spatial structure rather than to create it and highlights the rich variety of behaviour that is missed by continuum models which fail to represent such local dynamics.

Animals↗
PubMedDate not supplied

[Changes in the spatial structure of the destruction process under the conditions of atmospheric pollution of forest ecosystems].

Contamination of the southern taiga forests (Middle Urals) by discharges from cooper smelters (heavy metals combined with SO2) not only decreases the mean rate of decomposition of pure cellulose, but also radically changes the spatial structure of the destruction process. Heterogeneity of distribution of the destruction rate is sharply increased near the source of discharge due to differentiation of the space into microregions with high and low destruction rates. The characteristic size of spatial heterogeneity amounts to several tens of centimeters and the distribution of microregions with a high rate is accidental within several tens of meters. A hypothesis has been put forward that the described changes in the spatial structure of the destruction process are related, above all, to disturbed colonization of the substrate by soil myxomycetes.

Air Pollutants↗
PubMed2005

Local spatial structure and predator-prey dynamics: counterintuitive effects of prey enrichment.

The Lotka-Volterra predator-prey model with prey density dependence shows the final prey density to be independent of its vital rates. This result assumes the community to be well mixed so that encounters between predators and prey occur as a product of the landscape densities, yet empirical evidence suggests that over small spatial scales this may not be the normal pattern. Starting from an individual-based model with neighborhood interactions and movements, a deterministic approximation is derived, and the effect of local spatial structure on equilibrium densities is investigated. Incorporating local movements and local interactions has important consequences for the community dynamics. Now the final prey density is very much dependent on its birth, death, and movement rates and in ways that seem counterintuitive. Increasing prey fecundity or mobility and decreasing the coefficient of competition can all lead to decreases in the final density of prey if the predator is also relatively immobile. However, analysis of the deterministic approximation makes the mechanism for these results clear; each of these changes subtly alters the emergent spatial structure, leading to an increase in the predator-prey spatial covariance at short distances and hence to a higher predation pressure on the prey.

Animal Migration↗
PubMedDate not supplied

[Spatial structure of apamin in solution].

The calculation of the complete spatial structure of the bee venom peptide neurotoxin apamin has been carried out by means of a method elaborated earlier. It is based on the joint utilization of the molecular mechanics algorithms and NMR spectroscopy data. It was established that the molecule backbone conformation in solution may be represented as the combination of the beta-turn III (residues 2-5) and alpha-helical segment (9-18) both separated by the non-standard bend IV (5-8). The most probable system of the intramolecular hydrogen bonds in the apamin polypeptide backbone was proposed. Certain amino acid residues have been shown to be characterized by the lack of strict determination of the conformations of their side chains which may be realized in a few states providing approximately equal stabilization of the same form of the main chain. The conformational parameters of the proposed apamin structural model are appropriate to the NMR spectroscopy data derived from the literature and used in the calculations and are not contradictory to other experimental information.

Amino Acid Sequence↗
PubMed1998

Dynamics of a feline retrovirus (FeLV) in host populations with variable spatial structure.

The predictions of epidemic models are remarkably affected by the underlying assumptions concerning host population dynamics and the relation between host density and disease transmission. Furthermore, hypotheses underlying distinct models are rarely tested. Domestic cats (Felis catus) can be used to compare models and test their predictions, because cat populations show variable spatial structure that probably results in variability in the relation between density and disease transmission. Cat populations also exhibit various dynamics. We compare four epidemiological models of Feline Leukaemia Virus (FeLV). We use two different incidence terms, i.e. proportionate mixing and pseudo-mass action. Population dynamics are modelled as logistic or exponential growth. Compared with proportionate mixing, mass action incidence with logistic growth results in a threshold population size under which the virus cannot persist in the population. Exponential growth of host populations results in systems where FeLV persistence at a steady prevalence and depression of host population growth are biologically unlikely to occur. Predictions of our models account for presently available data on FeLV dynamics in various populations of cats. Thus, host population dynamics and spatial structure can be determinant parameters in parasite transmission, host population depression, and disease control.

Animals↗
PubMed2004

Spatial structure, dispersal, and management of a recovering raptor population.

The recovery of the peregrine falcon (Falco peregrinus anatum) in California has taken place amid strong geographical differences in habitat quality, potentially creating a sink population in the southern coastal habitat and source populations in the northern interior and urban habitats. We analyzed long-term monitoring data to investigate the mechanisms and consequences of spatial structuring for the recovery of this set of nonstable subpopulations. Dispersal rates between habitats were asymmetric, with extremely limited dispersal out of the interior habitat and a strong tendency for birds in the southern coast to disperse to the urban habitats. We used these dispersal estimates and habitat-specific productivity rates to build a set of regional population models that describe population growth within and dispersal between each subpopulation. We tested for the existence of habitat-specific survival and territory acquisition rates by comparing model projections with the number of breeding pairs censused annually in each subpopulation. Our analyses indicate a high rate of survival for interior birds and suggest that both the interior and urban subpopulations were regulated by territory availability over the study period. The inherent spatial structure of this regional peregrine falcon population has had a considerable influence on its recovery and management.

Animal Migration↗
PubMed2006

Scale-free extinction dynamics in spatially structured host-parasitoid systems.

Much of the work on extinction events has focused on external perturbations of ecosystems, such as climatic change, or anthropogenic factors. Extinction, however, can also be driven by endogenous factors, such as the ecological interactions between species in an ecosystem. Here we show that endogenously driven extinction events can have a scale-free distribution in simple spatially structured host-parasitoid systems. Due to the properties of this distribution there may be many such simple ecosystems that, although not strictly permanent, persist for arbitrarily long periods of time. We identify a critical phase transition in the parameter space of the host-parasitoid systems, and explain how this is related to the scale-free nature of the extinction process. Based on these results, we conjecture that scale-free extinction processes and critical phase transitions of the type we have found may be a characteristic feature of many spatially structured, multi-species ecosystems in nature. The necessary ingredient appears to be competition between species where the locally inferior type disperses faster in space. If this condition is satisfied then the eventual outcome depends subtly on the strength of local superiority of one species versus the dispersal rate of the other.

Animals↗
PubMed2000

Role of commensal relationships on the spatial structure of a surface-attached microbial consortium.

A flow cell-grown model consortium consisting of two organisms, Burkholderia sp. LB400 and Pseudomonas sp. B13(FR1), was studied. These bacteria have the potential to interact metabolically because Pseudomonas sp. B13(FR1) can metabolize chlorobenzoate produced by Burkholderia sp. LB400 when grown on chlorobiphenyl. The expected metabolic interactions in the consortium were demonstrated by high performance liquid chromatography (HPLC) analysis. The spatial structure of the consortium was studied by fluorescent in situ rRNA hybridization and scanning confocal laser microscopy. When the consortium was fed with medium containing a low concentration of chlorobiphenyl, microcolonies consisting of associated Burkholderia sp. LB400 and Pseudomonas sp. B13(FR1) bacteria were formed, and separate Pseudomonas sp. B13(FR1) microcolonies were evidently not formed. When the consortium was fed citrate, which can be metabolized by both species, the two species formed separate microcolonies. The structure development In the consortium was studied online using a gfp-tagged Pseudomonas sp. B13(FR1) derivative. After a shift In carbon source from citrate to a low concentration of chlorobiphenyl, movement of the Pseudomonas sp. B13(FR1) bacteria led to a change in the spatial structure of the consortium from the unassociated form towards the associated form within a few days. Experiments Involving a gfp-based Pseudomonas sp. B13(FR1) growth activity reporter strain Indicated that chlorobenzoate supporting growth of Pseudomonas sp. B13(FR1) is located close to the Burkholderia sp. LB400 microcolonies in chlorobiphenyl-grown consortia.

Adaptation, Physiological↗
PubMed1998

Spatial structure model of the CD4 receptor-binding site of the HIV envelope protein gp120.

In this study we have undertaken attempt to predict 3D structure of the CD4 receptor-binding site of the HIV envelope protein gp120. The structure of this site has been constructed by the analysis of low-energy conformers of peptide T, an HIV reproduction inhibitor with amino acid sequence corresponding to the fragment Ala-Ser-Thr-Thr-Thr-Asn-Tyr-Thr of protein gp120, ensuring the interaction of virus with T4 lymphocytes. To do this, the following researches have been carried out: i) the spatial structure models of peptide T and similar fragment 4-11 of an analogues of vasoactive intestinal peptide have been modeled by the restrained molecular mechanics method developed earlier, ii) conformational parameters of these models have been compared to geometrical characteristics of homologous segments of unrelated proteins with known spatial structures. The following major conclusions have been made based on the comparative analysis: i) the conformation of C-terminal fragment Thr-Thr-Asn-Tyr-Thr of peptide T, responsible for the biological activity of the molecule, does not undergo the essential distortions while embedding into the peptide chains of unrelated proteins; ii) this conformation, that is realized in isolated molecule and includes two consecutive reverse turns of the polypeptide chain, adequately describes the main conformational features of an appropriate site of the HIV protein gp120; iii) the fragment Ala-Ser-Thr-Thr-Thr-Asn-Tyr-Thr of protein gp120 accepts one of six spatial forms which are characteristic for peptide T.

Binding Sites↗
PubMed2005

Variation in frequency of hybrids and spatial structure among Ipomopsis (Polemoniaceae) contact sites.

Rates of hybridization vary among angiosperm taxa. Among-taxon variation in hybridization rate has been used to compare the importance of pre- and post-zygotic reproductive isolating mechanisms. Variation in rates of hybridization within a single-species pair would suggest that local conditions also affect reproductive isolation within a single taxonomic context. In this study, contact sites of Ipomopsis aggregata-Ipomopsis tenuituba were surveyed for variation in frequency of hybrids, and spatial structure. Floral morphology was used to identify parent species and hybrids in seven contact sites in the western Rocky Mountains, USA. Contact sites varied widely in elevational range, the degree to which morphological variation was clinal rather than mosaic and the frequency of hybrids. Two sites provided a strong contrast between a clinal, unimodal site and a mosaic, bimodal site. This natural variation among contact sites of the same species pair provides an opportunity to assess the effect of local ecological conditions and spatial structure of parent populations on reproductive isolation, while controlling for between-taxon variation.

Demography↗
PubMed2005

[New method for spatial structure quantification of biofilm with GFP tagged bacteria].

Green fluorescent protein (GFP) gene was applied to mark E. coli JM109 by the method of CaCl2 transformation. The GFP-tagged cells was inoculated into the LB medium containing 50 microg/mL of Ampicillin to develop biofilm on the surface of lava by mix-culture with lava particles in flask under 37 degrees C, 120 r/min for 16 h. The fluorescent images stack of different layer at specific area (250 microm x 250 microm) on lava covered with biofilm was obtained by confocal scanning laser microscopy. The quantifying parameters of biofilm spatial structure could be acquired after the image information was calculated by COMSTAT program, i. e., the average thickness of specific area of biofilm after 16 h was 0.120 844 microm, the maximum thickness was 10.5 microm, the volume was 0.136 986 microm3 /microm2, the surface area was 21 338.1 microm2, and the surface area per volume biofilm was 3.36 854 microm2 /microm3. This method might be expanded to reflect the spatial structure of biofilm with any other GFP tagged bacteria.

Biodegradation, Environmental↗