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At least 1,405 records · Page 78Linked to original sources
PubMed2006

Comparative analysis of the within-population genetic structure in wild cherry (Prunus avium L.) at the self-incompatibility locus and nuclear microsatellites.

Gametophytic self-incompatibility (SI) systems in plants exhibit high polymorphism at the SI controlling S-locus because individuals with rare alleles have a higher probability to successfully pollinate other plants than individuals with more frequent alleles. This process, referred to as frequency-dependent selection, is expected to shape number, frequency distribution, and spatial distribution of self-incompatibility alleles in natural populations. We investigated the genetic diversity and the spatial genetic structure within a Prunus avium population at two contrasting gene loci: nuclear microsatellites and the S-locus. The S-locus revealed a higher diversity (15 alleles) than the eight microsatellites (4-12 alleles). Although the frequency distribution of S-alleles differed significantly from the expected equal distribution, the S-locus showed a higher evenness than the microsatellites (Shannon's evenness index for the S-locus: E = 0.91; for the microsatellites: E = 0.48-0.83). Also, highly significant deviations from neutrality were found for the S-locus whereas only minor deviations were found for two of eight microsatellites. A comparison of the frequency distribution of S-alleles in three age-cohorts revealed no significant differences, suggesting that different levels of selection acting on the S-locus or on S-linked sites might also affect the distribution and dynamics of S-alleles. Autocorrelation analysis revealed a weak but significant spatial genetic structure for the multilocus average of the microsatellites and for the S-locus, but could not ascertain differences in the extent of spatial genetic structure between these locus types. An indirect estimate of gene dispersal, which was obtained to explain this spatial genetic pattern, indicated high levels of gene dispersal within our population (sigma(g) = 106 m). This high gene dispersal, which may be partly due to the self-incompatibility system itself, aids the effective gene flow of the microsatellites, thereby decreasing the contrast between the neutral microsatellites and the S-locus.

Alleles↗
PubMedDate not supplied

[Spatially heterogeneous distribution of phytoplankton in a model of a regulated population].

With the help of the mathematical model is shown that the existence of the phitoplankton's mechanism of growth-speed regulation by means of excretion to the environment of biologically active substances, may lead to a considerable inhomogenity of it's spatial distribution, that is, patchness. There appears an intensive crowding of phitoplankton with homogenious distribution of nutrients, while some limitations of excretion rate and of dispersion coefficients are taken into account.

Mathematics↗
PubMedDate not supplied

A spatio-temporal analysis of BSE cases born before and after the reinforced feed ban in France.

A spatio-temporal analysis was carried out to see how the risk distribution of bovine spongiform encephalopathy (BSE) in France changed depending on the period of birth. The data concerned the 539 BSE cases born in France after the ban (BAB) of meat and bone meal (MBM) in 1990 and detected between July 1, 2001 and December 31, 2003, when the surveillance of BSE was comprehensive. Seventy-two of these cases were born after the reinforced (second) ban (BASB) in 1996, which involved the removal of BSE-risk materials and cadavers from the processing of MBM. The Ederer-Myers-Mantel (EMM) time and space cluster test was applied, after classifying the cases by trimester and region of birth, BAB or BASB status, and dairy or beef status. Then disease mapping was performed for four successive birth periods, three for the BAB cases (January 1991 through June 1994, July 1994 through June 1995, July 1995 through June 1996), and one for the BASB (July 1996 through October 1998). It was elaborated with the Bayesian graphical modelling methods and based on a Poisson distribution with spatial smoothing. The parameters were estimated by a Markov Chain Monte Carlo (MCMC) simulation method. The main finding was that the areas with the highest risk of BSE changed largely from one birth period to another; from the west, it reached the east of France for birth cohort 1994-1995 and the southwest for birth cohort 1995-1996. The EMM test identified a peak risk in this region both for dairy and beef cattle in the fall 1995. The spatial distribution of the risk for the BASB cases matched the spatial pattern of risk for the preceding BAB birth cohort quite well; this was in favour of a common origin of the infection of the BAB and BASB cases, despite the complementary control measures.

Animal Feed↗
PubMedDate not supplied

Temporally and spatially heterogeneous distribution of mTHPC in a murine tumor observed by two-color confocal fluorescence imaging and spectroscopy in a whole-mount model.

Efficient intratumor delivery of anticancer drugs and photosensitizers is an important factor in the success of chemotherapy and photodynamic therapy, respectively. Unfortunately, their adequate and uniform intratumor distribution is impeded by several physiological barriers and by binding to tissue components. Measurement of gross tumor drug accumulation is a routine method of investigating the uptake and clearance of chemotherapy agents and photosensitizers but tells little about their extravascular spatial distribution. We use whole-mount two-color confocal fluorescence imaging and imaging spectroscopy of unprocessed excised murine tumor fragments to investigate the intratumor distribution of the photosensitizer meso-tetrahydroxyphenyl chlorin (mTHPC) as a function of distance from blood vessels perfused with 0.2 mum diameter fluorescent microspheres. Significant mismatches between drug and perfused vasculature are caused by heterogeneities in tumor blood supply. We describe complex microscopic mTHPC gradients that reverse dramatically relative to the perfused vasculature with time after injection. This imaging technique can be applied to screen the dynamic intratumor distribution of other fluorescent photosensitizers and anticancer drugs.

Animals↗
PubMed1993

Functional topography of cat primary auditory cortex: responses to frequency-modulated sweeps.

The spatial distribution of neuronal responses to frequency-modulated (FM) sweeps was mapped with microelectrodes in the primary auditory cortex (AI) of barbiturate-anesthetized cats. Increasing and decreasing FM sweeps (upward- and downward-directed FM sweeps, respectively) covering a range of 0.25-64.0 kHz were presented at three different rates of frequency change over time (i.e, sweep speed). Using multiunit recordings, the high-frequency domain (between 3.2 and 26.3 kHz) of AI was mapped over most of its dorsoventral extent (as determined by the distribution of the excitatory bandwidth, Q10dB) for all six cases studied. The spatial distributions of the preferred sweep speed and the preferred sweep direction were determined for each case. Neuronal responses for frequency sweeps of different speeds appeared to be systematically distributed along the dorsoventral axis of AI. In the dorsal region, cortical cells typically responded best to fast and/or medium FM sweeps, followed more ventrally by cells that responded best to medium--then slow--, then medium-speed FM sweeps. In the more ventral aspect of AI (which in some cases may also have included cells located in the dorsal region of the second auditory field, AII), neurons generally preferred fast FM sweeps. However, a comparison of maps from different animals showed that there was more variability in the distribution of preferred speed responses in the ventral region of the cortex. The directional preference of units for FM sweeps was determined for the sweep speed producing the strongest response. Direction selectivity appeared to be nonrandomly distributed along the dorsoventral axis of AI. In general, units that responded best to upward-directed FM sweeps were located in the more dorsal and ventral aspects of AI while units that responded best to downward-directed FM sweeps were usually located in the mid-region of AI. Direction selectivity was also determined for multiunit responses at each of the three FM sweep speeds. In general, there was a relatively close agreement between the spatial distributions of direction selectivity determined for the strongest response with those calculated for the fast and medium speeds. The spatial distribution of direction selectivity determined for slow FM sweeps deviated somewhat from that determined for the strongest response. Near the dorsoventral center of the mapped areas, the distribution of units that responded best to downward sweeps tended to overlay the distribution of units that responded best to slow speeds, suggesting some spatial covariance of the two parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed2005

Spatial electron distribution and its origin in the nanoporous TiO2 network of a dye solar cell.

Dye solar cells have been investigated by charge carrier extraction under short and open circuit conditions and an illumination intensity equivalent to 1 sun (AM 1.5). Under short circuit conditions, a surprisingly high amount of charge carriers stored in the nanoporous TiO2 network has been observed. A theoretical model was developed to describe the charge transport in the nanoporous TiO2 network of a dye solar cell, and the spatial distribution of the electron concentration was calculated. These results were compared with the experimental data of charge carriers stored in the TiO2 network under short and open circuit conditions. We were able to conclude that under short circuit conditions, the electrochemical potential of the electrons in the region far from the electrode is up to 550-570 meV higher than that of the electrons at the front electrode. This internal voltage is the driving force across the nanoporous TiO2 film under short circuit conditions.

Journal Article↗
PubMed2004

Coherence and discontinuity in the scaling of species' distribution patterns.

The spatial distribution of a species can be characterized at many different spatial scales, from fine-scale measures of local population density to coarse-scale geographical-range structure. Previous studies have shown a degree of correlation in species' distribution patterns across narrow ranges of scales, making it possible to predict fine-scale properties from coarser-scale distributions. To test the limits of such extrapolation, we have compiled distributional information on 16 species of British plants, at scales ranging across six orders of magnitude in linear resolution (1 m to 100 km). As expected, the correlation between patterns at different spatial scales tends to degrade as the scales become more widely separated. There is, however, an abrupt breakdown in cross-scale correlations across intermediate (ca. 0.5 km) scales, suggesting that local and regional patterns are influenced by essentially non-overlapping sets of processes. The scaling discontinuity may also reflect characteristic scales of human land use in Britain, suggesting a novel method for analysing the 'footprint' of humanity on a landscape.

Demography↗
PubMedDate not supplied

[Distribution and spatial ordering of biopolymer molecules in resting bacterial spores].

The presence, distribution and spatial arrangement of biopolymers in situ were studied in both a total intact spore and in a certain cellular layer using a spectroscopic technique of attenuated total refraction (ATR-IR) in the IR region. In contrast to vegetative cells, intact spores were characterized by isotropic distribution of protein components. This feature can be regarded as an index of the cryptobiotic state of spores. However, the distribution of protein components among individual layers of a spore was anisotropic. Bonds characterized by amide I and amide II bands were most often ordered in a layer which comprised cellular structures from the exosporium to the inner spore membrane.

Anaerobiosis↗
PubMedDate not supplied

Modeling population responses of rapidly-adapting mechanoreceptive fibers.

The population response of rapidly-adapting (RA) fibers is one component of the physiological substrate of the sense of touch. Herein, we describe a computational scheme based on the population-response model by K.O. Johnson (J. Neurophysiol. 37: 48-72, 1974) which we extended by permitting the capability to include the spatial distributions of receptors in the glabrous skin linked to RA fibers. The hypothetical cases simulated were rectangular, uniformly random and proximo-distally Gaussian distributions. Each spatial organization produced qualitatively distinct population-response profiles that also varied due to stimulus parameters. The effects of stimulus amplitude, average innervation density and contactor-probe location were studied by considering various response measures: number of active fibers, summated firing rate and the average firing rate of a subset of the modeled population. The outcome of the measures were statistically compared among simulated anatomical distributions. The response is the same for rectangular and uniformly random distributions, both of which have a homogeneous innervation density. However, the Gaussian distribution produced statistically different responses when the measure was not averaged over the subset population which represented the receptive field of a higher-order neuron. These results indicate that, as well as stimulus parameters, the anatomical organization is a significant determinant of the population response. Therefore, reconstructing population activity for testing psychophysical hypotheses must presently be done with care until the organization of the receptors within the skin has been clarified.

Action Potentials↗
PubMed1979

Extracellular potentials related to intracellular action potentials during impulse conduction in anisotropic canine cardiac muscle.

This paper considers a quantitative description of intracellular and transmembrane currents in anisotropic muscle, with emphasis on the factors that determine the extracellular potentials. Although Vmax of the intracellular action potential had no relation to changes in conduction velocity in anisotropic tissue with constant membrane properties, the extracellular waveforms were quite sensitive to velocity changes. Large amplitude biphasic deflection occurred in the fast areas, and in the slow areas the waveforms were of lower amplitude and triphasic in shape; i.e., negative potentials preceded the biphasic positive-negative deflection. The extracellular potentials were simulated on the bases of a model of intracellular currents, and the theoretical and measured results showed good agreement. In tissue with anisotropic conductivity, the relationship between the spatial intracellualr potential gradient and the magnitude of the extracellular potential of the excitation wave was opposite to the classical relationship in isotropic tissue. Due to the influence of the effective intracellular conductivity on the spread of intracellular currents and on conduction velocity, in anisotropic tissue the extracellular potential decreased as the intracellular potential gradient increased. The peak values of the positive and negative potentials and the spatial distribution of the potential gradients varied considerably along the activation front. These findings were accounted for by differences in the distribution and spatial extent of the transmembrane currents, which were determined by the intracellular currents. The theoretical analysis showed that intracellular and transmembrane currents were proportional to the local conduction velocities of the wavefront. Thereby, it was not possible to have a "uniform layer" of current when there were differences in conduction velocity along the length of the excitation wave. The implications of the analysis are considerable, since the gratifying agreement between the theoretical and measured results indicates that the details of the extracellular waveforms can be explained on the basis of the distribution of intracellular currents; i.e., extracellular potentials provide a sensitive index of intracellular current flow.

Action Potentials↗
PubMed2001

Temporal spatial differences observed by functional MRI and human intraoperative optical imaging.

Pre-operative functional magnetic resonance imaging (fMRI), cortical evoked potentials (EPs) and intraoperative optical imaging of intrinsic signals (iOIS) were employed to relate the temporal-spatial characteristics of sensorimotor responses in human brain. Peripheral somasthetic stimulation (2 s) was provided either by a 110 Hz finger vibrator or transcutaneous median nerve stimulation in eight patients undergoing neurosurgical procedures. Each technique provided unique spatial patterns and temporal response profiles. EPs and iOIS activities were observed over the surface of pre- and post-central gyri (at the level of the superior genu) with very similar spatial distributions. In contrast, fMRI spatial distributions depended upon the model used for statistical correlation analysis. Using a monophasic response model, fMRI primarily localized within the central sulcus and did not demonstrate large signal changes over the pre- and post-central gyri (areas with iOIS/EP activity). However, as initial negative responses were incorporated into the response model, fMRI progressively localized closer to the iOIS and somatosensory EP maps. Temporally, responses to single stimuli differed between the fMRI and iOIS techniques. Using a monophasic model for fMRI analysis, the total fMRI response was delayed by 2--3 s relative to iOIS. As initial negative responses were incorporated in the analysis, the fMRI time course developed temporal characteristics similar to iOIS. Ultimately, when fMRI time courses were examined over pixels co-localizing with iOIS activation (without using statistical correlation analysis), the fMRI temporal profile included an initial decrease in signal (an initial dip) that closely resembled the time course of iOIS response. This is the first study to experimentally co-localize (temporally and spatially) iOIS and fMRI signals in human subjects. The spatial/temporal differences in this study likely reflect the capillary versus venous contributions of iOIS and fMRI, respectively. The temporal/spatial co-localization of the iOIS signal and the fMRI initial dip suggests the initial fMRI dip and the iOIS signal may result from similar physiologic events.

Algorithms↗
PubMed2003

Spatial patterns of organic chlorine and chloride in Swedish forest soil.

The concentration of organic carbon, organic chlorine and chloride was determined in Swedish forest soil in the southern part of Sweden and the spatial distribution of the variables were studied. The concentration of organically bound chlorine was positively correlated to the organic carbon content, which is in line with previous studies. However, the spatial distribution patterns strongly indicate that some other variable adds structure to the spatial distribution of organic chlorine. The distribution patterns for chloride strongly resembled the distribution of organic chlorine. The spatial distribution of chloride in soil depends on the deposition pattern which in turn depends on prevailing wind-direction, amount of precipitation and the distance from the sea. This suggests that the occurrence of organic chlorine in soil is influenced by the deposition of chloride or some variable that co-varies with chloride. Two clearly confined strata were found in the area: the concentrations of organic chlorine and chloride in the western area were significantly higher than in the eastern area. No such difference among the two areas was seen regarding the carbon content.

Carbon↗
PubMed1987

Transverse relationships of the alpha rhythm on the scalp.

Using auto- and cross-spectral analyses on data of 3 normal adult subjects, two components of rhythmic alpha activity were distinguished with respect to their spatial distribution and spatial relationships on the scalp. Derivations from 21 scalp points vs. linked ears were analyzed. The alpha activity with power maxima in the bilateral occipital areas (component A) showed high coherence with activity in the anterior areas and somewhat lower coherence with activity in the central/parietal areas. The other component (B) appeared dominantly in the central areas, showing extremely low values of coherence between the anterior and posterior regions, and an unstable phase relation among recording points. Components A and B appear to correspond to the 'generalized' and 'localized' alpha components described earlier. It is suggested that these two components account for the major properties of the rhythmic alpha activity.

Adult↗
PubMed2003

Spatio-temporal growth dynamics of a subAlpine Pinus uncinata stand in the French Alps.

Natural forests are characterised by a high level of both spatial and temporal heterogeneity. The major processes involved in the creation and maintenance of forest heterogeneity in temperate climates are small-sized canopy disturbances. The resulting openings also modify the growth conditions for the remaining trees. The analysis of tree growth responses using dendrochronological techniques allows the reconstruction of the time sequence of the disturbances. The radial growth analysis is coupled here with both a spatial analysis and a demographic analysis of the forest structure. The age classes are temporally organised in cohorts and spatially distributed in spatial aggregates. The analysis of the most documented disturbance event allowed us to determine the size of the disturbance response patch (between 0.036 and 0.073 ha) and the duration of the disturbance (six years). We considered that this small-scale disturbance event may have been caused by the fall of a single tree and has freed the surrounding trees of its competition.

Altitude↗
PubMed2006

Neutron dosimetry with 6LiF-rich TL sheet.

A (6)LiF-rich thermoluminescent sheet-type dosemeter ((6)LiF-rich NTL sheet) was developed for neutron 2D dosimetry. The dosemeter utilises the (6)Li(n, alpha)(3)H reaction to detect thermal neutrons. Responses of the (6)LiF-rich NTL sheet to neutrons were measured at the neutron beam irradiation facility for BNCT in JRR-4 Research Reactor at the Japan Atomic Energy Research Institute. Placement of a multi-leaf collimator at the output port of the neutron (beam) irradiation facility, produced either stripe- or round-shaped neutron distributions; the spatial distribution was measured using the developed NTL sheets. Direct measurements of neutron attenuation in water were also carried out using the developed NTL sheet, submersed in a water phantom. In each experiment, NTL sheets having natural abundance (7.9%) of LiF, and (6)LiF-enriched NTL (18.94%) sheet were irradiated under the same conditions. The ratio of thermoluminescence intensities of the (6)LiF-rich NTL sheet to that of the normal NTL sheet was compared to a theoretically calculated value. The experimental measurements are shown to be in good agreement with the calculations.

Boron Neutron Capture Therapy↗
PubMed1999

Homologues of the Cf-9 disease resistance gene (Hcr9s) are present at multiple loci on the short arm of tomato chromosome 1.

The tomato Cf-4 and Cf-9 genes map at a genetically complex locus on the short arm of chromosome 1 and confer resistance against Cladosporium fulvum through recognition of different pathogen-encoded avirulence determinants. Cf-4 and Cf-9 are members of a large gene family (Hcr9s, Homologues of Cladosporium fulvum resistance gene Cf-9), some of which encode additional distinct recognition specificities. A genetic analysis of the majority of Hcr9s suggests that their distribution is spatially restricted to the short arm of chromosome 1. Two loci of clustered Hcr9 genes have been analyzed physically that mapped distal (Northern Lights) and proximal (Southern Cross) to the Cf-4/9 locus (Milky Way). Sequence homologies between intergenic regions at Southern Cross and Milky Way indicate local Hcr9 duplication preceded cluster multiplication. The multiplication of clusters involved DNA flanking Hcr9 sequences as indicated by conserved lipoxygenase sequences at Southern Cross and Milky Way. The similar spatial distribution of Hcr9 clusters in different Lycopersicon spp. suggests Hcr9 cluster multiplication preceded speciation.

Base Sequence↗
PubMed1996

Neighborhood Effects in Bird Distributions, Navarre, Spain

This study examines the role of neighborhood effects in the spatial distributions of selected bird species in Navarre, Spain. We employed a geographic information system (GIS) to organize the data on bird distributions and relevant environmental variables and to analyze their spatial patterns. Three bird species were selected for analysis: the European honey-buzzard (Pernis apivorus), the Eurasian hobby (Falco subbuteo), and the European pied flycatcher (Ficedula hypoleuca). Selected environmental variables of the study area were digitized to create a comprehensive data base and logistic regression models were used to evaluate the significance of each variable in the spatial distribution. The spatial patterns of bird distributions were used to extract topological relationships and to identify neighborhood effects. Although all the selected species illustrate a pattern of positive spatial autocorrelation in their distributions, the significance of neighborhood effects varies from species to species. Among the selected species, neighborhood effects are most evident in the distribution of the European pied flycatcher and are significant for the Eurasian hobby. The distribution of the European honey-buzzard is not much affected by neighborhood effects. The results suggest that examination of neighborhood effects is a prerequisite for modeling bird distributions.KEY WORDS: neighborhood effects, spatial autocorrelation, logistic regression, bird distributions.

Journal Article↗
PubMed1995

New thermoluminescent BaSO4:Eu sheet for in vivo measurement of spatial dose distribution in radiation therapy.

A new highly sensitive thermoluminescent (TL) sheet has been studied as a means of in vivo measurement of spatial dose distribution of radiation therapy. This TL sheet (40 cm x 50 cm x 0.2 mm), which is composed of teflon mixed with BaSO4:Eu doped powder, is very flexible and can be cut to the desired size. The TL sheet was found to have a linear response with a very wide dynamic range of at least 0.002 cGy to 5000 cGy absorbed dose. In addition, this sheet does not need be shielded, because of its insensitivity to room light, and therefore the sheet can detect low-energy electrons decreased by a few millimeter thick air for 3H(maximum beta-ray energy: 18 KeV). The spatial dose distribution was printed out with a newly developed digital readout system. In addition, another high-resolution dosimetry system, which exposes X-ray film with TL photons emitted from the irradiated TL sheet at constant room temperature, is reported. Clinically, the in vivo dose distribution on the surface of the rectal cancer for intracavitary radiation therapy was determined. The applicability of TL sheet for in vivo measurement of dose distribution is discussed.

Humans↗