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Results for “Environmental selection”

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At least 217 records · Page 12Linked to original sources

PubMed2004

Using the natural 15N abundance to assess the main nitrogen inputs into the sand dune area of the north-western Negev Desert (Israel).

The variation of the natural 15N abundance is often used to evaluate the origin of nitrogen or the pathways of N input into ecosystems. We tried to use this approach to assess the main input pathways of nitrogen into the sand dune area of the north-western Negev Desert (Israel). The following two pathways are the main sources for nitrogen input into the system: i. Biological fixation of atmospheric nitrogen by cyanobacteria present in biological crusts and by N2-fixing vascular plants (e.g. the shrub Retama raetam); ii. Atmospheric input of nitrogen by wet deposition with rainfall, dry deposition of dust containing N compounds, and gaseous deposition. Samples were taken from selected environmental compartments such as biological crusts, sand underneath these crusts (down to a depth of 90 cm), N2-fixing and non-N2-fixing plants, atmospheric bulk deposition as well as soil from arable land north of the sandy area in three field campaigns in March 1998, 1999 and 2000. The delta15N values measured were in the following ranges: grass -2.5/1000 to +1.5/1000; R. reatam: +0.5/1000 to +4.5/1000; non-N2-fixing shrubs +1/1000 to +7/1000; sand beneath the biological crusts +4/1000 to +20/1000 (soil depth 2-90 cm); and arable land to the north up to 10/1000. Thus, the natural 15N abundance of the different N pools varies significantly. Accordingly, it should be feasible to assess different input pathways from the various 15N abundances of nitrogen. For example, the biological N fixation rates of the Fabaceae shrub R. reatam from the 15N abundances measured were calculated to be 46-86% of biomass N derived from the atmosphere. The biological crusts themselves generally show slight negative 15N values (-3/1000 to -0.5/1000), which can be explained by biological N fixation. However, areas with a high share of lichens, which are unable to fix atmospheric nitrogen, show very negative values down to -10/1000. The atmospheric N bulk deposition, which amounts to 1.9-3.8 kg N/hayr, has a 15N abundance between 4.4/1000 and 11.6/1000 and is likely to be caused by dust from the arable land to the north. Thus, it cannot be responsible for the very negative values of lichens measured either. There must be an additional N input from the atmosphere with negative delta15N values, e.g. gaseous N forms (NOx, NH3). To explain these conflicting findings, detailed information is still needed on the wet, particulate and gaseous atmospheric deposition of nitrogen.

Desert Climate↗

PubMed1985

Diversity in the germline antibody repertoire. Molecular evolution of the T15 VN gene family.

The T15 heavy chain variable region (VH) gene family in BALB/c mice includes four elements each greater than 88% homologous with the other. One of these elements, V1, encodes virtually all of the VH regions in BALB/c antiphosphorylcholine antibodies, while another element, V3, is a pseudogene and cannot be transcribed or translated. We have examined the structural features of this VH gene family in other mouse strains and, in particular, have cloned and sequenced the alleles of these gene segments present in B10.P mice. Each of the four B10.P sequences can be matched with its allelic counterpart in BALB/c mice. This represents the first successful analysis of allelism in antibody variable region gene segments. The V1B10.P allele, like its BALB/c counterpart, encodes most of the known phosphorylcholine binding heavy chains from C37BL/6 mice. Similarly, the V3B10.P gene segment is a pseudogene like V3BALB, although only two of four abnormalities present in the BALB/c allele are also present in the B10.P allele. Careful analysis of the specific substitutions observed in the T15 VH gene family suggests that environmental selection for functional combining regions contributes significantly to the pattern of variation in the germline antibody repertoire. In addition, evidence is presented supporting frequent gene conversion events in the divergence of antibody genes.

Alleles↗

PubMed1993

Changes in the cytotoxic T-cell repertoire of HIV-1-infected individuals: relationship to disease progression.

The repertoire of antigen-specific receptors expressed on T lymphocytes is shaped by fixed genetic and variable environmental selective pressures. Recent technological advances have enabled the analysis of T-cell receptor (TCR) expression in the context of selective pressures arising through normal immune system development and also through pathological features of disease. The pathological features of acquired immune deficiency syndrome (AIDS) are reflected by selective depletion of particular T lymphocyte subsets and expansion of others. An important question concerning the immunopathogenesis of AIDS is whether or not the perturbation of the CD4+ and CD8+ T-cell subsets following infection with human immunodeficiency virus (HIV) is selective based on TCR variable (V) region gene expression. To address this question, we have functionally analyzed TCR V gene expression on CD8+ cytotoxic T lymphocytes from HIV-1-infected individuals. This was done using monoclonal antibodies against individual TCR V regions to trigger redirected cytolysis in 51Cr release assays. The percent specific lysis induced by each antibody functionally measures the representation of the TCR V region gene product it is specific for. Relative to non-HIV-infected controls and asymptomatic HIV-infected individuals with only moderate CD4 lymphocyte depletion, HIV-infected individuals with low CD4 lymphocyte counts exhibited skewed patterns of TCR V region representation. Therefore, the perturbation within the CD8+ cytotoxic T lymphocyte repertoire in HIV infection appears to be selective based on TCR V region usage, increasingly so as disease progresses. The TCR V genes affected varied between different HIV-infected individuals and skewing detected in functional assays was not always apparent by flow cytometric analysis. These results suggest that HIV infection causes generalized effects on the T-cell repertoire, which are reflected in the relative TCR V gene representation of the CD8+ cytotoxic T lymphocyte population in peripheral blood.

Acquired Immunodeficiency Syndrome↗

PubMed1992

The interactive effects of dietary quality on the growth and attained size of young Mexican children.

Predictors of attained size at 30 mo and growth rate between 18 and 30 mo (eg, diet, maternal size, morbidity, age at weaning, and selected environmental factors) were investigated by using longitudinal data from 67 Mexican children aged 18-30 mo. These children were small because of growth stunting in early infancy. Between 18 and 30 mo they grew on average at the 50th percentile of National Center for Health Statistics references values for weight, and the 25th for length. Size at 30 mo and growth rates were unrelated to energy or protein intakes during the previous year, but positively related to consumption of animal-origin foods. Maternal weight predicted size and growth only for children with larger intakes of animal products. Individual rates of weight and length growth were uncorrelated on average, correlated negatively when animal-product intake was low, and positively only when more animal products were consumed. Integrated weight and length growth, and the influence of maternal size on growth, may be disrupted by poor dietary quality. Different children would be identified as malnourished depending on whether weight or length growth was measured.

Analysis of Variance↗

PubMed2006

Relationship between seed bank expression, adult longevity and aridity in species of Chaetanthera (Asteraceae) in central Chile.

BACKGROUND AND AIMS: Broad surveys have detected inverse relationships between seed and adult longevity and between seed size and adult longevity. However, low and unpredictable precipitation is also associated with seed bank (SB) expression in semi-arid and arid areas. The relationship between adult longevity, SB formation, seed mass and aridity is examined in annual and perennial herbs of Chaetanthera (Asteraceae) from the Chilean Mediterranean-type climate and winter-rainfall desert areas over a precipitation range of one order of magnitude. METHODS: Seeds of 18 species and subtaxa (32 populations) were buried in field locations, and exhumed after two successive germination periods. Seeds not germinating in the field were tested in a growth chamber, and remnant intact seed tested for viability. Seed banks were classed as transient or persistent. The effect of life form, species, population and burial time on persistent SB size was assessed with factorial ANOVA. Persistent seed bank size was compared with the Martonne aridity index (shown to be a surrogate for inter-annual variation in precipitation) and seed size using linear regression. ANCOVA assessed the effect of life-form on SB size with aridity as covariate. KEY RESULTS: Three species had a transient SB and 15 a persistent SB. ANOVA revealed a significant effect of life-form on SB size with annuals having larger SB size and greater capacity to form a persistent SB than perennials. Significant inter-population variation in SB size was found in 64% of cases. Seed mass was negatively correlated with persistent SB size. Persistent seed bank size was significantly correlated with the Martonne aridity index in the perennial and annual species, with species from more arid areas having larger persistent SBs. However, when aridity was considered as a covariate, ANCOVA revealed no significant differences between the annual and perennial herbs. CONCLUSIONS: Persistent seed bank size in Chaetanthera appears to reflect environmental selection rather than any trade-off with adult longevity.

Asteraceae↗

PubMed1984

Change in household composition of the elderly: a preliminary investigation.

Selected environmental and life-cycle determinants of change in household composition were examined using longitudinal data. Although cross-sectional analysis would suggest that 12% of a random sample of 276 older community residents had experienced a change in type of household composition over a 30-month period, longitudinal analysis showed that 18% experienced a change in major type of household composition, some types of households being more stable than others. Longlinear analysis showed that these changes were best accounted for by earlier life cycle events that were largely beyond the individual's present control (i.e., by change in marital status, typically death of spouse, and by extent of help available from family and friends). The importance of such family life cycle events in explaining change in household composition was emphasized.

Aged↗

PubMed1990

Structure and evolution of mammalian VH families.

Antibodies are encoded by a limited number of germline gene segments that undergo somatic diversification through rearrangement and mutation. Because these mutation processes are efficient, it is widely believed that there is little environmental selection pressure for the maintenance of specific antibody gene sequences. We have performed pairwise comparisons of known germline (as opposed to somatically generated) antibody VH elements with the hope of identifying conserved structural features common to sets of VH gene segments. These studies reveal that VH families arose prior to the mammalian radiation and have since been conserved, that this conservation appears to reflect selection at the level of protein sequence, and that the conserved regions are discretely localized on a solvent-exposed face of the heavy chain, at some distance from the antibody combining site. A family-specific region was also identified within the recombinase recognition sequences. Our results provide a context for theories that address the physiological significance of variations in VH family utilization during the development of the immune repertoire.

Animals↗

PubMed1997

Density of larval Culicoides belkini (Diptera:Ceratopogonidae) in relation to physicochemical variables in different habitats.

Immature density and population size of the biting midge Culicoides belkini (Wirth & Arnaud) were estimated for habitats on Moorea Island, French Polynesia, by means of random, 2- and 3-stage sampling designs. Samples were taken in March 1993 from 5 strata of a large larval habitat: a sandy-mud surface of approximately 5,000 m2 (stratum 1) in which approximately 12,000 land crab burrows (stratum 2) were counted, a small pond surrounded by approximately 300 m2 of muddy bank (stratum 3), and a high organic muddy area (Kopara) of approximately 1,200 m2 (stratum 4) with approximately 3,500 crab burrows (stratum 5). Larval density was usually higher in the mud of crab burrows, especially those in the Kopara stratum. Larval density was significantly lower in the sediment of the sandy area as compared with pond banks or Kopara surface. The sampling designs and techniques were logistically adequate, statistically relevant, and were recommended for future studies on C. belkini larval density. Larval habitats were characterized by means of multivariate analysis. Comparison of larval densities with selected environmental variables indicated that larvae density was higher in wet sediments with high levels of organic matter (approximately 8% of dry weight of sediment) and low salinity (approximately 0.5-1.5% NaCl equivalents). These variables were considered significant if larval control by means of habitat modification has to be achieved. Nevertheless, C. belkini can tolerate a broad spectrum of variation in the other environmental variables measured and breed in a variety of ecological situations. Therefore, it has a high potential for colonizing new habitats.

Animals↗

PubMed1984

Retention of cryptic genes in microbial populations.

Cryptic genes are silenced genes that can still be reactivated by mutation. Since they can make no positive contribution to the fitness of their carriers, it is not clear why many cryptic genes in microbial populations have not degenerated into useless DNA sequences. Hall et al. (1983) have suggested that cryptic genes have persisted because of occasional strong environmental selection for reactivated genes. The present mathematical study supports their suggestion. It shows that a cryptic gene can be retained without having any selective advantage over a useless DNA sequence, if selection for the reactivated gene occasionally occurs for a substantially long time.

Bacteria↗

PubMed2005

Geographical distribution of road traffic deaths in England and Wales: place of accident compared with place of residence.

BACKGROUND: There are two sources of information on road traffic mortality in England and Wales: ONS records giving the place of residence and police records giving the place of the accident. Use of the police records has been limited by the lack of an obvious denominator to control for population at risk. This study compares the two measures and explores the implications of using the same population denominator for both. METHODS: The number of road traffic deaths occurring in 403 local authority districts in England and Wales during 1995-1999 was compared to the number of deaths to residents in the same period. Both numbers were related to the expected number of deaths to residents and selected environmental risk factors using regression techniques. RESULTS: Large differences were found between the number of deaths in each district and the number of deaths to residents. The expected number of deaths to residents was the strongest predictor of both observed totals. The number of deaths in the district and the number divided by expected deaths of residents were highly predictable from road accident risk factors, but the number of deaths to residents and the conventional SMR were not. CONCLUSION: Information on the place of residence of road traffic accident fatalities does not show true variations in accident risk. Police records are better for this purpose. The expected number of deaths to residents estimate provides a partial but effective and unbiased control for population effects.

Accidents, Traffic↗

PubMed1996

Growth indices and stomatal control of transpiration in Acacia koa stands planted at different densities.

We examined the influences of selected environmental variables on stomatal behavior and regulation of transpiration in 26-month-old Acacia koa Gray (koa) stands planted at spacings of 1 x 1 m or 2.5 x 2.5 m and grown without irrigation. Field measurements were made during recovery from an extended 60-day dry period with only 38 mm of precipitation. Biomass and leaf area were also measured at 3-month intervals over the first 24 months after planting and again following completion of the transpiration (T) and stomatal conductance (g(s)) measurements at about 26 months after planting. Transpiration was measured as sap flow through intact branches by a heat balance method. After a 22-day period during which 130 mm of rain were recorded, average T was substantially higher in the 2.5 x 2.5 m stand on both a leaf area and ground area basis even though leaf area index was about 3.5 times higher in the 1 x 1 m stand. After an additional 25 mm of rain during an 8-day period, T was still slightly higher on a leaf area basis in the 2.5 x 2.5 m stand but was about 3 times higher on a ground area basis in the 1 x 1 m stand. A strong stomatal response to humidity limited the increase in T with increasing evaporative demand. Values of g(s) in koa phyllodes were comparable to those reported for leaves of other mesic tropical forest trees, but were several times higher than those reported for Acacia species native to arid and semi-arid regions. The 1 x 1 m planting yielded three times more biomass per unit ground area than the 2.5 x 2.5 m planting. However, greater stand density, which resulted in more rapid depletion of soil water between rainfall inputs, was associated with lower growth efficiency and lower radiation conversion efficiency.

Journal Article↗

PubMed1985

Nosocomial infections with methicillin and tobramycin resistant Staphylococcus aureus--implication of physiotherapy in hospital-wide dissemination.

From January 1979 to July 1980, methicillin and tobramycin resistant Staphylococcus aureus was isolated from 45 patients at a Veteran's Administration hospital. Of these 45, 36 (80%) had clinical infections involving non-surgical wounds (16), urinary tract (10), blood (8), surgical wounds (7), and sputum (5). The epidemic strain had a single phage type (47/54/75/77/83A), a single, 16.4 Md plasmid, and produced an aminoglycoside inactivating enzyme (AAD[4']). Compared to a control group of patients with nosocomial antibiotic susceptible S. aureus infections, patients with the epidemic strain had more hospital days before a positive culture (p = .004), more severe underlying disease (p = .006), received antibiotics more often (p = .0018), and underwent physiotherapy more often (p = .00007). Although selected environmental and personnel cultures were negative for the epidemic strain, epidemiologic investigation suggested that hospital-wide dissemination of S. aureus may have occurred through patient contacts in the physiotherapy department with subsequent clusters of cases occurring on several wards.

Adult↗

PubMed1983

Acceptance of a volunteer first-responder system in rural communities: a field experiment.

This article describes a randomized control field experiment that was used to evaluate the impact of an organized volunteer-based emergency first-responder system in 36 rural, medically underserved communities in central Georgia. The system created an information network within communities, which allowed rapid contact with trained resident volunteers when emergencies occurred. The evaluation examined selected environmental variables related to creating an information network and their effect on the general public's willingness to use a first responder for medical emergencies. Measurements of community awareness and attitudes were made using a household telephone survey conducted immediately before project initiation, 3 months and 13 months after implementation. Willingness to use the system was greatest for individual respondents living in communities with less than 800 population and who were participants in the social network of the community. Implications for administrating this type of project through statewide EMS systems are discussed.

Adult↗

PubMed1995

MHC-dependent mate preferences in humans.

One substantial benefit of sexual reproduction could be that it allows animals (including humans) to react rapidly to a continuously changing environmental selection pressure such as coevolving parasites. This counteraction would be most efficient if the females were able to provide their progeny with certain allele combinations for loci which may be crucial in the parasite-host arms race, for example the MHC (major histocompatibility complex). Here we show that the MHC influences both body odours and body odour preferences in humans, and that the women's preferences depend on their hormonal status. Female and male students were typed for their HLA-A, -B and -DR. Each male student wore a T-shirt for two consecutive nights. The next day, each female student was asked to rate the odours of six T-shirts. They scored male body odours as more pleasant when they differed from the men in their MHC than when they were more similar. This difference in odour assessment was reversed when the women rating the odours were taking oral contraceptives. Furthermore, the odours of MHC-dissimilar men remind the test women more often of their own actual or former mates than do the odours of MHC-similar men. This suggests that the MHC or linked genes influence human mate choice today.

Adult↗

PubMed1991

Circadian Rhythms in Photosynthesis : Oscillations in Carbon Assimilation and Stomatal Conductance under Constant Conditions.

Net carbon assimilation and stomatal conductance to water vapor oscillated repeatedly in red kidney bean, Phaseolus vulgaris L., plants transferred from a natural photoperiod to constant light. In a gas exchange system with automatic regulation of selected environmental and physiological variables, assimilation and conductance oscillated with a free-running period of approximately 24.5 hours. The rhythms in carbon assimilation and stomatal conductance were closely coupled and persisted for more than a week under constant conditions. A rhythm in assimilation occurred when either ambient or intercellular CO(2) partial pressure was held constant, demonstrating that the rhythm in assimilation was not entirely the result of stomatal effects on CO(2) diffusion. Rhythms in assimilation and conductance were not expressed in plants grown under constant light at a constant temperature, demonstrating that the rhythms did not occur spontaneously but were induced by an external stimulus. In plants grown under constant light with a temperature cycle, a rhythm was entrained in stomatal conductance but not in carbon assimilation, indicating that the oscillators driving the rhythms differed in their sensitivity to environmental stimuli.

Journal Article↗

PubMed2004

Population structure of the malaria vector Anopheles funestus in Senegal based on microsatellite and cytogenetic data.

The study of chromosomal inversions distribution within natural Anopheles funestus populations from West Africa revealed high levels of genetic structuring. In Burkina Faso, this was interpreted as evidence for incipient speciation, and two chromosomal forms were described, namely 'Folonzo' and 'Kiribina'. Assignation of field collected specimens to one chromosomal form depends upon application of an algorithm based on chromosomal inversions. We assessed relevance and applicability of this algorithm on An. funestus populations from Senegal, where both forms occur. Furthermore, we estimated the level of genetic differentiation between populations using microsatellite loci spread over the whole genome. Significant genetic differentiation was revealed between geographical populations of An. funestus, and the pattern observed suggested isolation by distance. Chromosomal heterogeneity was not detected by microsatellite markers. Thus, although incipient speciation could not be ruled out by our data, our results suggest that differential environmental selection pressure acting on inversions should be considered a major factor in shaping their distribution in wild An. funestus populations.

Animals↗

PubMed2006

Bathymetric and geographic population structure in the pan-Atlantic deep-sea bivalve Deminucula atacellana (Schenck, 1939).

The deep-sea soft-sediment environment hosts a diverse and highly endemic fauna of uncertain origin. We know little about how this fauna evolved because geographic patterns of genetic variation, the essential information for inferring patterns of population differentiation and speciation are poorly understood. Using formalin-fixed specimens from archival collections, we quantify patterns of genetic variation in the protobranch bivalve Deminucula atacellana, a species widespread throughout the Atlantic Ocean at bathyal and abyssal depths. Samples were taken from 18 localities in the North American, West European and Argentine basins. A hypervariable region of mitochondrial 16S rDNA was amplified by polymerase chain reaction (PCR) and sequenced from 130 individuals revealing 21 haplotypes. Except for several important exceptions, haplotypes are unique to each basin. Overall gene diversity is high (h = 0.73) with pronounced population structure (Phi(ST) = 0.877) and highly significant geographic associations (P < 0.0001). Sequences cluster into four major clades corresponding to differences in geography and depth. Genetic divergence was much greater among populations at different depths within the same basin, than among those at similar depths but separated by thousands of kilometres. Isolation by distance probably explains much of the interbasin variation. Depth-related divergence may reflect historical patterns of colonization or strong environmental selective gradients. Broadly distributed deep-sea organisms can possess highly genetically divergent populations, despite the lack of any morphological divergence.

Animals↗

PubMed1996

Adhesion of Porphyromonas gingivalis fimbriae to human gingival cell line Ca9-22.

In this study, we examined the effects of selected environmental factors on the adhesion of Porphyromonas gingivalis fimbriae, an important structure involved in attachment of the bacteria to human gingival cells. The human gingival carcinoma cell line Ca9-22 was grown in microculture plates, and adherence was detected by use of 125I-labeled fimbriae. Adhesion was increased by changes in pH from 7.0-8.0, but was decreased by increase in the sodium chloride concentration above 0.15 M. Trypsin treatment of Ca9-22 cells also augmented adhesion of the fimbriae to the cells. These results indicate that fimbrial adhesion to gingival cells is controlled by various environmental factors, and the data on trypsin treatment suggest that elevated levels of protease in the gingival sulcus, such as can occur with poor oral hygiene and gingivitis, may expose adhesion molecules on the gingival cell surface, thereby promoting the attachment of P. gingivalis fimbriae.

Bacterial Adhesion↗