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Results for “balancing selection”

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PubMed1994

Human genetic polymorphisms.

Mutations in the human genome lead to genetic polymorphisms in the population. While many of these are considered to be normal variants, many lead to human disease and are usually maintained in the population by a balance between mutation and selection. The frequency and type of mutations have been determined for a number of loci in man. The frequency of mutation in transgenes is now receiving considerable attention, since proteins synthesized in recombinant DNA biological systems are subject to genetic alteration through mutation and selection. These changes can occur in the transgene, but mutations in the host cell genes can also produce structural modifications for example, by changes in post-translational modification. Mutation rates for human genes may be as high as 10(-5) per locus per generation. Spontaneous mutations that inactivate gene function in bacteria occur at a rate of 10(-5)-10(-6) per locus per generation. On the other hand the frequency of spontaneous mutations per nucleotide is two orders of magnitude less (usually less than 10(-7)).

DNA↗

PubMedDate not supplied

[Accepting stress. A contribution to research in coping].

A review of numerous studies on coping with stress points to the important role of the response class "accepting a stressful situation" in the daily lives of healthy persons and for patients coping with chronic disease and disability. For some extreme situations this response class is the only way to attain some affective balance and an adequate selection of adjustment processes. It is recommended that research on coping should not only focus on more or less complex response structures, but also on the analysis of specific response classes ("ways of coping").

Activities of Daily Living↗

PubMed1998

Adult olfactory epithelium contains multipotent progenitors that give rise to neurons and non-neural cells.

We have infused replication-incompetent retroviral vectors into the nasal cavity of adult rats 1 day after exposure to the olfactotoxic gas methyl bromide (MeBr) to assess the lineage relationships of cells in the regenerating olfactory epithelium. The vast majority of the retrovirus-labeled clones fall into three broad categories: clones that invariably contain globose basal cells (GBCs) and/or neurons, clones that always include cells in the ducts of Bowman's glands, and clones that are composed of sustentacular cells only. Many of the GBC-related clones contain sustentacular cells and horizontal basal cells as well. Most of the duct-related clones contain gland cells, and some also include sustentacular cells. Thus, the destruction of both neurons and non-neuronal cells that is caused by MeBr activates two distinct types of multipotent cells. The multipotent progenitor that gives rise to neurons and non-neuronal cells is a basal cell, whereas the progenitor that gives rise to duct, gland, and sustentacular cells resides within the ducts, based on the pattern of sparing after lesion and the analysis of early regeneration by using cell type-specific markers. We conclude that the balance between multipotency and selective neuropotency, which is characteristic of globose basal cells in the normal olfactory epithelium, is determined by which cell types have been depleted and need to be replenished rapidly.

Animals↗

PubMed2001

Cephalometric norms in an Arabic population.

AIM: To identify cephalometric norms for a Jordanian population. DESIGN: A cross sectional study using a sample selected on the basis of a balanced face and a Class I occlusion. MATERIALS AND METHODS: 65 subjects aged 14-17 years were selected from a larger sample on the basis of Class I incisors, a balanced profile and no previous orthodontic treatment. Lateral skull radiographs were digitised directly to produce values for SNA, SNB, ANB, UI/MX, LI/MN, UI/LI, MMPA and LI/A-Pog. RESULTS: SNA and SNB were very close to the Eastman Standards. MMPA was significantly lower in Jordanians whilst UI/MX and LI/MN were significantly higher. Lower incisors were 4.6 mm further forward in relation to A-Pog in Jordanians.

Adolescent↗

PubMed1984

Iron intake and status of men and women consuming self-selected diets.

Iron intakes and iron status (serum ferritin and iron balance) were determined in 16 women and 13 men consuming self-selected diets over a 1-yr period. Iron balance was determined during four 7-day balance periods corresponding to the four seasons of the year. The seasonal variation in iron intake and serum ferritin and the correlation among iron intake, iron balance, and serum ferritin was examined. No seasonal variation was found in iron intake or serum ferritin. No direct correlation could be found between iron status (ferritin levels and iron balance) and iron intake. However, the men who met or exceeded their recommended dietary allowance for iron intake had ferritin levels in a range indicative of adequate iron status. None of the women met their recommended dietary allowance and their ferritin levels indicated marginal iron stores.

Adult↗

PubMed2006

External postural perturbations induce multiple anticipatory postural adjustments when subjects cannot pre-select their stepping foot.

Previous research on human balance recovery suggests that, prior to an externally triggered postural perturbation, healthy subjects can pre-select their postural response based on the environmental context, but it is unclear whether this pre-selection includes the selection of a stepping leg when performing compensatory steps. We sought to determine how pre-selecting a stepping limb affects the compensatory steps and stability of young, healthy subjects when responding to postural perturbations. Nine healthy subjects (24-37 years of age) stepped in response to backward translations of a platform under their feet when, prior to the perturbations, the subjects either knew whether they were to step with their left or right leg to a visual target (the Predictable condition) or did not know whether to step with their left or right leg until one of two targets appeared at perturbation onset (the Unpredictable condition). The Unpredictable condition also included randomly inserted trials of toes-up rotations and backward translations without targets (catch trials). The results showed that, in the Predictable condition, the subjects consistently exhibited one anticipatory postural adjustment (APA; a lateral weight shift toward the stance limb) before stepping accurately to the target with the correct leg. In the Unpredictable condition, the subjects either (1) exhibited multiple APAs, late step onsets, and forward center-of-mass (CoM) displacements that were farther beyond their base of support, or (2) exhibited an early step with only one APA and kept their CoM closer to the base of support, but also stepped more often with the incorrect leg. Thus, when the subjects had to select a stepping leg at perturbation onset, they either became more unstable and used multiple APAs to delay stepping in order to provide enough time to select the correct stepping leg, or they stepped earlier to remain stable but often stepped with the incorrect leg. In addition, responses to catch trials in the Unpredictable condition included distorted step placements that resembled steps to anticipated targets, despite allowing the subjects to step with a leg of their choice and to a location of their choice. Lastly, the subjects' voluntary stepping latencies to visual targets presented without perturbations were twice as long as their stepping latencies to the backward platform translations. Therefore, healthy subjects appear to pre-select their stepping limb, even when the perturbation characteristics are unpredictable, because relying on visual input provided at perturbation onset requires a delayed response that leads to greater instability.

Adaptation, Physiological↗

PubMed1997

Branching process models for mutant genes in nonstationary populations.

A deleterious gene achieves a population balance between the opposing forces of selection and mutation. In this paper we explore the nature of this stochastic balance when the surrounding normal population is not at equilibrium. Assuming that new mutations occur according to a Poisson process and thereafter evolve by the rules of a continuous time branching process, we derive explicit formulas and recurrence relations determining the probability distribution of the current number of mutant individuals. In fact, we compute expectations for a variety of interesting random variables for genetic models involving autosomal dominant and X-linked diseases. We can also handle haplotype information on linked markers. This feature will be especially helpful in understanding the linkage disequilibrium strategy of positional cloning in population isolates. In the presence of exponential growth of the normal population, our formulas reduce to the evaluation of certain Laplace transforms.

Female↗

PubMed1986

Seasonal variation in food intake, pattern of physical activity and change in body weight in a group of young adult Dutch women consuming self-selected diets.

The effect of season on the energy balance was examined in 114 young adult Dutch women consuming self-selected diets. Energy intakes and patterns of physical activity were assessed monthly 14 times with the 24-h recall method. After this period of 14 months, in the second year the same estimates were made with intervals of 2-3 months to check if the seasonal variations observed were not accidental. The study did not demonstrate seasonal variations in the mean energy intake of the group under study. However, the intake of fat appeared to be higher in the winter and spring than in the summer and autumn, whereas for the intake of mono- and di-saccharides the reverse seemed to be true. The intake of dietary fibre was higher in the autumn than in the summer, with intermediate values for winter and spring. Small seasonal fluctuations were observed in body weight and time spent on various physical activities. On the one hand, these fluctuations are too small to indicate physiological significance, on the other hand they are wide enough to be taken into account in the design of many longitudinal studies on the relation between diet and disease.

Adult↗

PubMed1992

Electrostimulation effects of the vagus nerve on balance in epilepsy.

Preliminary results of selected postural measures in quiet standing indicate that stimulation of the vagus nerve appears not to be producing adverse effects. With this specific sample size, more testing is needed to determine long-term effects and future data analyses will examine correlations between electroencephalogram results, drug levels, and seizure frequency. In the present study three subjects have had old injuries to hips and ankles. Two subjects had normal values for postural control prior to stimulation, while other subjects were severely abnormal. In future, studies should include larger homogeneous sample sizes, as the current subjects show marked variability in age and premorbid health backgrounds. Future work should also control more vigorously for variables such as visual input (i.e., blindfolding subjects instead of simply closing the eyes). Evaluation of postural control mechanisms will be continued to assess stability changes in these patients as seizure frequency continues to subside.

Adult↗

PubMed1975

Quantitative variation in natural populations.

Quantitative variation is considered in natural populations using Drosophila as the example. A knowledge of such variation enables its rapid exploitation in directional selection experiments as shown for scutellar chaeta number. Where evidence has been obtained, genetic architectures are in qualitative agreement with Mather's concept of balance for traits under stabilizing selection. Additive genetic control is found for acute environmental stresses, but not for less acute stresses as shown by exposure to 60Co-gamma rays. D. simulans probably has a narrower ecological niche than its sibling species D. melanogaster associated with lower genetic heterogeneity. One specific environmental stress to which D. simulans is sensitive in nature is ethyl alcohol as shown by winery data.

Adaptation, Biological↗

PubMed1998

Differential effects of peptide diversity and stromal cell type in positive and negative selection in the thymus.

Thymocyte positive selection results in maturation to the single-positive stage, while negative selection results in death by apoptosis. Although kinetic analyses indicate only 3-5% of CD4+ 8+ cells reach the single-positive stage, the balance of positive and negative selection and the nature and quantity of cells mediating maximal negative selection are uncertain. Here, using a system where the number and type of stromal cells and thymocytes can be controlled, we investigated the maturation of CD4+ 8+ thymocytes in the presence or absence of thymic epithelium and dendritic cells (DC) from wild-type (wt) and H-2M(-/-) mice expressing different peptide arrays. We find that titration of wt DC into reaggregates of wt epithelium has a dramatic effect on the number of CD4+ cells generated, with 1% DC causing a maximal 80% reduction. Moreover, while addition of 1% wt DC into cultures of H-2M(-/-) epithelium causes a 90% reduction in CD4+ cells, no effect was observed when similar numbers of wt thymic epithelium were added. Collectively, these data provide the first accurate indication of the quantity and quality of stromal cells required for maximal negative selection in the thymus, demonstrate the importance of peptide diversity in T cell selection, and highlight a large degree of overlap between positive and negative selection events.

Animals↗

PubMed1985

Computer simulation of flagellar movement. VI. Simple curvature-controlled models are incompletely specified.

Computer simulation is used to examine a simple flagellar model that will initiate and propagate bending waves in the absence of viscous resistances. The model contains only an elastic bending resistance and an active sliding mechanism that generates reduced active shear moment with increasing sliding velocity. Oscillation results from a distributed control mechanism that reverses the direction of operation of the active sliding mechanism when the curvature reaches critical magnitudes in either direction. Bend propagation by curvature-controlled flagellar models therefore does not require interaction with the viscous resistance of an external fluid. An analytical examination of moment balance during bend propagation by this model yields a solution curve giving values of frequency and wavelength that satisfy the moment balance equation and give uniform bend propagation, suggesting that the model is underdetermined. At 0 viscosity, the boundary condition of 0 shear rate at the basal end of the flagellum during the development of new bends selects the particular solution that is obtained by computer simulations. Therefore, the details of the pattern of bend initiation at the basal end of a flagellum can be of major significance in determining the properties of propagated bending waves in the distal portion of a flagellum. At high values of external viscosity, the model oscillates at frequencies and wavelengths that give approximately integral numbers of waves on the flagellum. These operating points are selected because they facilitate the balance of bending moments at the ends of the model, where the external viscous moment approaches 0. These mode preferences can be overridden by forcing the model to operate at a predetermined frequency. The strong mode preferences shown by curvature-controlled flagellar models, in contrast to the weak or absent mode preferences shown by real flagella, therefore do not demonstrate the inapplicability of the moment-balance approach to real flagella. Instead, they indicate a need to specify additional properties of real flagella that are responsible for selecting particular operating points.

Flagella↗

PubMed1994

Management of dietary essential metals (iron, copper, zinc, chromium and manganese) by Wistar and Zucker obese rats fed a self-selected high-energy diet.

The balances and content of essential elements (iron, copper, zinc, chromium and manganese) in the body of Wistar, Zucker lean and Zucker obese rats fed a reference or cafeteria diet from day 30 to 60 after birth have been studied. Intestinal iron absorption compensated for low iron content of the cafeteria diet and the extra needs of growth and fat deposition. It can be assumed that the altered energy regulation processes that afflict the genetically obese rat are not directly related to altered iron metabolism. Obese Zucker rats had lower copper tissue concentrations than lean rats, but when fed a cafeteria diet the differences between Zucker rats strains disappear. This cannot be traced to large differences in diet copper concentration. A low diet availability of zinc--such as that of cafeteria-fed fa/fa rats--is easily compensated for by increasing absorption. So, as a consequence, we can conclude that genetic obesity did not impair zinc absorption. There was no deficit of zinc in any of the groups studied; the rats have enough capacity to extract zinc within a wide range of dietary concentrations. The absorption of dietary chromium was inversely proportional to its concentration. The ability to extract chromium from the diet and the very low urinary losses are a consequence of its scarcity in most dietary items. Despite wide variations in the manganese of the diets, the absorption rates were practically unchanged except for obese rats fed the cafeteria diet. It seems that this low absorptive capacity is enough to supply the rat with the manganese it needs, since a sizeable--but subjected to 8-fold-span variations--proportion is lost in the urine. This alone points towards a considerable excess of manganese in both diets studied. Obesity does not have a significant effect on the abilities to absorb and retain minerals, since these processes were more related to dietary availability. Management of essential metals by obese rats depends whether this condition is genetic or induced by diet. Most of the differences observed can be related to differences in diet concentration, to the excess fat content or different metabolic attitude to use substrates of obese animals. The data presented show that the cafeteria diet used adequately serves the mineral needs of the rat, since the rat adapts its absorbing and retaining strategies to match the dietary availability of these minerals.

Animals↗

PubMed1999

Estimating the contributions of selection and self-organization in RNA secondary structure.

In addition to characteristic structural properties imposed by evolutionary modification, evolved, single-stranded RNAs also display characteristic structural properties imposed by intrinsic physical constraints on RNA polymer folding. The balance of intrinsic and functionally selected characters in the folded conformation of evolved secondary structures was determined by comparing the predicted secondary structures of evolved and unevolved (random) RNA sequences. Though evolved conformations are significantly more ordered than conformations of random-sequence RNA, this analysis demonstrates that the majority of conformational order within evolved structures results not from evolutionary optimization but from constraints imposed by rules intrinsic to RNA polymer folding.

Algorithms↗

PubMed1996

Spatial frequency and attention: effects of level-, target-, and location-repetition on the processing of global and local forms.

Is attentional selection between local and global forms based on spatial frequency? This question was examined by having subjects identify local or global forms of stimuli that had been "contrast balanced," a technique that eliminates low spatial frequencies. Response times (RTs) to global (but not local) forms were slowed for contrast-balanced stimuli, suggesting that low spatial frequencies mediate the global RT advantage typically reported. In contrast, the beneficial effect of having targets appear at the same, as opposed to a different, level as that on the immediately preceding trial was unaffected by contrast balancing. This suggests that attentional selection between different levels of structure is not based on spatial frequency. The data favor an explanation in terms of "priming," rather than in terms of adjustments in the diameter of an attentional "spotlight."

Adolescent↗

PubMed2001

Water acquisition and partitioning in Drosophila melanogaster: effects of selection for desiccation-resistance.

We examined physiological features related to water balance in five replicate populations of Drosophila melanogaster that have undergone selection for enhanced resistance to desiccation (D populations) and in five replicate control (C) populations. Adult D flies contain 34 % more water than the control flies. We examined two hypotheses for increased water acquisition in the D flies: (i) that they accumulate more water early in development and (ii) that they have a reduced post-eclosion diuretic water loss. We found no evidence of differential water or dry mass acquisition between the C and D populations prior to adulthood. We also found no evidence of differential post-eclosion diuresis, i.e. both C and D groups showed insignificant changes in water volume in the 4 h period immediately after eclosion. In addition, we quantified water content in the intra- and extracellular compartments of the C and D populations and were able to identify the hemolymph as the primary storage site of the 'extra' water carried by the desiccation-resistant flies. We estimated that 68 % of the increased water volume observed in the D flies was contained in the hemolymph. Desiccation-resistance was strongly correlated with hemolymph volume and only weakly with intracellular water volume. Survival during desiccation was also strongly related to the carbohydrate content of the D flies. It has been presumed that the D flies accumulate carbohydrate primarily as intracellular glycogen, which would result in a significant increase in intracellular water volume. We found that carbohydrate content was weakly correlated with intracellular water volume and more strongly with hemolymph volume. The carbohydrate pool in the D flies may, therefore, be contained in the extracellular compartment as well as in cells. These results are suggestive of the importance of modifications in hemolymph volume and hemolymph solute concentrations in the evolution of enhanced desiccation-tolerance in populations of Drosophila melanogaster.

Animals↗

PubMed1990

A depression of active tissue plasminogen activator in plasma characterizes patients with unstable angina pectoris who develop myocardial infarction.

The balance between the coagulation system generating fibrin and its subsequent removal by the fibrinolytic system determines the fate of fibrin deposited in the vascular system. In a prospective study, selected haemostatic variables assessing this balance were determined in plasma samples from 20 consecutive patients admitted with unstable angina pectoris. Over a follow-up period of 6 years, eight patients developed myocardial infarction, whereas 12 patients did not. There was no significant difference between the two groups in the median plasma concentrations of thrombin-antithrombin III complexes reflecting the coagulant activity. The infarction group was characterized by a significantly lower median activity of tissue plasminogen activator in plasma euglobulins (P less than 0.05), a higher median concentration of tissue plasminogen activator antigen in plasma (P less than 0.05) and a tendency to higher plasma levels of antigenic and functional plasminogen activator inhibition. In all patients, the activities of tissue plasminogen activator inhibitor and of tissue plasminogen activator were significantly associated (rs = -0.4811, P less than 0.05). We conclude that a depressed fibrinolytic capacity attributable to a low tissue plasminogen activator activity is of pathogenetic importance for the development of myocardial infarction in patients with unstable angina pectoris.

Aged↗

PubMedDate not supplied

Anesthetic considerations for pericardectomy in uremic pericardial effusion.

Fifteen patients with impending uremic cardiac tamponade underwent either anterior pericardectomy or pericardiostomy under general anesthesia. Anesthetic agents and technics were selected in the light of physiopathologic derangements involving fluids and electrolyte balance, excretory and cardiovascular disturbances, and pharmacodynamics. Anesthesia was induced with diazepam and maintained with N2O-O2 (70:30) and fentanyl. Pancuronium was used for muscle relaxation. Adequate preoperative assessment of patient, careful monitoring of vital signs, maintenance of the critical balance of fluid and blood replacement, and selection of anesthetic agents for minimal depression of vital multiorgan systems provided excellent results. No postanesthetic morbidity or mortality occurred.

Anesthesia↗