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Results for “balancing selection”

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PubMed1989

The divergence of a polygenic system subject to stabilizing selection, mutation and drift.

Polygenic variation can be maintained by a balance between mutation and stabilizing selection. When the alleles responsible for variation are rare, many classes of equilibria may be stable. The rate at which drift causes shifts between equilibria is investigated by integrating the gene frequency distribution W2N II (pq)4N mu-1. This integral can be found exactly, by numerical integration, or can be approximated by assuming that the full distribution of allele frequencies is approximately Gaussian. These methods are checked against simulations. Over a wide range of population sizes, drift will keep the population near an equilibrium which minimizes the genetic variance and the deviation from the selective optimum. Shifts between equilibria in this class occur at an appreciable rate if the product of population size and selection on each locus is small (Ns alpha 2 less than 10). The Gaussian approximation is accurate even when the underlying distribution is strongly skewed. Reproductive isolation evolves as populations shift to new combinations of alleles: however, this process is slow, approaching the neutral rate (approximately mu) in small populations.

Alleles↗
PubMedDate not supplied

Balanced intraintestinal nutrition: comparison of absorption degree of selected fats and emulsions of fats.

The absorption of emulsion of soybean oil, soybean oil and molted butter was compared with that of the Intralipid preparation manufactured by Vitrum. The experiments were carried out on a dog and rats using the method of isolated intestinal fistulae. It was found that the emulsion of soybean oil prepared by the authors was absorbed in a degree similar to that of Intralipid fat emulsion and that it could be used as a component of a preparation intended for balanced non-residual intraintestinal nutrition.

Animals↗
PubMed2000

Selective microstimulation of central nervous system neurons.

The goal of this study was to identify stimulus parameters and electrode geometries that were effective in selectively stimulating targeted neuronal populations within the central nervous system (CNS). Cable models of neurons that included an axon, initial segment, soma, and branching dendritic tree, with geometries and membrane dynamics derived from mammalian motoneurons, were used to study excitation with extracellular electrodes. The models reproduced a wide range of experimentally documented excitation patterns including current-distance and strength-duration relationships. Evaluation of different stimulus paradigms was performed using populations of fifty cells and fifty fibers of passage randomly positioned about an extracellular electrode(s). Monophasic cathodic or anodic stimuli enabled selective stimulation of fibers over cells or cells over fibers, respectively. However, when a symmetrical charge-balancing stimulus phase was incorporated, selectivity was greatly diminished. An anodic first, cathodic second asymmetrical biphasic stimulus enabled selective stimulation of fibers, while a cathodic first, anodic second asymmetrical biphasic stimulus enabled selective stimulation of cells. These novel waveforms provided enhanced selectivity while preserving charge balancing as is required to minimize the risk of electrode corrosion and tissue injury. Furthermore, the models developed in this study can predict the effectiveness of electrode geometries and stimulus parameters for selective activation of specific neuronal populations, and in turn represent useful tools for the design of electrodes and stimulus waveforms for use in CNS neural prosthetic devices.

Animals↗
PubMed2001

Genotype-by-environment interaction and the fitness of plant hybrids in the wild.

Natural hybrid zones between related species illustrate processes that contribute to genetic differentiation and species formation. A common viewpoint is that hybrids are essentially unfit, but they exist in a stable tension zone where selection against them is balanced by gene flow between the parent species. An alternative idea is that selection depends on the environment, for example, by favoring opposite traits in the two parental habitats or favoring hybrids within a bounded region. To determine whether selection of hybrids is environment dependent, we crossed plants of naturally hybridizing Ipomopsis aggregata and I. tenuituba in the Colorado Rocky Mountains and reciprocally planted the seed offspring into a suite of natural environments across the hybrid zone. All types of crosses produced similar numbers and weights of seeds. However, survival of the offspring after 5 years differed markedly among cross types. On average, the F1 hybrids had survival and growth rates as high as the average for their parents. But hybrid survival depended strongly on the direction of a cross, that is, on which species served as the maternal parent. This fitness difference between reciprocal hybrids appeared only in the parental environments, suggesting cytonuclear gene interactions that are environment specific. These results indicate that complex genotype-by-environment interactions can contribute to the evolutionary outcome of hybridization.

Colorado↗
PubMed2000

On selection mechanisms during initial evolution and on the possible role of ionic balance.

Initial evolution at the molecular level can proceed through mechanisms that enhance either the generation or the survival of certain molecules. Phenomena related to ionic balance can be the basis for both generative and survival selection mechanisms that may have driven the initial evolution of macromolecules. An important advantage of mechanisms based on ionic balance phenomena is that they can conceivably generate spatial and temporal inhomogeneities in semi-enclosed spaces within cavities of porous materials and/or membranes. Under certain circumstances, such inhomogeneities are required for initial evolution to proceed.

Animals↗
PubMed2005

On the worldwide spread of an insecticide resistance gene: a role for local selection.

Adaptation occurs by gene replacement (or transient balanced polymorphism). Replacement may be caused by selection (local or global) and/or genetic drift among alleles. In addition, historical events may blur the respective effects of selection and drift during the course of replacement. We address the relative importance of these processes in the evolution of insecticide resistance genes in the mosquito Culex pipiens. The resistance allele, Ester2, has a broad geographic distribution compared to the other resistance alleles. To distinguish between the different processes explaining this distribution, we reviewed the literature and analysed updated data from the Montpellier area of southern France. Overall, our data indicate that Ester2 prevails over other Ester resistance alleles in moderately treated areas. Such conditions are common and favour the hypothesis of selection acting at a local level. This places an emphasis on the importance of ecological conditions during the evolution of resistance. Finally, we highlight that historical events have contributed to its spread in some areas.

Adaptation, Physiological↗
PubMed2000

Investigation and comparison of the effects of rehabilitation on balance and coordination problems in patients with posterior fossa and cerebellopontine angle tumours.

BACKGROUND: This study was planned to investigate and compare the effects of acute rehabilitation programme on balance and coordination problems in patients with posterior fossa and cerebellopontine angle tumours. METHODS: This study was carried out at Hacettepe University, School of Physical Therapy and Rehabilitation, neurosurgical rehabilitation unit on 40 patients whose ages varied between 18-60 years. Subjects were subdivided into two groups. Group I consisted of 20 patients with posterior fossa tumour, while the group II was composed of 20 patients with cerebellopontine angle tumour. Patients were evaluated according to the localisation of the tumour, the number of the surgical operations, the level of the surgical excision of the tumour, percent of having shunt, radiotherapy and chemotherapy. Balance problems were assessed according to Mokken's functional independence assessment scale in 5 selected positions and standing balance was also evaluated according to the method of Bohannon. The rehabilitation of the patients included balance and coordination training. The patients were re-evaluated before their discharge and the results of the two evaluations were compared. RESULTS: A significant improvement was found in the second group when pre- and post-treatment evaluations were compared. There were more coordination problems in group 1 in the pretreatment period. Both groups showed significant improvement of coordination problems after the treatment. However, this improvement was more obvious in the second group (p<0.05). There was a negative significant correlation between the decrease of balance problems and the amount of the tumour excided (I Group r=-0.51, II Group r=-0.57). CONCLUSIONS: It can be concluded that rehabilitation approaches are effective in balance and coordination problems observed in both tumour groups, but the responses to treatment are more obvious in the cerebellopontine angle tumours, which have better prognosis.

Adolescent↗
PubMed2003

Supplementation of methionine and selection of highly digestible rumen undegradable protein to improve nitrogen efficiency for milk production.

Metabolizable protein (MP) supply and amino acid balance were manipulated through selection of highly digestible rumen-undegradable protein (RUP) sources and methionine (Met) supplementation. Effects on production efficiency and N utilization of lactating dairy cows were determined. Thirty-two multiparous (647 kg) and 28 primiparous (550 kg) Holstein cows were assigned during the fourth week of lactation to one of four dietary treatments. Treatments were 1) 18.3% crude protein (CP) with low estimated intestinal digestibility of RUP (HiCP-LoDRUP), 2) 18.3% CP with high digestibility RUP (HiCP-HiDRUP), 3) 16.9% CP with high digestibility RUP (LoCP-HiDRUP), and 4) 17.0% CP with high digestibility RUP and supplemental Met (LoCP-HiDRUP + Met). Diets were balanced to have equal concentrations of net energy for lactation (NE(L)), acid detergent fiber (ADF), neutral detergent fiber (NDF), and ash. Milk yields (40.8, 46.2, 42.9, 46.6 kg/d), protein percentages (2.95, 2.98, 2.99, 3.09%), and fat percentages (3.42, 3.64, 3.66, 3.73%) are reported here for HiCP-LoDRUP, HiCP-HiDRUP, LoCP-HiDRUP, and LoCP-HiDRUP + Met, respectively. Milk urea N and BUN decreased when feeding a lower CP diet. Efficiency of use of N for milk protein production was higher when feeding higher digestibility RUP, especially with the LoCP-HiDRUP + Met diet. A digestibility study followed the production trial, with six cows per treatment group continuing on the same treatment for an additional week. The experimental periods were 5 d long, with 1 d of adjustment and 4 d of total collection of urine and feces. Dry matter intake, milk production, milk protein production, and N digestibility were not significantly different among treatments during the collection trial, whereas N intake and N absorbed increased with the higher CP diets. The quantity of N in feces did not change with diet, but quantity of N in urine decreased in the low CP diets. Milk N as a percentage of intake N and milk N as a percentage of N absorbed showed a trend toward increasing as CP concentration in the diet decreased. The supplementation of Met did not improve the efficiency of N utilization during the digestibility study, in contrast to what was estimated during the production trial. Supplementing the highly digestible RUP source with rumen available and rumen escape sources of Met resulted in maximal milk and protein production and maximum N efficiency by cows during the production trial, indicating that postruminal digestibility of RUP and amino acid balance can be more important than total RUP supplementation.

Animal Nutritional Physiological Phenomena↗
PubMed2002

Deleterious mutations in a hybrid zone: can mutational load decrease the barrier to gene flow?

The aim of this paper is to investigate the effect of deleterious mutations in a hybrid zone maintained by selection against hybrids. In such zones, linkage disequilibria among hybrid depression loci, resulting from a balance between migration and selection, are crucial in maintaining the barrier because they allow each locus, in addition to its own selection coefficient, to cumulate indirect selective effects from other loci. Deleterious alleles produce heterosis and increase by this means the effective migration rate in structured populations. In a hybrid zone, they therefore contribute to decrease linkage disequilibria as well as the barrier to gene flow imposed by hybrid depression. However, deleterious mutations have no effect: (i) when selection against hybrids is weak, because linkage disequilibria are small even without heterosis in this case, or (ii) when selection against hybrids is so strong that it overwhelms heterosis. On the other hand, with moderate selection against hybrids, the decrease in the strength of the barrier due to heterosis may reach detectable levels, although it requires relatively small population sizes and/or migration rates. The effect is expected to be small and only within small genomes where loci are tightly linked can it become strong. Nevertheless, neglecting mutational load may to some extent obscure the estimations of selective parameters based either on artificial F1 crosses or on cline characteristics.

Computer Simulation↗
PubMed2003

Selective induction of high avidity CTL by altering the balance of signals from APC.

High avidity CTL are most effective at clearing viruses and cancer cells. Therefore, understanding the mechanisms involved in induction of high avidity CTL is critical for effective vaccines. However, no vaccine approach to selectively induce high avidity CTL in vivo has been discovered. In a new approach, signals from MHC class I (signal 1) and costimulatory molecules (signal 2) were adjusted by varying Ag dose and by use of recombinant poxvirus expressing a triad of costimulatory molecules (B7-1, ICAM-1, and LFA-3), respectively. Independent of CTL avidity, a strong signal 1 resulted in an increased frequency of CD8(+) CTL. However, a strong signal 2 was necessary for the induction of high avidity CD8(+) CTL that killed target cells more efficiently, and signal 2 played a more crucial role in the absence of a strong signal 1. Only CTL induced with strong signal 2 killed tumor cells endogenously expressing low levels of Ag. Signal 2 contributed to the induction of high avidity CD8(+) CTL in both primary and secondary responses. Thus, although signal 2 has been known to increase the quantity of CTL response, in this study we show that it also improves the quality of CTL response. Our data also suggested that dendritic cells play an important role in induction of high avidity CD8(+) CTL in vivo. This strategy to selectively induce higher avidity CTL may lead to more effective vaccines for viruses and cancer.

3T3 Cells↗
PubMed1994

The effects of spontaneous mutation on quantitative traits. I. Variances and covariances of life history traits.

We have accumulated spontaneous mutations in the absence of natural selection in Drosophila melanogaster by backcrossing 200 heterozygous replicates of a single high fitness second chromosome to a balancer stock for 44 generations. At generations 33 and 44 of accumulation, we extracted samples of chromosomes and assayed their homozygous performance for female fecundity early and late in adult life, male and female longevity, male mating ability early and late in adult life, productivity (a measure of fecundity times viability) and body weight. The variance among lines increased significantly for all traits except male mating ability and weight. The rate of increase in variance was similar to that found in previous studies of egg-to-adult viability, when calculated relative to trait means. The mutational correlations among traits were all strongly positive. Many correlations were significantly different from 0, while none was significantly different from 1. These data suggest that the mutation-accumulation hypothesis is not a sufficient explanation for the evolution of senescence in D. melanogaster. Mutation-selection balance does seem adequate to explain a substantial proportion of the additive genetic variance for fecundity and longevity.

Animals↗
PubMed1987

Effect of a selective beta 2-adrenergic agonist (clenbuterol) on energy balance and body composition in normal and protein deficient rats.

In rats fed a normal (22% protein) diet, injection of clenbuterol (1 mg/kg/d for 21 d) did not affect energy intake, energy expenditure or weight gain, but reduced energetic efficiency, and fat and energy gains and increased body protein content. Presenting a low-protein (8%) diet reduced energy intake, gain and efficiency, body protein content and the mass of the gastrocnemius muscle when compared to rats fed the control diet. Injection of the protein-deficient rats with clenbuterol (1 mg/kg/d for 21 d) caused hypophagia and reduced body weight and energy gains, energy expenditure and total body fat. However, the total body content of protein was not significantly reduced and the percentage of body protein in this protein deficient, clenbuterol-treated group was greater than that of untreated rats on both the high- and low-protein diets. The ratio of body protein to fat following clenbuterol treatment was increased by over 50% in both normal and protein-deficient rats. The results show that in protein deficient animals, clenbuterol treatment may help conserve body protein at the expense of fat, resulting in a smaller, but leaner body mass.

Animals↗
PubMed2006

Gender preferences and demand for preconception sex selection: a survey among pregnant women in Pakistan.

BACKGROUND: In its recent report 'Human Reproductive Technologies and the Law', the House of Commons' Select Committee on Science and Technology called for greater efforts to establish the potential demographic impact of sex selection across all sectors of UK society. Given the well-known preference for boys over girls among some communities, there is concern that a readily available service for social sex selection may upset the balance of the sexes. Of particular interest are the gender preferences and the demand for sex selection among Pakistanis. METHODS: We conducted a social survey on gender preferences and potential demand for preconception sex selection among 301 pregnant women in Karachi, Pakistan, using a self-report questionnaire consisting of 14 questions. RESULTS: About 41.5% wish to have a family with an equal number of boys and girls; 3.3% would like to have only boys, 1.0% only girls, 27.6% more boys than girls and 4.3% more girls than boys, and 22.3% stated that they do not care about the sex composition of their family. Whereas 6.3% could imagine employing cytometric sperm separation for social sex selection, 76.1% could not and 17.6% were undecided. About 27.2% felt that social sex selection ought to be legal, 48.8% thought it ought to be illegal and 23.9% were undecided. CONCLUSIONS: Although Pakistani women do show a statistically significant preference for boys over girls, the number of women willing to subject themselves to cytometric sperm separation appears to be too small to cause a severe imbalance of the sexes. However, further research among British citizens of Pakistani origin is needed to establish whether sex selection poses a serious threat to the sex ratio of UK communities.

Adolescent↗
PubMed1993

Dynamic selection of models for a ventilator-management advisor.

A ventilator-management advisor (VMA) is a computer program that monitors patients who are treated with a mechanical ventilator. A VMA implements a patient-specific physiologic model to interpret patient data and to predict the effects of alternative control settings for the ventilator. Because a VMA evaluates its physiologic model repeatedly during each cycle of data interpretation, highly complex models may require more computation time than is available in this time-critical application. On the other hand, less complex models may be inaccurate if they are unable to represent a patient's physiologic abnormalities. For each patient, a VMA should select a model that balances the tradeoff of prediction accuracy and computation-time complexity. I present a method to select models that are at an appropriate level of detail for time-constrained decision tasks. The method is based on a local search in a graph of models (GoM) for a model that maximizes the tradeoff of computation-time complexity and prediction accuracy. For each model under consideration, a belief network computes a probability of model adequacy given the qualitative prior information, and the goodness of fit of the model to the data provides a measure of the conditional probability of adequacy given the quantitative observations. I apply this method to the problem of model selection for a VMA. I describe an implementation of a graph of physiologic models that range in complexity from VentPlan, a simple model with 3 compartments, to VentSim, a multicompartment model with detailed airway, circulation and mechanical ventilator components.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer Simulation↗
PubMed1997

Balance characteristics of persons with osteoporosis.

OBJECTIVE: People affected by osteoporosis are at particular risk for bone fractures caused by falls. Preventive intervention depends on first describing the risk factors for falls present in this population as a group and as individuals. In this preliminary study, balance characteristics of women with and without osteoporosis were measured with computerized dynamic posturography (CDP). DESIGN: A case control design was selected to compare the balance characteristics of each group of patients with osteoporosis. SETTING: Testing was performed in the vestibular assessment area of our multispecialty clinic. SUBJECTS: Patient groups were selected from within our case load. Ten women with osteoporosis were compared with six women with osteoporosis and kyphosis (Cobb angle more than 54 degrees) and with five age-matched normal subjects. INTERVENTIONS: Because this was an observational study, no interventions were used. MAIN OUTCOME MEASURE: Averaged results from all trials of sensory organization tests 5 and 6, with use of sway amplitude and balance strategy scores, were used to compare the performance of each patient group. RESULTS: Both groups with osteoporosis had different balance control strategies than the group without osteoporosis. Specifically, those with osteoporosis had greater use of hip strategies for maintaining balance than did the normal group. Those with kyphosis also had greater postural sway than either of the other two groups. CONCLUSION: Results of this study suggest that there are differences in balance control strategies and sway amplitude between patients with and those without osteoporosis. Further study is recommended in which CDP is used to clarify and confirm these differences. Individual CDP results can be used to optimize habilitative management of these patients.

Accidental Falls↗