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Results for “selection pressure”

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At least 19 recordsLinked to original sources
PubMed1977

Spontaneous implantation of antibiotic-resistant Enterobacteriaceae in the digestive tract of chickens in the absence of selective pressure.

In the absence of selective pressure by antibiotics, resistant enterobacteria implanted rapidly in the intestinal tract of chickens, where these organisms subsequently persisted in high numbers. Food could be an important source of this contamination: resistant Escherichia coli present in small numbers in the diet became rapidly and persistently established in the gut. The human caretaker played a passive role in the spread of antibiotic-resistant bacteria between separate groups of chickens. Resistant enteric organisms colonized the gut of animals, with different population sizes. Some strains were able to reach high numbers (10(7) to 10(9)/g), and other strains established themselves at a lower level (10(3) to 10(5)/g), whereas a third type seemed to be only transient inhabitants, unable to persist.

Animals↗
PubMed1977

Some population genetic models combining artificial and natural selection pressures in the presence of assortative mating.

We have attempted quantitatively through a series of assortative mating models to gain insight into the interaction between the usually antagonistic tendencies of artifical and natural selection pressures. We summarize some of the robust conclusions. In cases where natural selection is expressed only through the phenotype and acts in the opposite direction to the culling incline, then fixation of the dominant or recessive type can be achieved and which occurs depends critically on the initial composition of the population and the magnitude of the degree of culling compared to the selection coefficients. With traits determined at two loci in the case that the double heterozygote is the desired kind, the effect of selfing can only be overcome by very strong artificial selection pressures (high culling order). The degree of culling to achieve its objective can be relaxed with weakening of linkage. The relevant comparison is r2+(1-r)2less than 2(1-c) indicating the precise extent of culling needed, to prevent fixation. The relationships are more complex when natural selection forces are also involved (see Model IV).

Alleles↗
PubMed1977

Maintenance of genetic variability under mutation and selection pressures in a finite population.

Formulas are developed for the distribution of allele frequencies and the mean and variance of heterozygosity under mutation and selection pressures. In large populations, even slight selection drastically changes the shape of the distribution of allele frequencies and reduces heterozygosity. On the other hand, the number of rare alleles in a sample is much less affected by selection. Under genic selection, heterozygosity may decrease with increasing population size. As a test statistic, the variance of heterozygosity can be used to detect the presence of selection, though it is not efficient when selection is very slight.

Alleles↗
PubMed2026

Evaluating Patient Satisfaction and Oral Health Impact Profile-14 (OHIP-14): A Multicenter Crossover Study Comparing Selective Pressure Impression Conventional Dentures with Mucostatic Digital Dentures.

PURPOSE: To compare patient satisfaction and oral health impact between individuals receiving complete dentures made by digital methods and those using conventional techniques. MATERIALS AND METHODS: In this randomized crossover clinical trial, 23 patients aged 40 years and older with completely edentulous arches were enrolled at three treatment centers. Each participant received two sets of complete dentures: one set created using conventional methods (selective pressure impression) and the other through digital techniques (mucostatic digital impression). The order of denture placement was randomized, with each set used for 4 weeks. A trained specialist administered treatments alongside research tools, including a general information questionnaire, a denture satisfaction survey, and the OHIP-14 interview tool. Statistical analysis was conducted using Mann-Whitney U test. RESULTS: Participants with digital dentures reported significantly higher satisfaction regarding treatment duration, comfort, confidence, chewing ability, esthetics, and overall satisfaction compared to those with conventional dentures. There were no significant differences in satisfaction concerning speech and pronunciation. Overall, the oral health impact on quality of life was similar between denture types, but participants indicated improved quality of life while using dentures compared to being edentulous. CONCLUSIONS: Patients with digital dentures exhibited greater satisfaction across various domains compared to those with conventional dentures, despite similar satisfaction levels in speech and pronunciation. The impact on quality of life was comparable between both types, as measured by the OHIP-14.

Humans↗
PubMed1992

What is the selective pressure that maintains the gene loci encoding the antigen receptors of T and B cells? A hypothesis.

The dominant view is that the gene loci encoding the B cell antigen receptor (BAr) or the T cell antigen receptor (TAr) specify a vast array of combining sites. The 'germline' repertoire is estimated to be > 10(10) by multiplying numbers of subunit complements by DN-region variability. This implies that the germline can be maintained by a selection imposed by all or most of the antigenic universe. Its unchallenged popularity, notwithstanding, this neo-germline view is untenable and hence the need for a competing concept, as presented here. The immunoglobulin (Ig) loci are under a totally different selection from the T loci. The Ig loci are selected upon largely by carbohydrate determinants on pathogens that vary more slowly than the proteins produced by the Ig loci, which are necessary to rid these selective antigens. By contrast, the T loci are selected to recognize the allele-specific determinants on restricting elements encoded in the major histocompatibility complex (MHC). The expression of the germline results in a high copy number (HCN) repertoire; this repertoire is the substrate for 'mutation' that yields the low copy number (LCN) repertoire. For the B cell, these two repertoires interact to optimize the response to the unexpected. For the T cell, only the LCN repertoire is functional. The immunoglobulin (Ig) loci are selected upon as light(L)-heavy (H) pairs; the T loci are selected upon as single units alpha or beta (i.e. the VT-gene segments act as a single pool). This competing concept carries with it many important and testable consequences.

Alleles↗
PubMed1975

Gene flow versus selection pressure and ancestral differentiation in the composition of species: analysis of population variation of Juniperus ashei Buch. using terpenoid data.

Twenty-four populations of Juniperus ashei were sampled throughout the range of this taxon and the terpenoids of the foliage were analyzed by gas/liquid chromatography. Population differentiation was investigated by analysis of variance and numerical taxonomy. Three south Texas and one Mexican population clustered together with the rest of the taxon appearing fairly uniform. No evidence was found of hybridization or introgression with other taxa. Disjunct populations in Oklahoma and the Ozarks, which have been genetically isolated from the central population for thousands of years, showed no signs of differentiation nor genetic drift. The present pattern of distribution probably dates from the Pleistocene. The south Texas and Mexican populations appear to be the primitive elements of the species. Populational differences have apparently been maintained in adjacent populations in spite of seemingly large gene flow and conversely, chemical uniformity is being maintained in many disjunct populations where there is little or no gene flow.

Biological Evolution↗
PubMed1976

R-plasmid transfer in vivo in the absence of antibiotic selection pressure.

The persistence of an O18 Escherichia coli strain resistant to tetracycline, streptomycin, and sulfonamide has been followed in the fecal flora of a single individual over a period of 9 months. The strain in question carrying an R plasmid was detectable from the beginning of the survey, but it was only after a 10-day period of tetracycline therapy that it reached an all but permanent dominance in the fecal flora. No transfer of the R plasmid carried by the strain to any other coliform could be detected for 202 days after the end of tetracycline treatment. At this point, however, an O88 E. coli carrying the same plasmid as the O18 strain appeared briefly as a predominant component of the flora. The two plasmids isolated from the O18 and the O88 E. coli strains have been characterized in molecular terms and found to be similar. This suggests that R-plasmid transfer between two E. coli strains occurred in an individual who was living a normal daily life and who was not receiving antibiotics.

Anti-Bacterial Agents↗
PubMed1976

Selective pressure of tetracyclines on the faecal flora. A comparison between tetracycline and doxycycline.

The aerobic faecal flora was studied in two groups of healthy adult persons, each group consisting of 18 individuals, before a course of either doxycycline or tetracycline HCI, at the cessation of therapy and 5 weeks afterwards. The main object of this study was to evaluate the effect of antibacterial treatment on the occurrence of mutagenic resistant and plasmid carried resistant strains in the faecal flora, and in particular development of resistant E. coli. The frequency of resistant strains increased after treatment with both of the drugs without significant differences.

Adult↗
PubMed1979

Muscle and adipose cell development in mice selected for postweaning growth rate.

Genetic selection for postweaning growth rate between 3 and 6 weeks of age resulted in significant changes in body weight, semimembranosus muscle weight, and epididymal fat pad weights. Body weights at 15 weeks of age ranged from 24.9 gm for the Low line (slow growth) mice and 59.4 gm for the High line mice (rapid growth). Total muscle DNA content was decreased in Low line mice and increased in High line mice when compared to Control mice (random bred). RNA to DNA and protein to DNA ratios were not significantly affected by growth rate selection pressure. Selection for high or low growth rate appeared to affect the development of muscle by altering the amount of DNA synthesized, suggesting that growth hormone status was altered by this type of selection. Adipose cell number was reduced in the Low line and increased in the High line when compared to Control mice. Adipose cell size was increased in High line mice when compared to Low and Control lines. The role of adipose cell development in supporting growth selection success is suggested.

Adipose Tissue↗
PubMed1979

A mutation in Caenorhabditis elegans that increases recombination frequency more than threefold.

In higher organisms the rate of recombination between genetic loci is presumably responsive to selective pressure. Recently, selective pressures and mutational events that influence recombination have been reviewed. Mutational sites and chromosomal rearrangements that enhance or suppress recombination frequency in specific regions are known, but general mechanisms that enhance recombination have not yet been discovered. We describe here the isolation and characterisation of a strain of the hermaphroditic nematode, Caenorhabditis elegans, that has a recombination frequency at least threefold higher than that found in the wild type. In this strain, rec-1, the number of reciprocal recombination events between linked loci is increased. This is true for all pairs of linked loci studies so far. The high recombination strain behaves as if it carries a classical recessive mutation, although a second mutation exists which can alter the recessive behaviour of rec-1.

Animals↗
PubMed2024

Characterization of West Nile virus Koutango lineage from phlebotomine sandflies in Kenya.

The West Nile virus (WNV), primarily transmitted by mosquitoes, is one of the most widespread flaviviruses globally, with past outbreaks occurring in the USA and Europe. Recent studies in parts of Africa, including Kenya, have identified the West Nile virus Koutango lineage (WN-KOUTV) among phlebotomine sandfly populations, however, our understanding of this virus remains limited. This study aimed to characterize WN-KOUTV from phlebotomine sandflies. Sandflies were sampled between 12th -16th March 2021 and 16th -20th March 2023 from six villages each in Baringo and Isiolo Counties, using CDC light traps. Female sandflies were taxonomically identified and pooled based on genus and site of collection. Virus isolation was performed in Vero cells. Viral genomes were determined using next-generation sequencing. Phylogenetic and molecular clock analyses were done to decipher the virus's evolutionary relationships. Comparative analyses of amino acid sequences were performed to determine variations. Protein modeling in Pymol was conducted to elucidate variations in key protein regions. Evolutionary pressure analysis investigated the selection pressures on the virus. In vitro experiments were done to investigate the virus growth kinetics in mammalian Vero E6 and mosquito C6/36 cells. We report the isolation of WN-KOUTV from Salabani in Baringo and Aremet in Isiolo, Kenya. The isolated WN-KOUTVs clustered with previously identified WN-KOUTV strains. Comparative analysis revealed a unique amino acid at NS5 653. The WN-KOUTV lineage as a whole is under purifying selective pressure, with diversifying pressure acting at site NS3 267. The current WN-KOUTV replicated in Vero E6 and C6/36 cells comparable to West Nile virus Lineage 1a, isolated from mosquitoes. Subsequent isolations of WN-KOUTV in phlebotomine sandflies suggest potential vectors, however, vector competence studies would confirm this. Replication in mammalian and insect cell lines suggests there may exist a vector/host relationship. We speculate the close genetic relationship of WN-KOUTV strains from East and West Africa may potentially be enabled by bird migratory routes between the two regions. If proven, this could point to a potential future pandemic pathway for this virus.

Animals↗