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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↗
PubMed1992

Equal G and C contents in histone genes indicate selection pressures on mRNA secondary structure.

Protein-specific versus taxon-specific patterns of nucleotide frequencies were studied in histone genes. The third positions of codons have a (well-known) taxon-specific G+C level and a histone type-specific G/C ratio. This ratio counterbalances the G/C ratio in the first and second positions so that the overall G and C levels in the coding region become approximately equal. The compensation of the G/C ratio indicates a selection pressure at the mRNA level rather than a selection pressure or mutation bias at the DNA level or a selection pressure on codon usage. The structure of histone mRNAs is compatible with the hypothesis that the G/C compensation is due to selection pressures on mRNA secondary structure. Nevertheless, no specific motifs seem to have been selected, and the free energy of the secondary structures is only slightly lower than that expected on the basis of nucleotide frequencies.

Animals↗
PubMed1988

Amplified expression constructs for human tissue-type plasminogen activator in Chinese hamster ovary cells: instability in the absence of selective pressure.

By linking an expression cassette for human tissue-type plasminogen activator (t-PA) to an amplifiable marker gene, its introduction into Chinese hamster ovary dhfr- cells and subsequent amplification with methotrexate, we have generated cell lines that overproduce the heterologous protein and contain 300-1100 copies of the expression constructs integrated into the hamster genome. We present a detailed investigation of the fate of amplified sequences in the presence and absence of selective pressure by parallel examination of three producer cell lines with respect to relevant parameters. These include the determination of t-PA production upon continuous propagation in culture, the genomic organization of the integrated expression constructs by Southern blotting, and the localization of homogeneously staining regions by in-situ hybridization with biotinylated probes and visualization by interference reflection microscopy. We conclude that in the three cell lines examined, the decrease in production of t-PA in the absence of methotrexate selection is accompanied by decreases in the number of integrated expression constructs and the size of the amplified regions, whereas all these parameters are stable when selective pressure is maintained. The instability is probably due to the head-to-tail mode of integration of the expression constructs in the hamster genome, which increases the frequency of homologous recombination between the integrated plasmids in recombination-proficient cells in the absence of selective pressure.

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↗
PubMed1992

Transfer of defined numbers of chloroplasts into albino protoplasts by subprotoplast/protoplast microfusion: chloroplasts can be "cloned", by using suitable plastome combinations or selective pressure.

Defined numbers (1-5) of (donor) chloroplasts were transferred into (acceptor) protoplasts of plastid albino mutants by subprotoplasts/protoplast microfusion. Single transferred plastids gave rise to new organelle populations in the progeny of the fusion products when suitable combinations of plastomes were used or when selective pressure for the plastome transferred was applied. This process is termed "chloroplast cloning" and is the first reported case of "cloning" a cell organelle. The plastome combination and the presence or absence of selective pressure were found to influence the frequencies with which cell lines, containing both plastomes or acceptor or donor only, were obtained, and the number of cell generations needed for complete segregation - as measured by the duration of culture before the green donor plastome could be detected. The high frequency of cell lines and regenerated shoots recovered with donor plastome only, even when only a single chloroplast was transferred, leads to the conclusion that all organelles present in the fusion product contribute to the organelle population of the progeny, i.e. organelle death or loss are not regularly occurring events during plant regeneration from protoplasts in Nicotiana tabacum.

Cells, Cultured↗
PubMed1991

Random nature of coronavirus RNA recombination in the absence of selection pressure.

RNA-RNA recombination is thought to occur preferentially at certain selected sites and in only a few RNA viruses; the mechanism for these restrictions is unknown. In this paper we report the development of a recombination assay for coronavirus, using polymerase chain reaction, in the absence of selection pressure. Our results showed that within a 1-kb region of the peplomer gene, RNA recombination occurred at almost every potential crossover site. Thus, coronavirus RNA recombination appears to be more random than previously realized. However, after serial passages of the recombinant viruses in tissue culture, the recombination sites among the progeny viruses became clustered in the region which contains the previously reported "hot spot" for coronavirus recombination. These results suggest that RNA recombination is common and random in nature, but only certain recombinants can be selected. Thus, the presence of recombinational "hot spots" for coronavirus or other RNA viruses most likely resulted from selection of certain recombinant viruses and not restriction on the occurrence of RNA recombination. The failure to detect recombinants in other RNA viruses may therefore be due to unfavorable properties of recombinant viruses. This approach can be used to detect recombinants in these viruses.

Animals↗
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↗
PubMed1992

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in area 18 of the visual cortex of the cat.

Recordings were made from single neurons in area 18 of anesthetized cats (N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate) in which one optic nerve had been pressure blocked to selectively block conduction in the largest (Y-type) fibers. Cortical neurons were stimulated visually via the normal eye or via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields such as their spatial organization (S or C cells), orientation tuning, and the presence and strength of end-zone inhibition appear to be unaffected by removal of the Y input. By contrast, the removal of the Y input resulted in a small but significant reduction in the size of the discharge field and in the direction-selectivity index. In three respects, peak response discharge rate, eye dominance, and velocity sensitivity, removal of the Y input had strong and highly significant effects. Thus, the mean peak discharge frequency of responses evoked by the stimulation of binocular neurons via the Y-blocked eye was significantly lower than that of responses evoked by the stimulation via the normal eye. Accordingly, the eye-dominance histogram was shifted markedly towards the normal eye (more so than in the homologous experiment conducted on area 17-Burke et al., 1992). Finally, the mean preferred velocity of responses of cells activated via the normal eye was in the vicinity of 145 deg/s, whereas for cells activated via the Y-blocked eye the value was about 35 deg/s. Overall, the results of the present study imply that (1) apart from Y-type excitatory input there are significant excitatory non-Y-inputs to area 18; these inputs at least partially consist of indirect X-type input relayed via area 17; (2) in neurons of area 18 that receive both Y-type and non-Y-type excitatory inputs, the Y-type input has a major influence on strength of the response and velocity sensitivity and a lesser influence on the direction selectivity and size of the discharge fields; and (3) area 18 contains mechanisms determining such receptive-field properties as S- or C-type organization, orientation tuning, and direction selectivity which can be accessed either by the Y input or by non-Y input.

Afferent Pathways↗
PubMed1989

Effect of selection pressure on the cholinesterase of Boophilus microplus (Acari: Ixodidae) resistant to coumaphos.

The effect of selection pressure on the cholinesterase (AChE) activity of two strains of Boophilus microplus (Canestrini) resistant to coumaphos was monitored. Total AChE and protein was determined from three generations of resistant ticks and a susceptible strain. The effect of an AChE inhibitor, coroxon (the oxygen analog of coumaphos), was also determined. The resistance of the susceptible strain (Escondido) to coumaphos remained relatively unchanged throughout the study. The Tuxpan strain lost some of its resistance to coumaphos as the generations proceeded (AChE increased instead of decreased). The Tuxtla strain became more resistant to coumaphos as the generations proceeded (AChE increased).

Acetylcholinesterase↗
PubMed1974

Some population genetic models combining artificial and natural selection pressures.

The evolutionary behavior of a diploid population characterized by a trait determined at one or two major loci subject to the combined effects of artificial and natural selection pressures is investigated. A number of different genotypic phenotypic associations are set forth, including additive allelic effects and additive loci effects with a variety of culling programs. Threshold selection schemes as well as culling favoring intermediate phenotypic values are considered. For these formulations results are reported concerning the dynamic progress of the population, a delineation of the numbers and properties of the stable equilibria outcomes, and a discussion of their qualitative and quantitative dependence on the two kinds of selection forces. The level of culling can be used as a control through which natural selection parameters can be estimated.

Alleles↗
PubMed1990

Quantitative analysis of the selective pressure exerted on homologous proteins.

Evolution analysis is used to locate the regions of a protein that are important for its function or structure. The rate of evolution is generally constant for a given family of homologous sequences. From the starting point of this observation, an algorithm is proposed to establish quantitatively the sequence zones where selective pressure is maximal. A program that computes this pressure has been written in PASCAL. Analysis of results on some sequences validate this theoretical approach, and this knowledge can be used as a starting-point for carrying out site-directed mutagenesis.

Algorithms↗
PubMed1986

Pesticide resistance mechanisms produced by field selection pressures on Anopheles nigerrimus and A. culicifacies in Sri Lanka.

In Sri Lanka, Anopheles nigerrimus is resistant to a range of organophosphate and carbamate insecticides at both the larval and adult stages. Biochemical studies indicate that an alteration in acetylcholinesterase is the basis of resistance rather than increased metabolic breakdown of the insecticides. In contrast, A. culicifacies is resistant only to malathion and closely related compounds containing a carboxylate ester bond. Agricultural pesticides are the sole source of selection pressure for resistance in A. nigerrimus, while in A. culicifacies pressure arises predominantly from antimalarial spraying.

Animals↗
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↗
PubMed1988

Selection pressure and altered hepatocellular islands after a single injection of aflatoxin B1.

Development of glutathione S-transferase placental form (GST-P)-positive focal populations was investigated subsequent to single injections of the hepatocarcinogens aflatoxin B1 (AfB1), dimethylnitrosamine (DMN) and diethylnitrosamine (DEN). While DEN proved far more potent at inducing putative initiated hepatocytes, the AfB1 treatment was associated with a very rapid (3 weeks) development of lesions approaching nodular proportions. Autoradiographic investigation revealed selective incorporation of label into GST-P-positive hepatocytes and oval cells at the day 7 time point following AfB1 treatment. Administration of butylated hydroxyanisole (BHA) subsequent to carcinogen injection was associated with a decrease in the final yield of lesions and increased tritiated thymidine incorporation in perivenular zone 3 background hepatocytes. The results suggest that 'selection pressure', resulting in rapid growth and development of putative preneoplastic lesions, is inherent in a single injection of the mycotoxin and indicate that variations of the present short-term model may be useful for elucidating the mechanisms underlying AfB1-induced hepatocarcinogenesis.

Aflatoxin B1↗
PubMed1990

Mutations with multiple independent origins in surface antigens mark the targets of biological selective pressure.

This manuscript documents the existence and importance of a small subset of mutations in Plasmodium falciparum genes that continually reoccur in separated populations. Here we describe how to identify recurrent mutations using a novel application of pre-existing computer programs designed to trace genealogy. Perhaps the most striking example of this phenomenon occurs in the cytotoxic T-cell epitope of a malaria surface antigen, the circumsporozoite (CS) protein of P. falciparum, where identical sequence types clearly have multiple independent origins. The importance of this observation is that it conclusively demonstrates selective pressure on these sequences and indicates that there is a significant natural mechanism relating to the circumsporozoite protein that affects the success of malaria sporozoites.

Animals↗
PubMed1990

Enhanced preneoplastic liver lesion development under 'selection pressure' conditions after administration of deoxycholic or lithocholic acid in the initiation phase in rats.

The initiating potential of the secondary bile acids, deoxycholic acid (DCA) and lithocholic acid (LCA), for rat hepatocarcinogenesis was investigated using the development of hyperplastic nodules and/or glutathione S-transferase placental form (GST-P)-positive liver foci as the end point. Five week old male F344 rats were given either basal diet, or diets containing 0.5% DCA or 0.5% LCA for 3 weeks in conjunction with partial hepatectomy performed midway, followed by a selection regimen consisting of 2 weeks feeding of 0.02% 2-acetylaminofluorene diet and a single gastric intubation of carbon tetrachloride. The animals were then placed on either basal diet or a diet containing 0.05% phenobarbital (PB) for 52 weeks. Significantly higher numbers of hyperplastic liver nodules developed in the DCA-treated rats irrespective of PB promotion as compared with the respective control groups. No such increase was evident in the LCA-treated rats. In contrast, both DCA and LCA treatments enhanced the development of GST-P-positive liver foci with or without subsequent PB promotion. Only one hepatocellular carcinoma was diagnosed in a control group animal. The present data indicate that a short period of feeding of DCA and LCA in the initiation stage in conjunction with partial hepatectomy results in enhanced development of preneoplastic liver lesions under selection pressure conditions with or without subsequent PB promotion. They thus confirm and extend our previous finding of enhanced gamma-glutamyltranspeptidase-positive liver foci development in a short-term assay of DCA and LCA, and suggest that these secondary bile acids either possess possible initiating activity or some other priming effect for rat hepatocarcinogenesis.

Animals↗
PubMed1986

Role of selective pressure by screening organisms in the development of producer mutant isolates of different antibiotic markers.

The phenotypes of producer mutant isolates when obtained after mutagenesis of the spontaneous degenerate Aspergillus versicolor N5 by screening on Trichophyton rubrum and those on A. niger G3Br were found to be markedly different. In respect of phenotypic character anti-T. rubrum producer mutant isolates resembled the parental type more closely (which is specifically active against T. rubrum and generally against dermatophytes) than anti-A. niger mutant isolates. This clearly indicates not only the role of selective pressure of the screening organism in the development of a given producer mutant isolate but also shows the possibility of recovery of a producer type of the lost antibiotic marker.

Anti-Bacterial Agents↗
PubMed1990

DNA recombination and natural selection pressure sustain genetic sequence diversity of the feline MHC class I genes.

Sequence comparisons of seven distinct MHC class I cDNA clones revealed that feline class I molecules have a remarkable similarity to human HLA genes in their organization of functional domains as well as in the nonrandom partitioning of genetic variability according to the functional constraints ascribed to different regions of the MHC molecule. The distribution of the pattern of sequence polymorphism in the cat as compared with genetic diversity of human and mouse class I genes provides evidence for four coordinate factors that contribute to the origin and sustenance of abundant allele diversity that characterizes the MHC in the species. These include: (a) a gradual accumulation of spontaneous mutational substitution over evolutionary time; (b) selection against mutational divergence in regions of the class I molecule involved in T cell receptor interaction and also in certain regions that interact with common features of antigens; (c) positive selection pressure in favor of persistence of polymorphism and heterozygosity at 57 nucleotide residues that comprise the antigen recognition site; and (d) periodic intragenic (interallelic) and intergenic recombination within the class I genes. We describe a highly conserved 23-bp nucleotide sequence within the coding region of the first alpha-helix that separates two relatively polymorphic segments located in the alpha 1 domain that may act as a template or "hot spot" for homologous recombination between class I alleles.

Amino Acid Sequence↗