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Results for “selection pressure”

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At least 181 records · Page 10Linked to original sources
PubMed2003

Determination of 39 polybrominated diphenyl ether congeners in sediment samples using fast selective pressurized liquid extraction and purification.

In order to reduce time of analysis, a new pressurized liquid extraction (PLE) method that automatically and rapidly achieves quantitative and selective extraction of 39 polybrominated diphenyl ether (PBDE) congeners in sediment samples was optimized. It consists of on-line cleanup by inclusion of sorbents in the extraction cell. The new method was compared with a conventional method based on the use of Soxhlet extraction followed by solid-phase extraction (SPE) with cartridges. The instrumental determination was performed by GC-MS, using negative chemical ionization in the selected-ion monitoring mode. Recoveries from 47 to 82% were obtained for spiked tri- to hepta-PBDE congeners in sediment sample. The repeatability of replicate extractions was better than 15% relative standard deviation. The detection limits obtained with the new developed method were between 1 and 46 pg/g dry weight. The reduction in the sample preparation (extraction + cleanup) time (from days to 30 min) with a similar efficiency than that afforded by the conventional Soxhlet extraction-SPE cleanup technique indicates the suitability of this method. The method was applied to sediment samples where the analytes were detected in the range of 0.86-2.49 ng/g dry mass.

Ethers↗
PubMed2000

Evidence for variable selective pressures at MC1R.

It is widely assumed that genes that influence variation in skin and hair pigmentation are under selection. To date, the melanocortin 1 receptor (MC1R) is the only gene identified that explains substantial phenotypic variance in human pigmentation. Here we investigate MC1R polymorphism in several populations, for evidence of selection. We conclude that MC1R is under strong functional constraint in Africa, where any diversion from eumelanin production (black pigmentation) appears to be evolutionarily deleterious. Although many of the MC1R amino acid variants observed in non-African populations do affect MC1R function and contribute to high levels of MC1R diversity in Europeans, we found no evidence, in either the magnitude or the patterns of diversity, for its enhancement by selection; rather, our analyses show that levels of MC1R polymorphism simply reflect neutral expectations under relaxation of strong functional constraint outside Africa.

Africa↗
PubMed2002

Habitat-dependent song divergence in the little greenbul: an analysis of environmental selection pressures on acoustic signals.

Bird song is a sexual trait important in mate choice and known to be shaped by environmental selection. Here we investigate the ecological factors shaping song variation across a rainforest gradient in central Africa. We show that the little greenbul (Andropadus virens), previously shown to vary morphologically across the gradient in fitness-related characters, also varies with respect to song characteristics. Acoustic features, including minimum and maximum frequency, and delivery rate of song notes showed significant differences between habitats. In contrast, we found dialectal variation independent of habitat in population-typical songtype sequences. This pattern is consistent with ongoing gene flow across habitats and in line with the view that song variation in the order in which songtypes are produced is not dependent on habitat characteristics in the same way physical song characteristics are. Sound transmission characteristics of the two habitats did not vary significantly, but analyses of ambient noise spectra revealed dramatic and consistent habitat-dependent differences. Matching between low ambient noise levels for low frequencies in the rainforest and lower minimal frequencies in greenbul songs in this habitat suggests that part of the song divergence may be driven by habitat-dependent ambient noise patterns. These results suggest that habitat-dependent selection may act simultaneously on traits of ecological importance and those important in prezygotic isolation, leading to an association between morphological and acoustic divergence. Such an association may promote assortative mating and may be a mechanism driving reproductive divergence across ecological gradients.

Animals↗
PubMed1993

Decline of pyrethroid resistance in the absence of selection pressure in a population of German cockroaches (Dictyoptera: Blattellidae).

The level of resistance to seven pyrethroids was followed in a laboratory-selected population of Blattella germanica (L.) for 15 generations, during which time it was not exposed to any insecticide. As estimated by LT50 resistance ratios (RRs), resistance declined in a stepwise manner to preselection levels with all of the pyrethroids except pyrethrins. I also compared the levels of resistance as indicated by RRs and by gene frequency (GF) estimates. Results showed that GF estimates provide a more precise indication of the changes in resistance that occur over time than do RRs alone, especially when combined with RR and end-point mortality data. I discuss my results from the standpoints of the number of genetic mechanisms involved in pyrethroid resistance in this insect, and what the declines in resistance mean for future cockroach control.

Animals↗
PubMed2004

Evolution of the human ABO polymorphism by two complementary selective pressures.

The best-known example of terminal-glycan variation is the ABO histo-blood group polymorphism in humans. We model two selective forces acting on histo-blood group antigens that may account for this polymorphism. The first is generated by the invasion of opportunistic bacterial or other pathogens that interact with the epithelial-mucosal surfaces. The bacteria adapt to the microenvironments of common host phenotypes and so create frequency-dependent selection for rarer host alleles. The second is generated by intracellular viruses, and accounts for the observed differentials between the ABO-phenotype frequencies. It is thought that viruses acquire histo-blood group structures as part of their envelope from their previous host. The presence of host antigens on the viral envelope causes differential transmission of the virus between host types owing to the asymmetric action of ABO natural antibodies. Our model simulations show that these two forces acting together can account for the major features of the ABO polymorphism in humans.

ABO Blood-Group System↗
PubMed2004

Sarcocystis neurona major surface antigen gene 1 (SAG1) shows evidence of having evolved under positive selection pressure.

The major surface antigen gene 1 (SAG1) is conserved among members of Sarcocystidae and may play an important role in parasite pathogenesis. Additionally, generation and selection of different antigenic variants of SAG1 has the potential for inclusion in a subunit vaccine or in the development of a diagnostic assay. In this study, patterns of nucleotide polymorphism were used to test the hypothesis that natural selection promotes diversity in different parts of SAG1 of Sarcocystis neurona. Nucleotide and amino acid sequence analysis of SAG1 from multiple S. neurona isolates identified two alleles. Sequences were identical intra-allele and highly divergent inter-alleles. Also, phylogenetic reconstruction showed sequences clustering into two clades. Tajima's and Fu and Li's neutrality tests indicated that selection is more likely to be acting on SAG1. Moreover, a sliding window analysis based on the ratio of silent substitutions to amino acid replacements provided strong evidence that two short segments in the central and 3' domain of SAG1 have been under positive selection in the divergence of the two alleles, suggesting that it may be important for the evasion of host immune responses and would be a suitable target for vaccine development.

Animals↗
PubMed1981

Transfer of heritable properties by cell hybridization: specificity and the role of selective pressure.

Enucleated chloramphenicol (CAP) resistant mouse L-cells (LEA-2A) were fused with the mouse hepatoma cells (BW1J). The resultant cybrids expressed CAP resistance (the property used in selection of the cybrids), and also expressed the hepatic-specific functions of the BW1J parent. Hybrids between these same cells, on the other hand, exhibited chloramphenicol resistance and extinction of the hepatoma-specific properties. Cybrids were also prepared between enucleated rat hepatoma cells (FT-2) and mouse erythroleukemia cells (C19TK). The resultant cybrids selected in tyrosine-free medium expressed phenylalanine hydroxylase, an enzyme normally appeared to be the result of activation of the previously silent gene of the C19TK cells. These cybrids, however, did not express any other liver-specific functions present in the FT-2 cytoplast donor. These experiments suggest that the transfer of heritable properties by cell cybridization is selection specific and that activation or extinction observed in hybrids may not occur in cybrids of the same cells.

Animals↗
PubMedDate not supplied

Selection pressures on codon usage in the complete genome of bacteriophage T7.

We searched the complete 39,936 base DNA sequence of bacteriophage T7 for nonrandomness that might be attributed to natural selection. Codon usage in the 50 genes of T7 is nonrandom, both over the whole code and among groups of synonymous codons. There is a great excess of purine- any base-pyrimidine (RNY) codons. Codon usage varies between genes, but from the pooled data for the whole genome (12,145 codons) certain putative selective constraints can be identified. Codon usage appears to be influenced by host tRNA abundance (particularly in highly expressed genes), tRNA-mRNA (one such interaction being perhaps responsible for maintaining the excess of RNY codons) and a lack of short palindromes. This last constraint is probably due to selection against host restriction enzyme recognition sites; this is the first report of an effect of this kind on codon usage. Selection against susceptibility to mutational damage does not appear to have been involved.

Base Sequence↗
PubMed2003

Paternity analysis reveals opposing selection pressures on crown coloration in the blue tit (Parus caeruleus).

In socially monogamous species, extra-pair paternity can increase the variance in reproductive success and thereby the potential for sexual selection on male ornaments. We studied whether male secondary sexual ornaments are selected through within- and/or extra-pair reproductive success in the blue tit (Parus caeruleus). Male blue tits display a bright blue crown plumage, which reflects substantially in the ultraviolet (UV) and previously has been indicated to be an important sexual signal. We show that males with a more UV-shifted crown hue were less cuckolded, which probably resulted from female preference for more ornamented mates. By contrast, however, older males and males with a less UV-shifted hue sired more extra-pair young. This probably did not reflect direct female preference, since cuckolders were not less UV-ornamented than the males they cuckolded. Alternatively, a trade-off between UV ornamentation and other traits that enhance extra-pair success could explain this pattern. Our results might reflect two alternative male mating tactics, where more UV-ornamented males maximize within-pair success and less UV-ornamented males maximize extra-pair success. Since crown colour was selected in opposite directions by within-pair and extra-pair paternity, directional selection through extra-pair matings seemed weak, at least in this population and breeding season. Reduced intensity of sexual selection due to alternative mating tactics constitutes a potential mechanism maintaining additive genetic variance of male ornaments.

Animals↗
PubMed1985

Continuous positive airway pressure selectively reduces obstructive apnea in preterm infants.

Apnea in preterm infants has been classified as obstructive, central (nonobstructive), and mixed, based on the presence or absence of upper airway obstruction. Continuous positive airway pressure (CPAP) is widely used in apneic infants, although its mechanism of action is still unclear. To determine whether CPAP is equally effective in obstructive and nonobstructive apnea, we compared the types of apnea observed in 14 preterm infants during sequential 45-minute periods with and without CPAP. CPAP markedly decreased the incidence of both mixed and obstructive apnea episodes of greater than or equal to 5 seconds (P less than 0.01 and less than 0.03, respectively). In contrast, central apnea episodes of greater than or equal to 5 seconds were entirely unaffected by CPAP. Although minute ventilation was unchanged, transcutaneous PO2 increased by 11 +/- 11 mm Hg during CPAP whether or not apnea was present. We postulate that CPAP reduces apnea in preterm infants by relief of upper airway obstruction, possibly via splinting of the pharyngeal airway.

Apnea↗
PubMed2005

Datamonkey: rapid detection of selective pressure on individual sites of codon alignments.

UNLABELLED: Datamonkey is a web interface to a suite of cutting edge maximum likelihood-based tools for identification of sites subject to positive or negative selection. The methods range from very fast data exploration to the some of the most complex models available in public domain software, and are implemented to run in parallel on a cluster of computers. AVAILABILITY: http://www.datamonkey.org. In the future, we plan to expand the collection of available analytic tools, and provide a package for installation on other systems.

Algorithms↗
PubMed2003

Ketones as building blocks for dynamic combinatorial libraries: highly active neuraminidase inhibitors generated via selection pressure of the biological target.

New and potent inhibitors of neuraminidase, a key enzyme in the influenza virus activity, have been discovered in dynamic combinatorial libraries based on ketones and amines as building blocks. Selective synthesis of a number of inhibitors among multiple theoretically possible combinations of building blocks is driven by the presence of the target enzyme.

Amines↗
PubMed2001

[Estimation of blood pressure, selected biochemical parameters and indices of left ventricular heart function in patients with mild or moderate essential hypertension treated with potassium losartan].

This study was conducted in 19 patients with mild or moderate essential hypertension randomised open trial to evaluate the influence of potassium losartan dosed 50 mg per day on blood pressure, metabolic processes and left ventricular heart function. The following intervals of examination were accepted-at entry (before therapy with losartan) and then after 10 days; after 30 days and after 60 days of treatment. Mean values of arterial blood pressure and heart rate measured by conventional technics and obtained from 24-hour ambulatory monitoring data, biochemical results (serum levels of glucose, uric acid, creatinine, sodium, potassium, total cholesterol, HDL-cholesterol, LDL-cholesterol, triglycerides, bilirubin and serum activity of aminotransferases) and selected indices of left ventricular heart function were evaluated as intragroup differences using Student's paired t test. Afterwards the body mass index was compared. Statistically significant decrease of systolic arterial pressure after 60 days of therapy was revealed and it concerned both diurnal period and sleep time and during whole 24 hours. During treatment with losartan mean creatinine serum level was increased and the serum level of bilirubin was decreased in serum after 10 days of therapy. No significant changes were noted in any other laboratory parameters. After 30 and 60 days of treatment with potassium losartan thickness of interventricular septum end-diastole (IVS-D) diminished and E/A ratio (transmitral flow during early ventricular diastole to peak velocity of late filling) augmented statistically significantly after 30 days of treatment. In addition body mass index was increased after 10 days of therapy with losartan when compared with this index at the entry. This study confirmed profitable antihypertensive losartan effect as significant decrease of systolic blood pressure and demonstrated improve diastolic function of left ventricle during therapy in patients with mild or moderate essential hypertension.

Adult↗