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PubMed1986

Genetic divergence under uniform selection. II. Different responses to selection for knockdown resistance to ethanol among Drosophila melanogaster populations and their replicate lines.

We have tested the hypothesis that genetic differences among conspecific populations may result in diverse responses to selection, using natural populations of Drosophila melanogaster. Selection for ethanol tolerance in a tube measuring knockdown resistance was imposed on five West Coast populations. In 24 generations the selected lines increased their mean knockdown times, on average, by a factor of 2.7. An initially weak latitudinal cline was steepened by selection. The two southernmost populations showed the same increases in the selected character, but differed consistently in their correlated responses in characters related to ethanol tolerance. This result indicates that the populations responded to selection by different genetic changes. Selection decreased female body weight and increased resistance to acetone, suggesting components of the response unrelated to ethanol metabolism. The Adhs allele was favored by selection in all populations at the onset, but increased in frequency only in the selected lines of the southernmost population. There was a correlation between latitude and Adh frequency changes, suggesting that fitnesses of the Adh alleles were dependent on the genetic background. Genetic background also had a large effect on the loss of fitness due to selection. Genetic drift between replicate lines caused more variation in selection response than initial genetic differences between populations. This result demonstrates the importance of genetic drift in divergence among natural populations undergoing uniform selection, since the effective population sizes approached those of small natural populations. Drift caused greater divergence between selected replicates than control replicates. Implications of this result for the genetic model of selection response are discussed.

Alcohol Dehydrogenase↗
PubMed2025

Evaluating selection at intermediate scales within genes provides robust identification of genes under positive selection in M. tuberculosis clinical isolates.

Multiple studies have reported genes in the M. tuberculosis (Mtb) genome that are under diversifying selection, based on genetic variants among Mtb clinical isolates. These might reflect adaptions to selection pressures associated with modern clinical treatment of TB. Many, but not all, of these genes under selection are related to drug resistance. Most of these studies have evaluated selection at the gene-level. However, positive selection can be evaluated on different scales, including individual sites (codons) and local regions within an ORF. In this paper, we use GenomegaMap, a Bayesian method for calculating selection, to evaluate selection of genes in the Mtb genome at all three levels. We present evidence that the intermediate analysis (windows of codons) yields the most credible list of candidate genes under selection (excluding PPE and PE_PGRS genes, which are predicted less reliably due to frequent sequencing errors). A further advantage of this approach is that it identifies specific regions within proteins that are under selective pressure, which is useful for structural and functional interpretation. In an analysis of two separate collections of Mtb clinical isolates (from Moldova; and a globally-representative set), we observed 53 and 173 significant genes under selection, with 36% overlap. The lists of genes under selection include many drug-resistance genes, as well as other genes that have previously been reported to be under selection (resR, phoR). The specific regions under selection identified within drug-resistance genes are shown to correspond to protein structural features known to be involved in resistance, supporting accuracy of the method. Positive selection in several ESX-1-related genes was also observed, suggesting adaptation to immune pressure.

adaptation↗
PubMed1997

Degree of carotid artery stenosis. Comparison of selective and non-selective angiographic findings with surgical specimens.

OBJECTIVE: To compare the degree of vessel narrowing seen on selective and non-selective carotid artery catheter angiograms using criteria set by NASCET and ECST with the results obtained from corresponding surgical specimens. SUBJECTS: In 40 preoperative angiograms (20 non-selective, 20 selective) the 'distal' degree of internal carotid artery (ICA) stenosis according to NASCET criteria and the 'local' degree of stenosis according to ECST criteria was assessed. These data were compared with the 'distal' and 'local' degree of ICA stenosis obtained by measuring the specimens and the diameter of the distal ICA intraoperatively. RESULTS: The median 'local' degree of stenosis was 86.5% in the specimen and 83.5% in the selective angiograms (difference not significant). In non-selective angiography the median 'local' degree of stenosis was 77.5% compared to 84% in the corresponding specimens (P < 0.01). The median 'distal' degree of stenosis in selective angiography was 76.5 versus 75.5% in the specimens (n.s.). The median 'distal' degree of non-selective angiography was 67% compared to 77.5% in the corresponding specimens (P = 0.02). The trend to underestimate high grade stenosis (above 90%) was more pronounced in non-selective than in selective angiography. Medium grade stenosis (60-80%) was slightly overestimated in selective angiography. CONCLUSION: Selective angiography is more accurate in determining the 'true' degree of stenosis in internal carotid artery disease, taking into account a slight overestimation of medium grade stenosis. High grade stenosis is underestimated in both selective and non-selective angiography. These observations extend to both the ECST and NASCET criteria of measuring the degree of stenosis, which differ by about 10%.

Aged↗
PubMed2001

Selective photocoagulation of communicating vessels in the treatment of monochorionic twins with selective growth retardation.

OBJECTIVE: Current treatment of patients with selective intrauterine growth retardation in monochorionic twins includes expectant management, termination of pregnancy, or umbilical-cord occlusion. The purpose of this study was to assess the outcome of monochorionic twins with selective intrauterine growth retardation who were treated with selective laser photocoagulation of the communicating vessels. STUDY DESIGN: Monochorionic twin pregnancies with selective intrauterine growth retardation at less than 26 weeks were eligible for the study. Selective intrauterine growth retardation was defined as <10th percentile for gestational age. Absent or reverse end-diastolic velocity in the umbilical artery of the twin with selective intrauterine growth retardation was required for eligibility after January 2000. RESULTS: Thirty patients met the criteria for the study: 17 patients were treated expectantly (group I); 2 patients underwent umbilical-cord ligation of the twin with selective intrauterine growth retardation, and 11 patients underwent selective laser photocoagulation of the communicating vessels (group II). Survival rates for at least 1 fetus were no different between groups I and II (14/17 [82.3%] vs 8/11 [72.3%]; P = .4). However, concomitant demise of the co-twin occurred in 4 of 7 patients, and iatrogenic premature delivery for deterioration of the twin with selective intrauterine growth retardation was necessary in 2 patients in group I, which resulted in significant neonatal morbidity. Of the live-born babies, neurologic handicap was present in 3 of 22 babies (13.6%) versus 0 of 12 in groups I and II, respectively (P < .0001). CONCLUSION: Selective intrauterine growth retardation in monochorionic twins can be effectively treated with selective laser photocoagulation of the communicating vessels. By unlinking the circulations between the fetuses, the pregnancy is rendered "functionally" dichorionic, which improves pregnancy treatment and results in decreased neonatal morbidity. This approach constitutes a new valuable alternative in the treatment of monochorionic twin pregnancies with selective intrauterine growth retardation. A randomized clinical trial of expectant treatment versus selective laser photocoagulation of the communicating vessels for monochorionic selective intrauterine growth retardation can be considered.

Blood Vessels↗
PubMed2005

Attentional selection and action selection in the ventral and orbital prefrontal cortex.

Different accounts of the ventral and orbital prefrontal cortex (PFv+o) have emphasized either its role in learning conditional rules for action selection or the attentional selection of behaviorally relevant stimuli. Although the accounts are not mutually exclusive, it is possible that the involvement of PFv+o in conditional action selection is a consequence of its role in selecting relevant stimuli or that its involvement in attentional selection is a consequence of the conditional rules present in many attentional paradigms. Five macaques learned a conditional action-selection task in which the difficulty of identifying the stimulus relevant for guiding action selection was varied in a simple manner by either altering its distance from the action or presenting additional distracting stimuli. Simply increasing the spatial separation between the instructing stimulus led to slower responses. Experiment 1 showed that bilateral PFv+o lesions impaired conditional action selection even when attentional demands were kept to a minimum, but there was evidence that the impairment was exacerbated by manipulating stimulus selection difficulty. Experiment 2 confirmed the importance of PFv+o for conditional action selection even when stimulus selection difficulty was minimal. Experiments 3 and 4 demonstrated that the action-selection impairment was significantly increased by making identification of the behaviorally relevant stimulus difficult. PFv+o is central to the use of conditional rules when selecting courses of action, but conditional rules are also represented in premotor and striatal regions. A special contribution of PFv+o may be initial selection of behaviorally relevant stimuli.

Animals↗
PubMed2006

Population development by phenotypic selection with subsequent marker-assisted selection for line extraction in cucumber (Cucumis sativus L.).

Cucumber (Cucumis sativus L.; 2n = 2x = 14) has a narrow genetic base, and commercial yield of US processing cucumber has plateaued in the last 15 years. Yield may be increased by altering plant architecture to produce unique early flowering (days to flower, DTF), female (gynoecious, GYN), highly branched (multiple lateral branching, MLB), long-fruited (length:diameter ratio, L:D) cultivars with diverse plant statures. The genetic map position of QTL conditioning these quantitatively inherited yield component traits is known, and linked molecular markers may have utility in marker-assisted selection (MAS) programs to increase selection efficiency, and effectiveness. Therefore, a base population (C0), created by intermating four unique but complementary lines, was subjected to three cycles (C1-C3) of phenotypic (PHE) mass selection for DTF, GYN, MLB, and L:D. In tandem, two cycles of marker-assisted backcrossing for these traits began with selected C2 progeny (C2S) to produce families (F1[i.e., C2S x C2S], and BC(1) [i.e., F1 x C2S]) for line extraction, and for comparative analysis of gain from selection by PHE selection, and MAS. Frequencies of marker loci were used to monitor selection-dependent changes during PHE selection, and MAS. Similar gain from selection was detected as a result of PHE selection, and MAS for MLB (approximately 0.3 branches/cycle), and L:D (approximately 0.1 unit increase/cycle) with concomitant changes in frequency at linked marker loci. Although genetic gain was not realized for GYN during PHE selection, the percentage of female flowers of plants subjected to MAS was increased (5.6-9.8% per cycle) depending upon the BC1 population examined. Selection-dependent changes in frequency were also detected at marker loci linked to female sex expression during MAS. MAS operated to fix favorable alleles that were not exploited by PHE selection in this population, indicating that MAS could be applied for altering plant architecture in cucumber to improve its yield potential.

Biomarkers↗
PubMed1997

The effects of local selection, balanced polymorphism and background selection on equilibrium patterns of genetic diversity in subdivided populations.

Levels of neutral genetic diversity in populations subdivided into two demes were studied by multilocus stochastic simulations. The model includes deleterious mutations at loci throughout the genome, causing 'background selection', as well as a single locus at which a polymorphism is maintained, either by frequency-dependent selection or by local selective differences. These balanced polymorphisms induce long coalescence times at linked neutral loci, so that sequence diversity at these loci is enhanced at statistical equilibrium. We study how equilibrium neutral diversity levels are affected by the degree of population subdivision, the presence or absence of background selection, and the level of inbreeding of the population. The simulation results are compared with approximate analytical formulae, assuming the infinite sites neutral model. We discuss how balancing selection can be distinguished from local selection, by determining whether peaks of diversity in the region of the polymorphic locus are seen within or between demes. The width of such diversity peaks is shown to depend on the total species population size, rather than local deme sizes. We show that, with population subdivision, local selection enhances between-deme diversity even at neutral sites distant from the polymorphic locus, producing higher FST values than with no selection; very high values can be generated at sites close to a selected locus. Background selection also increases FST, mainly because of decreased diversity within populations, which implies that its effects may be distinguishable from those of local selection. Both effects are stronger in selfing than outcrossing populations. Linkage disequilibrium between neutral sites is generated by both balancing and local selection, especially in selfing populations, because of linkage disequilibrium between the neutral sites and the selectively maintained alleles. We discuss how these theoretical results can be related to data on genetic diversity within and between local populations of a species.

Alleles↗
PubMed2000

Direct and correlated responses to selection for milk yield: results and conclusions of regional project NC-2, "improvement of dairy cattle through breeding, with emphasis on selection". NC-2 Technical Committee.

Measurement of direct and correlated responses to single-trait selection for milk yield was the major objective of regional project NC-2. The NC-2 Technical Committee included representatives from Alaska, Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Nebraska, South Dakota, Wisconsin, and the USDA. All representatives, except Illinois, Kansas and Nebraska, maintained a selection line formed by using AI sires selected for high estimated transmitting abilities for milk and a second line that served as some type of a control. Stations varied in criteria for selection of bulls for control lines. Farms were managed similarly, including feeding and management of selection and control lines as one herd, random mating within line, and restricted culling policies. Selection for milk yield effectively increased milk production. All selection lines increased milk and net income per lactation more than control lines. Realized gains matched or exceeded gains expected from estimates of breeding values. Yields of milk components increased, but component percentages decreased appreciably for selection lines. Reproduction of nulliparous animals was not affected, but days open for lactating selection cows increased in some of the individual projects. Selected cows tended to have larger health costs, specifically for mammary treatment. Udder and conformation traits did not deteriorate for selection lines, although control lines with selection of sires on genetic evaluations for type received higher type scores. There should be few reservations about undesirable responses correlated with selection for milk yield.

Animal Feed↗
PubMed2004

A convenient method for positive selection of retroviral producing cells generating vectors devoid of selectable markers.

Early retroviral vectors containing both a therapeutic gene and a dominant selectable marker gene, offered some distinct advantages with respect to gene therapy, in that they simplified the generation, isolation, and titration of retroviral producer cell clones, as well as the evaluation and selection of successfully targeted cells. However, a number of problems were engendered by this strategy: the promoter driving the selectable marker gene could interfere with transcription of the therapeutic gene, and immune responses could be induced to cells expressing foreign proteins of selection marker origin. Simplified retroviral vectors, which lack a selection marker gene, were constructed to address these problems, but the inability to use a selection marker has made identification and cloning of virus producing transfected cells a heavy burden. To maintain the benefits of simplified retroviral vectors, while providing a facile means to select packaging cells transfected with retroviral DNA, we cloned the bacterial selection marker gene encoding neomycin phosphotransferase (neo) into the plasmid backbone of the vector, but outside of the provirus, resulting in efficient selection of transfected packaging cells and generation of packaged virus which lacks the neo gene. This novel approach generates greater numbers of high infectious titer producing clones, after selection in G418 media, than does a co-transfection approach, due to integration of higher construct copy numbers per cell. No transmission of the selection marker gene to target cells was observed following retroviral transduction. Thus, our strategy eliminates the adverse consequences of a selection-based method, while diminishing the burden of identification of packaging cells transfected with vectors devoid of selectable markers.

DNA, Viral↗
PubMed2002

Induction of thymocyte positive selection does not convey immediate resistance to negative selection.

The acquisition of functional competence represents a critical phase during intrathymic development of T cells. Thymocytes reaching this stage represent cells which have been positively selected on the basis of major histocompatibility complex reactivity, but which have also been purged of potentially autoreactive T-cell receptor specificities by negative selection. While the developmental window in which thymocytes are subjected to positive selection is now well defined, the precise developmental timing of negative selection, in relation to positive selection events, is less clear. Moreover, the underlying mechanism allowing single-positive thymocytes to respond to T-cell receptor ligation by activation rather than death, remains controversial. Here we have analysed the developmental timing of negative selection in relation to positive selection, using measurement of thymocyte susceptibility to dendritic cell presentation of the superantigen staphylococcal enterotoxin B (SEB). We show that thymocytes which have received initial positive selection signals, namely CD4+ CD8+ CD69+ thymocytes, like their CD4+ CD8+ CD69minus sign precursors, are susceptible to negative selection, indicating that induction of positive selection does not convey immediate resistance to negative selection. In contrast, newly generated CD4+ CD8minus sign CD69+ cells are not only resistant to deletion by SEB, but respond to SEB-mediated T-cell receptor-ligation by activation, indicating that the acquisition of functional competence occurs at the newly generated CD4+ CD8minus sign CD69+ stage. Finally, by using direct retroviral infection of primary CD4+ CD8+ thymocytes, we also show that Notch-1 activation in CD4+ CD8+ thymocytes does not correlate with, nor convey resistance to superantigen-mediated negative selection. Thus, our data suggest that although Notch-1 has been implicated in resistance to thymocyte apoptosis, the acquisition of resistance to negative selection occurs independently of Notch-1 signalling.

Animals↗
PubMed1989

Quantitative genetic variability maintained by mutation-stabilizing selection balance: sampling variation and response to subsequent directional selection.

A model of genetic variation of a quantitative character subject to the simultaneous effects of mutation, selection and drift is investigated. Predictions are obtained for the variance of the genetic variance among independent lines at equilibrium with stabilizing selection. These indicate that the coefficient of variation of the genetic variance among lines is relatively insensitive to the strength of stabilizing selection on the character. The effects on the genetic variance of a change of mode of selection from stabilizing to directional selection are investigated. This is intended to model directional selection of a character in a sample of individuals from a natural or long-established cage population. The pattern of change of variance from directional selection is strongly influenced by the strengths of selection at individual loci in relation to effective population size before and after the change of regime. Patterns of change of variance and selection responses from Monte Carlo simulation are compared to selection responses observed in experiments. These indicate that changes in variance with directional selection are not very different from those due to drift alone in the experiments, and do not necessarily give information on the presence of stabilizing selection or its strength.

Gene Frequency↗
PubMed1999

Improving the efficiency of artificial selection: more selection pressure with less inbreeding.

The use of population genetic variability in present-day selection schemes can be improved to reduce inbreeding rate and inbreeding depression without impairing genetic progress. We performed an experiment with Drosophila melanogaster to test mate selection, an optimizing method that uses linear programming to maximize the selection differential applied while at the same time respecting a restriction on the increase in inbreeding expected in the next generation. Previous studies about mate selection used computer simulation on simple additive genetic models, and no experiment with a real character in a real population had been carried out. After six selection generations, the optimized lines showed an increase in cumulated phenotypic selection differential of 10.76%, and at the same time, a reduction of 19.91 and 60.47% in inbreeding coefficient mean and variance, respectively. The increased selection pressure would bring greater selection response, and in fact, the observed change in the selected trait was on average 31.03% greater in the optimized lines. These improvements in the selection scheme were not made at the expense of the long-term expectations of genetic variability in the population, as these expectations were very similar for both mate selection and conventionally selected lines in our experiment.

Animals↗
PubMed1975

Hybridization and selection for increased penicillin titre in wild-type isolates of Aspergillus nidulans.

Repeated hybridization and selection among wild-type isolates produced strains of Aspergillus nidulans with increased penicillin titre. Four independent selection lines were established, each originating from a sexual cross between two different heterokaryon-incompatible wild-type isolates. In each generation, two selected high-titre sister strains were crossed to produce the next generation. An initial increase in titre was obtained in each line, but after four or five generations of selection the genetic variation was considerably reduced and the rate of response to selection had decreased. From a base population of wild-type isolates with a mean titre of 8-6 units/ml the progeny mean titre was raised to between 16 and 20 units/ml in each line. The gradual nature of the response suggests that a number of genes determine penicillin titre in the wild-type isolates used. The gene action throughout the selection programme was predominantly additive.

Aspergillus nidulans↗
PubMed1986

Selection in Hereford cattle. I. Selection intensity, generation interval and indexes in retrospect.

Selection intensity and generation interval were evaluated in a Hereford cattle herd made up of 14 inbred lines and 14 linecross groups corresponding to the lines of inbred sires at the Suan Juan Basin Research Center, Hesperus, Colorado. Selection indexes practiced were calculated in retrospect. Analyses of the records collected from 1946 through 1973 involved weaning weight (WW) and postweaning traits in males and females. Analyses by line were performed for the inbreds, while pooled analyses were done on the inbred and linecross populations. From records of 1,239 calves weaned, age of sire averaged 3.75 yr compared with 4.52 yr for age of dam, showing faster generation turnover for sires than for dams. Generation interval determined as actual age of midparent was 4.13 yr. Selection applied for WW, evaluated as annual selection differentials within inbred lines and then pooled over all lines, averaged .55 standard deviations (sigma)/generation for sires. For females, selection was much less intense. Midparent selection differential amounted to .33 sigma/generation. For sires, pooled standardized selection differentials per generation over all lines during the postweaning gain period were .49 sigma, .46 sigma, .40 sigma, -.20 sigma, -.10 sigma and .69 sigma, respectively, for initial weight, final weight, feed consumed, feed efficiency (FE, unadjusted and adjusted) and average daily gain (ADG). Selection of females for postweaning traits was not intense. Selection index actually practiced in retrospect for sires was: IS = .4461 (WW) - .0092 (FE) + .6126 (ADG). The indexes for dams included WW, 12-mo weight (12W), 18-mo weight (18W), mature spring weight (SPW) and mature fall weight (FAW) and were: for inbred dams, ID = .1824 (WW) - .0284 (12W) + .0736 (18W) - .1097 (SPW) - .1097 (FAW); for linecross dams, ID = .2693 (WW) - .2960 (12W) + .0147 (18W) + .1185 (SPW) - .0354 (FAW). The corresponding index selection differentials were .818, .203 and .209. Sire index selection differentials represent about 79% of the total selection differentials.

Animals↗
PubMed2003

Selection responses for clinical mastitis and protein yield in two Norwegian dairy cattle selection experiments.

Inferences from two dairy cattle selection experiments, in which sires were selected from external sources, were drawn by using an animal model to analyze data from the entire population. The first selection experiment was carried out in the period from 1978 to 1989 and included groups selected for high milk production (HMP) and low milk production (LMP). Each year, the highest ranking proven sires for milk production, from the most recent group of Norwegian Dairy Cattle (NRF) test bulls, were selected and mated to the cows in the HMP group. A group of sires with low milk production indices from progeny testing in 1978 and 1979 were used as sires in the LMP group during the entire experiment. The second selection experiment, which started in 1989, included one high protein yield (HPY) group and one low clinical mastitis (LCM) group. The highest ranking proven NRF sires for protein yield and mastitis resistance were selected each year from the most recent group of progeny tested bulls and used as sires in the HPY and LCM groups, respectively. Genetic trends for protein yield were positive (as expected) for HMP and HPY cows, and negative for LMP and LCM cows. Estimates of annual genetic trends for clinical mastitis were +0.23, -0.02, +0.04, and -0.91% per year for HMP, LMP, HPY, and LCM cows, respectively. The difference in genetic trend of clinical mastitis between HMP and HPY groups, both selected for increased milk production, reflects the gradual change in the NRF breeding objective towards more weight on health relative to milk over the last 20 yr. After four cow generations, the genetic difference in mastitis between HMP and LMP group cows was 3.1% clinical mastitis, a correlated response to selection for increased milk production. The genetic difference between LCM and HPY cows of 8.6% clinical mastitis after three cow generations is mainly a result of direct selection against clinical mastitis in the LCM group. In the NRF population, an approximately flat genetic trend for clinical mastitis was found for cows born from 1976 to 1990, whereas cows born after 1990 showed a genetic improvement equivalent to a reduction of 0.19% clinical mastitis per year. The results show that it is possible to obtain considerable selection response for clinical mastitis, and that selection for increased milk production will result in an unfavorable correlated increase in mastitis incidence, if mastitis is ignored in the breeding program.

Animals↗
PubMed1996

Divergent selection for body weight and yolk precursor in Coturnix coturnix japonica. 10. Response to selection over thirty generations.

Japanese quail lines were divergently selected for 30 generations for 4-wk BW (HW, LW) or plasma yolk precursor (HP, LP) as measured by total plasma phosphorus (TPP) at the beginning of lay. The HW, LW, HP, and LP lines were initiated from a randombred control line (R1). In the ninth generation of selection in the HW line, sublines were developed by continuing to select males for increased 4-wk BW whereas females were selected for increased (HW-HP line) or decreased (HW-LP) TPP. Individual selection was used in the HW, LW, HW-HP, and HW-LP lines. In the HP and LP lines, females were individually selected whereas males were selected on their full-sisters' performance. Realized heritabilities (h2), based on the linear regression of selection responses (corrected for environmental changes) on accumulated selection differentials (weighted for the number of offspring produced) over generations, declined with selection in the HW, LW, HP, and LP lines. The h2 of 4-wk BW in the HW line was 0.42 during the first 10 generations of selection and then declined to 0.30 during the next 2 generations of selection. In the LW line, realized h2 of 4-wk BW was 0.40, 0.16, and -0.12 in Generations 1 through 10, 11 through 20, and 21 through 30, respectively. The respective h2 for TPP in the HP line were 0.29, 0.17, and 0.14. The h2 of TPP in the LP line was 0.35 during the first 10 generations and essentially zero during the next 20 generations. Four-week BW and TPP were negatively associated in the HW, HP, and LP lines. There was no significant relationship between these two traits in the LW line.

Animals↗
PubMed1995

Growth, proline accumulation and water relations of NaCl-selected and non-selected callus lines of Dactylis glomerata L.

Sodium chloride-tolerant calli were selected from leaf-derived embryogenic calli of Dactylis glomerata L. on agar solidified medium supplemented with 200 mM NaCl, a concentration lethal to non-selected calli. Growth characteristics, water relations and proline accumulation pattern were compared in selected and non-selected lines. The objective was to gain an understanding of the mechanism(s) of tolerance in the NaCl-tolerant line. Growth in the selected line, as expressed in terms of tolerance index (ratio of fresh wt. on NaCl medium:fresh wt. on NaCl free medium x 100), was greater than that of the non-selected line at all levels of NaCl between 50 and 300 mM. There was no significant difference in proline accumulation in the selected and non-selected lines. Maintenance of turgor by osmotic adjustment was observed in the non-selected line despite decreased growth. In contrast, the selected line lost either the need or the ability to adjust osmotically. There was little or no increase in symplastic osmolality in the selected line when exposed to NaCl. Presumably, selection was made for a salt-excluding tissue that has lost the ability to accumulate solutes and adjust turgor with NaCl stress.

Cell Line↗
PubMed2002

Cation-selective mutations in the M2 domain of the inhibitory glycine receptor channel reveal determinants of ion-charge selectivity.

Ligand-gated ion channel receptors mediate neuronal inhibition or excitation depending on their ion charge selectivity. An investigation into the determinants of ion charge selectivity of the anion-selective alpha1 homomeric glycine receptor (alpha1 glycine receptor [GlyR]) was undertaken using point mutations to residues lining the extra- and intracellular ends of the ion channel. Five mutant GlyRs were studied. A single substitution at the intracellular mouth of the channel (A-1'E GlyR) was sufficient to convert the channels to select cations over anions with P(Cl)/P(Na) = 0.34. This result delimits the selectivity filter and provides evidence that electrostatic interactions between permeating ions and pore residues are a critical factor in ion charge selectivity. The P-2'Delta mutant GlyR retained its anion selectivity (P(Cl)/P(Na) = 3.81), but it was much reduced compared with the wild-type (WT) GlyR (P(Cl)/P(Na) = 27.9). When the A-1'E and the P-2'Delta mutations were combined (selectivity double mutant [SDM] GlyR), the relative cation permeability was enhanced (P(Cl)/P(Na) = 0.13). The SDM GlyR was also Ca(2+) permeable (P(Ca)/P(Na) = 0.29). Neutralizing the extracellular mouth of the SDM GlyR ion channel (SDM+R19'A GlyR) produced a more Ca(2+)-permeable channel (P(Ca)/P(Na) = 0.73), without drastically altering monovalent charge selectivity (P(Cl)/P(Na) = 0.23). The SDM+R19'E GlyR, which introduces a negatively charged ring at the extracellular mouth of the channel, further enhanced Ca(2+) permeability (P(Ca)/P(Na) = 0.92), with little effect on monovalent selectivity (P(Cl)/P(Na) = 0.19). Estimates of the minimum pore diameter of the A-1'E, SDM, SDM+R19'A, and SDM+R19'E GlyRs revealed that these pores are larger than the alpha1 GlyR, with the SDM-based GlyRs being comparable in diameter to the cation-selective nicotinic acetylcholine receptors. This result provides evidence that the diameter of the ion channel is also an important factor in ion charge selectivity.

Amino Acid Sequence↗