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      Glyphosate resistance: state of knowledge

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          Abstract

          Studies of mechanisms of resistance to glyphosate have increased current understanding of herbicide resistance mechanisms. Thus far, single-codon non-synonymous mutations of EPSPS (5-enolypyruvylshikimate-3-phosphate synthase) have been rare and, relative to other herbicide mode of action target-site mutations, unconventionally weak in magnitude for resistance to glyphosate. However, it is possible that weeds will emerge with non-synonymous mutations of two codons of EPSPS to produce an enzyme endowing greater resistance to glyphosate. Today, target-gene duplication is a common glyphosate resistance mechanism and could become a fundamental process for developing any resistance trait. Based on competition and substrate selectivity studies in several species, rapid vacuole sequestration of glyphosate occurs via a transporter mechanism. Conversely, as the chloroplast requires transporters for uptake of important metabolites, transporters associated with the two plastid membranes may separately, or together, successfully block glyphosate delivery. A model based on finite glyphosate dose and limiting time required for chloroplast loading sets the stage for understanding how uniquely different mechanisms can contribute to overall glyphosate resistance.

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          Evolution by gene duplication: an update

           Jianzhi Zhang (2003)
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            The evolution of gene duplications: classifying and distinguishing between models.

            Gene duplications and their subsequent divergence play an important part in the evolution of novel gene functions. Several models for the emergence, maintenance and evolution of gene copies have been proposed. However, a clear consensus on how gene duplications are fixed and maintained in genomes is lacking. Here, we present a comprehensive classification of the models that are relevant to all stages of the evolution of gene duplications. Each model predicts a unique combination of evolutionary dynamics and functional properties. Setting out these predictions is an important step towards identifying the main mechanisms that are involved in the evolution of gene duplications.
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              Darwinian evolution can follow only very few mutational paths to fitter proteins.

              Five point mutations in a particular beta-lactamase allele jointly increase bacterial resistance to a clinically important antibiotic by a factor of approximately 100,000. In principle, evolution to this high-resistance beta-lactamase might follow any of the 120 mutational trajectories linking these alleles. However, we demonstrate that 102 trajectories are inaccessible to Darwinian selection and that many of the remaining trajectories have negligible probabilities of realization, because four of these five mutations fail to increase drug resistance in some combinations. Pervasive biophysical pleiotropy within the beta-lactamase seems to be responsible, and because such pleiotropy appears to be a general property of missense mutations, we conclude that much protein evolution will be similarly constrained. This implies that the protein tape of life may be largely reproducible and even predictable.
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                Author and article information

                Journal
                Pest Manag Sci
                Pest Manag. Sci
                ps
                Pest Management Science
                John Wiley & Sons, Ltd (Chichester, UK )
                1526-498X
                1526-4998
                September 2014
                12 March 2014
                : 70
                : 9
                : 1367-1377
                Affiliations
                [a ]Monsanto Company St Louis, MO, USA
                [b ]Department of Bioagricultural Sciences and Pest Management, Colorado State University Fort Collins, CO, USA
                Author notes
                *Correspondence to: R Douglas Sammons, Monsanto Company, St Louis, MO, USA. E-mail: r.douglas.sammons@ 123456monsanto.com
                10.1002/ps.3743
                4260172
                25180399
                © 2014 Society of Chemical Industry

                This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

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