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  1. In agriculture, various chemicals are used to control the weeds. Out of which, glyphosate is an important herbicide invariably used in the cultivation of glyphosate-resistant crops to control weeds. Overuse of glyphosate results in the evolution of glyphosate-resistant weeds. Evolution of glyphosate resistance (GR) in Amaranthus palmeri (AP) is a serious concern in the USA. Investigation of the mechanism of GR in AP identified different resistance mechanisms of which 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene amplification is predominant. Molecular analysis of GR AP identified the presence of a 5- to >160-fold increase in copies of the EPSPS gene than in a glyphosate-susceptible (GS) population. This increased copy number of the EPSPS gene increased the genome size ranging from 3.5 to 11.8%, depending on the copy number compared to the genome size of GS AP. FISH analysis using a 399-kb EPSPS cassette derived from bacterial artificial chromosomes (BACs) as probes identified that amplified EPSPS copies in GR AP exist in extrachromosomal circular DNA (eccDNA) in addition to the native copy in the chromosome. The EPSPS gene-containing eccDNA having a size of ∼400 kb is termed EPSPS-eccDNA and showed somatic mosacism in size and copy number. EPSPS-eccDNA has a genetic mechanism to tether randomlymore »to mitotic or meiotic chromosomes during cell division or gamete formation and is inherited to daughter cells or progeny generating copy number variation. These eccDNAs are stable genetic elements that can replicate and exist independently. The genomic characterization of the EPSPS locus, along with the flanking regions, identified the presence of a complex array of repeats and mobile genetic elements. The cytogenomics approach in understanding the biology of EPSPS-eccDNA sheds light on various characteristics of EPSPS-eccDNA that favor GR in AP.« less
  2. Gene transfer from wild wheat relatives to bread wheat is restricted to homologous recombination. The presence of the Pairing homoeologous 1 (Ph1) gene in the long arm of wheat chromosome 5B allows only homologous chromosomes to pair and recombine, resulting in diploid inheritance of polyploid wheat. Previously, we identified a potent homoeologous pairing promotor gene(s) (Hpp-5Mg); its carrier chromosome 5Mg derived from Aegilops geniculata and its wheat homoeologous chromosome 5D freely recombined in the presence of the Ph1 gene. In this study, we investigated the effect of Hpp-5Mg on homoeologous recombination in the absence of Ph1. In Hpp-5Mg/ph1bph1b plants, we observed a vast genome-wide increase in homoeologous recombination and multiple crossovers (CO), including CO breakpoints in proximal regions of the chromosomes where recombination is known to be suppressed. We tested the efficacy of Hpp-5Mg/ph1bph1b-induced homoeologous recombination by producing new recombinants for the wheat streak mosaic virus resistance gene, Wsm3, present in the wheat-Thinopyrum intermedium Robertsonian translocation (RobT T7BS.7S#3L). A recombination frequency of 6.5% was detected by screening the progenies double monosomic for T7BS.7S#3L and 7B by genomic in situ hybridization. This recombination frequency was about 100-fold higher compared with the recombinant frequency of 0.06% observed by using ph1b-induced homoeologous recombinationmore »alone. Our results indicate that chromosome 5Mg promotes homoeologous recombination between wheat and wild wheat relative chromosomes, which helps in the generation of pre-breeding materials thereby accelerating wheat crop improvement.« less
  3. Abstract

    Genebanks are valuable resources for crop improvement through the acquisition,ex-situconservation and sharing of unique germplasm among plant breeders and geneticists. With over seven million existing accessions and increasing storage demands and costs, genebanks need efficient characterization and curation to make them more accessible and usable and to reduce operating costs, so that the crop improvement community can most effectively leverage this vast resource of untapped novel genetic diversity. However, the sharing and inconsistent documentation of germplasm often results in unintentionally duplicated collections with poor characterization and many identical accessions that can be hard or impossible to identify without passport information and unmatched accession identifiers. Here we demonstrate the use of genotypic information from these accessions using a cost-effective next generation sequencing platform to find and remove duplications. We identify and characterize over 50% duplicated accessions both within and across genebank collections ofAegilops tauschii, an important wild relative of wheat and source of genetic diversity for wheat improvement. We present a pipeline to identify and remove identical accessions within and among genebanks and curate globally unique accessions. We also show how this approach can also be applied to future collection efforts to avoid the accumulation of identical material. When coordinatedmore »across global genebanks, this approach will ultimately allow for cost effective and efficient management of germplasm and better stewarding of these valuable resources.

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  4. Gene amplification has been observed in many bacteria and eukaryotes as a response to various selective pressures, such as antibiotics, cytotoxic drugs, pesticides, herbicides, and other stressful environmental conditions. An increase in gene copy number is often found as extrachromosomal elements that usually contain autonomously replicating extrachromosomal circular DNA molecules (eccDNAs). Amaranthus palmeri , a crop weed, can develop herbicide resistance to glyphosate [ N -(phosphonomethyl) glycine] by amplification of the 5-enolpyruvylshikimate-3-phosphate synthase ( EPSPS ) gene, the molecular target of glyphosate. However, biological questions regarding the source of the amplified EPSPS , the nature of the amplified DNA structures, and mechanisms responsible for maintaining this gene amplification in cells and their inheritance remain unknown. Here, we report that amplified EPSPS copies in glyphosate-resistant (GR) A. palmeri are present in the form of eccDNAs with various conformations. The eccDNAs are transmitted during cell division in mitosis and meiosis to the soma and germ cells and the progeny by an as yet unknown mechanism of tethering to mitotic and meiotic chromosomes. We propose that eccDNAs are one of the components of McClintock’s postulated innate systems [McClintock B (1978) Stadler Genetics Symposium ] that can rapidly produce soma variation, amplify EPSPS genesmore »in the sporophyte that are transmitted to germ cells, and modulate rapid glyphosate resistance through genome plasticity and adaptive evolution.« less