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Role of recombinaison proteins in crossover formation, pairing and synapsis in Arabidopsis meiosis : Physiologie et génétique moléculaires

Abstract : The visible manifestation of genetic crossing-over, chiasmata link homologous chromosome pairs to permit them to properly orient on the meiotic Anaphase I spindle. They are the result of an intricate and tightly regulated process involving induction of DNA double- strand breaks and their repair through invasion of a homologous template DNA duplex. Recombination is thus essential for the synapsis and accurate segregation of meiotic chromosomes at the first meiotic division, and in doing so, generates genetic variation. Although the processes permitting a chromosome to pair only with its homologue are not fully understood, successful pairing of homologous chromosomes is tightly linked to recombination catalysed by the DNA strand exchange enzymes RAD51 and DMC1. Both proteins share very similar capabilities in vitro, but are functionally distinct in vivo. The first part of my thesis shows the impact of eliminating the strand exchange activity of RAD51 in Arabidopsis meiosis, while retaining its function as an accessory factor for the action of DMC1. Recombination can give rise to both crossover (CO) and non-crossover (NCO) outcomes and the meiosis-specific recombinase DMC1 has been thought to be of particular importance in the production of inter-homolog CO. Recent results however suggest strongly that that DMC1 is the only active recombinase in wild-type meiosis and thus must be responsible for both CO and NCO outcomes. Approximately 95% of meiotic homologous recombination in Arabidopsis does not result in inter-homologue crossovers and Arabidopsis is thus a particularly sensitive model for testing the relative importance of the two proteins - even minor effects on the non-crossover event population should produce detectable effects on crossing-over. DMC1 catalyses repair of all meiotic DNA breaks in the presence of the catalytically inactive RAD51 (RAD51-GFP fusion) and the results of my work show that this has no detectable effect on the relative rates of CO and NCO recombination, both locally and chromosome- and genome-wide, nor on the progression of the meiotic division. This work has resulted in a publication in the journal PLoS One (Singh G, Da Ines O, Gallego ME & White CI (2017) Analysis of the impact of the absence of RAD51 strand exchange activity in Arabidopsis meiosis. PLoS ONE 12: e0183006–16).Previous publications show partial, incomplete homolog synapsis in the absence of rad51 and xrcc3 in Arabidopsis meiosis. This is accompanied by the presence of many short ZYP1 fibres in these nuclei, possibly indicating short stretches of Synaptonemal Complex (SC). The partial synapsis is both SPO11- and DMC1-dependent and involves peri-centromeres, showing that DMC1 is able to (at least partially) drive synapsis in peri-centromeres in the absence of RAD51. In an effort to better characterize this and to test the hypothesis that the short ZYP1 fibres show the presence of initiation of SC at these sites, immunofluorescence and SIM imaging with DAPI staining and ASY1, ZYP1 and CENH3 antisera were carried out for cytogenetic analyses of synapsis in rad51 and xrcc3 mutants and the WT in the second part of my thesis work. Although I do observe short ZYP1 fibres including centromeres in the mutants, these are not the rule, so synapsis does not necessarily begin at centromeres or peri-centromeres. The superresolution imaging does confirm the presence of stretches of 4-chromatid fibres in xrcc3 plants and this approach will be extended in future work of the group to probe the nature of the RAD51-independent partial meiotic chromosome synapsis.Finally, I have designed and built CRISPR/CAS9 constructs with the aim of creating meiotic DSB hotspots at specific genomic loci. Taking advantage of single nucleotide polymorphism data, these constructs were designed to specifically cleave sites in the Arabidopsis Col-0 ecotype, and not in Ler-0 plants. (...)
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Gunjita Singh. Role of recombinaison proteins in crossover formation, pairing and synapsis in Arabidopsis meiosis : Physiologie et génétique moléculaires. Plants genetics. Université Clermont Auvergne, 2017. English. ⟨NNT : 2017CLFAC037⟩. ⟨tel-01781616⟩

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