Investigating Meiotic Recombination and the Synaptonemal Complex in Arabidopsis thaliana

Meiotic crossovers reshuffle genetic material and are essential for accurate chromosome segregation. Crossovers are a product of meiotic recombination, a complex and highly regulated process, which occurs through the co-ordinated induction and repair of DNA double strand breaks. In the majority of sexually reproducing eukaryotes, the synaptonemal complex (a tripartite ultrastructure compromised of two closely apposed lateral elements adjoined by rod like transverse filaments) is installed between homologs during prophase I; this is essential for normal crossover formation. Using cytological techniques, fluorescent marker assays and novel CRISPR/Cas9 derived mutants the roles of the Arabidopsis thaliana transverse filament protein (ZYP1) were investigated. Additionally, this work also examined a candidate meiotic regulatory protein, the general transcription factor subunit TFIIFb2.

This thesis reveals that in the absence of ZYP1, homologs are completely asynaptic yet chromosomes extensively and intimately align during prophase I. Furthermore, zyp1 retains near wild-type fertility and in the vast majority of cases homologs successfully and accurately crossover, though infrequently the obligate crossover is lost. This work shows that in zyp1 mutants HEI10 foci (a cytological marker of class I crossovers), counted at diplotene, are increased by ~2 fold compared to the wild type. Similarly, map lengths measured using fluorescently tagged lines were increased by ~1.5 fold in zyp1 compared to the wild-type. Despite this increase in recombination, zyp1 provides no rescue when crossed to msh5, instead zyp1 msh5 mutants show ~50% fewer chiasmata than msh5 single mutants, indicating that increased crossovers derive from the ZMM dependant class I pathway. In summary, this work reveals that zyp1 is an essential regulator of crossover control. Firstly, zyp1 is required to ensure all chromosomes receive an obligate crossover. Secondly, zyp1 is essential for the normal imposition of crossover interference which limits the occurrence of ZMM derived double crossovers.