ZYP1 is required for obligate cross-over formation and cross-over interference in Arabidopsis

The synaptonemal complex is a tripartite proteinaceous ultrastructure that forms between homologous chromosomes during prophase I of meiosis in the majority of eukaryotes. It is characterized by the coordinated installation of transverse filament proteins between two lateral elements and is required for wild-type levels of crossing over and meiotic progression. We have generated null mutants of the duplicatedArabidopsistransverse filament geneszyp1aandzyp1busing a combination of T-DNA insertional mutants and targeted CRISPR/Cas mutagenesis. Cytological and genetic analysis of thezyp1null mutants reveals loss of the obligate chiasma, an increase in recombination map length by 1.3- to 1.7-fold and a virtual absence of cross-over (CO) interference, determined by a significant increase in the number of double COs. At diplotene, the numbers of HEI10 foci, a marker for Class I interference-sensitive COs, are twofold greater in thezyp1mutant compared to wild type. The increase in recombination inzyp1does not appear to be due to the Class II interference-insensitive COs as chiasmata were reduced by ∼52% inmsh5/zyp1compared tomsh5. These data suggest that ZYP1 limits the formation of closely spaced Class I COs inArabidopsis. Our data indicate that installation of ZYP1 occurs at ASY1-labeled axial bridges and that loss of the protein disrupts progressive coalignment of the chromosome axes.