The Arabidopsis synaptonemal complex protein ZYP1 is required for chromosome synapsis and normal fidelity of crossing over

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Abstract
The duplicated Arabidopsis genes ZYP1a/ZYP1b encode closely related proteins with structural similarity to the synaptonemal complex (SC) transverse filament proteins from other species. Immunolocalization detects ZYP1 foci at late leptotene, which lengthen until at pachytene fluorescent signals extending the entire length of the fully synapsed homologs are observed. Analysis of zyp1a and zyp1b T-DNA insertion mutants indicates that the proteins are functionally redundant. The SC is not formed in the absence of ZYP1 and prophase I progression is significantly delayed suggesting the existence of an intraprophase I surveillance mechanism. Recombination is only slightly reduced in the absence of ZYP1 such that the chiasma frequency at metaphase I is ∼80% of wild type. Moreover cytological analysis indicates that chiasma distribution within zyp1 bivalents is indistinguishable from wild type, providing evidence that the SC is not required for the imposition of interference. Importantly in the absence of ZYP1, recombination occurs between both homologous and nonhomologous chromosomes suggesting the protein is required to ensure the fidelity of meiotic chromosome associations.