The synaptonemal complex central element SCEP3 interlinks synapsis initiation and crossover formation in Arabidopsis thaliana

The synaptonemal complex (SC) forms between homologous chromosomes during meiosis. In Arabidopsis thaliana, its central region (CR) is composed of the transverse filament protein ZYP1 and the central element proteins SCEP1 and SCEP2. Here we identify SCEP3 as a CR protein that is evolutionarily conserved across plant species. SCEP3 spatiotemporally overlaps with other CR proteins and localizes to the SC CR. The loss of SCEP3 prevents SC assembly, abolishes crossover (CO) assurance and interference, and eliminates sex-specific differences in CO rates (heterochiasmy) through increased CO in females. SCEP3 is required for a subset of COs in SC-deficient mutants, such as zyp1. Although SCEP3 physically interacts with ZYP1, it loads independently of other CR proteins. We propose that SCEP3 may associate with certain recombination intermediates, stabilizing them and/or recruiting additional factors, such as ZYP1, to a subset of these intermediates, thereby promoting and interlinking SC assembly and CO formation.