Parallel recruitment pathways contribute to synaptonemal complex assembly during mammalian meiosis

During meiosis, the synaptonemal complex (SC) assembles between paired chromosomes, binding them together in close apposition, and facilitating recombination. SC assembly is thought to occur through the hierarchical zipper-like recruitment of axial elements, followed by transverse filaments and then central elements. However, the rapidity of SC formation in mammals has hitherto hindered investigation of its assembly mechanisms and their relationship with recombination. Using super-resolution imaging of separation-of-function mouse mutants, we show that, contrary to the hierarchical assembly model, central element protein SYCE2 is recruited to recombination sites early in SC assembly, and independently of SYCP1-containing transverse filaments. Further, SYCE2-TEX12 binds DNA in vitro , and SYCE2-containing bridges physically link paired chromosomes at recombination sites prior to transverse filament recruitment and chromosome synapsis. These data suggest that mammals integrate parallel recruitment pathways to assemble a mature SC: one recruiting central element proteins to recombination sites, and another recruiting transverse filaments to chromosomes.