ZSL Orchestrates Synaptonemal Complex Assembly as a Central Region Scaffold to Ensure Synapsis Fidelity and Crossover Control in Polyploid Meiosis

The synaptonemal complex (SC) is essential for accurate homologous chromosome pairing, recombination, and segregation during meiosis. Although several core SC components have been identified in plants, the molecular mechanism coordinating their assembly remains poorly understood. Here, through a refined temporal transcriptomic analysis of another development in the allotetraploid Brassica napus, this study identifies ZYP1-SCEP1/2 Linker (ZSL) as a central element scaffold that bridges the transverse filament protein ZYP1 with the heterodimeric central element proteins SCEP1/2. It is shown that ZYP1 loading onto chromosomes occurs independently of ZSL and SCEP1/2, whereas ZYP1 is required for their recruitment. Loss of ZSL impedes SCEP1/2 recruitment but not vice versa, and zsl mutants completely lack continuous SC central region assembly, leading to synapsis failure and chromosome mis-segregation. This study further demonstrates that ZSL directly interacts with both ZYP1 and SCEP1/2. These findings define a hierarchical assembly cascade of ZYP1 → ZSL → SCEP1/2 during SC formation. Furthermore, analysis of HEI10 foci and genome-wide crossover (CO) mapping in zsl mutants reveals an ≈100% increase in both male and female COs, accompanied by a loss of interference and elimination of sex-specific CO differences. Together, the results establish ZSL as a key molecular adaptor coordinating SC central region assembly and CO patterning, providing new mechanistic insight into meiotic fidelity and genome stability in polyploid species.