Alternative Synaptonemal Complex Structures: Too Much of a Good Thing?

The synaptonemal complex (SC), a highly conserved structure built between homologous meiotic chromosomes, is required for crossover formation and ensuring proper chromosome segregation. In many organisms, SC components can also form alternative structures, including repeating SC structures that are known as polycomplexes (PCs), and extensively modified SC structures that are maintained late in meiosis. PCs display differences in their ability to localize with lateral element proteins, recombination machinery, and DNA. They can be created by defects in post-translational modification, suggesting that these modifications have roles in preventing alternate SC structures. These SC-like structures provide insight into the rules for building and maintaining the SC by offering an ‘in vivo laboratory’ for models of SC assembly, structure, and disassembly. Here, we discuss what these structures can tell us about the rules for building the SC and the roles of the SC in meiotic processes. Synaptonemal complex (SC) components can assemble into alternative structures, including polycomplexes (PCs), observed as repeating stacks of SC monomers, aggregates, and other modified SC structures.SC components must be actively regulated to ensure assembly between homologs rather than into PCs. PCs can form due to environmental changes, when the lateral element is disrupted, as well as in mutants implicated in post-translational modification or recombination, indicating organisms have multiple mechanisms to ensure linear SC assembly.Some PCs can still interact with DNA, lateral element proteins, and recombination proteins to affect meiotic processes, suggesting that PCs retain characteristics of normal SC. Alternate SC structures can be used as tools to understand how the SC interacts with other meiotic protein complexes, as well as the function and regulation of the SC.