Crossovers break SC symmetry

Abstract

At the start of meiotic prophase I, homologous chromosomes become aligned lengthwise. After recombination, chromosomes reconfigure to reduce contacts between homologues and prepare for bipolar spindle attachment. On page 683, Nabeshima et al. illustrate these rearrangements during worm meiosis and provide evidence that crossovers are symmetry-breaking events that promote the ungluing of lengthwise contacts between homologues.During homologous recombination, proteins of the synaptonemal complex (SC) link aligned homologues. Later, the SC disassembles, leaving exchanged DNA and sister chromatid cohesion to link homologues. Figure After recombination, SC proteins (green) remain only on the short chromosome axis. The new images reveal that this SC disassembly is asymmetric. Crossovers usually occur at an off-center position, leaving a short and a long axis emerging from the spot of the crossover. SC components departed from the long axis first, but lingered on the short axis. The crossover, or a precursor structure, seems to cue the unequal SC disassembly. Recombination mutants underwent more symmetric SC disassembly, but asymmetry was restored when crossovers were initiated by radiation-induced DNA breaks. The authors imagine that SC disassembly increases with distance from a crossover. As SC proteins bind cooperatively, loss of a few from the long end may start a chain reaction that clears that axis of the SC. SC proteins lingered at the same spots where sister chromatid cohesion must be released in anaphase I. This partial cohesion release allows homologues to separate even as sister chromatids are still held together on the long axis. The lingering SC proteins may direct local release by recruiting the aurora kinase AIR-2, which concentrates on the short axis at the end of prophase.