Crossovers break SC symmetry

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.

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,...