Crossover patterning through condensation and coarsening of pro-crossover factors.

Meiotic recombination mixes genetic information from parental genomes, creating unique combinations of alleles. During meiotic prophase, each homologue pair must undergo at least one crossover to segregate faithfully. Only a few recombination intermediates become crossovers, and these are widely spaced or limited to one per chromosome pair. Mechanisms that regulate crossover number and spacing remain poorly understood. Here we show that, in Caenorhabditis elegans, 'recombination nodules', protein assemblies that stabilize recombination intermediates and promote crossover formation, assemble in part through biomolecular condensation and are stabilized by CDK-2 kinase activity. We further demonstrate that essential components of these nodules move along the synaptonemal complex (SC) and do not freely exchange between SCs in the same nucleus. Our findings reveal that recombination nodules behave as active droplets and support a model in which coarsening of these droplets via protein translocation along liquid crystalline SCs underlies crossover patterning.