The synaptonemal complex assembles between meiotic chromosomes by wetting.

Exchange of genetic information between the parental chromosomes during sexual reproduction is controlled by a conserved structure called the synaptonemal complex. It is composed of axes (stiff chromosomal backbones), and a central region that assembles between two parallel axes. To form exchanges, the parental chromosomes must be drawn together and aligned by the synaptonemal complex. However, its mechanism of assembly remains unknown. Here we identify an axis-central region interface in C. elegans composed of the axis component HIM-3 and the central region component SYP-5. Weaker interface prevented complete synaptonemal complex assembly, and crucially, altered its canonical layered ultrastructure. Informed by these phenotypes, we built a thermodynamic model for synaptonemal complex assembly. The model recapitulates our experimental observations, indicating that the liquid-like central region can move chromosomes by wetting the axes without active energy consumption. More broadly, our data show that condensation can bring about tightly regulated nuclear reorganization.