Abstract

When cells enter meiosis, their chromosomes reorganize as linear arrays of chromatin loops anchored to a central axis. Meiotic chromosome axes form a platform for the assembly of the synaptonemal complex (SC), and play central roles in other meiotic processes, including homologous pairing, recombination, and chromosome segregation. However, little is known about the three-dimensional organization of components within the axes, which consist of cohesin complexes and additional meiosis-specific proteins. Here we investigate the molecular organization of meiotic chromosome axes in C. elegans through STORM and PALM superresolution imaging of intact germline tissue. By tagging one axis protein (HIM-3) with a photoconvertible fluorescent protein, we established a spatial reference for other components, which were localized using antibodies against epitope tags inserted by CRISPR/Cas9 genome editing. Using three-dimensional averaging, we determined the 3D-organization of all known components within synapsed chromosome axes to a precision of 2-5 nanometers. We find that meiosis-specific HORMA-domain proteins span a gap between cohesin complexes and the central region of the SC, consistent with their essential roles in SC assembly. Our data further suggest that the two different meiotic cohesin complexes are distinctly arranged within the axes: Cohesin complexes containing COH-3 or -4 kleisins form a central core in the central plane of the axes, whereas complexes containing REC-8 kleisin protrude above and below the plane defined by the SC. This splayed organization may help to explain the role of the chromosome axes in promoting inter-homolog repair of meiotic double strand breaks by inhibiting inter-sister repair.