The cohesin REC8 prevents illegitimate inter-sister synaptonemal complex assembly

Abstract

During meiosis, a specialized chromosome structure is assembled to promote pairing/synapsis of homologous chromosomes and meiotic recombination, a process yielding chiasmata between homologs to ensure accurate segregation. Meiosis‐specific cohesin complexes mediating sister chromatid cohesion play pivotal roles in almost all these events, including synaptonemal complex (SC) formation. In this issue of EMBO Reports, Agostinho and colleagues have examined chromosome axes and SC structures by taking advantage of a hypomorphic Stag3 mutant in which the levels of the cohesin subunit REC8 are partly reduced 6. Using super‐resolution microscopy, the authors illuminate previously unforeseen chromosome axis structures, showing locally separated axes in regions where REC8 is absent, regardless of RAD21L or RAD21 cohesin localization. Furthermore, they assessed the relationship between sister chromatid cohesion and inter‐sister SC formation, demonstrating that “axial opening” in the REC8‐free region is accompanied by illegitimate SC formation between sister chromatids. This study highlights the physiological importance of REC8 in sister chromatid cohesion and proper SC formation during meiosis, suggesting a new model in which a high density of REC8 deposition along the chromosome prevents illegitimate inter‐sister SC formation.

Figure 1. Synaptonemal complex (SC) assembly in the cohesin mutant mice

(A) The meiosis‐specific cohesin complex contains four core subunits, SMC1β, SMC3, STAG3, and one of the kleisin subunits REC8, RAD21L, or RAD21. (B) In wild‐type cells, the chromosome axis in the unsynapsed region is recognized as a single line of SYCP3‐labeled structures along which a high density of REC8 foci can be detected. In Rec8 KO spermatocytes, the axis of the univalent chromosome is resolved as two separate SYCP3‐labeled structures. In Stag3 mutant and Smc1β KO cells, the chromosome axis is regionally separated in those areas where REC8 is absent. The “axial opening” is flanked by REC8 and is accompanied by inter‐sister SC formation.