(A) Projections of confocal Z-sections through entire pachytene nuclei. A single LacI::GFP focus is detected in pachytene nuclei of wild-type, coh-4 coh-3, and spo-11 mutant worms, indicating that sister chromatids are tethered by SCC. In contrast, sisters are separated in most pachytene nuclei of kleisin triple mutants but still remain close together, suggesting that residual SCC persists. Partial depletion of SCC-1 in kleisin triple mutant animals increases both the frequency of sister separation and the distance between sisters, demonstrating a meiotic role for SCC-1. Surprisingly, sisters could be resolved in only ∼10% of nuclei in rec-8 and spo-11 rec-8 worms (white circles). The robust synaptonemal complex (SC) assembly in spo-11 rec-8 worms suggested that SC proteins may tether sister chromatids independently of cohesin. Indeed, disrupting the axial element (AE) protein HIM-3 severely compromised SCC in both rec-8 and spo-11 rec-8 mutants. Disrupting the central region (CR) protein SYP-1 had a lesser effect, suggesting that AE proteins can tether sisters together independently of CR proteins and cohesin. (B) Quantification of sister separation in pachytene nuclei. no. = number of nuclei scored. (C) Z-projected confocal images of wild-type gonads stained with DAPI and antibodies to SCC-1. Similar to REC-8, SCC-1 was detected in premeiotic nuclei and became enriched in axial structures of transition zone and pachytene nuclei. Nucleoplasmic staining obscured any chromosomal signal from pachytene exit until prometaphase; however, SCC-1 was undetectable following nuclear envelope breakdown in prometaphase, indicating that SCC-1 cohesin was removed from chromosomes during diplotene or diakinesis. (D) Similar sets of kleisins function during meiosis in C. elegans, mammals and plants. (E) A schematic of SC structure. Studies in worms have identified four components of the axial/lateral element, or LE (HTP-3, HIM-3, and the functionally redundant proteins HTP-1 and HTP-2) and four components of the CR (SYP-1, SYP-2, SYP-3, and SYP-4). (F and G) Two models of SC-dependent linkages between sisters. (F) CR proteins link AEs formed along each sister. (G) AE proteins hold sisters together independently of CRs. (H) REC-8 cohesin and COH-3/4 cohesin load onto chromosomes at different times and establish SCC by different mechanisms.