Figure 6: Divergent kleisin subunits of cohesin specify distinct mechanisms that tether and release meiotic chromosomes. Establishment of cohesion (top).  REC-8 and COH-3/4 cohesins load onto chromosomes at different times, using shared and separate loading factors, and establish sister chromatid cohesion by different mechanisms. Cohesion mediated by REC-8 is established after pre-meiotic DNA replication and before the onset of meiosis.  In contrast, bound COH-3/4 cohesins require the DNA double strand breaks made by the meiosis-specific SPO-11 endonuclease to trigger cohesion.  The sole other example of replication-independent cohesion establishment occurs in mitotically proliferating yeast that suffer DNA damage. 

Release of cohesion (bottom cartoon).  In C. elegans, a single asymmetrically positioned crossover forms between each pair of homologous chromosomes, and the site of the crossover, rather than a centromere, determines where sister chromatid cohesion will be released during the meiotic divisions. In the cartoon, sisters of one homolog are red and orange, and sisters of the other are light and dark blue.  After recombination is complete, the chromosomes are restructured around the crossover to form a cruciform structure.  Just prior to the first meiotic division, SCC is released at the short arm to allow homologs to separate.  SCC persists at the long arm until just before the second meiotic division, when it is released to allow sisters to separate. 

REC-8 and COH-3/4 cohesins (represented schematically by orange and green ovals) appear distributed all along sister chromatids in recombined homologous chromosomes (see Figure 5) and in chromosomes restructured around the crossover. Separase-independent removal of REC-8 and COH-3/4 cohesins from reciprocal chromosomal territories then occurs in prometaphase, long before the first meiotic division.  Confocal micrographs of cruciform chromosomes stained with antibodies to the axis protein HTP-3 and kleisin subunits REC-8 or COH-3/4 show the selective removal of REC-8 cohesin from the short arms and its persistence along the long arms.  The reciprocal pattern occurs for COH-3/4 cohesins, removal from the long arm and persistence on the short arm.  The pattern of kleisin removal in prometaphase is consistent with the phenotypes caused by null mutations. Loss of rec-8 causes all sisters to separate prematurely in the first meiotic division.  In contrast, loss of coh-3/4 causes random segregation of homologous chromosomes.  Our work suggests that COH-3/4 must be destroyed by separase to allow homologs to separate and then REC-8 must be destroyed to allow sisters to separate.