Accurate segregation of meiotic chromosomes requires the removal of linkages between homologous chromosomes and sister chromatids prior to anaphase. Moreover, proper attachment of meiotic chromosomes to their spindle microtubules requires that the chromosomes be correctly organized. We have shown that a C. elegans condensin complex is required for both the resolution of homologs and sister chromatids and the condensation of meiotic chromosomes into properly organized bivalents. The prototypical condensin complex is composed of two Structural Maintenance of Chromosomes (SMC) subunits, Smc2 and Smc4, and three non-SMC subunits, called CAP-D2, -G, and -H. The single C. elegans Smc2 homolog MIX-1 functions in two condensin-like complexes. MIX-1 associates with the Smc4 homolog DPY-27 to mediate dosage compensation, a process that equalizes the expression of X-linked genes between XX hermaphrodites and XO males. MIX-1 associates with a second SMC partner, Ce SMC-4, to mediate the condensation of mitotic chromosomes. Through biochemical analysis of MIX-1 protein complexes, we identified HCP-6, the C. elegans ortholog of the condensin subunit XCAP-D3, and demonstrated the exclusive association of HCP-6 with the mitotic condensin complex. Chromosome condensation is a conserved property of meiotic prophase I, but the role of the condensin complex in meiosis has not been well characterized in metazoans. We therefore examined the requirements for condensin in the C. elegans meiotic germline. Reducing HCP-6 function disrupted chromosome segregation in meiosis I and II. Condensin was also required for the compaction and reorganization of meiotic chromosomes that occur after exit from pachytene and prepare chromosomes for their segregation. Homologs in pachytene nuclei were resolved in
hcp-6 mutants as in wild-type, but chromosomes inappropriately decondensed in diplotene and individualized bivalents were rarely observed in diakinesis oocytes of animals with severely compromised HCP-6 function. In contrast, chromosomes in wild-type animals rapidly compacted to form six resolved bivalents in diplotene and diakinesis. Finally, condensin was necessary for the removal of linkages between diakinesis chromatids. In animals with wild-type HCP-6 function, homologs and sister chromatids separated prematurely following RNAi-mediated depletion of the meiosis-specific cohesin subunit REC-8. A partial loss-of-function
hcp-6 allele prevented this precocious separation. Taken together, our results suggest that de novo linkages between homologs and sister chromatids arise in diplotene/diakinesis nuclei of
hcp-6 mutants. We propose that one function of meiotic condensin is to maintain the resolution of chromosomes as the chromosomal axis is reorganized around the chiasma following breakdown of the synaptonemal complex in diplotene. Consistent with the genetic requirement for HCP-6 in late meiotic prophase, HCP-6 was first detectable on diplotene/diakinesis chromosomes, where it colocalized with MIX-1. Surprisingly, we found different loading dependencies for HCP-6 and MIX-1 on meiotic and mitotic chromosomes: while MIX-1 and HCP-6 loaded independently on mitotic chromosomes, MIX-1 required HCP-6 for its association with meiotic chromosomes. Additionally, the centromeric histone variant CENP-A was required for the association of HCP-6 with mitotic chromosomes but not with meiotic chromosomes. Our RNAi conditions effectively depleted CENP-A: meiotic chromosome segregation and the accumulation of the kinetochore proteins HIM-10 and MCAK on diakinesis chromosomes were disrupted. We conclude that condensin functions late in meiotic prophase, and that different rules govern the loading of HCP-6 and MIX-1 on chromosomes during meiotic and mitotic divisions.