Accurate chromosome segregation during cell division requires not only the establishment, but also the precise, regulated release of chromosome cohesion. Chromosome dynamics during meiosis are more complicated, because homologues separate at anaphase I whereas sister chromatids remain attached until anaphase II. How the selective release of chromosome cohesion is regulated during meiosis remains unclear. We show that the aurora-B kinase AIR-2 regulates the selective release of chromosome cohesion during Caenorhabditis elegans meiosis. AIR-2 localizes to subchromosomal regions corresponding to last points of contact between homologues in metaphase I and between sister chromatids in metaphase II. Depletion of AIR-2 by RNA interference (RNAi) prevents chromosome separation at both anaphases, with concomitant prevention of meiotic cohesin REC-8 release from meiotic chromosomes. We show that AIR-2 phosphorylates REC-8 at a major amino acid in vitro. Interestingly, depletion of two PP1 phosphatases, CeGLC-7α and CeGLC-7β, abolishes the restricted localization pattern of AIR-2. In Ceglc-7α/β(RNAi) embryos, AIR-2 is detected on the entire bivalent. Concurrently, chromosomal REC-8 is dramatically reduced and sister chromatids are separated precociously at anaphase I in Ceglc-7α/β(RNAi) embryos. We propose that AIR-2 promotes the release of chromosome cohesion via phosphorylation of REC-8 at specific chromosomal locations and that CeGLC-7α/β, directly or indirectly, antagonize AIR-2 activity.

Oocytes are released from meiotic prophase I arrest through a process termed oocyte maturation. We present here a genetic characterization of oocyte maturation, using C. elegans as a model system. We show that two TIS11 zinc finger-containing proteins, OMA-1 and OMA-2, express specifically in maturing oocytes and function redundantly in oocyte maturation. Oocytes in oma-1;oma-2 mutants initiate but do not complete maturation and arrest at a defined point in prophase I. Two maturation signal-induced molecular events, including the maintenance of activated MAP kinase, do not occur in Oma oocytes. The Oma prophase arrest is released by inactivation of a MYT-1-like kinase, suggesting that OMA-1 and OMA-2 function upstream of MYT-1 as positive regulators of prophase progression during meiotic maturation.

During Caenorhabditis elegans embryogenesis, anteroposterior (AP) polarity is established by a hierarchy of PAR proteins (for a review, see Kemphues, 2000). Many of these PAR proteins are asymmetrically localized at the cortex along the AP axis. The asymmetric distribution of PAR proteins determines the position of the first mitotic spindle and the asymmetric localization of key cytoplasmic determinants (for reviews, see Cowan and Hyman, 2004;Lyczak et al., 2002). This results in two daughters that are different in both size and developmental fate: the larger anterior somatic cell, AB, and the smaller posterior germline blastomere, P1. P1 then undergoes a series of three more asymmetric divisions, each giving rise to a germline blastomere (sequentially P2 to P4, termed the P lineage) and a corresponding somatic cell (Fig. 1).PAR proteins regulate spindle position via a G-protein signaling pathway (Colombo et al., 2003;Hess et al., 2004), and cytoplasmic polarity via two maternally supplied proteins, MEX-5 and MEX-6 (Schubert et al., 2000). MEX-5 and MEX-6 are closely related proteins and are both preferentially localized toward the anterior cytoplasm of the one-cell embryo and are enriched in the somatic daughter after the division of each germline blastomere (Cuenca et al., 2003;Schubert et al., 2000). Whereas mex-5 mutants exhibit 100% embryonic lethality, mex-6 mutant embryos are 100% viable with no observable defects (Schubert et al., 2000). However, many molecular defects in mex-5 mutant embryos are dramatically enhanced when mex-6 is also mutated or depleted, suggesting partially redundant functions for these two genes (Schubert et al., 2000). For simplicity, unless specifically noted, we will use MEX-5/6 to refer to MEX-5 and MEX-6.One major function of MEX-5/6 is to restrict the localization of maternally supplied germline proteins, such as PIE-1, POS-1 and MEX-1, to germline blastomeres (Guedes and Priess, 1997;Mello et al., 1996;Schubert et al., 2000;Tabara et al., 1999). In the onecell embryo, as MEX-5/6 become asymmetrically localized anteriorly, PIE-1 becomes localized posteriorly (Cuenca et al., 2003;Mello et al., 1996;Schubert et al., 2000). After cell division, PIE-1 is enriched in P1, and this pattern reiterates in each subsequent Plineage division. The small amount of PIE-1 segregated to the somatic sister after each division is degraded by a ZIF-1-containing CUL-2 E3 ligase complex (DeRenzo et al., 2003;Reese et al., 2000). Both asymmetric distribution of PIE-1 before division, as well as asymmetric degradation after division, require the function of MEX-5/6 (DeRenzo et al., 2003;Schubert et al., 2000). MEX-5/6 are themselves also substrates for this ZIF-1-containing E3 ligase complex (DeRenzo et al., 2003).Before meiosis II, high levels of both PIE-1 and MEX-5/6 proteins are detected uniformly throughout the cytoplasm of oocytes and one-cell embryos ( Fig. 1) (Cuenca et al., 2003;Schubert et al., 2000). This suggests that localization of PIE-1 by MEX-5/6 is a developmentally regulated event ...

SYP-3 is a new structural component of the synaptonemal complex (SC) required for the regulation of chromosome synapsis. Both chromosome morphogenesis and nuclear organization are altered throughout the germlines of syp-3 mutants. Here, our analysis of syp-3 mutants provides insights into the relationship between chromosome conformation and the repair of meiotic double-strand breaks (DSBs). Although crossover recombination is severely reduced in syp-3 mutants, the production of viable offspring accompanied by the disappearance of RAD-51 foci suggests that DSBs are being repaired in these synapsis-defective mutants. Our studies indicate that once interhomolog recombination is impaired, both intersister recombination and nonhomologous end-joining pathways may contribute to repair during germline meiosis. Moreover, our studies suggest that the conformation of chromosomes may influence the mode of DSB repair employed during meiosis.