The Anaphase Promoting Complex or Cyclosome (APC/C) is a multisubunit E3 ubiquitin ligase, which causes the destruction of cell-cycle regulators leading to the segregation of chromosomes at anaphase and exit from mitosis. Previously, we and others identified nine genes which encode subunits of the C. elegans APC/C, based on sequence homology, RNA-mediated interference (RNAi), and/or temperature-sensitive (ts) mutational analysis (1-3). The analysis of these genes was the first demonstration of a requirement for the APC/C for progression through meiosis I in eukaryotes. This study focuses on the function of APC/C during meiosis I. Peptide-specific antisera raised against C. elegans CDC-23 (APC-8) suggest that during meiotic prophase, the APC/C is diffusely localized to the nucleus, with some concentration on chromosomes in diakinesis. At metaphase I, APC/C localization shifts to the meiotic spindle. A similar pattern is observed in mitotically dividing embryos, in which APC/C is nuclear-associated during interphase but centrosome-associated at metaphase and anaphase. These localization patterns are consistent with a role for APC/C in chromosome segregation during mitosis and meiosis. Some temperature-sensitive alleles of
cdc-23 arrest only at the metaphase-to-anaphase transition of meiosis I in oocytes, whereas others have mitotic and/or spermatogenesis defects as well. This suggests that the APC/C might have oocyte meiotic-specific functions in addition to its well-characterized mitotic functions. To address this issue, we conducted a screen for suppressors of
mat-3 (
or180), an oocyte meiosis-specific ts allele of
cdc-23. At the non-permissive temperature of 250 ,
mat-3 (
or180) hermaphrodites arrest as one-cell meiotic embryos. Among 250,000 mutagenized haploid genomes, we identified 11 alleles that partially suppress the oocyte meiotic-specific defect of
mat-3 (
or180). One allele,
som-1 (
av4), is a dominant extragenic suppressor of
mat-3 (
or180) on LG I. At 250 ,
mat-3 (
or180);
som-1 (
av4) animals gives rise to ca. 10% viable embryos within a given brood, whereas nearly 100% of the embryos are multicellular. Many of these multicellular embryos are clearly dividing abnormally. In addition,
mat-3 (
or180);
som-1 (
av4) animals display a high-incidence-of-males (him) phenotype at 250 . Taken together, these phenotypes suggest that
som-1 (
av4) allows aberrant chromosome segregation to occur in order for
mat-3 (
or180) animals to reproduce. We postulate that the gene represented by
som-1 (
av4) encodes a novel, meoitic-specific substrate or regulator of the APC/C.