| Our current understanding of early development in the nematode Caenorhabditis elegans comes largely from the analysis of temperature-sensitive, maternal effect lethal mutants. These genetic studies have revealed much about the mechanisms of cell differentiation, but little about the genes essential for meiotic maturation, egg activation and early zygotic development. In an attempt to understand the one-cell stage, a detailed phenotypic analysis of emb-27(g48), whose embryos were known to arrest at the one-cell stage, was undertaken in this study. The mutant emb-27 embryos proved to arrest after fertilization but prior to the onset of meiotic chromosome segregation. In fact, detailed cytogenetic analysis revealed that the emb-27 embryos arrest at metaphase I and fail to complete meiotic maturation.;Primary spermatocytes in mat mutants also arrest in metaphase of meiosis I. Nevertheless, they undergo a budding division much like wild type spermatocytes. Although the resulting spermatozoa lack DNA, these "anucleate" sperm can activate, crawl toward and fertilize maturing primary oocytes. The resulting haploid paternal effect lethal (Pel) embryos die prior to morphogenesis, but key one-cell stage events such as egg activation, meiotic maturation and the specification of zygotic polarity occur normally. Thus, the physical presence of DNA is required for neither sperm production nor early zygotic development. These studies also demonstrate that APC/C is not required for the sperm budding division in C. elegans .;The molecular analysis of g48 showed that emb-27 encodes an ortholog of Cdc16, a core component of the anaphase promoting complex (APC/C) (Sudakin et al., 1995, King et al., 1995). In yeast, the APC/C is required for the metaphase to anaphase transition and mitotic exit. In collaboration with three other laboratories, we identified and characterized over thirty-two new mutants with identical one-cell arrest phenotypes. These mutants represent a new mutant class called metaphase to anaphase transition defective or mat mutants, and they identify six different complementation groups. All map near genes that encode APC/C subunits. |
