Metaphase to anaphase (mat) transition-defective Mutants in Caenorhabditiselegans

The metaphase to anaphase transition is a critical stage of the eukaryotic cell cycle, and, thus, it is highly regulated. Errors during this transitio n can lead to chromosome segregation defects and death of the organism. In genetic screens for temperature-sensitive maternal effect embryonic lethal (Me1) mutants, we have identified 32 mutants in the nematode Caenorhabditis elegans in which fertilized embryos arrest as one-cell embryos. In these m utant embryos, the oocyte chromosomes arrest in metaphase of meiosis I with out transitioning to anaphase or producing polar bodies. An additional bloc k in M phase exit is evidenced by the failure to form pronuclei and the per sistence of phosphohistone H3 and MPM-2 antibody staining. Spermatocyte mei osis is also perturbed; primary spermatocytes arrest in metaphase of meiosi s I and fail to produce secondary spermatocytes. Analogous mitotic defects cause M phase delays in mitotic germline proliferation. We have named this class of mutants "mat" for metaphase to anaphase transition defective. Thes e mutants, representing six different complementation groups, all map near genes that encode subunits of the anaphase promoting complex or cyclosome, and, here, we show that one of the genes, emb-27, encodes the C. elegans CD C16 ortholog.