STAGE-SPECIFIC EFFECTS OF X-IRRADIATION ON YEAST MEIOSIS

Previous work has shown that cdc13 causes meiotic arrest of Saccharomyces cerevisiae following DNA replication by a RAD9-dependent mechanism. In the present work, we have further investigated the implicit effects of chromosomal lesions on progression through meiosis by exposing yeast cells to X-irradiation at various times during sporulation. We find that exposure of RAD9 cells to X-irradiation early in meiosis prevents sporulation, arresting the cells at a stage prior to premeiotic DNA replication. rad9 meiotic cells are much less responsive to X-irradiation damage, completing sporulation after treatment with doses sufficient to cause arrest of RAD9 strains. These findings thereby reveal a RAD9-dependent checkpoint function in meiosis that is distinct from the G2 arrest previously shown to result from cdc13 dysfunction. Analysis of the spores that continued to be produced by either RAD9 or rad9 cultures that were X-irradiated in later stages of sporulation revealed most spores to be viable, even after exposure to radiation doses sufficient to kill most vegetative cells. This finding demonstrates that the lesions induced by X-irradiation at later times fail to trigger the checkpoint function revealed by cdc13 arrest and suggests that the lesions may be subject to repair by serving as intermediates in the recombination process. Strains mutant for chromosomal synapsis and recombination, and therefore defective in meiotic disjunction, were tested for evidence that X-ray-induced lesions might alleviate inviability by promoting recombination. Enhancement of spore viability when spo11 (but not hop1) diploids were X-irradiated during meiosis indicates that induced lesions may partially substitute for SPO11-dependent functions that are required for the initiation of recombination.