CROSSOVER AND NONCROSSOVER RECOMBINATION DURING MEIOSIS - TIMING AND PATHWAY RELATIONSHIPS

During meiosis, crossovers occur at a high level, but the level of non crossover recombinants is even higher. The biological rationale for th e existence of the latter events is not known, It has been suggested t hat a noncrossover-specific pathway exists specifically to mediate chr omosome pairing. Using a physical assay that monitors both crossovers and noncrossovers in cultures of yeast undergoing synchronous meiosis, we find that both types of products appear at essentially the same ti me, after chromosomes are fully synapsed at pachytene. We have also an alyzed a situation in which commitment to meiotic recombination and fo rmation of the synaptonemal complex are coordinately suppressed (merl versus merl MER2(++)). We find that suppression is due primarily to re storation of meiosis-specific double-strand breaks, a characteristic o f the major meiotic recombination pathway. Taken together, the observa tions presented suggest that there probably is no noncrossover;specifi c pathway and that restoration of intermediate events in a single pair ing/recombination pathway promotes synaptonemal complex formation. The biological significant of noncrossover recombination remains to be de termined, however.