Genetic recombination and commitment to meiosis in Saccharomyces.

Diploid cells of the yeast Saccharomyces cerevisiae become committed to recombination at meiotic levels without becoming committed to the meiotic disjunction of chromosomes. These two events of the meiotic process can be separated by removing cells from a meiosis-inducing medium and returning them to a medium that promotes vegetative cell division. Cells removed at an appropriate time remain diploid, revert to mitosis, and display recombination with meiotic-like frequencies. Those removed after this time are committed to the completion of meiosis. Diploids of three conditional sporulation-deficient mutants (spo1-1, spo2-1, and spo3-1) have been examined for recombination at restrictive temperatures. All exhibit commitment to recombination without commitment to meiotic disjunction as in the wild type. Cells of spo1-1/spo1-1 do not replicate the spindle pole body for meiosis I; thus, recombination ability can be acquired by cells that do not proceed beyond this cytological stage.