Timing of ribosome synthesis during ascosporogenesis of yeast cells: evidence for early function of haploid daughter genomes.

During meiosis and sporulation in Saccharomyces cerevisiae, the recessive genetic marker for cycloheximide resistance, believed to be due to an altered ribosomal protein (C. S. McLaughlin, p. 815-827, in M. Nomura et al., ed., Ribosomes, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.), is expressed as early as meiosis II. Ribosomal ribonucleic acid synthesis peaks near the time that cycloheximide resistance begins to appear. Less than 25% of the 17S and 25S ribonucleic acid of the vegetative cells persists in spores, but pulse-labeling studies indicate that greater than 90% of the stable ribonucleic acid made after 6 h survives in spores. These results indicate that the haploid daughter genomes begin to function near the time of meiosis II.