Homologous Recombination and Repair Functions Required for Mutagenicity during Yeast Meiosis.

Several meiotic events reshape the genome prior to its transfer (via gametes) to the next generation. The occurrence of new meiotic mutations is tightly linked to homologous recombination (HR) and firmly depends on Spo11-induced DNA breaks. To gain insight into the molecular mechanisms governing mutagenicity during meiosis, we examined the timing of mutation and recombination events in cells deficient in various DNA HR-repair genes, which represent distinct functions along the meiotic recombination process. Despite sequence similarities and overlapping activities of the two DNA translocases, Rad54 and Tid1, we observed essential differences in their roles in meiotic mutation occurrence: in the absence of Rad54, meiotic mutagenicity was elevated 8-fold compared to the wild type (WT), while in the tid1Δ mutant, there were few meiotic mutations, nine percent compared to the WT. We propose that the presence of Rad54 channels recombinational repair to a less mutagenic pathway, whereas repair assisted by Tid1 is more mutagenic. A 3.5-fold increase in mutation level was observed in dmc1∆ cells, suggesting that single-stranded DNA (ssDNA) may be a potential source for mutagenicity during meiosis. Taken together, we suggest that the introduction of de novo mutations also contributes to the diversification role of meiotic recombination. These rare meiotic mutations revise genomic sequences and may contribute to long-term evolutionary changes.

Mutagenicity in haploid yeast meiosis resulting from repair of DSBs by the sister chromatid.

Mutations in diploid budding yeast occur in meiosis at higher frequencies than in cells grown vegetatively. Such meiotic mutations are thought to result from the repair of double-strand breaks (DSBs) in meiosis, during the process of recombination. Here, we report studies of mutagenicity in haploid strains that may undergo meiosis due to the expression of both mating-type alleles, MATa and MATα. We measure the rate of mutagenicity in the reporter gene CAN1, and find it to be fivefold higher than in mitotic cells, as determined by fluctuation analysis. This enhanced meiotic mutagenicity is shown to depend on the presence of SPO11, the gene responsible for meiotic DSBs. Mutations in haploid meiosis must result from repair of the DSBs through interaction with the sister chromatid, rather than with non-sister chromatids as in diploids. Thus, mutations in diploid meiosis that are not ostensibly associated with recombination events can be explained by sister-chromatid repair. The spectrum of meiotic mutations revealed by Sanger sequencing is similar in haploid and in diploid meiosis. Compared to mitotic mutations in CAN1, long Indels are more frequent among meiotic mutations. Both, meiotic and mitotic mutations are more common at G/C sites than at A/T, in spite of an opposite bias in the target reporter gene. We conclude that sister-chromatid repair of DSBs is a major source of mutagenicity in meiosis.

Timing of appearance of new mutations during yeast meiosis and their association with recombination.

Mutations in budding yeast occur in meiosis at higher frequencies than in cells grown vegetatively. In contrast to mutations that occur in somatic cells, meiotic mutations have a special, long-range impact on evolution, because they are transferred to the following generations through the gametes. Understanding the mechanistic basis of meiotic mutagenicity is still lacking, however. Here, we report studies of mutagenicity in the reporter gene CAN1, in which forward mutation events in meiosis are sevenfold higher than in mitotic cells, as determined by fluctuation analysis. Meiotic mutations appear approximately at the same time as heteroallelic-recombination products and as meiotic DSBs. Recombination-associated timing of meiotic mutagenicity is further augmented by the absence of meiotic mutations in cells arrested after pre-meiotic DNA synthesis. More than 40% of the mutations generated in meiosis in CAN1 are found on chromosomes that have recombined in the 2.2 kb covering the reporter, implying that the mutations have resulted from recombination events and that meiotic recombination is mutagenic. The induced expression in yeast meiosis of low-fidelity DNA polymerases coded by the genes REV1, REV3, RAD30, and POL4 makes them attractive candidates for introducing mutations. However, in our extensive experiments with polymerase-deleted strains, these polymerases do not appear to be the major source of meiotic mutagenicity. From the connection between meiotic mutagenicity and recombination, one may conclude that meiotic recombination has another diversification role, of introducing new mutations at the DNA sequence level, in addition to reshuffling of existing variation. The new, rare meiotic mutations may contribute to long-range evolutionary processes and enhance adaptation to challenging environments.