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Characterizing the contribution of mutators to mutation accumulation is essential for understanding cellular adaptation and diseases like cancer. By measuring single and double mutation rates, including point mutations, segmental duplications, and reciprocal translocations, we found that wild-type yeast colonies exhibit double mutation rates up to 17 times higher than expected from experimentally determined single mutation rates. These double mutants retained wild-type mutation rates, indicating they originated from genetically normal cells that transiently expressed a mutator phenotype. Numerical simulations suggest that transient mutator subpopulations likely consist of less than a few thousand cells, and experience high-intensity mutational bursts for less than five generations. Most double mutations accumulated sequentially across cell cycles, with simultaneous acquisition being rare and likely linked to systemic genomic instability. Additionally, we explored the genetic control of transient hypermutation and found that the excess of double mutants can be modulated by replication stress and the DNA damage tolerance pathway. Our findings suggest that transient mutators play a significant role in genomic instability and contribute to the mutational load accumulating in growing isogenic populations. Synopsis This study shows that small cell subpopulations experience transient, high-intensity mutational bursts, greatly increasing double mutation rates in wild-type yeast. These mutator episodes are genetically regulated by the DNA damage tolerance pathway and can lead to systemic genomic instability. WT yeast colonies produce double mutants at a rate 17 times higher than expected. Double mutants originate from cell subpopulations that transiently express a mutator phenotype before reverting to a wild-type mutation rate. Mutator subpopulations likely consist of tens to a few thousand cells experiencing intense mutational bursts for five or less generations. Transient hypermutation can lead to systemic genome instability, where multiple mutations accumulate rapidly during one or a few cell cycles. Transient hypermutation is influenced by replication stress and modulated by the DNA damage tolerance pathway. This study shows that small cell subpopulations experience transient, high-intensity mutational bursts, greatly increasing double mutation rates in wild-type yeast. These mutator episodes are genetically regulated by the DNA damage tolerance pathway and can lead to systemic genomic instability.

Recombination is often suppressed at sex-determining loci in plants and animals, and at self-incompatibility or mating-type loci in plants and fungi. In fungal ascomycetes, recombination suppression around the mating-type locus is associated with pseudo-homothallism, i . e . the production of self-fertile dikaryotic sexual spores carrying the two opposite mating types. This has been well studied in two species complexes from different families of Sordariales : Podospora anserina and Neurospora tetrasperma . However, it is unclear whether this intriguing association holds in other species. We show here that Schizothecium tetrasporum , a fungus from a third family in the order Sordariales , also produces mostly self-fertile dikaryotic spores carrying the two opposite mating types. This was due to a high frequency of second meiotic division segregation at the mating-type locus, indicating the occurrence of a single and systematic crossing-over event between the mating-type locus and the centromere, as in P . anserina . The mating-type locus has the typical Sordariales organization, plus a MAT1-1-1 pseudogene in the MAT1-2 haplotype. High-quality genome assemblies of opposite mating types and segregation analyses revealed a suppression of recombination in a region of 1.47 Mb around the mating-type locus. We detected three evolutionary strata, indicating a stepwise extension of recombination suppression. The three strata displayed no rearrangement or transposable element accumulation but gene losses and gene disruptions were present, and precisely at the strata margins. Our findings indicate a convergent evolution of self-fertile dikaryotic sexual spores across multiple ascomycete fungi. The particular pattern of meiotic segregation at the mating-type locus was associated with recombination suppression around this locus, that had extended stepwise. This association between pseudo-homothallism and recombination suppression across lineages and the presence of gene disruption at the strata limits are consistent with a recently proposed mechanism of sheltering deleterious alleles to explain stepwise recombination suppression.

Characterizing the pace of mutation accumulation is crucial for understanding how populations adapt to their environment and for unraveling the intricate dynamics between gradual processes and more sudden burst-like events occurring during cancer development. We engineered the genome of Saccharomyces cerevisiae to measure the rates of single and double mutations, including point mutations, segmental duplications and reciprocal translocations. We found that during the development of wild-type yeast colonies, double mutations occur at rates that are up to 17-fold higher than those expected on the basis of single mutation rates. We found that this excess of double mutations is partially dependent on the ELG1/ATAD5 clamp unloader. Additionally, the double mutants retain wild-type mutation rates, suggesting that they originated from genetically wild-type cells that transiently expressed a mutator phenotype. Numerical simulations based on the experimentally measured mutation rates, confirmed that the excess of double mutations can be accounted for by subpopulations of transient mutators within the colony. These subpopulations would be limited to less than a few thousand cells and temporarily adopt mutation rates multiplied by hundreds or thousands for less than five generations. We found that the majority of double mutations would accumulate sequentially in different cell cycles. The simultaneous acquisition of both mutations during the same cell cycle would be rare and possibly associated with systemic genomic instability. In conclusion, our results suggest that transient hypermutators play a major role in genomic instability and contribute significantly to the mutational load naturally accumulating during the growth of isogenic cell populations.Competing Interest StatementThe authors have declared no competing interest.Footnotes* mostly text changes

Recombination is often suppressed at sex-determining loci in plants and animals, and at self-incompatibility or mating-type loci in plants and fungi. In fungal ascomycetes, recombination suppression around the mating-type locus is associated with pseudo-homothallism, i.e., the production of self-fertile dikaryotic sexual spores carrying the two opposite mating types. This has been well studied in two species complexes from different families of Sordariales: Podospora anserina and Neurospora tetrasperma. However, it is unclear whether this intriguing convergent association holds in other species. We show here that Schizothecium tetrasporum, a fungus from a third family in the order Sordariales, also produces mostly self-fertile dikaryotic spores carrying the two opposite mating types. This was due to a high frequency of second meiotic division segregation at the mating-type locus, indicating the occurrence of a single and systematic crossing-over event between the mating-type locus and the centromere, as in P. anserina. The mating-type locus has the typical Sordariales organization, plus a MAT1-1-1 pseudogene in the MAT1-2 haplotype. High-quality genome assemblies of opposite mating types and segregation analyses revealed a suppression of recombination in a region of 1.3 Mb around the mating-type locus. We detected three evolutionary strata, displaying a stepwise extension of recombination suppression, but no rearrangement or transposable element accumulation in the non-recombining region. Our findings indicate a convergent evolution of self-fertile dikaryotic sexual spores across multiple ascomycete fungi. The particular pattern of meiotic segregation at the mating-type locus was associated with recombination suppression around this locus, that had extended stepwise. This association is consistent with a recently proposed mechanism of deleterious allele sheltering through recombination suppression around a permanently heterozygous locus. Competing Interest Statement The authors have declared no competing interest. Footnotes * Author affiliation information