Heterogeneous Mutation Rates and Spectra in Yeast Hybrids

Emergence of fungal hybrids - Potential threat to humans

Fungal hybrids arise through the interbreeding of distinct species. This hybridization process fosters increased genetic diversity and the emergence of new traits. Mechanisms driving hybridization include the loss of heterozygosity, copy number variations, and horizontal gene transfer. Genetic mating barriers, changes in ploidy, chromosomal instability, and genomic diversity influence hybridization. These factors directly impact the fitness and adaptation of hybrid offspring. Epigenetic factors, including DNA methylation, histone modifications, non-coding RNAs, and chromatin remodelling, play a role in post-mating isolation in hybrids. In addition to all these mechanisms, successful hybridization in fungi is ensured by cellular mechanisms like mitochondrial inheritance, transposable elements, and other genome conversion mechanisms. These mechanisms support hybrid life and enhance the virulence and pathogenicity of fungal hybrids, which provoke diseases in host organisms. Recent advancements in sequencing have uncovered fungal hybrids in pathogens like Aspergillus, Candida, and Cryptococcus. Examples of these hybrids, such as Aspergillus latus, Candida metapsilosis, and Cryptococcus neoformans, induce severe human infections. Identifying fungal hybrids is challenging due to their altered genome traits. ITS sequencing has emerged as a promising method for diagnosing these hybrids. To prevent the emergence of novel hybrid fungal pathogens, it is crucial to develop effective diagnostic techniques and closely monitor pathogenic fungal populations for signs of hybridization. This comprehensive review delves into various facts about fungal hybridization, including its causes, genetic outcomes, barriers, diagnostic strategies, and examples of emerging fungal hybrids. The review emphasises the potential threat that fungal hybrids pose to human health and highlights their clinical significance.

Hybrid adaptation is hampered by Haldane's sieve

Hybrids between species exhibit plastic genomic architectures that could foster or slow down their adaptation. When challenged to evolve in an environment containing a UV mimetic drug, yeast hybrids have reduced adaptation rates compared to parents. We find that hybrids and their parents converge onto similar molecular mechanisms of adaptation by mutations in pleiotropic transcription factors, but at a different pace. After 100 generations, mutations in these genes tend to be homozygous in the parents but heterozygous in the hybrids. We hypothesize that a lower rate of loss of heterozygosity (LOH) in hybrids could limit fitness gain. Using genome editing, we first demonstrate that mutations display incomplete dominance, requiring homozygosity to show full impact and to entirely circumvent Haldane's sieve, which favors the fixation of dominant mutations. Second, tracking mutations in earlier generations confirmed a different rate of LOH in hybrids. Together, these findings show that Haldane's sieve slows down adaptation in hybrids, revealing an intrinsic constraint of hybrid genomic architecture that can limit the role of hybridization in adaptive evolution.

An integrative taxonomy approach reveals Saccharomyces chiloensis sp. nov. as a newly discovered species from Coastal Patagonia

Species delineation in microorganisms is challenging due to the limited markers available for accurate species assignment. Here, we applied an integrative taxonomy approach, combining extensive sampling, whole-genome sequence-based classification, phenotypic profiling, and assessment of interspecific reproductive isolation. Our work reveals the presence of a distinct Saccharomyces lineage in Nothofagus forests of coastal Patagonia. This lineage, designated Saccharomyces chiloensis sp. nov., exhibits 7% genetic divergence from its sister species S. uvarum, as revealed by whole-genome sequencing and population analyses. The South America-C (SA-C) coastal Patagonia population forms a unique clade closely related to a previously described divergent S. uvarum population from Oceania (AUS, found in Australia and New Zealand). Our species reclassification is supported by a low Ortho Average Nucleotide Identity (OANI) of 93% in SA-C and AUS relative to S. uvarum, which falls below the suggested species delineation threshold of 95%, indicating an independent evolutionary lineage. Hybrid spore viability assessment provided compelling evidence that SA-C and AUS are reproductively isolated from S. uvarum. In addition, we found unique structural variants between S. chiloensis sp. nov. lineages, including large-scale chromosomal translocations and inversions, together with a distinct phenotypic profile, emphasizing their intraspecies genetic distinctiveness. We suggest that S. chiloensis sp. nov diverged from S. uvarum in allopatry due to glaciation, followed by post-glacial dispersal, resulting in distinct lineages on opposite sides of the Pacific Ocean. The discovery of S. chiloensis sp. nov. illustrates the uniqueness of Patagonia's coastal biodiversity and underscores the importance of adopting an integrative taxonomic approach in species delineation to unveil cryptic microbial species. The holotype of S. chiloensis sp. nov. is CBS 18620T.

Two independent DNA repair pathways cause mutagenesis in template switching deficient Saccharomyces cerevisiae

Upon DNA replication stress, cells utilize the postreplication repair pathway to repair single-stranded DNA and maintain genome integrity. Postreplication repair is divided into 2 branches: error-prone translesion synthesis, signaled by proliferating cell nuclear antigen (PCNA) monoubiquitination, and error-free template switching, signaled by PCNA polyubiquitination. In Saccharomyces cerevisiae, Rad5 is involved in both branches of repair during DNA replication stress. When the PCNA polyubiquitination function of Rad5 s disrupted, Rad5 recruits translesion synthesis polymerases to stalled replication forks, resulting in mutagenic repair. Details of how mutagenic repair is carried out, as well as the relationship between Rad5-mediated mutagenic repair and the canonical PCNA-mediated mutagenic repair, remain to be understood. We find that Rad5-mediated mutagenic repair requires the translesion synthesis polymerase ζ but does not require other yeast translesion polymerase activities. Furthermore, we show that Rad5-mediated mutagenic repair is independent of PCNA binding by Rev1 and so is separable from canonical mutagenic repair. In the absence of error-free template switching, both modes of mutagenic repair contribute additively to replication stress response in a replication timing-independent manner. Cellular contexts where error-free template switching is compromised are not simply laboratory phenomena, as we find that a natural variant in RAD5 is defective in PCNA polyubiquitination and therefore defective in error-free repair, resulting in Rad5- and PCNA-mediated mutagenic repair. Our results highlight the importance of Rad5 in regulating spontaneous mutagenesis and genetic diversity in S. cerevisiae through different modes of postreplication repair.

Loss of Heterozygosity and Its Importance in Evolution

Loss of heterozygosity (LOH) is a mitotic recombination event that converts heterozygous loci to homozygous loci. This mutation event is widespread in organisms that have asexual reproduction like budding yeasts, and is also an important and frequent mutation event in tumorigenesis. Mutation accumulation studies have demonstrated that LOH occurs at a rate higher than the point mutation rate, and can impact large portions of the genome. Laboratory evolution experiments of heterozygous yeasts have revealed that LOH often unmasks beneficial recessive alleles that can confer large fitness advantages. Here, I highlight advances in understanding dominance, fitness, and phenotypes in laboratory evolved heterozygous yeast strains. I discuss best practices for detecting LOH in intraspecific and interspecific evolved clones and populations. Utilizing heterozygous strain backgrounds in laboratory evolution experiments offers an opportunity to advance our understanding of this important mutation type in shaping adaptation and genome evolution in wild, domesticated, and clinical populations.

Late Pleistocene-dated divergence between South Hemisphere populations of the non-conventional yeast L. cidri

Most organisms belonging to the Saccharomycotina subphylum have high genetic diversity and a vast repertoire of metabolisms and lifestyles. Lachancea cidri is an ideal yeast model for exploring the interplay between genetics, ecological function and evolution. Lachancea cidri diverged from the Saccharomyces lineage before the whole-genome duplication and is distributed across the South Hemisphere, displaying an important ecological success. We applied phylogenomics to investigate the genetic variation of L. cidri isolates obtained from Australia and South America. Our approach revealed the presence of two main lineages according to their geographic distribution (Aus and SoAm). Estimation of the divergence time suggests that SoAm and Aus lineages diverged near the last glacial maximum event during the Pleistocene (64-8 KYA). Interestingly, we found that the French reference strain is closely related to the Australian strains, with a recent divergence (405-51 YA), likely associated to human movements. Additionally, we identified different lineages within the South American population, revealing that Patagonia contains a similar genetic diversity comparable to that of other lineages in S. cerevisiae. These findings support the idea of a Pleistocene-dated divergence between South Hemisphere lineages, where the Nothofagus and Araucaria ecological niches likely favoured the extensive distribution of L. cidri in Patagonia.

Recent insights into the evolution of mutation rates in yeast

Mutation is the origin of all genetic variation, good and bad. The mutation process can evolve in response to mutations, positive or negative selection, and genetic drift, but how these forces contribute to mutation-rate variation is an unsolved problem at the heart of genetics research. Mutations can be challenging to measure, but genome sequencing and other tools have allowed for the collection of larger and more detailed datasets, particularly in the yeast-model system. We review key hypotheses for the evolution of mutation rates and describe recent advances in understanding variation in mutational properties within and among yeast species. The multidimensional spectrum of mutations is increasingly recognized as holding valuable clues about how this important process evolves.

The evolutionary and ecological potential of yeast hybrids

Recent findings in yeast genetics and genomics have advanced our understanding of the evolutionary potential unlocked by hybridization, especially in the genus Saccharomyces. We now have a clearer picture of the prevalence of yeast hybrids in the environment, their ecological and evolutionary history, and the genetic mechanisms driving (and constraining) their adaptation. Here, we describe how the instability of hybrid genomes determines fitness across large evolutionary scales, highlight new hybrid strain engineering techniques, and review tools for comparative hybrid genome analysis. The recent push to take yeast research back 'into the wild' has resulted in new genomic and ecological resources. These provide an arena for quantitative genetics and allow us to investigate the architecture of complex traits and mechanisms of adaptation to rapidly changing environments. The vast genetic diversity of hybrid populations can yield insights beyond those possible with isogenic lines. Hybrids offer a limitless supply of genetic variation that can be tapped for industrial strain improvement but also, combined with experimental evolution, can be used to predict population responses to future climate change - a fundamental task for biologists.

Hybridization drives mitochondrial DNA degeneration and metabolic shift in a species with biparental mitochondrial inheritance

Mitochondrial DNA (mtDNA) is a cytoplasmic genome that is essential for respiratory metabolism. Although uniparental mtDNA inheritance is most common in animals and plants, distinct mtDNA haplotypes can coexist in a state of heteroplasmy, either because of paternal leakage or de novo mutations. mtDNA integrity and the resolution of heteroplasmy have important implications, notably for mitochondrial genetic disorders, speciation, and genome evolution in hybrids. However, the impact of genetic variation on the transition to homoplasmy from initially heteroplasmic backgrounds remains largely unknown. Here, we use Saccharomyces yeasts, fungi with constitutive biparental mtDNA inheritance, to investigate the resolution of mtDNA heteroplasmy in a variety of hybrid genotypes. We previously designed 11 crosses along a gradient of parental evolutionary divergence using undomesticated isolates of Saccharomyces paradoxus and Saccharomyces cerevisiae Each cross was independently replicated 48 to 96 times, and the resulting 864 hybrids were evolved under relaxed selection for mitochondrial function. Genome sequencing of 446 MA lines revealed extensive mtDNA recombination, but the recombination rate was not predicted by parental divergence level. We found a strong positive relationship between parental divergence and the rate of large-scale mtDNA deletions, which led to the loss of respiratory metabolism. We also uncovered associations between mtDNA recombination, mtDNA deletion, and genome instability that were genotype specific. Our results show that hybridization in yeast induces mtDNA degeneration through large-scale deletion and loss of function, with deep consequences for mtDNA evolution, metabolism, and the emergence of reproductive isolation.