Adaptive Gene Content and Allele Distribution Variations in the Wild and Domesticated Populations of Saccharomyces cerevisiae

Graph-Based Pan-Genome Reveals the Pattern of Deleterious Mutations during the Domestication of Saccharomyces cerevisiae

The "cost of domestication" hypothesis suggests that the domestication of wild species increases the number, frequency, and/or proportion of deleterious genetic variants, potentially reducing their fitness in the wild. While extensively studied in domesticated species, this phenomenon remains understudied in fungi. Here, we used Saccharomyces cerevisiae, the world's oldest domesticated fungus, as a model to investigate the genomic characteristics of deleterious variants arising from fungal domestication. Employing a graph-based pan-genome approach, we identified 1,297,761 single nucleotide polymorphisms (SNPs), 278,147 insertion/deletion events (indels; <30 bp), and 19,967 non-redundant structural variants (SVs; ≥30 bp) across 687 S. cerevisiae isolates. Comparing these variants with synonymous SNPs (sSNPs) as neutral controls, we found that the majority of the derived nonsynonymous SNPs (nSNPs), indels, and SVs were deleterious. Heterozygosity was positively correlated with the impact of deleterious SNPs, suggesting a role of genetic diversity in mitigating their effects. The domesticated isolates exhibited a higher additive burden of deleterious SNPs (dSNPs) than the wild isolates, but a lower burden of indels and SVs. Moreover, the domesticated S. cerevisiae showed reduced rates of adaptive evolution relative to the wild S. cerevisiae. In summary, deleterious variants tend to be heterozygous, which may mitigate their harmful effects, but they also constrain breeding potential. Addressing deleterious alleles and minimizing the genetic load are crucial considerations for future S. cerevisiae breeding efforts.

Annotation of 2,507 Saccharomyces cerevisiae genomes

Saccharomyces cerevisiae (baker's yeast, budding yeast) is one of the most important model organisms for biological research and is a crucial microorganism in industry. Currently, a huge number of Saccharomyces cerevisiae genome sequences are available at the public domain. However, these genomes are distributed at different websites and a large number of them are released without annotation information. To provide one complete annotated genome data resource, we collected 2,507 Saccharomyces cerevisiae genome assemblies and re-annotated 2,506 assemblies using a custom annotation pipeline, producing a total of 15,407,164 protein-coding gene models. With a custom pipeline, all these gene sequences were clustered into families. A total of 1,506 single-copy genes were selected as marker genes, which were then used to evaluate the genome completeness and base qualities of all assemblies. Pangenomic analyses were performed based on a selected subset of 847 medium-high-quality genomes. Statistical comparisons revealed a number of gene families showing copy number variations among different organism sources. To the authors' knowledge, this study represents the largest genome annotation project of S. cerevisiae so far, providing rich genomic resources for the future studies of the model organism S. cerevisiae and its relatives.IMPORTANCESaccharomyces cerevisiae (baker's yeast, budding yeast) is one of the most important model organisms for biological research and is a crucial microorganism in industry. Though a huge number of Saccharomyces cerevisiae genome sequences are available at the public domain, these genomes are distributed at different websites and most are released without annotation, hindering the efficient reuse of these genome resources. Here, we collected 2,507 genomes for Saccharomyces cerevisiae, performed genome annotation, and evaluated the genome qualities. All the obtained data have been deposited at public repositories and are freely accessible to the community. This study represents the largest genome annotation project of S. cerevisiae so far, providing one complete annotated genome data set for S. cerevisiae, an important workhorse for fundamental biology, biotechnology, and industry.

Tracking alternative versions of the galactose gene network in the genus Saccharomyces and their expansion after domestication

When Saccharomyces cerevisiae grows on mixtures of glucose and galactose, galactose utilization is repressed by glucose, and induction of the GAL gene network only occurs when glucose is exhausted. Contrary to reference GAL alleles, alternative alleles support faster growth on galactose, thus enabling distinct galactose utilization strategies maintained by balancing selection. Here, we report on new wild populations of Saccharomyces cerevisiae harboring alternative GAL versions and, for the first time, of Saccharomyces paradoxus alternative alleles. We also show that the non-functional GAL version found earlier in Saccharomyces kudriavzevii is phylogenetically related to the alternative versions, which constitutes a case of trans-specific maintenance of highly divergent alleles. Strains harboring the different GAL network variants show different levels of alleviation of glucose repression and growth proficiency on galactose. We propose that domestication involved specialization toward thriving in milk from a generalist ancestor partially adapted to galactose consumption in the plant niche.

Horizontal transfer and recombination fuel Ty4 retrotransposon evolution in Saccharomyces

Horizontal transposon transfer (HTT) plays an important role in the evolution of eukaryotic genomes, however the detailed evolutionary history and impact of most HTT events remain to be elucidated. To better understand the process of HTT in closely-related microbial eukaryotes, we studied Ty4 retrotransposon subfamily content and sequence evolution across the genus Saccharomyces using short- and long-read whole genome sequence data, including new PacBio genome assemblies for two S. mikatae strains. We find evidence for multiple independent HTT events introducing the Tsu4 subfamily into specific lineages of S. paradoxus, S. cerevisiae, S. eubayanus, S. kudriavzevii and the ancestor of the S. mikatae/S. jurei species pair. In both S. mikatae and S. kudriavzevii, we identified novel Ty4 clades that were independently generated through recombination between resident and horizontally-transferred subfamilies. Our results reveal that recurrent HTT and lineage-specific extinction events lead to a complex pattern of Ty4 subfamily content across the genus Saccharomyces. Moreover, our results demonstrate how HTT can lead to coexistence of related retrotransposon subfamilies in the same genome that can fuel evolution of new retrotransposon clades via recombination.

Lateral gene transfer generates accessory genes that accumulate at different rates within a grass lineage

Lateral gene transfer (LGT) is the movement of DNA between organisms without sexual reproduction. The acquired genes represent genetic novelties that have independently evolved in the donor's genome. Phylogenetic methods have shown that LGT is widespread across the entire grass family, although we know little about the underlying dynamics. We identify laterally acquired genes in five de novo reference genomes from the same grass genus (four Alloteropsis semialata and one Alloteropsis angusta). Using additional resequencing data for a further 40 Alloteropsis individuals, we place the acquisition of each gene onto a phylogeny using stochastic character mapping, and then infer rates of gains and losses. We detect 168 laterally acquired genes in the five reference genomes (32-100 per genome). Exponential decay models indicate that the rate of LGT acquisitions (6-28 per Ma) and subsequent losses (11-24% per Ma) varied significantly among lineages. Laterally acquired genes were lost at a higher rate than vertically inherited loci (0.02-0.8% per Ma). This high turnover creates intraspecific gene content variation, with a preponderance of them occurring as accessory genes in the Alloteropsis pangenome. This rapid turnover generates standing variation that can ultimately fuel local adaptation.

Whole genome sequencing of Canadian Saccharomyces cerevisiae strains isolated from spontaneous wine fermentations reveals a new Pacific West Coast Wine clade

Vineyards in wine regions around the world are reservoirs of yeast with oenological potential. Saccharomyces cerevisiae ferments grape sugars to ethanol and generates flavor and aroma compounds in wine. Wineries place a high-value on identifying yeast native to their region to develop a region-specific wine program. Commercial wine strains are genetically very similar due to a population bottleneck and in-breeding compared to the diversity of S. cerevisiae from the wild and other industrial processes. We have isolated and microsatellite-typed hundreds of S. cerevisiae strains from spontaneous fermentations of grapes from the Okanagan Valley wine region in British Columbia, Canada. We chose 75 S. cerevisiae strains, based on our microsatellite clustering data, for whole genome sequencing using Illumina paired-end reads. Phylogenetic analysis shows that British Columbian S. cerevisiae strains cluster into 4 clades: Wine/European, Transpacific Oak, Beer 1/Mixed Origin, and a new clade that we have designated as Pacific West Coast Wine. The Pacific West Coast Wine clade has high nucleotide diversity and shares genomic characteristics with wild North American oak strains but also has gene flow from Wine/European and Ecuadorian clades. We analyzed gene copy number variations to find evidence of domestication and found that strains in the Wine/European and Pacific West Coast Wine clades have gene copy number variation reflective of adaptations to the wine-making environment. The "wine circle/Region B", a cluster of 5 genes acquired by horizontal gene transfer into the genome of commercial wine strains is also present in the majority of the British Columbian strains in the Wine/European clade but in a minority of the Pacific West Coast Wine clade strains. Previous studies have shown that S. cerevisiae strains isolated from Mediterranean Oak trees may be the living ancestors of European wine yeast strains. This study is the first to isolate S. cerevisiae strains with genetic similarity to nonvineyard North American Oak strains from spontaneous wine fermentations.

Macroevolutionary diversity of traits and genomes in the model yeast genus Saccharomyces

Species is the fundamental unit to quantify biodiversity. In recent years, the model yeast Saccharomyces cerevisiae has seen an increased number of studies related to its geographical distribution, population structure, and phenotypic diversity. However, seven additional species from the same genus have been less thoroughly studied, which has limited our understanding of the macroevolutionary events leading to the diversification of this genus over the last 20 million years. Here, we show the geographies, hosts, substrates, and phylogenetic relationships for approximately 1,800 Saccharomyces strains, covering the complete genus with unprecedented breadth and depth. We generated and analyzed complete genome sequences of 163 strains and phenotyped 128 phylogenetically diverse strains. This dataset provides insights about genetic and phenotypic diversity within and between species and populations, quantifies reticulation and incomplete lineage sorting, and demonstrates how gene flow and selection have affected traits, such as galactose metabolism. These findings elevate the genus Saccharomyces as a model to understand biodiversity and evolution in microbial eukaryotes.

Phylogenomics of a Saccharomyces cerevisiae cocoa strain reveals adaptation to a West African fermented food population

Various yeast strains have been proposed as candidate starter cultures for cocoa fermentation, especially strains of Saccharomyces cerevisiae. In the current study, the genome of the cocoa strain S. cerevisiae IMDO 050523 was unraveled based on a combination of long- and short-read sequencing. It consisted of 16 nuclear chromosomes and a mitochondrial chromosome, which were organized in 20 contigs, with only two small gaps. A phylogenomic analysis of this genome together with another 105 S cerevisiae genomes, among which 20 from cocoa strains showed a geographical distribution of the latter, including S. cerevisiae IMDO 050523. Its genome clustered together with that of a West African fermented food population, indicating a wider adaptation to West African food niches than cocoa. Furthermore, S. cerevisiae IMDO 050523 contained genetic signatures involved in sucrose hydrolysis, pectin degradation, osmotolerance, and conserved amino acid changes in key ester-producing enzymes that could point toward specific niche adaptations.

The teenage years of yeast population genomics trace history, admixing and getting wilder

Population genomics studies the evolutionary processes that shape intraspecies genetic variations. In this review, I explore the insights into yeast-population genomics that have emerged from recent advances in sequencing. Genomes of the model Saccharomyces cerevisiae and many new yeast species from around the world are being used to address various aspects of population biology, including geographical origin, the level of introgression, domestication signatures, and outcrossing frequency. New long-read sequencing has enabled a greater capacity to quantify these variations at a finer resolution from complete de novo genomes at the population scale to phasing subgenomes of different origins. These resources provide a platform to dissect the relationship between phenotypes across environmental niches.

Yeasts from temperate forests

Yeasts are ubiquitous in temperate forests. While this broad habitat is well-defined, the yeasts inhabiting it and their life cycles, niches, and contributions to ecosystem functioning are less understood. Yeasts are present on nearly all sampled substrates in temperate forests worldwide. They associate with soils, macroorganisms, and other habitats, and no doubt contribute to broader ecosystem-wide processes. Researchers have gathered information leading to hypotheses about yeasts' niches and their life cycles based on physiological observations in the laboratory as well as genomic analyses, but the challenge remains to test these hypotheses in the forests themselves. Here we summarize the habitat and global patterns of yeast diversity, give some information on a handful of well-studied temperate forest yeast genera, discuss the various strategies to isolate forest yeasts, and explain temperate forest yeasts' contributions to biotechnology. We close with a summary of the many future directions and outstanding questions facing researchers in temperate forest yeast ecology. Yeasts present an exciting opportunity to better understand the hidden world of microbial ecology in this threatened and global habitat.

The Ecology and Evolution of the Baker's Yeast Saccharomyces cerevisiae

The baker's yeast Saccharomyces cerevisiae has become a powerful model in ecology and evolutionary biology. A global effort on field survey and population genetics and genomics of S. cerevisiae in past decades has shown that the yeast distributes ubiquitously in nature with clearly structured populations. The global genetic diversity of S. cerevisiae is mainly contributed by strains from Far East Asia, and the ancient basal lineages of the species have been found only in China, supporting an 'out-of-China' origin hypothesis. The wild and domesticated populations are clearly separated in phylogeny and exhibit hallmark differences in sexuality, heterozygosity, gene copy number variation (CNV), horizontal gene transfer (HGT) and introgression events, and maltose utilization ability. The domesticated strains from different niches generally form distinct lineages and harbor lineage-specific CNVs, HGTs and introgressions, which contribute to their adaptations to specific fermentation environments. However, whether the domesticated lineages originated from a single, or multiple domestication events is still hotly debated and the mechanism causing the diversification of the wild lineages remains to be illuminated. Further worldwide investigations on both wild and domesticated S. cerevisiae, especially in Africa and West Asia, will be helpful for a better understanding of the natural and domestication histories and evolution of S. cerevisiae.

Highly diverged lineages of Saccharomyces paradoxus in temperate to subtropical climate zones in China

The wild yeast Saccharomyces paradoxus has become a new model in ecology and evolutionary biology. Different lineages of S. paradoxus have been recognized across the world, but the distribution and genetic diversity of the species remain unknown in China, where the origin of its sibling species S. cerevisiae lies. In this study, we investigated the ecological and geographic distribution of S. paradoxus through an extensive field survey in China and performed population genomic analysis on a set of S. paradoxus strains, including 27 strains, representing different geographic and ecological origins within China, and 59 strains representing all the known lineages of the species recognized in the other regions of the world so far. We found two distinct lineages of S. paradoxus in China. The majority of the Chinese strains studied belong to the Far East lineage, and six strains belong to a novel highly diverged lineage. The distribution of these two lineages overlaps ecologically and geographically in temperate to subtropical climate zones in China. With the addition of the new China lineage, the Eurasian population of S. paradoxus exhibits higher genetic diversity than the American population. We observed more possible lineage-specific introgression events from the Eurasian lineages than from the American lineages. Our results expand the knowledge on ecology, genetic diversity, biogeography, and evolution of S. paradoxus.