Molecular genetic diversity of the Saccharomyces yeasts in Taiwan: Saccharomyces arboricola, Saccharomyces cerevisiae and Saccharomyces kudriavzevii

Genetic hybridization, sequence and karyotypic analyses of natural Saccharomyces yeasts isolated in different regions of Taiwan revealed three biological species: Saccharomyces arboricola, Saccharomyces cerevisiae and Saccharomyces kudriavzevii. Intraspecies variability of the D1/D2 and ITS1 rDNA sequences was detected among S. cerevisiae and S. kudriavzevii isolates. According to molecular and genetic analyses, the cosmopolitan species S. cerevisiae and S. kudriavzevii contain local divergent populations in Taiwan, Malaysia and Japan. Six of the seven known Saccharomyces species are documented in East Asia: S. arboricola, S. bayanus, S. cerevisiae, S. kudriavzevii, S. mikatae, and S. paradoxus.     

Taxonomy,ecology, and genetics of the yeast <Emphasis Type="Italic">Saccharomyces bayanus</Emphasis>: A new object for science and practice

The review considers various aspects of the biology of the yeast Saccharomyces bayanus, which is distantly related to the cultured yeast S. cerevisiae. The cryotolerant S. bayanus strains found in wine-making became the second most important yeast for basic and applied studies. Introduction of natural and experimental hybrids of S. cerevisiae × S. bayanus in a range of fermentation processes indicates the high breeding importance of S. bayanus. The biological species S. bayanus acts as a new gene pool for the scientific and breeding projects.     

Enhancing yeast cell viability after dehydration by modification of the lipid profile

In the present study, we analysed metabolite features during the dehydration-rehydration process for different yeast species genetically closely related to S. cerevisiae, in order to determine whether metabolites might play a role in cell viability. We ranked the species S. cerevisiae, S. paradoxus, S. kudriavzevii, L. kluyveri, N. castellii, S. mikatae, S. bayanus, and S. servazzii according to their viability rate after the dehydration-rehydration process, and showed that desiccation tolerance across the species did not correlate with the intracellular content of trehalose or glycogen. Cell lipid composition was also investigated during this process, to see whether the content of triacylglycerols and phosphatidylcholine showed significant variations across the species. The increase of phosphatidylcholine level increase in both S. paradoxus and S. bayanus cells grown in supplemented media enhanced both their cell viability after stress imposition and lipid storage.     

Molecular phylogeny of pectinase genes <Emphasis Type="Italic">PGU</Emphasis> in the yeast genus <Emphasis Type="Italic">Saccharomyces</Emphasis>

Using yeast genome databases and literature data, phylogenetic analysis of pectinase PGU genes from 112 Saccharomyces strains assigned to the biological species S. arboricola, S. bayanus (var. uvarum), S. cariocanus, S. cerevisiae, S. kudriavzevii, S. mikatae, S. paradoxus, and the hybrid taxon S. pastorianus (syn. S. carlsbergensis) was carried out. A superfamily of divergent PGU genes was found. Natural interspecies transfer of the PGU gene both from S. cerevisiae to S. bayanus and from S. paradoxus to S. cerevisiae may, however, occur. Within the Saccharomyces species, identity of the PGU nucleotide sequences was 98.8–100% for S. cerevisiae, 86.1–95.7% for S. bayanus (var. uvarum), 94–98.3% for S. kudriavzevii, and 96.8–100% for S. paradoxus/S. cariocanus. For the first time, a family of polymeric PGU1b, PGU2b, PGU3b and PGU4b genes is documented for the yeast S. bayanus var. uvarum, a variety important for winemaking.     

Metabolomic Comparison of Saccharomyces cerevisiae and the Cryotolerant Species S. bayanus var. uvarum and S. kudriavzevii during Wine Fermentation at Low Temperature

Temperature is one of the most important parameters affecting the length and rate of alcoholic fermentation and final wine quality. Wine produced at low temperature is often considered to have improved sensory qualities. However, there are certain drawbacks to low temperature fermentations such as reduced growth rate, long lag phase, and sluggish or stuck fermentations. To investigate the effects of temperature on commercial wine yeast, we compared its metabolome growing at 12°C and 28°C in a synthetic must. Some species of the Saccharomyces genus have shown better adaptation at low temperature than Saccharomyces cerevisiae. This is the case of the cryotolerant yeasts Saccharomyces bayanus var. uvarum and Saccharomyces kudriavzevii. In an attempt to detect inter-specific metabolic differences, we characterized the metabolome of these species growing at 12°C, which we compared with the metabolome of S. cerevisiae (not well adapted at low temperature) at the same temperature. Our results show that the main differences between the metabolic profiling of S. cerevisiae growing at 12°C and 28°C were observed in lipid metabolism and redox homeostasis. Moreover, the global metabolic comparison among the three species revealed that the main differences between the two cryotolerant species and S. cerevisiae were in carbohydrate metabolism, mainly fructose metabolism. However, these two species have developed different strategies for cold resistance. S. bayanus var. uvarum presented elevated shikimate pathway activity, while S. kudriavzevii displayed increased NAD⁺ synthesis.     

Mitochondrial DNA polymorphism of the yeast <Emphasis Type="Italic">Saccharomyces bayanus</Emphasis> var. <Emphasis Type="Italic">uvarum</Emphasis>

Genetic relationships among forty-one strains of Saccharomyces bayanus var. uvarum isolated in different wine regions of Europe and four wild isolates were investigated by restriction analysis (RPLP) of mitochondrial DNA (mtDNA) with four restriction endonucleases, AluI, DdeI, HinfI and RsaI. No clear correlation between origin and source of isolation of S. bayanus var. uvarum strains and their mtDNA restriction profiles was found. On the whole, the mtDNA of S. bayanus var. uvarum is much less polymorphic than that of S. cerevisiae. This observation is in good agreement with results obtained by electrophoretic karyotyping. Unlike wine S. cerevisiae, strains of S. bayanus var. uvarum display a low level of chromosome length polymorphism.     

Mitochondrial–nuclear co‐evolution leads to hybrid incompatibility through pentatricopeptide repeat proteins

Mitochondrial–nuclear incompatibility has a major role in reproductive isolation between species. However, the underlying mechanism and driving force of mitochondrial–nuclear incompatibility remain elusive. Here, we report a pentatricopeptide repeat‐containing (PPR) protein, Ccm1, and its interacting partner, 15S rRNA, to be involved in hybrid incompatibility between two yeast species, Saccharomyces cerevisiae and Saccharomyces bayanus. S. bayanus‐Ccm1 has reduced binding affinity for S. cerevisiae‐15S rRNA, leading to respiratory defects in hybrid cells. This incompatibility can be rescued by single mutations on several individual PPR motifs, demonstrating the highly evolvable nature of PPR proteins. When we examined other PPR proteins in the closely related Saccharomyces sensu stricto yeasts, about two‐thirds of them showed detectable incompatibility. Our results suggest that fast co‐evolution between flexible PPR proteins and their mitochondrial RNA substrates may be a common driving force in the development of mitochondrial–nuclear hybrid incompatibility.     

A Structural and Phylogenetic Study of the HO Gene from Saccharomyces bayanus var. uvarum

A novel HO gene (Uv-HO) was cloned from the Saccharomyces bayanus var. uvarum (abbreviated as S. uvarum in this study) type strain. The coding region of Uv-HO showed relatively high homology (95%) to that of the Sb-HO gene (S. bayanus var. bayanus HO), but not to the HO genes of other Saccharomyces sensu stricto species. However, the 5′ and 3′ non-coding region of Uv-HO showed less similarity (79% and 76% respectively) even to those of the most homologous gene Sb-HO. Motifs of the mating-type control and the cell-cycle control were conserved in the 5′ non-coding region of Uv-HO, but numbers and positions of motifs were different from those of Sb-HO. CHEF-Southern analysis showed that all tested strains of S. bayanus species, including S. uvarum, carried the HO gene on the 1,100-kb chromosome. By HO-typing PCR using mixed primers, which provided a rapid and convenient tool for yeast identification, either the Uv-HO gene or the Sb-HO gene was detected in strains of S. bayanus species, but two strains were found to have both types of HO gene in each genome. These results suggest that S. uvarum has a unique sequence, but might share the same chromosome constitution within S. bayanus species, and that S. bayanus is a heterogeneous species, of which some strains might be natural hybrid.     

Retention of Browning Compounds by Yeasts Involved in the Winemaking of Sherry Type Wines

Wine model solutions were used to study the ability of dehydrated yeasts to retain the brown products formed in the reaction between (+)-catechin and acetaldehyde. Saccharomyces cerevisiae races capensis and bayanus, two typical flor yeasts involved in the biological aging of sherry wines, had a higher capacity to retain coloured compounds than S. cerevisiae fermentative yeast. Of the flor yeasts, capensis exhibited a higher colour reduction capacity than bayanus. Such differences may account for the different rate at which browning compounds are removed at different times of year during the biological aging of wines.     

Natural polymorphism of the plasmid double-stranded RNA of the <Emphasis Type="Italic">Saccharomyces</Emphasis> yeasts

The distribution and peculiarities of viral double-stranded RNA in natural Saccharomyces strains were studied. It is for the first time that the presence of the L and M fractions in the species S. kudriavzevii and S. mikatae has been documented. S. kudriavzevii has two types of M-dsRNA: M₁ and M₄, whereas the yeast S. mikatae is characterized by three types of plasmids: M₂–M₄. Plasmid dsRNAs are absent in S. cariocanus strains. A total of eleven types of M-dsRNA were identified; some of them were specific to particular species. Plasmids M₅–M₇ were revealed only in S. paradoxus strains and the yeast S. bayanus is characterized by M₈–M₁₁ double-stranded RNA. According to the results of phenotypic analysis, all the M-dsRNAs revealed were cryptic.     

Four <Emphasis Type="Italic">Saccharomyces</Emphasis> species differ in their tolerance to various stresses though they have similar basic physiological parameters

Saccharomyces species, which are mostly used in the food and beverage industries, are known to differ in their fermentation efficiency and tolerance of adverse fermentation conditions. However, the basis of their difference has not been fully elucidated, although their genomes have been sequenced and analyzed. Five strains of four Saccharomyces species (S. cerevisiae, S. kudriavzevii, S. bayanus, and S. paradoxus), when grown in parallel in laboratory conditions, exhibit very similar basic physiological parameters such as membrane potential, intracellular pH, and the degree to which they are able to quickly activate their Pma1 H⁺-ATPase upon glucose addition. On the other hand, they differ in their ability to proliferate in media with a very low concentration of potassium, in their osmotolerance and tolerance to toxic cations and cationic drugs in a growth-medium specific manner, and in their capacity to survive anhydrobiosis. Overall, S. cerevisiae (T73 more than FL100) and S. paradoxus are the most robust, and S. kudriavzevii the most sensitive species. Our results suggest that the difference in stress survival is based on their ability to quickly accommodate their cell size and metabolism to changing environmental conditions and to adjust their portfolio of available detoxifying transporters.     

Chimeric Genomes of Natural Hybrids of Saccharomyces cerevisiae and Saccharomyces kudriavzevii

Recently, a new type of hybrid resulting from the hybridization between Saccharomyces cerevisiae and Saccharomyces kudriavzevii was described. These strains exhibit physiological properties of potential biotechnological interest. A preliminary characterization of these hybrids showed a trend to reduce the S. kudriavzevii fraction of the hybrid genome. We characterized the genomic constitution of several wine S. cerevisiae × S. kudriavzevii strains by using a combined approach based on the restriction fragment length polymorphism analysis of gene regions, comparative genome hybridizations with S. cerevisiae DNA arrays, ploidy analysis, and gene dose determination by quantitative real-time PCR. The high similarity in the genome structures of the S. cerevisiae × S. kudriavzevii hybrids under study indicates that they originated from a single hybridization event. After hybridization, the hybrid genome underwent extensive chromosomal rearrangements, including chromosome losses and the generation of chimeric chromosomes by the nonreciprocal recombination between homeologous chromosomes. These nonreciprocal recombinations between homeologous chromosomes occurred in highly conserved regions, such as Ty long terminal repeats (LTRs), rRNA regions, and conserved protein-coding genes. This study supports the hypothesis that chimeric chromosomes may have been generated by a mechanism similar to the recombination-mediated chromosome loss acting during meiosis in Saccharomyces hybrids. As a result of the selective processes acting during fermentation, hybrid genomes maintained the S. cerevisiae genome but reduced the S. kudriavzevii fraction.The genus Saccharomyces consists of seven biological species: S. arboricolus, S. bayanus, S. cariocanus, S. cerevisiae, S. kudriavzevii, S. mikatae, and S. paradoxus (29, 59) and the partially allotetraploid species S. pastorianus (46, 58).The hybrid species S. pastorianus, restricted to lager brewing environments, arose from two or more natural hybridization events between S. cerevisiae and a S. bayanus-like yeast (7, 16, 28, 46). Recent studies of S. bayanus have also revealed the hybrid nature of certain strains of this species, which has subsequently been subdivided into two groups, S. bayanus var. bayanus, containing a variety of hybrid strains, and S. bayanus var. uvarum, also referred to as S. uvarum, that contains nonhybrid strains (45, 46).New hybrids of other species from the genus Saccharomyces have recently been described. Hybrid yeasts of S. cerevisiae and S. kudriavzevii have been characterized among wine (6, 20, 33) and brewing yeasts (21); even triple hybrids of S. cerevisiae, S. bayanus, and S. kudriavzevii have been identified (20, 41).The first natural Saccharomyces interspecific hybrid identified, the lager brewing yeast S. pastorianus (S. carlsbergensis) (42, 57), has become one of the most investigated types of yeast hybrids. The genome structure of these hybrids has been examined by competitive array comparative genome hybridization (aCGH) (5, 16, 28), complete genome sequencing (28), and PCR-restriction fragment length polymorphism (RFLP) analysis of 48 genes and partial sequences of 16 genes (46). The aCGH analyses of several S. pastorianus strains with S. cerevisiae-only DNA arrays (5, 28) revealed the presence of aneuploidies due to deletions of entire regions of the S. cerevisiae fraction of the hybrid genomes. A recent aCGH analysis of S. pastorianus strains with S. cerevisiae and S. bayanus DNA arrays (16) showed two groups of strains according to their genome structure and composition. These groups arose from two independent hybridization events, and each one is characterized by a reduction and an amplification of the S. cerevisiae genome fraction, respectively.The genetic characterization of the wine S. cerevisiae and S. kudriavzevii hybrids by restriction analysis of five nuclear genes located in different chromosomes, 5.8S-ITS rDNA region and the mitochondrial COX2 gene, revealed the presence of three types of hybrids in Swiss wines, thus indicating the presence of different hybrid genomes (20). In a recent study (21), we identified six new types of S. cerevisiae and S. kudriavzevii hybrids among brewing strains, which were compared to wine hybrids by a genetic characterization based on RFLP analysis of 35 protein-encoding genes. This analysis confirmed the presence of three different genome types among wine hybrids that contain putative chimeric chromosomes, probably generated by a recombination between homeologous chromosomes of different parental origins.The aim of the present study is to investigate the genome composition and structure of wine hybrids of S. cerevisiae and S. kudriavzevii. This has been achieved by a combined approach based on the RFLP analysis of 35 gene regions from our previous study, comparative genome hybridizations using S. cerevisiae DNA macroarrays, a ploidy analysis by flow cytometry, and gene dose determinations by quantitative real-time PCR. This multiple approach allowed us to confirm the presence of chimeric chromosomes and define the mechanisms involved in their origins.     

Enhanced Enzymatic Activity of Glycerol-3-Phosphate Dehydrogenase from the Cryophilic Saccharomyces kudriavzevii

During the evolution of the different species classified within the Saccharomyces genus, each one has adapted to live in different environments. One of the most important parameters that have influenced the evolution of Saccharomyces species is the temperature. Here we have focused on the study of the ability of certain species as Saccharomyces kudriavzevii to grow at low temperatures, in contrast to Saccharomyces cerevisiae. We observed that S. kudriavzevii strains isolated from several regions are able to synthesize higher amounts of glycerol, a molecule that has been shown to accumulate in response to freeze and cold stress. To explain this observation at the molecular level we studied the expression of glycerol biosynthetic pathway genes and we observed a higher expression of GPD1 gene in S. kudriavzevii compared to S. cerevisiae in micro-vinification conditions. We observed higher enzymatic activity of Gpd1p in S. kudriavzevii in response to osmotic and cold stress. Also, we determined that S. kudriavzevii Gpd1p enzyme presents increased catalytic properties that will contribute to increase glycerol production. Finally, we evaluated the glycerol production with S. cerevisiae, S. kudriavzevii or a recombinant Gpd1p variant in the same background and observed that the S. kudriavzevii enzyme produced increased glycerol levels at 12 or 28°C. This suggests that glycerol is increased in S. kudriavzevii mainly due to increased V _max of the Gpd1p enzyme. All these differences indicate that S. kudriavzevii has changed the metabolism to promote the branch of the glycolytic pathway involved in glycerol production to adapt to low temperature environments and maintain the NAD⁺/NADH ratio in alcoholic fermentations. This knowledge is industrially relevant due to the potential use, for example, of S. cerevisiae-S. kudriavzevii hybrids in the wine industry where glycerol content is an important quality parameter.     

RFLP analysis of the ribosomal internal transcribed spacers and the 5.8S rRNA gene region of the genus Saccharomyces: a fast method for species identification and the differentiation of flor yeasts

The PCR amplification and subsequent restriction analysis of the region spanning the internal transcribed spacers (ITS1 and ITS2) and the 5.8S rRNA gene was applied to the identification of yeasts belonging to the genus Saccharomyces. This methodology has previously been used for the identification of some species of this genus, but in the present work, this application was extended to the identification of new accepted Saccharomyces species (S. kunashirensis, S. martiniae, S. rosinii, S. spencerorum, and S. transvaalensis), as well as to the differentiation of an interesting group of Saccharomyces cerevisiae strains, known as flor yeasts, which are responsible for ageing sherry wine. Among the species of the Saccharomyces sensu lato complex, the high diversity observed, either in the length of the amplified region (ranged between 700 and 875 bp) or in their restriction patterns allows the unequivocal identification of these species. With respect to the four sibling species of the Saccharomyces sensu stricto complex, only two of them, S. bayanus and S. pastorianus, cannot be differentiated according to their restriction patterns, which is in accordance with the hybrid origin (S. bayanus × S. cerevisiae) of S. pastorianus. The flor S. cerevisiae strains exhibited restriction patterns different from those typical of the species S. cerevisiae. These differences can easily be used to differentiate this interesting group of strains. We demonstrate that the specific patterns exhibited by flor yeasts are due to the presence of a 24-bp deletion located in the ITS1 region and that this could have originated as a consequence of a slipped-strand mispairing during replication or be due to an unequal crossing-over. A subsequent restriction analysis of this region from more than 150 flor strains indicated that this deletion is fixed in flor yeast populations.     

New family of pectinase genes <Emphasis Type="Italic">PGU1</Emphasis>b–<Emphasis Type="Italic">PGU3b</Emphasis> of the pectinolytic yeast <Emphasis Type="Italic">Saccharomyces bayanus</Emphasis> var. <Emphasis Type="Italic">uvarum</Emphasis>

Using yeast genome databases and literature data, we have conducted a phylogenetic analysis of pectinase PGU genes from Saccharomyces strains assigned to the biological species S. arboricola, S. bayanus (var. uvarum), S. cariocanus, S. cerevisiae, S. kudriavzevii, S. mikatae, S. paradoxus, and hybrid taxon S. pastorianus (syn. S. carlsbergensis). Single PGU genes were observed in all Saccharomyces species, except S. bayanus. The superfamily of divergent PGU genes has been documented in S. bayanus var. uvarum for the first time. Chromosomal localization of new PGU1b, PGU2b, and PGU3b genes in the yeast S. bayanus var. uvarum has been determined by molecular karyotyping and Southern hybridization.     

Systematics of the species of the yeast genus Saccharomyces associated with the fermentation industry

Summary The so-called wine yeasts Saccharomyces cerevisiae, S. chevalieri, S. bayanus, S. italicus and S. uvarum are characterized by high ethanol tolerance and fermentation velocity. They are ecologically related, being predominantly associated with grape must and wine, and are taxonomically indistinguishable. The only significant physiological differences are between the ability to ferment certain sugars. A taxonomic revision of more than 1,000 strains isolated during the past 50 years and belonging to the above species showed extreme instability in the ability to ferment different sugars. The relationships between these yeasts were examined for DNA base composition and DNA-DNA reassociation. The G+C ranged from 37.6% to 39.0% while optical reassociation experiments defined a first group of species (Saccharomyces cerevisiae, S. chevalieri and S. italicus) exhibiting high base sequence complementarity (>90%). S. bayanus and S. uvarum also showed a high degree of relatedness. Low homology values (30%) indicate that the two groups of species are not closely related. While it is proposed to combine S. cerevisiae, S. chevalieri and S. italicus into one single species under the oldest epithet Saccharomyces cerevisiae, a study of a larger number of strains is recommended before considering the taxonomic position of S. bayanus and S. uvarum.     

Genetic relationship and biological status of the industrially important yeast <Emphasis Type="Italic">Saccharomyces eubayanus</Emphasis> Sampaio et al.

The genomes of the recently discovered yeast Saccharomyces eubayanus and traditional S. cerevisiae are known to be found in the yeast S. pastorianus (syn. S. carlsbergensis), which are essential for brewing. The cryotolerant yeast S. bayanus var. uvarum is of great importance for production of some wines. Based on ascospore viability and meiotic recombination of the control parental markers in hybrids, we have shown that there is no complete interspecies post-zygotic isolation between the yeasts S. eubayanus, S. bayanus var. bayanus and S. bayanus var. uvarum. The genetic data presented indicate that all of the three taxa belong to the same species.     

Adaptive evolution in the Saccharomyces kudriavzevii Aro4p promoted a reduced production of higher alcohols

The use of unconventional yeast species in human-driven fermentations has attracted a lot of attention in the last few years. This tool allows the alcoholic beverage industries to solve problems related to climate change or the consumer demand for newer high-quality products. In this sense, one of the most attractive species is Saccharomyces kudriavzevii, which shows interesting fermentative traits such as the increased and diverse aroma compound production in wines. Specifically, it has been observed that different isolates of this species can produce higher amounts of higher alcohols such as phenylethanol compared with Saccharomyces cerevisiae. In this work, we have shed light on this feature relating it to the S. kudriavzevii aromatic amino acid anabolic pathway in which the enzyme Aro4p plays an essential role. Unexpectedly, we observed that the presence of the S. kudriavzevii ARO4 variant reduces phenylethanol production compared with the S. cerevisiae ARO4 allele. Our experiments suggest that this can be explained by increased feedback inhibition, which might be a consequence of the changes detected in the Aro4p amino end such as L₂₆Q₂₄ that have been under positive selection in the S. kudriavzevii specie.     

Comparative Molecular Genetic Analysis of β-Fructosidases of Yeasts <Emphasis Type="Italic">Saccharomyces</Emphasis>

To study the evolution of the polymeric β-fructosidase (invertase) genes (SUC) of yeasts Saccharomyces, new SUC gene of S. cariocanus was cloned and sequenced and the nucleotide and amino acid sequences were compared for all known β-fructosidases of Saccharomyces species. The proteins showed 90–97% homology. The most divergent was S. bayanus β-fructosidase. The results testified again to high conservation of yeast β-fructosidases. Transitions C-T prevail in the total spectrum of nucleotide substitutions observed in the coding regions of the SUC genes; most of these transitions are in the third codon position and cause no changes in the amino acid sequences of the encoded proteins. The six Saccharomyces species each carry one (probably, non-telomeric) β-fructosidase gene. SUC is on chromosome IX in S. cerevisiae, S. bayanus, S. kudriavzevii, S. mikatae, and S. paradoxus and in a translocation region on chromosome XV in S. cariocanus.__________Translated from Molekulyarnaya Biologiya, Vol. 39, No. 3, 2005, pp. 413–419.Original Russian Text Copyright © 2005 by Korshunova, Naumova, Naumov.     

Pure and Mixed Genetic Lines of Saccharomyces bayanus and Saccharomyces pastorianus and Their Contribution to the Lager Brewing Strain Genome

The yeast species Saccharomyces bayanus and Saccharomyces pastorianus are of industrial importance since they are involved in the production process of common beverages such as wine and lager beer; however, they contain strains whose variability has been neither fully investigated nor exploited in genetic improvement programs. We evaluated this variability by using PCR-restriction fragment length polymorphism analysis of 48 genes and partial sequences of 16. Within these two species, we identified “pure” strains containing a single type of genome and “hybrid” strains that contained portions of the genomes from the “pure” lines, as well as alleles termed “Lager” that represent a third genome commonly associated with lager brewing strains. The two pure lines represent S. uvarum and S. bayanus, the latter a novel group of strains that may be of use in strain improvement programs. Hybrid lines identified include (i) S. cerevisiae/S. bayanus/Lager, (ii) S. bayanus/S. uvarum/Lager, and (iii) S. cerevisiae/S. bayanus/S. uvarum/Lager. The genome of the lager strains may have resulted from chromosomal loss, replacement, or rearrangement within the hybrid genetic lines. This study identifies brewing strains that could be used as novel genetic sources in strain improvement programs and provides data that can be used to generate a model of how naturally occurring and industrial hybrid strains may have evolved.