Chris Todd Hittinger

Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast

Domestication of plants and animals promoted humanitys transition from nomadic to sedentary lifestyles, demographic expansion, and the emergence of civilizations. In contrast to the well-documented successes of crop and livestock breeding, processes of microbe domestication remain obscure, despite the importance of microbes to the production of food, beverages, and biofuels. Lager-beer, first brewed in the 15th century, employs an allotetraploid hybrid yeast, Saccharomyces pastorianus (syn. Saccharomyces carlsbergensis), a domesticated species created by the fusion of a Saccharomyces cerevisiae ale-yeast with an unknown cryotolerant Saccharomyces species. We report the isolation of that species and designate it Saccharomyces eubayanus sp. nov. because of its resemblance to Saccharomyces bayanus (a complex hybrid of S. eubayanus, Saccharomyces uvarum, and S. cerevisiae found only in the brewing environment). Individuals from populations of S. eubayanus and its sister species, S. uvarum, exist in apparent sympatry in Nothofagus (Southern beech) forests in Patagonia, but are isolated genetically through intrinsic postzygotic barriers, and ecologically through host-preference. The draft genome sequence of S. eubayanus is 99.5% identical to the non-S. cerevisiae portion of the S. pastorianus genome sequence and suggests specific changes in sugar and sulfite metabolism that were crucial for domestication in the lager-brewing environment. This study shows that combining microbial ecology with comparative genomics facilitates the discovery and preservation of wild genetic stocks of domesticated microbes to trace their history, identify genetic changes, and suggest paths to further industrial improvement.

Gene duplication and the adaptive evolution of a classic genetic switch.

How gene duplication and divergence contribute to genetic novelty and adaptation has been of intense interest, but experimental evidence has been limited. The genetic switch controlling the yeast galactose use pathway includes two paralogous genes in Saccharomyces cerevisiae that encode a co-inducer (GAL3) and a galactokinase (GAL1). These paralogues arose from a single bifunctional ancestral gene as is still present in Kluyveromyces lactis. To determine which evolutionary processes shaped the evolution of the two paralogues, here we assess the effects of precise replacement of coding and non-coding sequences on organismal fitness. We suggest that duplication of the ancestral bifunctional gene allowed for the resolution of an adaptive conflict between the transcriptional regulation of the two gene functions. After duplication, previously disfavoured binding site configurations evolved that divided the regulation of the ancestral gene into two specialized genes, one of which ultimately became one of the most tightly regulated genes in the genome.

Identifying genes of agronomic importance in maize by screening microsatellites for evidence of selection during domestication.

Crop species experienced strong selective pressure directed at genes controlling traits of agronomic importance during their domestication and subsequent episodes of selective breeding. Consequently, these genes are expected to exhibit the signature of selection. We screened 501 maize genes for the signature of selection using microsatellites or simple sequence repeats (SSRs). We applied the Ewens–Watterson test, which can reveal deviations from a neutral-equilibrium model, as well as two nonequilibrium tests that incorporate the domestication bottleneck. We investigated two classes of SSRs: those known to be polymorphic in maize (Class I) and those previously classified as monomorphic in maize (Class II). Fifteen SSRs exhibited some evidence for selection in maize and 10 showed evidence under stringent criteria. The genes containing nonneutral SSRs are candidates for agronomically important genes. Because demographic factors can bias our tests, further independent tests of these candidates are necessary. We applied such an additional test to one candidate, which encodes a MADS box transcriptional regulator, and confirmed that this gene experienced a selective sweep during maize domestication. Genomic scans for the signature of selection offer a means of identifying new genes of agronomic importance even when gene function and the phenotype of interest are unknown.

The Awesome Power of Yeast Evolutionary Genetics: New Genome Sequences and Strain Resources for the Saccharomyces sensu stricto Genus

High-quality, well-annotated genome sequences and standardized laboratory strains fuel experimental and evolutionary research. We present improved genome sequences of three species of Saccharomyces sensu stricto yeasts: S. bayanus var. uvarum (CBS 7001), S. kudriavzevii (IFO 1802T and ZP 591), and S. mikatae (IFO 1815T), and describe their comparison to the genomes of S. cerevisiae and S. paradoxus. The new sequences, derived by assembling millions of short DNA sequence reads together with previously published Sanger shotgun reads, have vastly greater long-range continuity and far fewer gaps than the previously available genome sequences. New gene predictions defined a set of 5261 protein-coding orthologs across the five most commonly studied Saccharomyces yeasts, enabling a re-examination of the tempo and mode of yeast gene evolution and improved inferences of species-specific gains and losses. To facilitate experimental investigations, we generated genetically marked, stable haploid strains for all three of these Saccharomyces species. These nearly complete genome sequences and the collection of genetically marked strains provide a valuable toolset for comparative studies of gene function, metabolism, and evolution, and render Saccharomyces sensu stricto the most experimentally tractable model genus. These resources are freely available and accessible through www.SaccharomycesSensuStricto.org.

Remarkably ancient balanced polymorphisms in a multi-locus gene network

Local adaptations within species are often governed by several interacting genes scattered throughout the genome. Single-locus models of selection cannot explain the maintenance of such complex variation because recombination separates co-adapted alleles. Here we report a previously unrecognized type of intraspecific multi-locus genetic variation that has been maintained over a vast period. The galactose (GAL) utilization gene network of Saccharomyces kudriavzevii, a relative of brewer’s yeast, exists in two distinct states: a functional gene network in Portuguese strains and, in Japanese strains, a non-functional gene network of allelic pseudogenes. Genome sequencing of all available S. kudriavzevii strains revealed that none of the functional GAL genes were acquired from other species. Rather, these polymorphisms have been maintained for nearly the entire history of the species, despite more recent gene flow genome-wide. Experimental evidence suggests that inactivation of the GAL3 and GAL80 regulatory genes facilitated the origin and long-term maintenance of the two gene network states. This striking example of a balanced unlinked gene network polymorphism introduces a remarkable type of intraspecific variation that may be widespread.

A Gondwanan imprint on global diversity and domestication of wine and cider yeast Saccharomyces uvarum.

In addition to Saccharomyces cerevisiae, the cryotolerant yeast species S. uvarum is also used for wine and cider fermentation but nothing is known about its natural history. Here we use a population genomics approach to investigate its global phylogeography and domestication fingerprints using a collection of isolates obtained from fermented beverages and from natural environments on five continents. South American isolates contain more genetic diversity than that found in the Northern Hemisphere. Moreover, coalescence analyses suggest that a Patagonian sub-population gave rise to the Holarctic population through a recent bottleneck. Holarctic strains display multiple introgressions from other Saccharomyces species, those from S. eubayanus being prevalent in European strains associated with human-driven fermentations. These introgressions are absent in the large majority of wild strains and gene ontology analyses indicate that several gene categories relevant for wine fermentation are overrepresented. Such findings constitute a first indication of domestication in S. uvarum.

Saccharomyces diversity and evolution: a budding model genus

Saccharomyces cerevisiae is one of the best-understood and most powerful genetic model systems. Several disciplines are now converging to turn Saccharomyces into an exciting model genus for evolutionary genetics and genomics. Yeast taxonomists and ecologists have dramatically expanded and clarified Saccharomyces diversity, more than doubling the number of bona fide species since 2000. High-quality genome sequences are available (or soon will be) for all seven known species. Haploid laboratory strains are enabling a deep integration of classic genetic approaches with modern genomic tools. Population genomic surveys and quantitative trait mapping of variation within species are underway across the genus. Finally, several case studies have illuminated general and novel genetic mechanisms of evolution. Expanding strain collections, low-cost genome sequencing, and tools for precise genetic manipulation promise to usher in a golden era for this surprisingly diverse genus as an evolutionary model.

Leveraging skewed transcript abundance by RNA-Seq to increase the genomic depth of the tree of life

Assembling the tree of life is a major goal of biology, but progress has been hindered by the difficulty and expense of obtaining the orthologous DNA required for accurate and fully resolved phylogenies. Next-generation DNA sequencing technologies promise to accelerate progress, but sequencing the genomes of hundreds of thousands of eukaryotic species remains impractical. Eukaryotic transcriptomes, which are smaller than genomes and biased toward highly expressed genes that tend to be conserved, could potentially provide a rich set of phylogenetic characters. We sampled the transcriptomes of 10 mosquito species by assembling 36-bp sequence reads into phylogenomic data matrices containing hundreds of thousands of orthologous nucleotides from hundreds of genes. Analysis of these data matrices yielded robust phylogenetic inferences, even with data matrices constructed from surprisingly few sequence reads. This approach is more efficient, data-rich, and economical than traditional PCR-based and EST-based methods and provides a scalable strategy for generating phylogenomic data matrices to infer the branches and twigs of the tree of life.

The Genome Sequence of Saccharomyces eubayanus and the Domestication of Lager-Brewing Yeasts

The dramatic phenotypic changes that occur in organisms during domestication leave indelible imprints on their genomes. Although many domesticated plants and animals have been systematically compared with their wild genetic stocks, the molecular and genomic processes underlying fungal domestication have received less attention. Here, we present a nearly complete genome assembly for the recently described yeast species Saccharomyces eubayanus and compare it to the genomes of multiple domesticated alloploid hybrids of S. eubayanus × S. cerevisiae (S. pastorianus syn. S. carlsbergensis), which are used to brew lager-style beers. We find that the S. eubayanus subgenomes of lager-brewing yeasts have experienced increased rates of evolution since hybridization, and that certain genes involved in metabolism may have been particularly affected. Interestingly, the S. eubayanus subgenome underwent an especially strong shift in selection regimes, consistent with more extensive domestication of the S. cerevisiae parent prior to hybridization. In contrast to recent proposals that lager-brewing yeasts were domesticated following a single hybridization event, the radically different neutral site divergences between the subgenomes of the two major lager yeast lineages strongly favor at least two independent origins for the S. cerevisiae × S. eubayanus hybrids that brew lager beers. Our findings demonstrate how this industrially important hybrid has been domesticated along similar evolutionary trajectories on multiple occasions.

Comparative genomics of biotechnologically important yeasts

Significance The highly diverse Ascomycete yeasts have enormous biotechnological potential. Collectively, these yeasts convert a broad range of substrates into useful compounds, such as ethanol, lipids, and vitamins, and can grow in extremes of temperature, salinity, and pH. We compared 29 yeast genomes with the goal of correlating genetics to useful traits. In one rare species, we discovered a genetic code that translates CUG codons to alanine rather than canonical leucine. Genome comparison enabled correlation of genes to useful metabolic properties and showed the synteny of the mating-type locus to be conserved over a billion years of evolution. Our study provides a roadmap for future biotechnological exploitations. Ascomycete yeasts are metabolically diverse, with great potential for biotechnology. Here, we report the comparative genome analysis of 29 taxonomically and biotechnologically important yeasts, including 16 newly sequenced. We identify a genetic code change, CUG-Ala, in Pachysolen tannophilus in the clade sister to the known CUG-Ser clade. Our well-resolved yeast phylogeny shows that some traits, such as methylotrophy, are restricted to single clades, whereas others, such as l-rhamnose utilization, have patchy phylogenetic distributions. Gene clusters, with variable organization and distribution, encode many pathways of interest. Genomics can predict some biochemical traits precisely, but the genomic basis of others, such as xylose utilization, remains unresolved. Our data also provide insight into early evolution of ascomycetes. We document the loss of H3K9me2/3 heterochromatin, the origin of ascomycete mating-type switching, and panascomycete synteny at the MAT locus. These data and analyses will facilitate the engineering of efficient biosynthetic and degradative pathways and gateways for genomic manipulation.