Brettanomyces bruxellensis: Overview of the genetic and phenotypic diversity of an anthropized yeast.

Human-associated microorganisms are ideal models to study the impact of environmental changes on species evolution and adaptation because of their small genome, short generation time, and their colonization of contrasting and ever-changing ecological niches. The yeast Brettanomyces bruxellensis is a good example of organism facing anthropogenic-driven selective pressures. It is associated with fermentation processes in which it can be considered either as a spoiler (e.g., winemaking, bioethanol production) or as a beneficial microorganism (e.g., production of specific beers, kombucha). In addition to its industrial interests, noteworthy parallels and dichotomies with Saccharomyces cerevisiae propelled B. bruxellensis as a valuable complementary yeast model. In this review, we emphasize that the broad genetic and phenotypic diversity of this species is only beginning to be uncovered. Population genomic studies have revealed the coexistence of auto- and allotriploidization events with different evolutionary outcomes. The different diploid, autotriploid and allotriploid subpopulations are associated with specific fermented processes, suggesting independent adaptation events to anthropized environments. Phenotypically, B. bruxellensis is renowned for its ability to metabolize a wide variety of carbon and nitrogen sources, which may explain its ability to colonize already fermented environments showing low-nutrient contents. Several traits of interest could be related to adaptation to human activities (e.g., nitrate metabolization in bioethanol production, resistance to sulphite treatments in winemaking). However, phenotypic traits are insufficiently studied in view of the great genomic diversity of the species. Future work will have to take into account strains of varied substrates, geographical origins as well as displaying different ploidy levels to improve our understanding of an anthropized yeast's phenotypic landscape.

Different trajectories of polyploidization shape the genomic landscape of the Brettanomyces bruxellensis yeast species.

Polyploidization events are observed across the tree of life and occur in many fungi, plant, and animal species. During evolution, polyploidy is thought to be an important source of speciation and tumorigenesis. However, the origin of polyploid populations is not always clear, and little is known about the precise nature and structure of their complex genome. Using a long-read sequencing strategy, we sequenced 71 strains from the Brettanomyces bruxellensis yeast species, which is found in anthropized environments (e.g., beer, contaminant of wine, kombucha, and ethanol production) and characterized by several polyploid subpopulations. To reconstruct the polyploid genomes, we phased them by using different strategies and found that each subpopulation had a unique polyploidization history with distinct trajectories. The polyploid genomes contain either genetically closely related (with a genetic divergence <1%) or diverged copies (>3%), indicating auto- as well as allopolyploidization events. These latest events have occurred independently with a specific and unique donor in each of the polyploid subpopulations and exclude the known Brettanomyces sister species as possible donors. Finally, loss of heterozygosity events has shaped the structure of these polyploid genomes and underline their dynamics. Overall, our study highlights the multiplicity of the trajectories leading to polyploid genomes within the same species.

nPhase: an accurate and contiguous phasing method for polyploids.

While genome sequencing and assembly are now routine, we do not have a full, precise picture of polyploid genomes. No existing polyploid phasing method provides accurate and contiguous haplotype predictions. We developed nPhase, a ploidy agnostic tool that leverages long reads and accurate short reads to solve alignment-based phasing for samples of unspecified ploidy ( https://github.com/OmarOakheart/nPhase ). nPhase is validated by tests on simulated and real polyploids. nPhase obtains on average over 95% accuracy and a contiguous 1.25 haplotigs per haplotype to cover more than 90% of each chromosome (heterozygosity rate ≥ 0.5%). nPhase allows population genomics and hybrid studies of polyploids.

The role of structural pleiotropy and regulatory evolution in the retention of heteromers of paralogs.

Gene duplication is a driver of the evolution of new functions. The duplication of genes encoding homomeric proteins leads to the formation of homomers and heteromers of paralogs, creating new complexes after a single duplication event. The loss of these heteromers may be required for the two paralogs to evolve independent functions. Using yeast as a model, we find that heteromerization is frequent among duplicated homomers and correlates with functional similarity between paralogs. Using in silico evolution, we show that for homomers and heteromers sharing binding interfaces, mutations in one paralog can have structural pleiotropic effects on both interactions, resulting in highly correlated responses of the complexes to selection. Therefore, heteromerization could be preserved indirectly due to selection for the maintenance of homomers, thus slowing down functional divergence between paralogs. We suggest that paralogs can overcome the obstacle of structural pleiotropy by regulatory evolution at the transcriptional and post-translational levels.

Author Correction: Hybridization is a recurrent evolutionary stimulus in wild yeast speciation.

The original version of the Supplementary Information associated with this Article contained errors in Supplementary Figures 2, 12, 20 and 22. The HTML has been updated to include a corrected version of the Supplementary Information; the original incorrect versions of these Figures can be found as Supplementary Information associated with this Correction.

Hybridization is a recurrent evolutionary stimulus in wild yeast speciation.

Hybridization can result in reproductively isolated and phenotypically distinct lineages that evolve as independent hybrid species. How frequently hybridization leads to speciation remains largely unknown. Here we examine the potential recurrence of hybrid speciation in the wild yeast Saccharomyces paradoxus in North America, which comprises two endemic lineages SpB and SpC, and an incipient hybrid species, SpC*. Using whole-genome sequences from more than 300 strains, we uncover the hybrid origin of another group, SpD, that emerged from hybridization between SpC* and one of its parental species, the widespread SpB. We show that SpD has the potential to evolve as a novel hybrid species, because it displays phenotypic novelties that include an intermediate transcriptome profile, and partial reproductive isolation with its most abundant sympatric parental species, SpB. Our findings show that repetitive cycles of divergence and hybridization quickly generate diversity and reproductive isolation, providing the raw material for speciation by hybridization.

The Rapid Evolution of an Ohnolog Contributes to the Ecological Specialization of Incipient Yeast Species.

Identifying the molecular changes that lead to ecological specialization during speciation is one of the major goals of molecular evolution. One question that remains to be thoroughly investigated is whether ecological specialization derives strictly from adaptive changes and their associated trade-offs, or from conditionally neutral mutations that accumulate under relaxed selection. We used whole-genome sequencing, genome annotation and computational analyses to identify genes that have rapidly diverged between two incipient species of Saccharomyces paradoxus that occupy different climatic regions along a south-west to north-east gradient. As candidate loci for ecological specialization, we identified genes that show signatures of adaptation and accelerated rates of amino acid substitutions, causing asymmetric evolution between lineages. This set of genes includes a glycyl-tRNA-synthetase, GRS2, which is known to be transcriptionally induced under heat stress in the model and sister species S. cerevisiae. Molecular modelling, expression analysis and fitness assays suggest that the accelerated evolution of this gene in the Northern lineage may be caused by relaxed selection. GRS2 arose during the whole-genome duplication (WGD) that occurred 100 million years ago in the yeast lineage. While its ohnolog GRS1 has been preserved in all post-WGD species, GRS2 has frequently been lost and is evolving rapidly, suggesting that the fate of this ohnolog is still to be resolved. Our results suggest that the asymmetric evolution of GRS2 between the two incipient S. paradoxus species contributes to their restricted climatic distributions and thus that ecological specialization derives at least partly from relaxed selection rather than a molecular trade-off resulting from adaptive evolution.

Speciation driven by hybridization and chromosomal plasticity in a wild yeast.

Hybridization is recognized as a powerful mechanism of speciation and a driving force in generating biodiversity. However, only few multicellular species, limited to a handful of plants and animals, have been shown to fulfil all the criteria of homoploid hybrid speciation. This lack of evidence could lead to the interpretation that speciation by hybridization has a limited role in eukaryotes, particularly in single-celled organisms. Laboratory experiments have revealed that fungi such as budding yeasts can rapidly develop reproductive isolation and novel phenotypes through hybridization, showing that in principle homoploid speciation could occur in nature. Here, we report a case of homoploid hybrid speciation in natural populations of the budding yeast Saccharomyces paradoxus inhabiting the North American forests. We show that the rapid evolution of chromosome architecture and an ecological context that led to secondary contact between nascent species drove the formation of an incipient hybrid species with a potentially unique ecological niche.

Distribution and population structure of the anther smut Microbotryum silenes-acaulis parasitizing an arctic-alpine plant.

Cold-adapted organisms with current arctic-alpine distributions have persisted during the last glaciation in multiple ice-free refugia, leaving footprints in their population structure that contrast with temperate plants and animals. However, pathogens that live within hosts having arctic-alpine distributions have been little studied. Here, we therefore investigated the geographical range and population structure of a fungus parasitizing an arctic-alpine plant. A total of 1437 herbarium specimens of the plant Silene acaulis were examined, and the anther smut pathogen Microbotryum silenes-acaulis was present throughout the host's geographical range. There was significantly greater incidence of anther smut disease in more northern latitudes and where the host locations were less dense, indicating a major influence of environmental factors and/or host demographic structure on the pathogen distribution. Genetic analyses with seven microsatellite markers on recent collections of 195 M. silenes-acaulis individuals revealed three main genetic clusters, in North America, northern Europe and southern Europe, likely corresponding to differentiation in distinct refugia during the last glaciation. The lower genetic diversity in northern Europe indicates postglacial recolonization northwards from southern refugia. This study combining herbarium surveys and population genetics thus uniquely reveals the effects of climate and environmental factors on a plant pathogen species with an arctic-alpine distribution.

The genomics of wild yeast populations sheds light on the domestication of man's best (micro) friend.

The domestication of plants, animals and microbes by humans are the longest artificial evolution experiments ever performed. The study of these long-term experiments can teach us about the genomics of adaptation through the identification of the genetic bases underlying the traits favoured by humans. In laboratory evolution, the characterization of the molecular changes that evolved specifically in some lineages is straightforward because the ancestors are readily available, for instance in the freezer. However, in the case of domesticated species, the ancestor is often missing, which leads to the necessity of going back to nature in order to infer the most likely ancestral state. Significant and relatively recent examples of this approach include wolves as the closest wild relative to domestic dogs (Axelsson et al. 2013) and teosinte as the closest relative to maize (reviewed in Hake & Ross-Ibarra 2015). In both cases, the joint analysis of domesticated lineages and their wild cousins has been key in reconstructing the molecular history of their domestication. While the identification of closest wild relatives has been done for many plants and animals, these comparisons represent challenges for micro-organisms. This has been the case for the budding yeast Saccharomyces cerevisiae, whose natural ecological niche is particularly challenging to define. For centuries, this unicellular fungus has been the cellular factory for wine, beer and bread crafting, and currently for bioethanol and drug production. While the recent development of genomics has lead to the identification of many genetic elements associated with important wine characteristics, the historical origin of some of the domesticated wine strains has remained elusive due to the lack of knowledge of their close wild relatives. In this issue of Molecular Ecology, Almeida et al. (2015) identified what is to date the closest known wild population of the wine yeast. This population is found associated with oak trees in Europe, presumably its natural host. Using population genomics analyses, Almeida and colleagues discovered that the initial divergence between natural and domesticated wine yeasts in the Mediterranean region took place around the early days of wine production. Surprisingly, genomic regions that are key to wine production today appeared not to be derived from these natural populations but from genes gained from other yeast species.