Population genomics of domestic and wild yeasts

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Abstract
Baker's yeast, Saccharomyces cerevisiae, is one of the best studied model organisms, and has been associated with human activity for thousands of years. Two papers published in the 19 March 2009 issue of Nature provide a picture of its population structure and its relationship with other yeasts. Liti et al. compare genome variation in S. cerevisiae isolates with its closest wild cousin, S. paradoxus, which has never been associated with human activity. They find that variation in S. paradoxus closely follows geographic borders; S. cerevisiae shows less differentiation, consistent with opportunities for cross-breeding, rather than a few distinct domestication events, as the main human influence. Schacherer et al. compare 63 S. cerevisiae isolates from different ecological niches and geographic locations. They find evidence for genetic differentiation of three distinct subgroups based on where the strains were isolated: from vineyards, sake and related fermentations and lab strains. Their data support the hypothesis that these three groups represent separate domestication events, and that S. cerevisiae as a whole is not domesticated. By sequencing over seventy isolates of the domesticated baker's yeast Saccharomyces cerevisiae and its closest relative, S. paradoxus, this study describes variation in gene content, SNPs, indels, copy numbers and transposable elements, providing insights into the evolution of different lineages, phenotypic variation, domestication and population structure of Saccharomyces. Since the completion of the genome sequence of Saccharomyces cerevisiae in 1996 (refs 1, 2), there has been a large increase in complete genome sequences, accompanied by great advances in our understanding of genome evolution. Although little is known about the natural and life histories of yeasts in the wild, there are an increasing number of studies looking at ecological and geographic distributions3,4, population structure5,6,7,8 and sexual versus asexual reproduction9,10. Less well understood at the whole genome level are the evolutionary processes acting within populations and species that lead to adaptation to different environments, phenotypic differences and reproductive isolation. Here we present one- to fourfold or more coverage of the genome sequences of over seventy isolates of the baker’s yeast S. cerevisiae and its closest relative, Saccharomyces paradoxus. We examine variation in gene content, single nucleotide polymorphisms, nucleotide insertions and deletions, copy numbers and transposable elements. We find that phenotypic variation broadly correlates with global genome-wide phylogenetic relationships. S. paradoxus populations are well delineated along geographic boundaries, whereas the variation among worldwide S. cerevisiae isolates shows less differentiation and is comparable to a single S. paradoxus population. Rather than one or two domestication events leading to the extant baker’s yeasts, the population structure of S. cerevisiae consists of a few well-defined, geographically isolated lineages and many different mosaics of these lineages, supporting the idea that human influence provided the opportunity for cross-breeding and production of new combinations of pre-existing variations.