Population structure and reticulate evolution of S accharomyces eubayanus and its lager‐brewing hybrids

Reticulate evolution can be a major driver of diversification into new niches, especially in disturbed habitats and at the edges of ranges. Industrial fermentation strains of yeast provide a window into these processes, but progress has been hampered by a limited understanding of the natural diversity and distribution of S accharomyces species and populations. For example, lager beer is brewed with S accharomyces pastorianus , an alloploid hybrid of S . cerevisiae and S . eubayanus , a species only recently discovered in P atagonia, A rgentina. Here, we report that genetically diverse strains of S . eubayanus are readily isolated from P atagonia, demonstrating that the species is well established there. Analyses of multilocus sequence data strongly suggest that there are two diverse and highly differentiated P atagonian populations. The low nucleotide diversity found in the S . eubayanus moiety of hybrid E uropean brewing strains suggests that their alleles were drawn from a small subpopulation that is closely related to one of the P atagonian populations. For the first time, we also report the rare isolation of S . eubayanus outside P atagonia, in W isconsin, USA . In contrast to the clear population differentiation in P atagonia, the N orth A merican strains represent a recent and possibly transient admixture of the two P atagonian populations. These complex and varied reticulation events are not adequately captured by conventional phylogenetic methods and required analyses of B ayesian concordance factors and phylogenetic networks to accurately summarize and interpret. These findings show how genetically diverse eukaryotic microbes can produce rare but economically important hybrids with low genetic diversity when they migrate from their natural ecological context.