Evading domestication - evidence of ferality among diastatic S. cerevisiae

Certain lineages of the wine, beer and bread yeast Saccharomyces cerevisiae have diastatic activity. They contain the chimeric gene STA1 that codes for an extracellular glucoamylase which enables the strains to degrade starch and dextrins. Beer contaminations by diastatic yeasts can be dangerous because they can cause super- attenuation due to the consumption of otherwise non-fermentable oligosaccharides, gushing and off-flavours. Given that diastatic yeasts can be used for beer fermentation it is important to understand the relationship between production and contaminant diastatic yeasts, their natural reservoirs and entry routes into the brewery. Here we analyse real cases of contamination in a Portuguese craft brewery over a period of 18 months. By isolating and analysing with whole genome sequencing several contaminants, we show that recurrent contaminations by diastatic yeasts are caused by different environmental strains. Moreover, some beer contaminants were closely related with diastatic environmental strains isolated in Botswana. We observed thewidespread presence of domestication signatures in diastatic strains. Moreover, the combined phylogeny of STA1 and its ancestor, the gene SGA1, suggested a single STA1 origin, as ancient as the entire lineage of diastatic yeasts. Together our results suggest that diastatic yeasts are escaping from the domestication settings and becoming feral.