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Saccharomyces kudriavzevii hybrids

S. kudriavzevii was originally known from a few strains obtained from decayed leaves and soil in Japan (Naumov et al., 1995, 2000). Subsequently, a larger set of isolates was collected from oak trees in Europe (Sampaio and Gonsalves, 2008; Lopes et al., 2010; Erny et al., 2012) and from soil, leaves, and mushrooms in Taiwan (Naumov et al., 2013). Among the natural species of Saccharomyces, S. kudriavzevii can be considered a cryotolerant yeast since it can grow at low temperatures (=5°C) and grows poorly at 30°C (Belloch et al., 2008;

Sampaio and Gonsalves, 2008; Arroyo-Lopez et al., 2009). This is consistent with the observation that enrichment media incubated at low temperatures (10°C) readily yield S. kudriavzevii from oaks across Eurasia (Hittinger, 2013). By studying the glycolytic flux and the level of activity of individual enzymatic steps of the glycolysis, Gonsalves et al. (2011) concluded that S. kudriavzevii metabolism evolved towards a better performance at low temperatures, particularly in ethanol production, at the cost of heat resistance. Later, the metabolome at low temperatures was compared with the thermotolerant species S. cerevisiae and the main differences found were in carbohydrate metabolism, mainly in fructose metabolism (Lopez-Malo et al., 2013). Other metabolic data in favour of the cryotolerant nature of this species were a higher glycerol level and a lower ethanol content of S. kudriavzevii compared with S. cerevisiae at low temperatures (14°C), which was due to a higher glycerol-3-phosphate dehydrogenase activity (Arroyo-Lopez et al., 2010). Strains of S. kudriavzevii showed also higher percentages of medium-chain fatty acids and squalene regardless of the growth temperature in comparison to S. cerevisiae (Tronchoni et al., 2012). This differential lipid composition may partially explain the better adaptation of S. kudriavzevii at low temperatures. A similar comparison, made at the transcriptome level, led Tronchoni et al. (2014) to conclude that the cryotolerance of S. kudriavzevii is due to an enhanced ability to initiate a quick and efficient translation of crucial genes for cold adaptation (i.e. the cold stress marker gene NSR1 and lipid metabolism related genes).

The ethanol tolerance of S. kudriavzevii is relatively low in comparison with other Saccharomyces species, given that S. kudriavzevii shows weak or null growth above 5% ethanol (Belloch et al., 2008). Competitive exclusion of S. kudriavzevii by other mesophilic and/or more ethanol-tolerant Saccharomyces species has been experimentally demonstrated in laboratory mixed cultures (Sam- paio and Gonsalves, 2008; Arroyo-Lopez et al., 2011). The low stress tolerance and thus low competitiveness exhibited by S. kudriavzevii probably explains the lack of reports on the occurrence of this yeast in human-related fermentations or environments, with one exception - a few isolates from a brewery in New Zealand (Gonzalez et al., 2006). Hence, the absence of S. kudriavzevii in most brewing monographs is understandable. However, as already mentioned for S. eubayanus, S. kudria- vzevii might be relevant to brewing as a contributor to hybrid strains rather than as a pure lineage.

Indeed, hybrid strains combining the genomes of S. cerevisiae and S. kudriavzevii have been isolated and characterized from fermenting environments, mostly from wine and cider fermented at low temperatures (Groth et al., 1999; Lopandic et al.,

  • 2007) . More recently, similar hybrids have been associated to beer and seem to be common in Belgian-style beers (Gonzalez et al., 2008). With the implementation of genome sequencing studies, strains originally assumed to be S. cerevisiae are being recognized as S. cerevisiae x S. kudriavzevii hybrids. For example, Gonzalez et al. (2008) analysed the genomic composition of 24 brewing strains labelled as S. cerevisiae and found that 25% of them were in fact S. cerevisiae x S. kudriavzevii hybrids. Even triple hybrids of S. cerevisiae, S. uvarum, and S. kudriavzevii have been identified in cider and wine (Naumova et al., 2005; Gonzalez et al., 2006). On the other hand, Lopes et al. (2010) performed several genome-wide molecular analyses with European wild S. kudriavzevii strains and found intraspecific differences with respect to the Japanese population, in line with Hittinger et al. (2010), who showed that the two populations differed considerably in their GAL gene network. A European/Mediterranean origin for the brewing hybrids has been hypothesized (Gonzalez et al.,
  • 2008) , and molecular analyses of independently isolated hybrids lead to the conclusion that they are the result of multiple independent hybridization events (Erny et al., 2012; Peris et al., 2012). Together, these results suggest that an important fraction of brewing strains may correspond to S. cerevisiae x S. kudriavzevii hybrids.

In contrast with S. pastorianus, whose origin is directly connected with the brewing environment (see ‘Saccharomyces pastorianus and Saccharomyces carlsbergensis’, above), some authors believe that S. kudriavzevii hybrids might have originated in wild environments due to the low resistance of this species to the stress conditions of human-driven fermentations (Sipiczki, 2008; Arroyo-Lopez et al., 2011). However, despite the repeated isolation of S. kudriavzevii from natural sources, hybrid strains are only known from human-driven fermentations. Nevertheless, the role of S. eubayanus and S.

kudriavzevii subgenomes in the hybrids seems to be similar, guaranteeing a good fermentative performance at low temperatures. The hybrids retained by the industry exhibit the best properties of both parental species, such as the low-temperature fermentation abilities of S. kudriavzevii and the high ethanol resistance of S. cerevisiae (Giudici et al., 1998; Belloch et al., 2009; Arroyo-Lopez et al., 2009). The genomic contribution of S. kudriavzevii adds to the complexity of attributes provided by the genomes of S. cerevisiae and S. eubayanus in brewing environments and most probably to the properties of the final product. The limited data available on S. kudriavzevii x S. cerevisiae hybrids in brewing, starting from the low number of isolates so far analysed, precludes drawing conclusions about the prevalence of these newly discovered hybrids in beer, especially their actual contribution to the properties of the final product. Interestingly, half of the S. kudriavzevii x S. cerevisiae strains detected by Gonzalez et al. (2008) were recovered from Belgian specialty beers from Trappist monasteries (Trappist beers). Bottle re-fermentation or conditioning is a common practice in the production of these types of beers (van Landschoot et al., 2005), which allows adjusting and/or modifying the final flavour of beer, also known as bioflavouring (Van- derhaegen et al., 2003a). Further studies are needed to elucidate whether S. kudriavzevii hybrids have a role in the primary and/or secondary fermentation stages and to determine what is their actual contribution to the typical flavour complexity of Belgian specialty beers.

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