To overcome these difficulties, researchers have developed a molecular taxonomy based on the following tests: DNA–DNA reassociation, the similarity of DNA base composition, the similarity of enzymes, ultrastructure characteristics, and cell wall composition. The development of DNA sequence analysis has profoundly changed the way in which yeasts are identified. More specifically, the comparison of gene sequences coding for ribosome RNA now resolves the taxonomic position of closely related species, as well as the most distantly related taxa. The sequence analysis of variable domains 1 and 2 (D1/D2) has been successfully applied to the identification of ascomycetous yeasts (Kurtzman and Robnett, 1998). Then, Fell et al. (2000) developed the method by bringing in an additional database to identify basidiomycetous yeasts. Moreover, analysis of internal transcribed spacer (ITS) sequences is often used to identify species (Granchi et al., 1999). Currently, the sequencing of all genomes is used to refine the taxonomy of yeasts in phylogenomic studies.
1.8.3 Successive Classifications of the Genus Saccharomyces and the Position of Wine Yeasts in the Current Classification
Due to many changes in yeast classification and nomenclature since the beginning of taxonomic studies, wine‐related yeast names and their positions in the classification have often changed. This has inevitably resulted in some confusion for winemakers. Even the most recent enological textbooks (Fleet, 1993; Delfini, 1995; Boulton et al., 1995) use a number of species names (cerevisiae, bayanus, uvarum, etc.) attached to the genus name Saccharomyces to designate yeasts responsible for alcoholic fermentation. Although still in use, this enological terminology is no longer accurate to designate the species currently delimited by taxonomists.
The evolution of species classification for the genus Saccharomyces since the early 1950s (Table 1.3) has created this difference between the naming of wine yeasts and current taxonomy. By taking a closer look at this evolution, the origin of the differences may be understood.
In Lodder and Kregger‐Van Rij (1952), the names cerevisiae, oviformis, bayanus, uvarum, etc., referred to a number of the 30 species of the genus Saccharomyces. Ribéreau‐Gayon and Peynaud (1960) considered that two principal fermentation species were found in wine: S. cerevisiae (formerly called ellipsoideus) and Saccharomyces oviformis, which was encountered especially toward the end of fermentation and was considered more ethanol resistant. The difference in their ability to ferment galactose distinguished the two species: S. cerevisiae (Gal+) fermented galactose, whereas S. oviformis (Gal−) did not. According to the same authors, the species Saccharomyces bayanus was rarely found in wines. Although it possessed the same physiological fermentation and sugar assimilation characters as S. oviformis, its cells were more elongated, its fermentation was slower, and it displayed specific behavior toward growth factors. As for the species Saccharomyces uvarum, identified in wine by many authors, it differed from S. cerevisiae, S. oviformis, and S. bayanus because it could ferment melibiose.
In Lodder's following edition (1970), the number of species of the genus Saccharomyces increased from 30 to 41. Some species formerly grouped with other genera were integrated into the genus Saccharomyces. Moreover, several species names were considered to be synonyms and disappeared altogether. Such was the case of S. oviformis, which was moved to the species S. bayanus. Ribéreau‐Gayon et al. (1975) considered, however, that the distinction between S. oviformis and S. bayanus was of enological interest because of the different technological characteristics of these two yeasts. Nevertheless, by the early 1980s, most enology texts had abandoned the name S. oviformis and replaced it with S. bayanus.
TABLE 1.3 Evolution of the Nomenclature for the Saccharomyces Genus, 1952–2011 (Libkind et al. 2011)
Source : From Barnett et al. ( 2000), Lodder and Kregger‐Van Rij ( 1952), Lodder ( 1970), Kregger‐Van Rij ( 1984), and Kurtzman et al. ( 2011).
| 1952 | 1970 | 1984 | 2011 |
|---|---|---|---|
| Saccharomyces cerevisiae ⇒ Saccharomyces pastorianus Saccharomyces bayanus ⇒ Saccharomyces oviformis Saccharomyces logos Saccharomyces chevalieri ⇒ Saccharomyces fructuum Saccharomyces lactis Saccharomyces elegans Saccharomyces heterogenicus Saccharomyces fermentati Saccharomyces mellis Saccharomyces italicus ⇒ Saccharomyces steineri Saccharomyces pastori Saccharomyces carlsbergensis Saccharomyces uvarum ⇒ |
Saccharomyces cerevisiae Saccharomyces aceti Saccharomyces bayanus Saccharomyces capensis Saccharomyces prostoserdovii Saccharomyces chevalieri Saccharomyces coreanus Saccharomyces diastaticus Saccharomyces globosus Saccharomyces heterogenicus
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