Evolutionary relationships between Saccharomyces cerevisiae and other fungal species as determined from genome comparisons

The increasing number of fungal genomes whose sequence has been completed permits their comparison both at the nucleotide and protein levels. The information thus obtained improves our knowledge on evolutionary relationships between fungi. Comparison of the Saccharomyces cerevisiae genome with other Hemiascomycetes genomes confirms that a whole-genome duplication occurred before the diversification between Candida glabrata and the Saccharomyces sensu stricto species and after separation from the branch leading to the other Hemiascomycetes. Duplication was followed by individual gene losses and rearrangements affecting extensive DNA regions. Although S. cerevisiae and C. glabrata are two closely related yeast species at an evolutionary scale, their different habitats and life styles correlate with specific gene differences and with more extensive gene loses having occurred in the parasitic C. glabrata. At a closer evolutive scale, diversification among the sensu stricto species began with nucleotide changes at the intergenic regions affecting sequences that are not relevant for gene regulation, together with more extensive genome rearrangements involving transposons and telomeric regions. One important characteristic of fungal genomes that is shared with other eukaryotes is the fusion of gene sequences coding for separate protein modules into a single open reading frame. This allows diversification of protein functions while saving gene information.     

Variability of HXT2 at the protein and gene level among the Saccharomyces sensu stricto group

Variability of HXT2 at the protein and gene level was investigated among Saccharomyces sensu stricto and other yeast species. Results showed that the HXT2 gene is probably present in yeast genera other than Saccharomyces, suggesting that this gene is widely distributed in the yeast world. Chromosomal analyses indicated the stable location of HXT2 on the same chromosome and with the same copy number throughout the entire sensu stricto group. Results of the immunoblotting assay demonstrated that all strains tested (with the exception of S. cerevisiae DBVPG 6042) exhibited a lower level of Hxt2p expression than that shown by laboratory wild-type. Moreover, Hxt2p expression seems to reinforce the taxonomical differences between the two pairs of species (S. cerevisiae and S. paradoxus vs. S. pastorianus and S. bayanus) within the sensu stricto group of the genus of Saccharomyces that also reflect their different ecological niche.     

Diversity in organization and the origin of gene orders in the mitochondrial DNA molecules of the genus Saccharomyces

Sequencing of the Saccharomyces cerevisiae nuclear and mitochondrial genomes provided a new background for studies on the evolution of the genomes. In this study, mitochondrial genomes of a number of Saccharomyces yeasts were mapped by restriction enzyme analysis, the orders of the genes were determined, and two of the genes were sequenced. The genome organization, i.e., the size, presence of intergenic sequences, and gene order, as well as polymorphism within the coding regions, indicate that Saccharomyces mtDNA molecules are dynamic structures and have undergone numerous changes during their evolution. Since the separation and sexual isolation of different yeast lineages, the coding parts have been accumulating point mutations, presumably in a linear manner with the passage of time. However, the accumulation of other changes may not have been a simple function of time. Larger mtDNA molecules belonging to Saccharomyces sensu stricto yeasts have acquired extensive intergenic sequences, including guanosine-cytosine-rich clusters, and apparently have rearranged the gene order at higher rates than smaller mtDNAs belonging to the Saccharomyces sensu lato yeasts. While within the sensu stricto group transposition has been a predominant mechanism for the creation of novel gene orders, the sensu lato yeasts could have used both transposition- and inversion-based mechanisms.     

Evidence for multiple interspecific hybridization in Saccharomyces sensu stricto species

Fluorescent amplified fragment length polymorphism analysis demonstrates a high level of gene exchange between Saccharomyces sensu stricto species, with some strains having undergone multiple interspecific hybridization events with subsequent changes in genome complexity. Two lager strains were shown to be hybrids between Saccharomyces cerevisiae and the alloploid species Saccharomyces pastorianus. The genome structure of CBS 380(T), the type strain of Saccharomyces bayanus, is also consistent with S. pastorianus gene transfer. The results indicate that the cider yeast, CID1, possesses nuclear DNA from three separate species. Mating experiments show that there are no barriers to interspecific conjugation of haploid cells. Furthermore, the allopolyploid strains were able to undergo further hybridizations with other Saccharomyces sensu stricto yeasts. These results demonstrate that introgression between the Saccharomyces sensu stricto species is likely.     

Sequence diversity, reproductive isolation and species concepts in Saccharomyces

Using the biological species definition, yeasts of the genus Saccharomyces sensu stricto comprise six species and one natural hybrid. Previous work has shown that reproductive isolation between the species is due primarily to sequence divergence acted upon by the mismatch repair system and not due to major gene differences or chromosomal rearrangements. Sequence divergence through mismatch repair has also been shown to cause partial reproductive isolation among populations within a species. We have surveyed sequence variation in populations of Saccharomyces sensu stricto yeasts and measured meiotic sterility in hybrids. This allows us to determine the divergence necessary to produce the reproductive isolation seen among species. Rather than a sharp transition from fertility to sterility, which may have been expected, we find a smooth monotonic relationship between diversity and reproductive isolation, even as far as the well-accepted designations of S. paradoxus and S. cerevisiae as distinct species. Furthermore, we show that one species of Saccharomyces--S. cariocanus--differs from a population of S. paradoxus by four translocations, but not by sequence. There is molecular evidence of recent introgression from S. cerevisiae into the European population of S. paradoxus, supporting the idea that in nature the boundary between these species is fuzzy.     

The Enrichment of TATA Box and the Scarcity of Depleted Proximal Nucleosome in the Promoters of Duplicated Yeast Genes

Population genetic theory of gene duplication suggests that the preservation of duplicate copies requires functional divergence upon duplication. Genes that can be readily modified to produce new gene expression patterns may thus be duplicated often. In yeast, genes exhibit dichotomous expression patterns based on their promoter architectures. The expression of genes that contain TATA box or occupied proximal nucleosome (OPN) tends to be variable and respond to external signals. On the other hand, genes without TATA box or with depleted proximal nucleosome (DPN) are expressed constitutively. We find that recent duplicates in the yeast genome are heavily biased to be TATA box containing genes and not to be DPN genes. This suggests that variably expressed genes, due to the functional organization in their promoters, have higher duplicability than constitutively expressed genes.     

Saccharomyces uvarum, a proper species within Saccharomyces sensu stricto

Saccharomyces uvarum is proposed as a proper species within the complex Saccharomyces sensu stricto. Molecular characteristics including the similarity of the restriction profile of the non-transcribed spacer 2 (NTS2) and of the D1/D2 sequences of the rDNA, as well as other genotypic and phenotypic characteristics confirm that this group of strains is highly homogeneous and distinguishable from other species of the Saccharomyces sensu stricto group.     

Identification of Saccharomyces sensu stricto and Torulaspora yeasts by PCR ribotyping

18S rDNA + ITS1 and 25S rDNA PCR products covering more than 95% of the nuclear ribosomal DNA repeat unit of 28 Saccharomyces sensu stricto and Torulaspora yeasts and their anamorph forms were digested with Hae III, Msp I, Hinf I and Cfo I. Using combinations of two restriction enzymes, specific ribotyping patterns of six species were found. PCR ribotyping offers a convenient tool for quick identification of yeast isolates, but specificity of ribotyping patterns should be checked with a larger number of strains to avoid misidentification because of lack of variation within different taxa or because of strain-specific ribotyping patterns of species type strains.     

Identification and characterization of Saccharomyces cerevisiae and Saccharomyces paradoxus strains isolated from Croatian vineyards

AIMS: The identification, differentiation and characterization of indigenous Saccharomyces sensu stricto strains isolated from Croatian vineyards and the evaluation of their oenological potential. METHODS AND RESULTS: A total of 47 Saccharomyces sensu stricto strains were isolated from Chardonnay grapes and identified by physiological and molecular genetic methods. By using the standard physiological and biochemical tests, six isolates were identified as Saccharomyces cerevisiae and 41 as Saccharomyces paradoxus. However, PCR-RFLP analyses of the internal transcribed spacer (ITS1) region of the 18S ribosomal DNA identified 12 of the isolates as S.cerevisiae and 35 as S. paradoxus. Fermentation trials in a grape juice medium showed that these isolates ferment vigorously at 18 degrees C and display tolerance to high levels of ethanol. None of these isolates appeared to produce either hydrogen sulphide or killer toxins. CONCLUSION: Saccharomyces paradoxus, possessing potentially important oenological characteristics, occurs in much higher numbers than S. cerevisiae in the indigenous population of Saccharomyces sensu stricto strains in Croatian vineyards. SIGNIFICANCE AND IMPACT OF THE STUDY: This study forms an essential step towards the preservation and exploitation of the hidden oenological potential of the untapped wealth of yeast biodiversity in the Croatian grape-growing regions. The results obtained demonstrate the value of using molecular genetic methods, such as PCR-RFLP analyses, in conjunction with the traditional taxonomic methods based on phenotypic characteristics in such ecotaxonomic surveys. The results also shed some light on the ecology and oenological potential of S.paradoxus, which is considered to be the natural parent species of the domesticated species of the Saccharomyces sensu stricto group.     

A rapid method for differentiating Saccharomyces sensu stricto strains from other yeast species in an enological environment

During programs for the selection of enological yeasts, several hundred natural isolates are usually screened. The scope of these operations is to isolate strains possessing good fermentative properties without necessarily arriving at a precise species designation: in other words, to detect strains belonging to the Saccharomyces sensu stricto complex. In the present study, a pair of primers, designed within the variable D1/D2 region of the 26S subunit of ribosomal yeast RNA, have been constructed. These generate an amplification fragment of 471 bp that is specific for the seven Saccharomyces sensu stricto species, while no signal was obtained for Saccharomyces sensu lato strains (17 species) or for another 18 selected species commonly found in enological environments. A second pair of primers was also constructed, within the 18S rRNA gene, composed of perfectly conserved sequences common for all 42 yeast species examined, which generate a 900 bp (c.) band for all strains. This was used as a positive experimental control in multiplex PCR analysis using all four primers.