Saccharomyces cerevisiae cells are generally ellipsoidal in shape, the vegetative reproduction is by multilateral budding and its vegetative phase is predominantly diploid, the only haploid stage is the ascospore. S. cerevisiae has a relatively small genome, a large number of chromosomes, little repetitive DNA, and few introns. Haploid strains contain approximately 12-13 megabases of nuclear DNA, distributed along 16 linear chromosomes whose sizes vary from 250 to 2000kb (Barre et al. 1992; Pretorius 2000). Most S. cerevisiae strains used in the laboratory are either haploid or diploid and have a defined set of chromosome lengths. However, wine strains are mainly diploid, aneuploid, or polyploid (Codon and Benitez 1995). Aneuploidy and polyploidy may confer advantages to adapt to variable external environments or, perhaps, is a way to increase the dosage of some genes important for fermentation (Bakalinsky and Snow 1990; Salmon 1997). The meiotic segregants from wine strains diploidise with high frequency, indicating a high frequency of homothallism. Heterozygosity has been observed in both homothallic and heterothallic strains (Barre et al. 1992; Codon and Benitez 1995).
Wine yeasts also show a high level of chromosome length polymorphism (Bidenne et al. 1992; Rachidi et al. 1999). This polymorphism is generated mainly by illegitimate recombination mediated by Ty transposons or subtelomeric repeated sequences. This feature has several practical consequences: sporulation ability is very variable, between 0 and 75% ascus formation can be observed on a sporulation medium and spore viability is also highly variable, ranging from 0 to 98% (Codon and Benitez 1995; Mortimer et al. 1994).
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