Introduction to Fungi, Third Edition

SACCHAROMYCES (SACCHAROMYCETACEAE)
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the enormous importance of S. cerevisiae for
fundamental cell biology. This fungus was the
first eukaryote (in 1996) to have its complete
genome sequenced, and this together with the
ease of genetic manipulation has further
enhanced the status of S. cerevisiae as a workhorse,
if not ‘model organism’, for eukaryote
research.
10.2.1 The life cycle of S. cerevisiae
Vegetative cells of S. cerevisiae are generally
diploid in nature, although tetraploid or aneuploid
strains also occur. Strains may be homoor
heterothallic. The chromosome number is 16
(Cherry et al., 1997). The life cycle of S. cerevisiae
is presented in Figs. 10.2 and 10.3. The haploid
ascospores often fuse within the ascus where
they were formed (Fig. 10.3d), or shortly after
release. However, if individual ascospores
become isolated, they can germinate and reproduce
as haploid cells by budding. Where two
haploid cells of opposite mating type are in close
contact, they secrete peptide hormones and produce
plasma membrane-bound receptors which
recognize the hormone of opposite mating type.
The binding of a hormone molecule to the
matching receptor sets a signalling chain in
motion (reviewed by Bardwell, 2004) which
arrests the mitotic cell cycle at G1, stimulates
transcription of mating-specific genes and
causes polarized growth of the two cells towards
each other (Leberer et al., 1997). Mating initially
involves an increased ability of the surfaces of
two cells to adhere to each other. This so-called
sexual agglutination is mediated by glycoproteins.
It seems that these are components of
fimbriae, i.e. long filaments radiating outwards
from the cell wall (see Fig. 23.15). Agglutination
is followed by co-ordinated digestion of the
walls separating the two cells. Plasmogamy and
karyogamy follow swiftly (Gammie et al., 1998).
The resulting diploid cell (Fig. 10.3e) can carry on
reproducing asexually by budding. In contrast
to Schizosaccharomyces pombe, there are therefore
two mitotic cycles in the life cycle of S. cerevisiae.
Under optimum conditions, the culture doubling
time by mitosis is about 100 min.
Diploid strains of S. cerevisiae can be induced
to form ascospores by suitable treatment, and
this yeast is therefore termed an ascosporogenous
yeast, in contrast to asporogenous yeasts
in which ascospores have not been observed.
Meiosis can be induced by growing the yeast on
a nutrient-rich presporulation medium containing
an assimilable sugar, a suitable nitrogen
source for good growth (nitrate is not utilized),
Fig10.2 The life cycle of S. cerevisiae. Both haploid and diploid cells can reproduce by budding.Open and closed circles represent
haploid nuclei of opposite mating type; diploid nuclei are larger and half-filled. Key events in the life cycle are plasmogamy (P),
karyogamy (K) and meiosis (M).