Introduction to Fungi, Third Edition

SCHIZOSACCHAROMYCETALES
255
important in agglutination, and fimbriae have
been observed at the surface of cells stimulated
by the appropriate pheromone (Johnson et al.,
1989). Using stable heterothallic haploid strains,
the purification of the pheromones of S. pombe
has been achieved; both are linear oligopeptide
hormones (Davey, 1992; Imai & Yamamoto, 1994).
The binding of a hormone released by cells of one
mating type to receptors in the membranes of
cells of opposite mating type initiates a signalling
chain which in turn triggers the cellular
response leading to agglutination and conjugation
(Davey, 1998). The principle of mating
factors will be discussed in more detail for
Saccharomyces cerevisiae (p. 266).
During agglutination, two cells come into
contact by a portion of their cell wall. In homothallic
strains, the fusing cells are often sister
cells formed by preceding mitotic division.
A pore is formed in the centre of the attachment
area and this widens and elongates to form a
conjugation canal (Fig. 9.3a). During this process
the nuclei, one from each cell, migrate towards
each other and fuse. Vacuoles may appear in the
young ascus following nuclear fusion. The fused
nucleus elongates and may reach half the length
of the ascus, and then divides by constriction,
the nuclear membrane remaining intact during
division. The two daughter nuclei migrate to
opposite ends of the ascus and divide further.
These two divisions constitute meiosis. A single
mitotic division usually follows so that eight
haploid nuclei result, and eight ascospores are
finally differentiated (Tanaka & Hirata, 1982).
Four-spored asci are also common. The ascospores
are released passively following disintegration
of the ascus wall.
The life cycle of Schizosaccharomyces (Fig. 9.4) is
thus interpreted as being based on haploid
vegetative cells which fuse to form asci, the
only diploid cells. Meiosis in the ascus restores
the haploid condition. Some variation in this
pattern may occur. For example, in S. japonicus
and S. pombe, limited division of the zygote in the
diploid state before ascospore formation may
take place, and it is possible to select diploid
strains (Tange & Niwa, 1995).
Physical and chemical analyses of the
cell walls of Schizosaccharomyces show that they
are principally composed of a b-(1,3)-glucan
with b-(1,6)-branches making up 50 54% of
the total cell wall carbohydrates, and of an
a-(1,3)-glucan with a-(1,4)-branches contributing
28 32%. There are also trace amounts of a
branched b-(1,6)-glucan (Manners & Meyer, 1977;
Kopecka et al., 1995). The b-(1,3)-glucan synthase is
involved in all aspects of wall synthesis, including
polarized growth, septum formation, and the
formation and germination of ascospores (Cortés
et al., 2002). A galactomannan linked to wall
matrix glycoprotein makes up about 9 14% of
the cell wall polysaccharides (Manners & Meyer,
1977). As in other Archiascomycetes, chitin is
present only in traces (Sietsma & Wessels, 1990),
but it seems to play an important role in
ascospore formation (Arellano et al., 2000). The
walls of ascospores contain amylose and give a
blue reaction with iodine.
Fig 9.4 The life cycle of the
homothallic yeast
Schizosaccharomyces octosporus.
Small open circles represent
haploid nuclei; diploid nuclei are
larger and split. Key events in the
life cycle are plasmogamy (P),
karyogamy (K) and meiosis (M).