Introduction to Fungi, Third Edition

254 ARCHIASCOMYCETES
as saprotrophic yeasts which reproduce asexually
by fission, i.e. by division of a vegetative cell
into two daughter cells of equal size (Fig. 9.3a).
Schizosaccharomyces is therefore called the fission
yeast. Occasionally, especially in S. japonicus, true
septate hyphae can be formed, and these may
fragment into arthrospores. Sexual reproduction
is by conjugation of two haploid vegetative
yeast cells, followed by karyogamy and meiosis
which gives rise to four or, more usually, eight
ascospores (Figs. 9.3a,b).
Schizosaccharomyces can be isolated from substrates
rich in soluble carbon sources, e.g. tree
exudates, fruits, honey and fruit juices. The bestknown
species are S. octosporus and S. pombe. The
latter is the fermenting agent of African millet
beer (pombe) and arak in Java. It can tolerate
ethanol levels up to 7% (v/v). Both species grow
well in liquid culture or on solid media such as
malt extract agar, developing ripe asci within
3 days at 25°C. All stages of the life cycle can be
readily seen if a preparation of cells from an agar
culture of S. octosporus is made in water (Fig. 9.3).
Individual cells are globose to cylindrical,
uninucleate and haploid. Cell division is
preceded by intranuclear mitosis, towards the
end of which the nucleus constricts and becomes
dumb-bell shaped (Tanaka & Kanbe, 1986). The
division of the cell into two daughter cells is
brought about by the centripetal development of
a septum which cuts the cytoplasm into two. The
two sister cells may remain attached to each
other for a while, or may separate by breakdown
of a layer of material in the middle of the septum
(Sipiczki & Bozsik, 2000).
Ascus formation in S. pombe is preceded by
copulation. Schizosaccharomyces octosporus is homothallic,
and quite often adjacent sister cells
may fuse together. In the case of S. pombe, both
homothallic and heterothallic strains are known,
the latter with a bipolar mating system (hþ and
h mating types). When cells of opposite mating
type of S. pombe are grown together in liquid
culture, especially under conditions of nitrogen
starvation, a strong sexual agglutination
occurs. This clumping together of the cells
becomes visible as a flocculation of the culture.
Changes in cell surface properties are
Fig 9.3 Schizosaccharomyces octosporus.
(a) Vegetative cells, three of which showing
transverse division.Two cells to the right of
the picture are conjugating. (b) Four- and
eight-spored asci.