Introduction to Fungi, Third Edition

10
Hemiascomycetes
10.1 Introduction
The class Hemiascomycetes contains the classical
ascomycete yeasts, exclusive of those which
belong to the Archiascomycetes (see the preceding
chapter) and the ‘black yeasts’ such as
Aureobasidium (see p. 486). Detailed descriptions
of the individual yeast genera and species are
given in Kurtzman and Fell (1998) and Barnett
et al. (2000). A useful taxonomic overview is that
by Kurtzman and Sugiyama (2001). There is only
one order, the Saccharomycetales, which has
been divided into 11 families and 276 species
(Kirk et al., 2001; Kurtzman & Sugiyama, 2001).
However, detailed phylogenetic analyses of the
Hemiascomycetes (Kurtzman & Robnett, 1998,
2003) indicate that this family arrangement is
likely to be modified in the future, and for this
reason we shall focus on selected genera.
The key feature that distinguishes the Hemiand
Archiascomycetes from the higher ascomycetes
(Euascomycetes) is that ascogenous
hyphae and an ascocarp, i.e. an investment of
sterile hyphae surrounding the asci, are lacking
in the first two groups. Instead, the asci are
formed freely and singly, either directly following
karyogamy or more rarely after a prolonged
diploid phase. Another distinguishing feature
is the composition of the cell wall, which
contains very little chitin in the Hemi- and
Archiascomycetes. Chitin is often confined to a
small ring around the site where the daughter
cell is produced (the bud scar). An ultrastructural
feature of distinction concerns the septal pore
of any hypha that may be produced. One or
several pores may be present, and these are
usually very small or plugged. They lack Woronin
bodies, in contrast to Euascomycete septa
which usually have only one large pore with
associated Woronin bodies (Alexopoulos et al.,
1996; M. E. Barr, 2001; see Fig. 8.3). Hence, the
Euascomycete septal pore permits passage of
organelles including nuclei (see Fig. 8.2), whereas
the micropore of the Hemiascomycete septum
does not. Cytoplasmic communication between
adjacent hyphal cells therefore does not seem
to be possible.
It is impossible to give a watertight set of
criteria by which Hemiascomycetes can be distinguished
from Archiascomycetes. The predominant
growth form of Hemiascomycetes in culture
as well as in nature is the yeast state, although
a limited mycelium or pseudomycelium may
also be present. Archiascomycetes may grow as
a mycelium in nature but as yeasts in the laboratory
(Taphrina, Protomyces). In Archiascomycetes,
asci may (Taphrina, Protomyces) or may not
(Pneumocystis, Schizosaccharomyces) forcibly discharge
their spores, whereas asci of Hemiascomycetes
generally have evanescent walls, i.e.
they release their ascospores passively.
In the absence of asci and ascospores, the
microscopic identification of yeasts is difficult
or impossible, and other methods have to
be employed, e.g. physiological tests based on
the ability of test species to grow on any of
a standard set of carbon or nitrogen sources