Introduction to Fungi, Third Edition

282 HEMIASCOMYCETES
form septa and quickly fragment into arthrospores
(Fig. 10.10), giving the colonies a creamy
appearance. The septa have micropores, like
those of true hyphae of C. albicans. Six species
are now known; by far the most important is
G. candidus (anamorph Geotrichum candidum),
formerly known as Galactomyces geotrichum (de
Hoog & Smith, 2004). It is a ubiquitous mould
which is common in soil, dairy products, sewage
and other substrata. It is also thought to be a
common constituent of the skin and gut flora
of humans and animals, although reports of
it being a human pathogen have generally
remained unsubstantiated (Kwon-Chung &
Bennett, 1992). In addition, G. candidus is a wellknown
cause of post-harvest rot in ripe fruits
and vegetables especially when these are kept
in plastic bags. Infected plant organs become
soft and eventually, upon puncturing, exude
a creamy mass of decaying tissue which has
a sour smell; hence the name ‘sour rot’ (Agrios,
2005).
The fungus grows readily in culture, forming
broad hyphae with finer lateral branches. The
vegetative cells contain 1 4 nuclei. Branching
is of two kinds, pseudodichotomous near the
apex, and lateral immediately behind a septum.
It is from such lateral branches that conidia
develop (Figs. 10.10a,b). Conidiophores are difficult
to differentiate from vegetative hyphae.
Prior to conidium formation, apical growth of
a hypha ceases, then septa are laid down in the
tip region. The septa are two-ply, and separation
of the two layers making up the septum
leads to the disarticulation of the terminal part
of a hypha into cylindrical segments termed
arthrospores or arthroconidia (Cole & Kendrick,
1969b). Conidia of other Galactomyces spp. are
virtually indistinguishable from those of
G. candidus.
Galactomyces candidus may be homo- or heterothallic,
but the sexual state is not frequently
seen. After mating, fertile hyphae are produced
and gametangia arise in pairs on either side of
a septum, in the broad main hyphae or short
side branches (Figs. 10.10c e). Fusion of the
gametangia gives rise to a globose fusion cell
which becomes transformed directly into an
ascus. The ascus contains only a single ascospore
which has two wall layers, a smooth inner layer
and a furrowed outer layer. Each ascospore
contains 1 2 nuclei. Whether and when meiosis
occurs is not yet known.
The Geotrichum arthroconidial state is found
also in the only other genus of the Dipodascaceae,
Dipodascus, which produces multispored
asci with 4 128 spores. A superficially similar
state, Saprochaete, is formed by a genus of phylogenetically
unrelated fungi now called Magnusiomyces.
Species descriptions and a key of
Galactomyces, Dipodascus and Magnusiomyces have
been provided by de Hoog and Smith (2004).
10.6 Saccharomycopsis
(Saccharomycopsidaceae)
Saccharomycopsis (formerly Endomycopsis) is a mycelial
yeast which reproduces by buds (blastospores
or yeast cells) and also forms asci parthenogenetically
or following isogamous fusion. About
10 species are known (Kurtzman & Smith, 1998;
Barnett et al., 2000). Saccharomycopsis fibuligera
grows in flour, bread, macaroni and other
starchy substrates, and produces a complex of
numerous active extracellular amylases, an
unusual property in yeasts (Hostinová, 2002).
This has been used to develop S. fibuligera as
a food yeast for cattle feed which can be grown
on potato starch processing wastes (Jarl, 1969).
This species is also used extensively for starch
hydrolysis by starter cultures in Far Eastern
fermented food (Beuchat, 1995).
In culture, S. fibuligera may form budding
yeast cells and branched septate hyphae which
produce blastospores laterally and terminally
(Fig. 10.11). Arthrospore formation has also
been demonstrated. Ascus formation in this
homothallic species can be induced by growing
the yeast for a few days on malt extract agar
and transferring it to distilled water. The asci
are mostly four-spored, and the spores are
hat-shaped (Fig. 10.11d), having a flange-like
extension of the wall.