Introduction to Fungi, Third Edition

INTRODUCTION
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Fig 24.1 Basidiomycete yeasts
as seen by light microscopy.
(a) Budding in Dioszegia sp.
(Heterobasidiomycetes).The yeast
cells are isodiametric with a slightly
pointed bud region. (b) Budding
cells of Rhodotorulaglutinis
(Urediniomycetes).This species
produces a prominent polysaccharide
capsule which has been revealed
by mounting yeast cells in Indian
ink. (c) Collarette at the site of
repeated bud formation in
Sakaguchia dacryoidea
(Urediniomycetes). (d) Annellidic
structure formed by a member of
the Dioszegia hungarica group
(Heterobasidiomycetes).
(e) Ballistoconidium formation
in Sporobolomycesroseus
(Urediniomycetes). (f) Septate
stalked basidium arising from a
teliospore of Sporidiobolus ruineniae
(Urediniomycetes). (a e) to same
scale.Original print of f kindly
provided by J. W. Fell.
caused by filamentous fungi (Janisiewicz &
Korsten, 2002).
Detailed descriptions of basidiomycete yeasts
along with their ascomycete counterparts may
be found in Kurtzman and Fell (1998) and
Barnett et al. (2000). A useful general introduction
to the topic is that by Fell et al. (2001).
24.1.2 Morphology and life cycles
Asexual reproduction is mainly by budding. In
contrast to ascomycete yeasts, the budding sites
are confined to either or both poles of the
vegetative cells which are usually ellipsoid or
elongated (Figs. 24.1b,d) but may be isodiametric
(Fig. 24.1a). Yeast cells may be coated by a
prominent polysaccharide capsule (Fig. 24.1b),
giving the culture a slimy appearance. Repeated
budding at the same site may lead to the
formation of collarettes (Fig. 24.1c) or even
annellide-like structures (Fig. 24.1d). The formation
of daughter cells is enteroblastic, i.e. only
the inner wall of the mother cell extends to form
the wall of the daughter cell. The whole budding
yeast cell can thus be viewed as a phialide. This is
in contrast to most ascomycete yeasts which bud
in a holoblastic manner (i.e. the daughter wall is
continuous with the entire wall of the mother
cell). Given a little practice, it is often possible
with the light microscope to recognize a basidiomycete
yeast as such. Transmission electron
microscopy has revealed differences between
the two-layered cell walls of ascomycetes and
the multi-layered lamellate walls of basidiomycete
yeasts (Kreger-van Rij & Veenhuis, 1971).
These differences have been correlated with the
reaction of basidiomycete but not ascomycete
yeasts with the diazonium blue B stain (Simmons
& Ahern, 1987).
Apart from budding, several genera of basidiomycete
yeasts such as Sporobolomyces and its
teleomorph Sporidiobolus (Urediniomycetes) or
Bullera and its teleomorph Bulleromyces