Introduction to Fungi, Third Edition

660 BASIDIOMYCETE YEASTS
(Heterobasidiomycetes) also produce conidia
which are actively liberated in the manner of
basidiospores, i.e. by means of the surfacetension
catapult mechanism involving Buller’s
drop (Fig. 24.1e). Since these are asexual propagules,
they are called ballistoconidia. Their
existence provided one of the first clues that
the yeasts producing them belong to the Basidiomycota
(Kluyver & van Niel, 1927).
Sexual reproduction is relatively rare in
basidiomycete yeasts except, of course, for those
species which have dominant mycelial diploid or
dikaryotic stages such as the smut fungi
discussed in Chapter 23, or jelly fungi and allies
(Chapter 21). Following mating between compatible
yeast cells, a limited dikaryotic mycelium
often bearing clamp connections may arise, and
this produces basidia either directly or, more
commonly, via thick-walled resting cells called
teliospores. There, karyogamy occurs, i.e. the
teliospores function as probasidia. Teliospores
germinate in a manner described in detail for
rust and smut fungi, namely by the production
of a promycelium (¼ metabasidium) which
may or may not undergo transverse septation.
The basidiospores thus produced germinate by
budding as yeast cells (Fig. 24.1f).
Where present, septa can be examined by
transmission electron microscopy for their ultrastructure,
and this is an important feature in
classification. In general, the septa of yeasts
belonging to Urediniomycetes and Ustilaginomycetes
contain simple pores whereas those of
heterobasidiomycete yeasts have dolipores, often
with a parenthesome (Fell et al., 2001).
24.1.3 Phylogeny of basidiomycete yeasts
In addition to examining morphological features
as outlined above, several biochemical tests can
be performed to characterize yeasts. Such tests,
e.g. carbon source utilization, the identity of
coenzyme Q or the spectrum of killer toxins
produced, have been employed extensively in
the past to identify yeasts and are still relevant
today (Yarrow, 1998; Fell et al., 2001). However, the
results are rarely clear-cut, and taxonomic confusion
has resulted due to the extensive overlap
of features between members of different taxa.
Hence, the names of many basidiomycete yeasts
are of descriptive rather than taxonomic value
and may be found in several phylogenetically
distinct clades (Table 24.1). For instance, the
main feature to distinguish Sporobolomyces from
Rhodotorula is the presence or absence (respectively)
of ballistoconidia, and it is now known
that these character states, and hence the two
generic names, are of little taxonomic relevance.
Major work is currently being carried out in
order to establish phylogenetically coherent
groups of yeasts, using especially ribosomal
DNA but also increasingly other gene sequences.
Valuable recent contributions to phylogeny at
higher taxonomic levels are those by Fell et al.
(2000, 2001) and Scorzetti et al. (2002) in which
the heterobasidiomycete, urediniomycete and
ustilaginomycete clades of yeasts have been
circumscribed (Table 24.1). The integration of
some of these clades into the taxonomy of
filamentous basidiomycetes has yet to be accomplished.
Additionally, numerous publications
have dealt with the analysis of taxa at the level
of genus or species. Hence, although it is
generally estimated that only 1 5% of all
basidiomycete yeasts have been discovered as
yet, a large database of DNA sequences is already
available. A convenient side effect of this work
is that the identification of new isolates or at
least their assignment to a given family or genus
is a relatively straightforward matter if their
rDNA sequences can be obtained. Biochemical
tests and microscopy can then be used to verify
and extend the identification.
24.2 Heterobasidiomycete yeasts
This is a large and morphologically diverse
group of yeasts. We shall focus on two species
which have been particularly well examined, the
serious human pathogen Filobasidiella neoformans
and the astaxanthin producer Phaffia rhodozyma.
Trichosporon spp., which are occasional opportunistic
human pathogens, are not further
discussed. They show a diversity of growth
forms, including hyphae, pseudohyphae, yeast
cells, blastoconidia and arthroconidia.