Introduction to Fungi, Third Edition

666 BASIDIOMYCETE YEASTS
and Xanthophyllomyces represent the same or
closely related species (Fell & Blatt, 1999;
Kucsera et al., 2000). Xanthophyllomyces appears
to be homothallic, vegetative cells being diploid.
Mating can occur between a mother cell and its
bud, giving rise to a tetraploid zygote (Kucsera
et al., 1998). A long thin aseptate metabasidium is
formed, and the fusion nucleus migrates to
the tip of it, undergoing meiosis in the course
of the journey. About 2 7 diploid basidiospores
are produced and released passively (Fig. 24.4c).
Vegetative reproduction is by budding (Fig.
24.4a), but the nuclear events are unusual in
that the mother nucleus migrates into the bud
and divides there, followed by the return of one
of the daughter nuclei to the mother cell
(Slaninova et al., 1999). Thick-walled chlamydospores
(Fig. 24.4b) are occasionally formed, and
these germinate by mitotic budding, i.e. they are
not equivalent to the teliospores of other
basidiomycete yeasts. The life cycle of
Xanthophyllomyces rhodozyma is summarized in
Fig. 24.3. In general, there is a considerable
variation between different isolates; for instance,
the number of chromosomes ranges from 7 to 17
(Kucsera et al., 2000).
24.3 Urediniomycete yeasts
Yeasts belonging to the Urediniomycetes appear
to fall into four phylogenetic groups (see
Table 24.1), some of which still need to be
integrated into higher taxa (families and
orders). Many species contain carotenoids, i.e.
they are red yeasts. Some of them, especially
species belonging to the Sporodiobolus clade
(provisional order Sporidiales), are extremely
common in the environment. For this reason,
we shall consider them here. There are two
important genera in the Sporidiales, Sporidiobolus
(anamorph Sporobolomyces) and Rhodosporidium
(anamorph Rhodotorula).
24.3.1 Sporidiales
In addition to budding, Sporobolomyces spp. also
form asexual spores which are ejected from a
sterigma into the air by the surface-tension
catapult mechanism and are therefore called
ballistoconidia. If an agar culture is incubated
upside down, a mirror image of the colony will
be deposited as spores on the Petri dish lid. For
this reason, Sporobolomyces is known as a mirror
yeast. One of the most frequently encountered
species, S. roseus, is now known to represent a
Fig 24.3 Life cycle of
Xanthophyllomyces dendrorhous.
Vegetative cells are diploid and
reproduce by budding.The fungus
is homothallic, and fusion between
mother and daughter cell establishes
a tetraploid zygote which forms an
elongate aseptate basidium in which
meiosis occurs.The diploid
basidiospores germinate by budding.
Xanthophyllomyces can survive
unfavourable conditions by means
of thick-walled chlamydospores.
Half-filled circles represent diploid
nuclei; tetraploid nuclei are larger and
divided into four segments. Key events
in the life cycle are plasmogamy (P),
karyogamy (K) and meiosis (M).