Introduction to Fungi, Third Edition

UREDINIOMYCETE YEASTS
667
Fig 24.4 Microscopic features of Phaffia rhodozyma (a) and
Xanthophyllomyces (b,c), a closely related teleomorph.
(a) Vegetatively dividing yeast cells. (b) Thick-walled
chlamydospore in the process of germination bybudding about
3 h after transfer to a fresh medium. (c) Aseptate elongate
basidia of Xanthophyllomyces sp. producing basidiospores at
their tips. All images to same scale.
complex of several species which are difficult to
distinguish by any means other than DNA
sequencing (Bai et al., 2002). Rhodotorula spp.
also reproduce vegetatively by budding but are
unable to produce ballistoconidia. Apart from
this difference, the two genera are very close to
each other, overlapping in phylogenetic trees
and also possessing similar life cycles.
Life cycle
The budding phase of Sporobolomyces roseus is
uninucleate, and nuclear division occurs at the
time of bud formation (Buller, 1933). If ballistoconidia
are to be formed, a conical sterigma
develops vertically and bears an asymmetric
spore which strongly resembles a basidiospore
(Figs. 24.1e and 24.5). A daughter nucleus passes
into this spore. Following the fusion of Buller’s
drop with the adaxial blob, the spore is flicked
away for a distance of about 0.1 mm. Colonies of
S. roseus begin to form ballistoconidia after
2 3 days on most agar media, and they are
recognized by the presence of numerous satellite
colonies outside of the margin of the parent
colony. A single sterigma may produce a second
or even a third spore, and occasionally two or
three sterigmata arise from one cell.
Although S. roseus can form pseudohyphae
and true hyphae, no sexual reproduction has
been observed. However, a related species,
Sporidiobolus (Sporobolomyces, Aessosporon) salmonicolor
(formerly also called S. odorus), does undergo
the full sexual cycle (Fig. 24.7). This species is
heterothallic with a bipolar (unifactorial) mating
system (Fell & Statzell-Tallman, 1981). Following
conjugation between compatible haploid uninucleate
cells, a dikaryotic mycelium with clamp
connections develops and eventually produces
globose binucleate teliospores (Fig. 24.6; Bandoni
et al., 1971). Karyogamy follows. Meiosis occurs
during teliospore germination, which gives
rise to an aseptate metabasidium. This buds
off haploid monokaryotic basidiospores at its
tip. Life cycles of this kind have also been
described in some other members of the
Sporidiales, with minor variations such as
the presence of transverse septa in the metabasidia,
or the absence of ballistoconidia (see
Fig. 24.1f).
There is, however, a complication in the life
cycle of S. salmonicolor because, according to
van der Walt (1970), yeast cells may be haploid
or diploid. The diploid cells are larger than the
haploid ones. Both types are capable of reproducing
by budding and by producing ballistoconidia,
but only the diploid cells can additionally
develop directly into thick-walled teliospores.
These germinate by means of a short aseptate
promycelium producing yeast-like basidiospores
at its tip (Fig. 24.7).
According to Fell and Statzell-Tallman (1998),
a similar life cycle is found in Rhodosporidium
toruloides (anamorph Rhodotorula glutinis). In this
species, diploid yeast cells are thought to arise if
there is a failure of meiosis during teliospore
germination. In contrast to the observations by