Introduction to Fungi, Third Edition

USTILAGINOMYCETE YEASTS
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Fig 24.8 Examples of the effect of aeration on carotenoid production in red yeasts belonging to the Basidiomycota. In most red
yeasts, carotenoid biosynthesis progresses via lycopene at least to the first end-group cyclization to give g-carotene. Several yeasts
adjust their carotenoid spectrum in response to the level of oxidative stress in their environment. In Sporobolomyces and Rhodotorula
spp. (Urediniomycetes), g-carotene and/or the bicyclic b-carotene accumulate under microaerophilic conditions (dotted arrow),
whereas torulene and oxygen-containing carotenoids (xanthophylls) such as torularhodin are produced under oxidative stress
(solid arrows). In contrast, in Phaffia rhodozyma (Heterobasidiomycetes) the b-carotene produced at low partial oxygen pressure
is directly oxidized to bicyclic xanthophylls (notably astaxanthin) under oxidative stress.
Here we shall briefly consider a third group
of Ustilaginomycetes, the Malasseziales, which
colonize the skin of warm-blooded animals,
i.e. mammals and birds. Good accounts of
this order have been written by Ashbee and
Evans (2002), Crespo Erchiga and Delgado
Florencio (2002), Inamadar and Palit (2003) and
Batra et al. (2005).
24.4.1 Malasseziales
This group has been named after the French
pathologist Louis C. Malassez who was one of the
pioneers in this field. Following an eventful
taxonomic history, there is now only one
genus, Malassezia (formerly Pityrosporum) with
seven species (Guillot & Guého, 1995; Guého
et al., 1996). All of them are haploid without
known sexual stages.
The yeast cells show several unusual characteristics
by which they can be recognized.
Budding is enteroblastic, with the daughter cell
arising in a unipolar fashion from an exceptionally
broad ring-like bud scar of the mother cell
(Guého & Meyer, 1989). The ultrastructure of
Malassezia is most unusual. The cell wall is thick,
with its inner surface sculptured into spiral
ridges (Fig. 24.9) which cast the plasma
membrane into corresponding grooves (David
et al., 2003). There is considerable morphological
plasticity in some Malassezia spp., notably
M. furfur, in that the shape of yeast cells may
change from globose to elongated after repeated
subculturing. Hyphae may also be formed under
certain conditions.
All but one species (M. pachydermatis) are
obligately lipophilic and are isolated from skin
samples if standard agar media are overlaid with
a thin film of olive oil. It is possible to isolate
most Malassezia spp. from the skin of humans
and other animals, with M. pachydermatis