Introduction to Fungi, Third Edition

670 BASIDIOMYCETE YEASTS
Fig 24.7 Possible life cycle of Sporidiobolussalmonicolor.Both diploid stages (large split nuclei) and haploid stages (small black or white
nuclei) exist and can reproduce by budding and ballistoconidium formation. Plasmogamy of two yeast cells of compatible mating
types or conversion of a diploid yeast cell leads to the formation of a teliospore which undergoes meiosis upon germination.
Failure of meiosis in the metabasidium or conjugation between two haploid yeast cells could give rise to diploid yeast cells.
Key events in the life cycle are plasmogamy (P), karyogamy (K) and meiosis (M).
with the biomonitoring of lichens (see p. 454),
and both groups of organisms are susceptible
to SO 2 .
Phylloplane yeasts including S. roseus are also
sensitive to UV radiation, and enhanced irradiation
leads to a decline in their abundance on
leaves (Newsham et al., 1997). However, some
protection may be afforded by the carotenoids
produced by red yeasts, since carotenoid-deficient
mutants are more sensitive to UV light
than the red wild-type strains. Pure-culture
studies with Sporobolomyces and Rhodotorula spp.
often show enhanced production of carotenoids
under oxidative stress such as high aeration or
the addition of radical generators. Further, the
carotenoid spectrum may also change; if it does,
the shift is commonly from g- and/or b-carotene
under reduced oxygen pressure to torulene
and oxidized carotenoids (e.g. torularhodin)
at oxidative stress (Fig. 24.8; Sakaki et al., 2002;
Davoli et al., 2004). This may represent an
adaptive strategy because many of the oxygencontaining
carotenoids (i.e. xanthophylls) have
superior anti-oxidant properties as compared
to b-carotene (Martin et al., 1999). Other metabolites
(e.g. vitamin E) or enzymes (superoxide
dismutase, catalase) with anti-oxidant activities
may also be produced by red yeasts so that
the correlation between carotenoid production
and anti-oxidant protection is not always
absolute.
24.4 Ustilaginomycete yeasts
Most yeasts belonging to the Ustilaginomycetes
are the monokaryotic stages of smut fungi
(Section. 23.2) or of Exobasidium (Section 23.4)
and live saprotrophically on plant surfaces.