Introduction to Fungi, Third Edition

MICROBOTRYALES (UREDINIOMYCETES)
653
Fig 23.14 Microbotryum violaceum.
(a) Germinating teliospore showing three cells
of the promycelium. (b) Detached three-celled
promycelium segment producing sporidia.
(c) Detached sporidia reproducing as yeast
cells.
In nutrient-rich conditions and at temperatures
above 25°C, the haploid sporidia reproduce by
yeast-like budding, whereas mating occurs at
lower temperatures in nutrient-poor media
such as tapwater agar. Microbotryum violaceum is
heterothallic and bipolar, and it was the first
smut fungus for which such an incompatibility
system was demonstrated (Kniep, 1919). During
mating, an a2 sporidium or yeast cell produces a
conjugation tube which grows towards the a1
cell. Provided that a-tocopherol (vitamin E) is
present, fusion is followed by the production of a
dikaryotic hypha which infects the flower of the
host plant (Castle & Day, 1984).
In the absence of a-tocopherol or on nutrientrich
media, de-dikaryotization occurs and
haploid or aneuploid yeast cells are formed by
the loss of chromosomes. Several other complications
exist in the life cycle of M. violaceum,
illustrating the wealth of non-meiotic mechanisms
of recombination encountered in fungi
(Day & Garber, 1988; Day, 1998). For instance,
nuclear fusion not associated with meiosis is
readily inducible in M. violaceum, and this
generates somatic diploids. Mitotic crossing-over
occurs at a high frequency in somatic diploids.
As already mentioned, M. violaceum has
dimorphic mating type chromosomes (Hood,
2002), and whilst the mating type locus is a
hot spot for mitotic recombination, meiotic
recombination does not occur in this region of
the genome.
Somatic diploids can be mated if they are
homozygous for mating type, so that triploid or
tetraploid strains can be generated. In nutrientrich
media, these strains grow as yeast cells but
are unstable due to the gradual loss of chromosomes.
The size of the yeast cells is correlated
with their ploidy level. There is also evidence for
the activity of transposons in enhancing genetic
recombination in M. violaceum (Garber & Ruddat,
2002). The genetic flexibility of M. violaceum may
explain the existence of numerous strains in
nature.
Fimbriae and conjugation
The process of sporidial conjugation has been
extensively studied in M. violaceum. Within 2 h of
placing sporidia of opposite mating type on a
suitable medium, a series of events occurs which
culminates in the formation of conjugation tubes
and plasmogamy. Poon and Day (1974) observed
an early adhesion between the two sporidia
before any wall-to-wall contact was established,
and they discovered that this initial contact
was mediated by fimbriae (Fig. 23.15) whose
existence in fungi had not been appreciated
previously. Subsequent investigations revealed
that fungi from most major groups (Zygomycota,
Ascomycota, Basidiomycota) possess such