Growth, Differentiation and Sexuality

Fig. 17.1. Life cycle of the dimorphic plant pathogen Ustilago maydis
pheromone signal induces the formation of long,
thin mating filaments that fuse at their tips. Once
the cells have fused, there is a switch to filamentous
mycelial growth and obligate pathogenicity. The
filamentous dikaryotic mycelium invades the
tissues of the host plant where it induces tumours,
which are filled with black diploid teliospores,
the smut from which the fungus takes its name.
These germinate to give a promycelium that buds
off basidiospores. Classical studies established
that mating filament formation was dependent on
compatible a genes whereas filamentous dikaryotic
growth required compatible b genes (see Banuett
1995; Kahmann et al. 2000).
C. cinereus is a saprophytic fungus that has
a typical mushroom life cycle. The roles of the
mating type genes in the mating pathway were
elucidated by Swiezynski and Day (1960). In all
homobasidiomycetes, hyphal fusion is sufficient
to initiate mating. There is no evidence that
Mating Type Genes in Basidiomycetes 359
pheromones are secreted into the environment to
attract mates; unlike U. maydis and C. neoformans,
cell fusion is mating type-independent in C.
cinereus, andpheromonesignallingisactivated
only after cells have fused (Olesnicky et al. 1999).
The asexual stage in C. cinereus is known as the
monokaryon (homokaryon) because the cells are
uninucleate. Provided mates have different alleles
of the B genes, following cell fusion there is an
exchange of nuclei and extensive migration of the
donor nucleus through the established cells of each
recipient monokaryon. Nuclear migration involves
major restructuring within the cell; the complex
dolipore septa separating the cells in the hyphae
normally preclude movement of organelles but
these are dissolved away, so that nuclear movement
is facilitated (Giesy and Day 1965). Once the tip
cells have both nuclei, this triggers a complex
division leading to the formation of the clamp
connection. The clamp cell forms on the side of the