Growth, Differentiation and Sexuality

282 R. Fischer and U. Kües
nitrogen sources (Rao and Niederpruem 1969;
Walser 1997). In monokaryons, oidia production
is constitutive, whilst in dikaryons oidiation is
repressed in the dark. In light, repression is
partially released and dikaryons indeed produce
spores but in much smaller quantities (cf. factors of
100 s–1000 s) than do the parental monokaryons.
Effective on the dikaryon is blue light. Elevated
temperatures (37–42 ◦ C) support spore production
better than do ambient temperatures
(25–30 ◦ C; Kertesz-Chaloupková et al. 1998; Kües
et al. 1998b), in an opposite manner to fruiting
body development (Kües 2000; Kües et al. 2004).
Negative control of oidiation on the dikaryon is
exerted by the two mating type loci (Tymon et al.
1992; Kües et al. 1998a, 2002b). The genes at the
A mating type locus encode two distinct classes
of homeodomain transcription factors (HD1
and HD2), which form heterodimeric complexes
functional in gene regulation when of compatible
mating type specificity. The mating type genes at
the B locus encode pheromones and pheromone
receptors. Functional are pheromones in combination
with pheromone receptors from different
mating type specificity (for more details on mating
type genes and their products, see reviews by Casselton
and Olesnicky 1998; Hiscock and Kües 1999;
Kües 2000; Kothe 2001; Chap. 17, this volume).
In the dikaryon, A mating type genes have been
shown to primarily be responsible for repression
of oidia formation in the dark (Tymon et al.
1992). If only the A mating pathway is activated,
then blue light is able to fully release A-mediated
repression (Kertesz-Chaloupková et al. 1998; Kües
et al. 1998a). A blue-light photoreceptor of the N.
crassa white collar 1-type has been cloned in C.
cinereus fromamutantdefectiveinlightregulation
of fruiting body development (Terashima
et al. 2005). Preliminary genetic data suggest
that this receptor also takes part in overriding
A mating type-mediated repression of oidiation
(W. Chaisaena, personal communication). The
B mating type genes counteract light-mediated
release of repression, resulting in spore production
in low numbers in the dikaryon. Activation of
the B pathway on its own can cause, in some
strains and on some media, reduction in general
colony growth and suppression of aerial mycelium
formation. However, sole activation of the B
mating type pathway has no severe effect on oidia
production (Kües et al. 2002b). There appears to
be a link between the B mating type genes and
nutritional control of growth, in particular for
nitrogen (Kües et al. 2004). Constitutive activation
of a Ras GTPase causes growth defects similar to
those resulting from activation of the B mating
type pathway, but oidia production is not affected
by constitutive Ras activation (Bottoli 2001; S.
Kilaru, personal communication).
Wessels and co-workers showed in the basidiomycete
Schizophyllum commune that, with
increasing distance of the two different haploid
nuclei in aerial cells of the dikaryotic mycelium,
nuclear communication eventually ceases. Consequently,
repression of monokaryon-specific genes
is released (Schuurs et al. 1998; see Chap. 19, this
volume). In the aerial mycelium of C. cinereus
dikaryons, there are possibly similar effects in
the control of oidia formation by mating type
genes. Spores produced on a wild-type dikaryon
are haploid and uninucleate, as are those on
monokaryons, indicating that nuclear communication
is interrupted. However, from the two
possible types of oidia on a dikaryon, usually one
dominates. This dominance is reversed when the
stronger nucleus is transformed by a compatible
A mating type gene. The interpretation of these
observations is that opposing but distinct length
gradients of A mating type proteins exist in
the dikaryotic cell, protein concentrations being
highest in the nucleus which carries the genes
for the corresponding proteins, and the stronger
nucleus producing the longer gradient. Functional
HD1-HD2 transcription factor complexes are
more likely to be produced in close vicinity of
the weaker nucleus which, consequently, will be
repressed in oidiation (Polak 1999; Kües et al.,
unpublished data). The model underlines that
the responsible sporulation genes are principally
monokaryon-specific. It also can explain the
hierarchy observed in the asexual formation of
uninucleate haploid spores on dikaryons of other
basidiomycetes (Hui et al. 1999; Kitamato et al.
2000). In the monokaryons of C. cinereus, A
mating type genes have no effect on oidiation.
Oidiophore formation is unaltered in C. cinereus
strains lacking all A mating type genes, and the
numbers of spores produced are similar to values
recorded in wild-type monokaryons (Polak 1999).
Chlamydospores occur in aging cultures of
some monokaryons of C. cinereus (Kües et al.
1998a) but they are much more common in
dikaryons (Anderson 1971; Kües et al. 2002a).
Regulation of production of chlamydospores
is opposite to that for oidiation and, in some
aspects, parallel to that for fruiting. Activation