Growth, Differentiation and Sexuality

398 H.A.B. Wösten and J.G.H. Wessels
Fig. 19.3. A–F Morphological appearances of 4-day-old
surface cultures of co-isogenic Schizophyllum commune
strains, inoculated as a lawn from a mycelial homogenate,
variously exhibiting the ability to produce aerial hyphae
and fruit bodies. A MATA41MATB41, B MATA41MATB41
thn, C MATA41/A43 MATB41/B43, D MATA41/A43
MATB41/B43 thn/thn, E MATA con MATB con , F MATA con
MATB con fbf
is accompanied by septal dissolution. In the apical
compartment, the donated and recipient nuclei
pair and hyphal dissolution is switched off. In fact,
new septa are formed which are more resistant to
dissolution and which physically prevent nuclear
migration (Wessels and Marchant 1974). The nuclei
in the dikaryotic hyphae divide synchronously.
Nuclear division is accompanied by the formation
of clamps. As a result, the apical and the subapical
compartments contain nuclei of both mating
type. The heterokaryotic dikaryon is stable and, unlike
the monokaryon, forms fruiting bodies under
appropriate environmental conditions. Coordination
of the mating-type regulation also manifests
itself in control of cAMP production (Swamy et al.
1985a), in cAMP-dependent protein kinase activity
(Swamy et al. 1985b) and in fruiting-body initiation
(Swamy et al. 1984).
The molecular structure of the mating-type
loci has been identified in, for instance, S. commune
and C. cinereus (for a detailed overview and references,
see Chap. 17, this volume). The A genes of
these fungi encode homeodomain proteins of the
HD1 and HD2 type. In heterokaryons with nuclei
containing different A genes, these proteins form
heterodimers which are active in development.
On the other hand, the B genes of C. cinereus and
S. commune encode pheromones and G-coupled
receptors for pheromones. In heterokaryons with
nuclei containing different B genes, pheromones
from one nucleus can interact with receptors
encoded by the other nucleus, and vice versa.
Homokaryotic mutant strains have been isolated
with a constitutive active A mating-type gene
(referred to as MATA con in S. commune, Amut in C.
cinereus) and/or B mating-type gene (referred to as
MATB con in S. commune, Bmut in C. cinereus;Raper
et al. 1965; Koltin 1970; Swamy et al. 1984). Activity
oftheselociisthusindependentofacompatible
locus donated by a sexual partner. The S. commune
MATA con MATB con homokaryon (Fig. 19.3E) is
a dikaryon which shares with the heterokaryotic
dikaryon (Fig. 19.3C) the propensity to form fruiting
bodies. Likewise, AmutBmut homokaryons of
C. cinereus do form fruiting bodies as dikaryons
(Swamy et al. 1984; Boulianne et al. 2000).
C. Other Regulatory Genes
Relatively little is known about genes involved
in establishment of the dikaryotic mycelium and
emergence of fruiting bodies, other than the
mating-type genes. Yet, many genes are expected
to play a role. Both establishment of the dikaryon
and emergence of fruiting bodies encompass
a number of complex processes which have to
be coordinated (see above). Before 1990, only
mutations were known and most of them have
hardly been studied. For instance, Raper and
Krongelb (1958) detected dominant alleles in
a natural population of S. commune affecting
the morphology of fruiting bodies, described as
corraloid (highly involuted hymenium), medusoid
(long stipes) and bug’s ear (numerous small
fruiting bodies without gills). By chemical and
UV mutagenesis of a normal dikaryon (Takemaru
and Kamada 1972) or a AmutBmut homokaryon
(Kanda and Ishikawa 1986), a remarkably high
number of variants of C. cinereus were isolated.
Many of the mutants had developmental defects
in initiation of fruiting-body development, in
primordia and fruiting-body maturation and/or
were defective in sporulation. More recently,
REMI (restriction enzyme mediated integration)
mutagenesis in homokaryotic fruiting strains gave
rise to similarly large collections of mutations with
defects in fruiting-body development and sporulation
(Granado et al. 1997; Cummings et al. 1999;
Muraguchi et al. 1999). Temperature-sensitive
sporulation mutants of S. commune have also been
isolated (Bromberg and Schwalb 1977). In the last
decade, several genes other than the mating-type
genes have been isolated which are involved in
establishment of the dikaryotic mycelium and