Growth, Differentiation and Sexuality

Rather, development is arrested in the initial stages
of hyphal aggregation. Still, how can the haploid
fruiting be explained when FRT1-1 is transformed
into a FRT1-2 strain? Horton et al. (1999) proposed
that in this case these proteins dimerise, relieving
the repression of the dikaryon-specific genes.
Moreover, the heterodimer would activate some
other genes, resulting in the formation of fruiting
bodies. However, fruiting in the dikaryon was not
affected when the FRT1 gene was deleted (Horton
et al. 1999). This suggests that FRT1 is not a crucial
component, if a component at all, of the pathways
encompassing the mating-type genes which lead
to formation of fruiting bodies in the dikaryon.
3. Regulatory Genes in Fruiting-Body Formation
but not in Establishment of the Dikaryotic
Mycelium
AmutationintheTHN (THIN)geneofS. commune
occurs spontaneously and has pleiotropic effects
(Raper and Miles 1958; Schwalb and Miles 1967;
Wessels et al. 1991b). The scanty presence of aerial
hyphae, the perfectly round edge of colonies, the
wavy or corkscrew-like appearance of submerged
hyphae, and a pungent smell easily score the mutation.
The suppression of formation of aerial hyphae
is best seen in cultures grown as a lawn from
mycelial fragments (Fig. 19.3B); in a dikaryon homozygous
for thn, formation of both aerial hyphae
and fruiting bodies is suppressed (Fig. 19.3D).
The THN1 gene was cloned (Fowler and Mitton
2000). It encodes a putative RGS protein (regulator
of G protein signalling) which is homologous
to Crg1 of Cryptococcus neoformans (Fraser et al.
2003; Wang et al. 2004), Sstp2p of S. cerevisae (Dietzel
and Kurjan 1987) and FlbA of Aspergillus nidulans
(Lee and Adams 1994). These proteins share
a domain of about 120 amino acids (de Vries et al.
1995) assumed to interact with the Gα subunit of
heterotrimeric G proteins. The interaction modulates
the conversion of GTP bound to the Gα subunit
to GDP. In this way, it regulates signals from an
activated receptor protein which are transferred via
a heterotrimeric G protein to downstream effector
molecules. It was hypothesized (Fowler and Mitton
2000) that THN1 regulates a heterotrimeric G protein
signalling pathway which, in turn, regulates hydrophobin
expression (see below). Schuren (1999)
reported that most of the pleiotropic effects of the
thn mutation were overcome by growing the mutant
near wild-type hyphae. A diffusible molecule
smaller than 8 kDa would be responsible for this
Fruiting in Basidiomycetes 401
effect, and may be part of the signalling cascade.
It is not yet clear whether this signalling pathway
is directly linked to the pheromone receptor encoded
by the MATB genes or whether it activates,
for example, cAMP production.
In contrast to thn of S. commune,aerialgrowth
is not affected in the ich1-1 mutant of C. cinereus.
Rather, cap differentiation is blocked at an early
stage of fruiting-body differentiation (Muraguchi
and Kamada 1998). In contrast to wild-type primorida,
no rudimentary pileus could be observed
in the primordial shaft of the ich1-1 mutant. ich1
mRNA accumulates in the cap of the wild-type
fruitingbody.Thepreciseroleofthegene,however,
is not yet known. The ich1-1 gene encodes a novel
protein of 1353 amino acids containing nuclear targeting
signals. Also notable, the protein contains
a S-adenosyl-L-methionine (SAM) binding motif
(Kües 2000), being characteristic for the enzyme
family of methyltransferases (Faumann et al. 1999).
D. Nuclear Positioning
The SC1, SC4 and SC6 hydrophobin genes as well
as SC7 and SC14 of S. commune (for their function,
see below) are expressed in dikaryons (MATAon
MATB-on) but not in monokaryons (MATA-off
MATB-off) and MATA-on MATB-off or MATA-off
MATB-on mycelia (Mulder and Wessels 1986; Wessels
et al. 1995). By contrast, the SC3 hydrophobin
gene is active in the monokaryon but it is downregulated
in a MATA-off MATB-on mycelium (Ásgeirsdóttir
et al. 1995). From this, it is expected
that SC3 would also be inactive in dikaryons (i.e.
MATA-on MATB-on). Indeed, SC3 mRNA was lowered
in a fruiting dikaryon. However, under nonfruiting
conditions (e.g. high CO2 and darkness),
high SC3 expression did occur in the dikaryon,
whereas expression of the dikaryon-specific hydrophobin
genes and the SC7 and SC14 genes was
relatively low (Wessels et al. 1987). Apparently, the
MATB pathway and, possibly, also the MATA pathwayarenotactiveinatleastpartofthedikaryotic
mycelium. How can this be explained? It was shown
that the distance between nuclei in the dikaryotic
hyphaevaries.Aerialhyphaehavealargenuclear
distance (>8 μm), correlating with high SC3 expression.
By contrast, the nuclear distance in hyphae
within the fruiting body is small (