DIFFERENTIATION AND DEVELOPMENT 107
mon B pairings) controls the pairing and synchronous
division of nuclei and also the formation of clamp
branches, whereas the B locuscontrols septal dissolu-
tion and the fusion of clamp branches. Septal dissolu-
tion coincides with a marked increase in the activity
of ββ-glucanasein the hyphae, indicating that the B
locus controls the derepression of glucanase genes.
Development of fruitbodies
The toadstools, brackets and other fruitbodies of
Basidiomycota are the largest and most complex
differentiated structures in the fungal kingdom. Their
development is correspondingly complex and still only
poorly understood. Here we consider one example
where a start has been made to dissect this process at
the biochemical and molecular level, and we end with
a discussion of commercial mushroom production
because of its economic importance.
Schizophyllum commune
Schizophyllum communeis ideally suited for laboratory
studies because it grows readily in agar culture and pro-
duces its small (about 1–2 cm) fan-shaped fruitbodies
in response to light. Actually, this trigger leads only
to the development of fruitbody primordia – compact
clusters of hyphae which are overarched by other
hyphae. Further development from the primordia
occurs when carbon nutrients are depleted from the
medium, and is then fuelled by carbon reserves within
the mycelium. Early in this process the mycelial
storage compounds such as glycogen are converted
to sugars, which are translocated to the developing
primordia. Then, as the sugar levels in the hyphae
decline, the hyphal walls begin to break down and the
breakdown products are translocated to the primordia.
The wall glucans seem to provide the major source of
sugars, because fruitbody development is associated
with a marked rise in glucanase activity in the mycelia.
We have already seen that synthesis of this enzyme
is derepressed by the B mating-type locus, but it is
still subject to catabolite repression by sugars; so its
generalized activity in the hyphae, as opposed to
its localized activity in degrading septa, depends on
depletion of the mycelial sugar reserves. The breakdown
of hyphal walls to recycle nutrients for differentiation
is, in fact, quite common in fungi. An example was
seen earlier in the production of sclerotia (Fig. 5.10).
The breakdown of wall glucans also fuels the develop-
ing ascocarps of Emericella nidulans.
Wessels and his colleagues (see Wessels 1992)
identified several differentiation-associated genes in
S. commune. In order to do this, they crossed and re-
peatedly back-crossed strains to generate monokaryons
that were essentially isogenic except for the mating-type
locus. Then the monokaryons, and the dikaryons
synthesized from them, were compared for their pro-
duction of polypeptides and messenger RNAs when
grown in different conditions. Any differentiation-
specific mRNAs in the dikaryon were confirmed by
making complementary DNA (cDNA) and testing
this for lack of hybridization to the mRNA from the
monokaryons. All these comparisons were made in
two sets of conditions: (i) for 2-day-old colonies, when
the monokaryons and dikaryons were growing as
mycelia with similar colony morphology, and (ii) for
4-day-old colonies grown in light, when the mono-
karyon had produced copious aerial hyphae but the
dikaryon had produced numerous small fruitbodies.
The following principal findings emerged from this
work.- For the 2-day colonies, 20 or so proteins were found
only in the monokaryon and 20 or so only in the
dikaryon. Yet there was no detectable difference
in the bands of mRNA, suggesting that the same
mRNAs are transcribed but their products undergo
different post-translational modification in mono-
karyons and dikaryons. - For the 4-day colonies, eight proteins were found only
in the monokaryon, and 37 only in the dikaryon.
Some of these 37 occurred only in the fruitbodies;
others were found in both the fruitbodies and the
mycelium of the dikaryon. These proteins included
some that were secreted into the growth medium. - For the 4-day colonies, about 30 unique mRNAs
were found only in the dikaryon, whereas no unique
mRNA was found in the monokaryon. cDNA was
used as a probe to assess the levels of the “fruiting-
associated mRNAs” during development. They were
scarce in young vegetative colonies of both strains,
and they remained scarce in the monokaryon, but
they increased in the dikaryon when this began to
fruit. - Some of the secreted proteins were the cysteine-
rich hydrophobins, mentioned earlier; in fact, the
hydrophobins were first discovered in this work on
S. commune. The gene (SC3) for one of these hydro-
phobins was expressed by both the monokaryon
and dikaryon during the emergence of aerial hyphae;
this hydrophobin is now known to cover the
aerial hyphae and the hydrophobic hyphae on the
fruitbody surface, but the gene is not expressed by
hyphae that make up the main fruitbody tissue.
Three other hydrophobin genes (SC1, SC4, and
SC6) were highly expressed only in the dikaryon
and especially in the developing fruitbody tissues
(Wessels 1996). Hydrophobin genes have been
shown to contain a putative signal peptide sequence
at the N-terminus, a feature associated with secretion
from the hyphal tips. It is suggested that the specific
properties of different hydrophobins might affect