Growth, Differentiation and Sexuality

396 H.A.B. Wösten and J.G.H. Wessels
sidiomycetes. For reviews in developmental patterns,
see Wessels (1965), Reijnders and Stafleu
(1992), Watling (1996) and Clémonçon (1997).
In colonies of S. commune (Perkins 1969; Raudaskoski
and Vauras 1982) and Coprinus (Coprinellus)
congregatus (Ross 1982; Durand 1983), it was
shown that light induction of primordia occurs
in the youngest growth zone, immediately behind
the advancing front of the colony. In S. commune,
light induction alone leads to the immediate appearance
of primordia (Perkins 1969; Yli-Mattila
et al. 1989b). In C. congregatus, some additional
stimulus emanating from the whole mycelium is
required to realize formation of primordia. Ross
(1982) noted that primordia formed only in the
growth zone after the colony front had reached the
edge of the Petri dish. Durand (1983) saw immediate
formation of primordia after light induction
in the growth zone of a half-colony growing on
non-nutritive medium, while the other part had already
fully colonized a nutrient medium. Thus, in
some cases, as in S. commune, induced initials may
immediately act as a sink for translocation of materials
from the vegetative mycelium whereas in other
cases, as in C. congregatus, vegetative mycelium has
to be checked in its growth before such a translocation
system becomes operative. Only part of the
hyphal aggregates eventually form mature fruiting
bodies. Possibly, stochastic processes and competition
for translocated materials determine which
initials will grow into primordia and, subsequently,
into mature fruiting bodies.
III. Regulation of Fruiting-Body
Formation
A. Environmental Signals
Much effort has gone into identifying environmental
factors conducive to fruiting in basidiomycetes.
On the one hand, such studies were done to provide
inroads to establish causative mechanisms of fruiting.
On the other hand, they have been very important
in establishing optimal conditions for commercial
mushroom growing. Apart from studies
on normal environmental conditions, a few studies
have been concerned with compounds in fungal
extracts which enhance fruiting. Sphingolipids
and cerebrosides appear to be effective inducers of
fruiting in S. commune and C. cinereus (Kawai and
Ikeda 1982; Kawai et al. 1986; Mizushina et al. 1998),
while cAMP was shown to stimulate fruiting in C.
cinereus (Uno and Ishikawa 1971, 1973, 1982). Also
in S. commune (Schwalb 1978; Yli-Mattila 1987; Kinoshita
et al. 2002), Phanerochaete chrysosporium
(Gold and Cheng 1979) and Lentinula edodes (Takagi
et al. 1988), a relation was found between high
levels of endogenous cAMP and fruiting. High levels
of intracellular cAMP could be obtained by expressing
dominant active heterotrimeric G protein
alpha subunits (SCGP-A and SCGP-C) in S. commune
(Yamagishi et al. 2002, 2004). However, this
resulted in reduced, rather than increased fruiting
in the dikaryon. Future research should elucidate
the exact role of cAMP and the heterotrimeric G
proteins.
It is self-evident that the emergence of fruiting
bodies is accompanied by a drastic change in
exposure to oxygen (limited availability in the substrate,
high in the air), carbon dioxide (high in the
substrate, low in the air) and light. It is therefore
not surprising that these environmental factors can
exert a profound influence on fruiting-body development
(Manachère 1980). Moreover, environmental
conditions like temperature, humidity and
availability of nutrients may play a decisive role
(Madelin 1956; Kües and Liu 2000). Of the environmental
factors least studied is the availability of
oxygen. In some basidiomycetes, higher mycelial
biomass production has been observed at 5% than
at 20% O2 (White and Boddy 1992). The high oxidative
activity of fruiting bodies of S. commune
(Wessels 1965) suggests that a high concentration
of O2 may be necessary for their development. Indeed,
at 2% O2, half of the maximum amount of
mycelium was formed but neither fruiting bodies
nor aerial hyphae appeared (J.G.H. Wessels, unpublished
data). A possible regulatory effect of oxygen
on emergent growth is the more interesting,
in view of the hypothesis that oxygen may induce
a hyperoxidant state involving oxidized proteins
which may operate a switch to aerial differentiation
(Hansberg and Aguirre 1990; Toledo and Hansberg
1990).
Light has been most intensively studied as
a modulating factor in fruiting-body development
(Lu 1974, 2000; Eger-Hummel 1980; Manachère
1980, 1988; Durand 1985). The effect of light on
S. commune is limited to induction of primordia
(Perkins 1969; Raudaskoski and Yli-Mattila 1985);
often, illumination for a few minutes suffices. This
inducing effect of light can sometimes be bypassed,
such as in C. cinereus, by low-temperature treatment
(Tsusué 1969). In, for example, C. congregatus