Growth, Differentiation and Sexuality

(Manachère 1988), C. cinereus (Tsusué 1969; Lu
1974; Kamada et al. 1978) and Flavolus arcularius
(Kitamoto et al. 1974), light is also required for
normal stipe and pileus (cap) development. In
fact, at least five light-sensitive phases can be
distinguished in fruiting-body formation in C.
cinereus (Kües 2000; Lu 2000). Light is needed
for the formation of initials, for maturation of
primordia and for karyogamy but it has a negative
effect on hyphal knot formation and completion
of meioses. Thus, for fruiting bodies to develop,
cycles of light and darkness are required. Light
effects are local and are not spread systemically
(Madelin 1956; Kertesz-Chaloupková et al. 1998).
Action spectra of light have been established
for several of these systems. The spectra show
differences, but all exhibit peaks in the UV-A
(320–400 nm) and blue (400–520 nm) regions,
suggesting that a flavin is the photoreceptor (Lu
1974; Elliott 1994). Recently, progress has been
made in C. cinereus in isolating a gene encoding
aputativeblue-lightreceptor.Thegenedst1 has
high similarity to the candidate photoreceptor
WC-1 of Neurospora crassa (Yuki et al. 2003).
Light caused formation of short, heavily
branched hyphal compartments in dikaryotic
strains of S. commune, an effect completely absent
from sealed cultures which do not develop primordia,
possibly due to accumulation of carbon dioxide
(Raudaskoski and Viitanen 1982; Raudaskoski and
Salonen 1983). The effect of sealing cultures on
fruiting-body formation in S. commune, attributed
to accumulation of carbon dioxide, was originally
detected by Niederpruem (1963). However, some
caution is necessary because this fungus also
releases large amounts of methylmercaptan and
dimethylsulfide (Birkinshaw et al. 1942), to the
extent that nearly all of the sulphate in the medium
whichisnotassimilatedisconvertedintothese
volatile compounds (O.M.H. de Vries and J.G.H.
Wessels, unpublished data).
In basidiomycete fruiting-body initiation, the
most rapid effects of blue light detected were increases
in contents of cAMP in C. cinereus (Uno
et al. 1974) and S. commune (Yli-Mattila 1987). The
light stimulus, combined with sufficient aeration,
also leads to activation of specific genes. Between 6
and 24 h after illumination of dark-grown colonies
of S. commune, levels of fruiting-associated mRNAs
rise (Yli-Mattila et al. 1989a). However, these increases
may have been a consequence, rather than
a cause of formation of fruiting-body primordia
(Wessels 1992). This is suggested from the fact that
Fruiting in Basidiomycetes 397
these genes are also activated in the dikaryon when
fruiting is suppressed by darkness or by a high concentration
of carbon dioxide (Wessels et al. 1987).
However, fruiting-body primordia maintain high
concentrations of these mRNAs, in contrast to the
vegetative mycelium (Mulder and Wessels 1986;
Ruiters and Wessels 1989b).
B. Mating-Type Genes as Master Regulators
In the heterothallic basidiomycetes, fruiting is
most regularly observed in the heterokaryon,
which is also called the secondary mycelium.
The heterokaryon arises from a mating between
two compatible homokaryons, that is, between
homokaryons carrying different mating-type
genes (in older literature referred to as incompatibility
factors; for further details, see Chap. 17,
this volume). With respect to morphological
differences between homokaryons and the derived
heterokaryon, there is a great deal of variation.
The most regular pattern is that exemplified by
the two most intensively studied species, namely,
S. commune and C. cinereus. In these species
the homokaryon contains one nucleus in each
hyphal compartment, and is therefore called
a monokaryon. The established heterokaryon
contains two (genetically different) nuclei in each
hyphal compartment, and is therefore called
a dikaryon. These dikaryons are typified by the
presence of a clamp connection at each septum,
which is formed during synchronous mitotic division
of the two nuclei (Chap. 17, this volume). To
cite a few deviating examples: in the occasionally
cultivated Agaricus bitorquis, the homokaryon is
multikaryotic, whereas the fertile heterokaryon
is dikaryotic but without clamp connections
(Raper 1976). In the commonly cultivated Agaricus
bisporus, the fertile heterokaryon grows directly
from a basidiospore which contains two nuclei
of different mating types. The heterokaryon is
multikaryotic and has no clamp connections
(Raper et al. 1972).
The mating-type genes are the master regulators
of sexual development (see also Chap. 10,
this volume). When two homokaryons with different
A and B mating-type genes (in S. commune,
here called MATA and MATB) fuse, a heterokaryon
is formed with the propensity to develop fruiting
bodies (see Fig. 19.3A,C). Following hyphal fusion,
nuclei are exchanged. These nuclei migrate to the
apical compartment of the recipient hypha, which