Growth, Differentiation and Sexuality

244 L.M. Corrochano and P. Galland
different developmental pathways in C. cinereus,
the presence or absence of light serving as the
major signal (reviewed in Kües 2000; Fischer and
Kües 2003; see Chap. 14, this volume).
a) Effect of Light on Fruiting Body Development
The presence of light determines the developmental
pathway followed by hyphal knots, since sclerotia
are produced in the dark, and initiation of
the fruiting body occurs only in the light. Fruiting
body development depends on dark/light cycles,
and several steps in this development are influenced
by light: (1) hyphal knot formation is inhibited
by light, (2) the formation of fruiting body
initials, maturation of primordia, and karyogamy
are induced by light, and (3) meiosis completion is
inhibited by light (reviewed in Kües 2000). Mutants
altered in different steps of fruiting body development
have been isolated, including blind mutants
that behave under dark/light cycles as the wild
type grown in the dark (Muraguchi et al. 1999).
The blind mutants suffered a blockage in fruiting
body development, such that only “dark stipes”
were formed. This blind phenotype suggested that
proteinsalteredinthemutantshadaroleinlight
signaling. One of the mutant genes has been cloned,
and its sequence resembles that of the Neurospora
wc-1 gene for the blue-light photoreceptor, suggesting
a similar role in C. cinereus photomorphogenesis
(Terashima et al. 2003).
Of particular interest is the role of light in
meiosis, since meiosis progression is controlled by
light/dark cycles (Lu 2000). The timing of karyogamy
and meiosis differed in different strains under
the same light/dark cycle, and depended on
thelightintensityused.Theeffectoflightwasrestrictedtoaperiodof16to6h
before karyogamy
but a subsequent dark period, or low-intensity exposure,
was required for the completion of meiosis.
The suppressing effect of light on the completion
of meiosis was not observed in a specific dikaryon
due to a mutation in a single gene. The effect of
light/dark cycles in meiosis, and possibly of the
whole fruiting body development, seems to allow
the maturation of fruiting bodies for spore dispersal
shortly after daybreak, regardless of night
duration (Lu 2000).
b) Other Effects of Light on Development
Inadditiontotheroleoflightonfruitingbody
development in dikaryons, it is possible to investigate
several developmental pathways in C. cinereus
monokaryons and its regulation by light after their
A and B mating type pathways have been activated.
Monokaryon mycelia develop oidia in high
numbers after growth in the dark or in the light.
However, a mutant monokaryon with both mating
type pathways activated produced a small number
of oidia in the dark, and a large amount of
oidia when exposed to light (Polak et al. 1997;
Kertesz-Chaloupková et al. 1998). Short illumination
times (1–2 min) were sufficient to induce oidia
above threshold levels, and the effect of light did
not spread to non-illuminated mycelia. An exposure
of 60 μmol/m 2 of blue light yielded a full oidia
induction, with other wavelengths being less effective
(Kertesz-Chaloupková et al. 1998). The role of
the A activated pathway was further explored after
transformation of monokaryons with heterologous
and compatible A mating type genes. The resulting
strains showed light induction of oidia production.
In addition, light incubation repressed the formation
of hyphal knots, sclerotia and chlamydospores,
suggesting a major role for the A activated program
in development and light regulation (Kües et al.
1998, 2002). This effect was partially modified by
an activated B pathway (Kües et al. 2002).
c) Light Transduction Pathway
The primary effect of light on basidiomycetes
may be transduced by G proteins. Fruiting body
formation in the related basidiomycete Coprinus
congregatus (Coprinellus congregatus), belonging
–asdoesC. cinereus – to the Psathyrellaceae,
requires light applied to dikaryon mycelia. Membrane
extracts from C. congregatus contain G
proteins showing a light-dependent nucleotide
analog binding. In addition, a G-protein α subunit
with similarity to transducin is expressed in lightsensitive
mycelia. These results suggest a role for
G proteins in light-mediated signal transduction
(Kozak and Ross 1991; Kozak et al. 1995).
Several blind mutants have been isolated
in C. cinereus (Muraguchi et al. 1999; Lu 2000)
that are blocked in specific light-dependent
stepsoffruitingbodydevelopment,withoutany
effect on other light responses. This indicates
that C. cinereus may use several light transduction
pathways. The genome of C. cinereus,
now available (C. cinereus Sequencing Project,
Broad Institute of MIT and Harvard, USA,
http://www.broad.mit.edu/annotation/fungi/copri
nus_cinereus/), holds the possibility of identifying
a whole array of photoreceptor genes that could