Growth, Differentiation and Sexuality

(Adams and Timberlake 1990). Carbon and nitrogen
limitation, for example, induces spore formation
in N. crassa and in A. nidulans when grown
in aerated liquid culture (Guignard et al. 1984;
Skromne et al. 1995; Madi et al. 1997). The phosphate
status, in combination with the density of the
mycelium, also influences developmental decisions
in A. nidulans. PhoA, a kinase similar to S. cerevisiae
Pho85, is involved in integrating these environmental
signals and transducing them into morphogenetic
pathways (Bussink and Osmani 1998).
A physical prerequisite for asexual sporulation
in several fungi is exposure to a water–air interphase.
Typically, N. crassa and A. nidulans do not
form spores in submerged culture but only when
they are exposed on a substrate surface (Siegel
et al. 1968; Rossier et al. 1973; Adams et al. 1988;
Springer and Yanofsky 1989). Nothing is known
about the sensing of this factor, but it is likely that
the redox status and, thus, the concentration of
reactive oxygen species (ROS) of the cell changes
upon air exposure. Acting as an intracellular antioxidantxs,
vitamin E reduces the accumulation of
ROS in A. nidulans and negatively affects sporulation
(Emri et al. 2004). There is also good evidence
in N. crassa that the amount of ROS plays
acriticalroleduringdevelopment,probablybyaffecting
light signalling (Peraza and Hansberg 2002;
Michan et al. 2003; Yoshida and Hasunua 2004).
Starvation-induced sporulation in liquid culture
by N. crassa is observed only under high oxygen
supply through continuous agitation (Maheshwari
1991; Madi et al. 1997). CO2, by contrast, inhibits
normal conidiogenesis already at a concentration
of 0.2% (Guignard et al. 1984). Fungal CO2 sensing
is considered one key factor serving to correctly
place sporophores into the aerial phase (Sage 2002).
Furthermore, in N. crassa, humidity plays a role
in successful conidiation (Guignard et al. 1984).
Raising CO2 concentrations was shown to suppress
conidiation in plant-pathogenic Alternaria species
whereasanincreaseinhumidityhadnonegative
effect (Smart et al. 1992). In humid C. cinereus microslide
cultures under micro-aeration (Polak et al.
1997a), oidiophore formation and oidia production
arrest as soon as humidity vanishes from the cultures
when lifting the cover slip (E. Polak, personal
communication).
Another important physical factor which triggers
several developmental decisions in fungi is
light (see Chap. 13, this volume; Blumenstein et al.
2005). In N. crassa and many other fungi, the most
effective light quality in asexual sporulation is that
Fungal Asexual Sporulation 267
of blue light (Kertesz-Chaloupková et al. 1998; Liu
et al. 2003). At least in N. crassa, bluelightalso
regulates the biological clock which, in turn, controls
conidiation (see The Mycota, Vol. III, 2nd edn.,
Chap. 11). On the other hand, in some ascomycetes
including A. nidulans, red light appears to be effective
(Tan 1974; Mooney and Yager 1990). Because
the red-light effect is reversible by far-red light, the
system is reminiscent of the plant phytochrome system
(see Chap. 13, this volume; Blumenstein et al.
2005). As in N. crassa, alsoA. nidulans has a biological
clock, but asexual sporulation appears not
to be under its control (Greene et al. 2003).
Fungi often form asexual spores over a broad
range of temperatures. Sometimes, but not always,
there are temperature optima for spore production
(Guignard et al. 1984; Kertesz-Chaloupková
et al. 1998; Copes and Hendrix 2004; Parra et al.
2004). In N. crassa, where temperature variations
in the range 4–37 ◦ C had no effect on conidiation
(Guignard et al. 1984), there is temperature compensation
via the biological clock (Nouwrousian
et al. 2003; Dunlap and Loros 2004). Furthermore,
the biological clock of N. crassa has a pH compensation
(Ruoff and Slewa 2002), and pH does not
influence the period length of sporulation (Ruoff
et al. 2000). By contrast, pH influences on sporulation
have been described in some other fungi (for
examples, see Murray and Walter 1991; Campbell
et al. 1996; Zhang et al. 2001).
In A. nidulans, all external factors (depletion
of nutrients, light, aeration) influencing
developmental decisions are without effect if
the fungus is not competent for development.
This means that germinated spores need to grow
vegetatively at least 18 h before they acquire this
developmental competence. The fact that mutants
with different competence times can be isolated
demonstrates that the phenomenon is genetically
controlled (Axelrod et al. 1973). Because this
particular time requirement is lacking in, e.g. N.
crassa (Siegel et al. 1968; Guignard et al. 1984),
it will be interesting to see how widespread this
phenomenon is.
Several low-molecular weight compounds, selfproduced
or from the environment, are involved in
the regulation of asexual sporulation (see Chap. 11,
this volume, and Roncal and Ugalde 2003), and the
spectrum of pheromones ranges from terpenoid
to fatty acid derivatives. In A. nidulans, asystem
of several interconvertible fatty acids, named PSI
(precocious sexual induction) factors, has been described
and has recently been studied on a molec-