Growth, Differentiation and Sexuality

of entomopathogens and other animal and human
pathogens are also present. Outdoor spore
flow seems widely to correlate with indoor spore
incidences but, for spores of moulds such as Aspergillus
and Penicillium, numbers indoors may
substantially exceed those outdoors. The relative
composition of outdoor fungal aerosols is influencedbythelocationofaplace,andalsoverymuch
by the season of the year and meteorological factors
such as temperature (maximum, minimum, mean),
wind speed (mean, maximum), relative humidity,
rainfall, snow and UV irradiation. The aerial spore
contents of some species follow the same reaction
pattern to changing climates, whereas others react
completely differently (Li and Kendrick 1995, 1996;
Angulo-Romero et al. 1999; Marchisio and Airaudi
2001; Troutt and Levetin 2001; Stennett and Beggs
2004; Ulevicius et al. 2004). Control of spore production
by physical climate parameters is expedient
for timely spore distribution and fungal survival
in relation to life styles, availability of growth
substrate, competition against other species, and
actual needs of dormancy. The pea pathogen Mycosphaerella
pinodes, for example, produces both
asexual and sexual spores in the evening and night
hoursbutspecificdaysofproductionaredifferently
determined by rainfall (Zhang et al. 2005). By contrast,
conidia of the aphid pathogenic zygomycete
Erynia neoaphidis are released after midnight up to
the early morning hours, except on humid, colder
days when spore release can be observed also in the
late afternoons (Hemmati et al. 2001).
During the day, proportions of UV-sensitive
spores commonly fluctuate. At midday and in the
afternoon, amounts of UV-resistant spores are
high (>50%) whereas in the evening UV-sensitive
spores dominate (>90%) in the air (Ulevicius
et al. 2004). These results suggest that spores are
released according to temporal environmental
conditions of the day, i.e. their production needs
to be time-controlled. In Neurospora crassa,
the well-understood circadian clock (see The
Mycota, Vol. III, 2nd edn., Chap. 11) ensures the
rhythmic appearance of spores at night (Gooch
et al. 2004). Spore production in species with
modes of spore distribution by animals may also
correlate to specific hours of the day. For example,
Brodie (1931) demonstrated that Coprinus cinereus
(Coprinopsis cinerea) monokaryons produce
abundantly wet, sticky oidia which are distributed
by flies. This fungus lives on horse dung, a locally
limited substrate, and it relies on the insects to
be transferred to fresh horse dung. Since flies
Fungal Asexual Sporulation 265
are active mostly in the early morning hours,
completion of oidia production occurs in the
early morning (Polak et al. 1997a; see Sect. IV.D),
a time of day ideal also because of the morning
dew which will support fast germination on new
substrate. Oidia are short-lived, and germination
abilities decline strongly already 2–3 days after
their proliferation (Hollenstein 1997). These
spores can act as spermatia to dikaryotize mycelia
of opposite mating type. To this end, they stimulate
hyphae to grow towards them, in a process called
oidial homing (Kemp 1977). In nature, the main
mycelial phase of basidiomycetes is the dikaryon
with two different haploid nuclei per cell, which
is formed upon fusion of two mating-compatible
monokaryons (see Chap. 17, this volume). Delay of
nuclear fusion upon formation of basidia within
fruiting bodies (Chaps. 19 and 20, this volume)
conserves the genetic status of both haploid nuclei
within the vegetative mycelium. Simultaneously,
the dikaryotic status enables genetic complementation,
as typically shown by a diploid nucleus. By
the formation of uninucleate haploid oidia, in C.
cinereus the nuclear association of the dikaryon
can break down, and the unaltered haploid nuclei
can be released to find new substrate and/or new
mating partners. This is advantageous for species
survival under climatic conditions in which fruiting
bodies cannot be formed. The balance (ratios)
between the two possible oidia types formed
in a dikaryon is highly in favour of a stronger
nucleus. Therefore, from another viewpoint, oidia
production on the dikaryon supports a selfish
character of haploid nuclei which, in the dikaryon,
take advantage of unrelated haploid genomes until
these are abandoned in the search for new partners
(Polak 1999; Kües 2002; Kües et al. 2002a). Oidia
have another fascinating ecological function in
the defence of resources. The spores attract not
only hyphae of their own species but also hyphae
from other species competing for the same limited
substrate. Upon fusion, somatic incompatibility
reactions are initiated, ending in the killing of the
fungal rival (Kemp 1977).
Unit-restriction of individual mycelia because
of limited resources, competition for these between
individuals, and dynamic heterogeneous environments
are clearly all integral factors controlling
fungal spore production and dispersal (Gourbière
et al. 1999; van Maanen and Gourbière 2000). Pine
needles in coniferous litter have been chosen as
a model ecosystem for spore dispersal properties
from ephemeral and periodically renewed