Growth, Differentiation and Sexuality

of the A mating type pathway strongly enhances
chlamydospore production when a dikaryon is
kept in the dark whereas constant incubation with
blue light represses chlamydospore production.
Additional activation of the B mating type pathway
promotes the process but activation on its own
also has an effect (Kües et al. 1998a, 2002b).
Chlamydospores are produced under conditions
favouring fruiting body development, as part of
thesexualmodeofreproductiononthedikaryon.
Fruiting body development occurs at 25–28 ◦ C
and needs repeated impulses of blue-light low
energy but it is suppressed by higher light energies
which are effective in oidiation (Kües 2000; Fischer
and Kües 2003; Kües et al. 2004). Chlamydospores
accumulate glycogen which can serve as energy
source for fruiting (Madelin 1960; Moore 1998).
Thepositiveregulationinthedikaryon,andthe
functional linkage to fruiting corroborate that
there are genes for chlamydospore production
which are primarily dikaryon-specific.
Genes acting in asexual sporulation in C.
cinereus other than the mating type genes have so
far not been identified. The homokaryotic mutant
strain AmutBmut has defects in both mating type
loci, which makes oidiation a light-controlled
process (Kertesz-Chaloupkova et al. 1998) and also
leads to fruiting body production without the need
to mate with another strain (Boulianne et al. 2000;
Walser et al. 2003). A mutant screening approach
in this strain identified many morphological
changes in oidiophore and spore production
and morphology. In most instances, the aerial
mycelium is simultaneously affected with alterations
in oidiation and, in about 85% of the cases,
also fruiting body formation (Polak 1999). To
separate genes acting generally in growth and
development as well as nonessential or marginally
important genes from functions acting specifically
and/or being essential in oidiation, a detailed
analysis of sexual and asexual phenotypes is
required. Strain AmutBmut produces mainly the
advanced type 1 and type 2A oidiophores (Polak
et al. 1997a, 2001), some of the mutants rather
the less-developed type 3 or type 4 structures or
irregularly structured oidiophores with altered
stems, branching or oidial hyphae production. In
other cases, branched and elongated oidial hyphae
with aberrant nuclear distributions were observed.
A considerable number of mutant phenotypes,
however, relate to the spores themselves. Spores
occur which are much longer or shorter than
normal, swollen spores, shrivelled spores and
Fungal Asexual Sporulation 283
branched spores. In a flocculating strain, the outer
fimbriae and the gelatinous layer were found to be
absent in the spores. In one mutant, apical oidia
separated not by schizolysis, the norm, but rather
through ripping of the cell below (rhexolysis). In
another mutant, spores are not released. There
are also interesting regulatory mutants including
constitutive producers and spore-negative strains
unable to initiate oidiophore production (Polak
1999). The pool of available mutants holds promise
for future work to identify many interesting genes.
D. Conserved Genetic Pathways
From the compilation of characterized genes from
variousfungiwepresentedinSect.IV.B,itisclear
that there are functions in asexual spore production
conserved between species. Even if the mechanisms
of spore formation and the environmental
requirements were variable among different fungi,
some genetic pathways or particular elements of
these pathway may be evolutionarily conserved.
How broadly will this be?
The availability of several fungal genome sequencesallowsustoaddressthisquestionfairly
easily by means of Blast searches, in contrast to
less-straightforward experimental methods such
as heterologous complementation or PCR-based
approaches (Prade and Timberlake 1994). In such
a computer-based approach, however, one should
bear in mind that sequence conservation does not
necessarily imply conserved cellular functions! It
is obvious that all fungi contain components of the
main signalling pathways, for example, various Gproteins,
proteins of the MAP cascade, and certain
transcription factors which can affect growth and
development in more than one way (Lengeler et al.
2000), including pathways of asexual sporulation
(see Sects. IV.A.2 and IV.B). Possible contributions
of such regular elements to asexual spore development
in a specific fungus certainly need to be addressed
experimentally. However, there are other
functions expected to act primarily during steps
of asexual sporulation, and unlikely to play roles
in other cellular pathways (see Sects. IV.A.2 and
IV.B).
Here, we used genes of A. nidulans central to
the conidiation process, and analysed the genomes
of a number of different fungi. The results are summarized
in Table 14.1. Probably not unexpected
because of the structurally related conidiophores
and the same phialidic mode of conidia produc-