Growth, Differentiation and Sexuality

eplicate metulae and, frequently, secondary conidiophores
on top of the primary conidiophores. Nevertheless,
those aberrant conidiophores are able to
produce conidia. Similarly to the stuA mutation, the
medA mutationhasaneffectonsexualreproduction.
Unlike stuA, however, medA mutants do form
Hülle cells, failing to produce only cleistothecia.
MedA is required for correct temporal expression
of both brlA transcripts and it regulates, together
with BrlA, the expression of abaA.Interestingly,the
lack of functional MedA can be overcome by an additionalcopyofbrlA
(Busby et al. 1996). A detailed
analysis of the medA gene locus and the interaction
of MedA with putative target promoter sequences
is unfortunately still pending.
The number of genes which, in one way or another,
influence spore formation is continuously
increasing. However, a link to the well-understood
regulatory cascade and the signalling components
described above is largely lacking, and mutations
in these genes cause pleiotropic phenotypes. The
question whether the conidiation phenotypes are
primary defects of the mutations or whether the effects
are secondary has to date not been fully solved.
One gene which comes into consideration is dopA.
It encodes a large protein (207 kDa)withseveralputative
domains, including three leucine zipper-like
domains (Pascon and Miller 2000). Mutant strains
have abnormal vegetative hyphae, delayed and synchronous
initiation of asexual development, and
also a defect in the sexual cycle. Another candidate
is the basic helix-loop-helix protein DevR (Tuncher
et al. 2004). Conidiophores appear to have a defect
at the phialide stage. There is evidence that
DevR is a component of the TcsA two-component
signalling pathway (Virginia et al. 2000). The last
example mentioned here, is the hymA gene (Karos
and Fischer 1996). Metulae fail to differentiate and
do not produce phialides – hence, the name hym,
hypha-like metulae. Expression is not developmentally
regulated and the gene encodes a highly conserved
protein (Karos and Fischer 1999). The corresponding
protein from mouse even partially complemented
the hymA defect in A. nidulans. Studies
in S. cerevisiae showed that the yeast homologue
Hym1 is involved in cell cycle regulation and polarity
establishment (Nelson et al. 2003). The hymA
mutant phenotype suggests that these functions are
likely to be the primary ones in the mycelial fungus,
too, because at the metula stage the cell cycle becomes
strictly coordinated to cytokinesis and the
reproductionmode changes to a pseudohyphal-like
process (Gimeno and Fink 1994).
Fungal Asexual Sporulation 275
g) Target Genes
The presence of receptors, signalling cascades and
transcription factors would not lead to any morphogenesis
without the concerted actions of enzymes
and structural proteins. Some of these have
been isolated in the mutant screening mentioned
above (Clutterbuck 1969). Two of them, yA and wA,
encode enzymes for pigment biosynthesis (Aramayo
and Timberlake 1990; Mayorga and Timberlake
1990, 1992). Other examples are the rodA and
dewA hydrophobins, which contribute to the hydrophobic
surface of the conidiospores. Whereas
RodA appears to form typical rodlet structures
at the spore surface, in dewA deletion mutants,
rodlets are still visible, suggesting that the main
spore-coating hydrophobin in A. nidulans is RodA
(Stringer et al. 1991; Stringer and Timberlake 1994;
Fig. 14.2). Another component of the cell wall and,
thus, also of the conidiospore wall is chitin. A.
nidulans contains several chitin synthases, which
appear to be expressed to different extents in different
cell types (Lee et al. 2004). Usually, deletion
ofasinglechitinsynthasegenedoesnotcauseany
phenotype, because the functions are redundant.
However, double deletion of, e.g. chsA and chsC,
caused conidiophore morphology alterations and
a reduction in spore production (Fujiwara et al.
2000). These results show that a fine regulation
ofthecellwallcompositionisessentialforhigher
yields of spores.
Other examples for differentially regulated
genes are the catalases. A. nidulans contains at
least four of these enzymes, of which CatA is
up-regulated during conidiation (Navarro and
Aguirre 1998; Kawasaki and Aguirre 2001). Interestingly,
up-regulation of catA was independent of
the regulator BrlA, suggesting a regulatory cascade
independent of brlA.
h) Interdependence of Vegetative Growth
and Development
There is increasing evidence that the different
developmental programmes are interlinked and,
to some extent, exclude each other. As described
above, the PSI factors influence the balance
between asexual and sexual development (Champe
et al. 1987). This phenomenon has been studied
recently at the molecular level, and it was found
that three fatty acid oxygenases (PpoA, PpoB and
PpoC) are involved in the biosynthesis of these
molecules. Deletion of ppoC caused a reduction of