Growth, Differentiation and Sexuality

276 R. Fischer and U. Kües
PsiB level, and increased the ratio between asexual
and sexual development (Tsitsigiannis et al. 2005).
Earlier on, it was already noticed that overexpression
of the central developmental regulator
brlA causes a block of vegetative growth, and that
hyphae produce single spore-like structures at their
tips (see above). By comparison, overexpression
of abaA also causes a block in hyphal extension
in liquid medium, but no differentiation of hyphae
(Adams and Timberlake 1990). Vice versa,
many mutants of a group named fluffy produce
masses of vegetative hyphae and are not able to
undergo asexual or sexual development (Fig. 14.4).
Recent experiments suggest a similar relationship
between sexual and asexual development. Wildtype
A. nidulans (veA+) develops mainly sexually
when grown in the dark, and asexually when grown
in the light. In the absence of the veA gene, strains
are completely asexual and develop asexually, independently
of light conditions. Therefore, it has been
contended that veA mediates the light response
(Käfer 1965; Kim et al. 2002). However, another explanation
could be that veA is required for sexual
development, and that the occurrence of the sexual
cycle inhibits the asexual cycle. Similarly, deletion
of nsdD causes this shift between the two developmental
cycles. Deletion mutants develop purely
asexual, independently of light (Han et al. 2001;
Vienken and Fischer, unpublished data).
Nice examples of how development is coupled
to cellular processes are presented by a number
of mutations which affect cell cycle regulation or
nuclear distribution. Initial evidence for the specific
requirement of genes necessary for vegetative
cell functions came from the analysis of two
mutants, apsA and apsB, isolatedinthescreenof
Clutterbuck (1969). These mutants developed conidiophores
until the metula stage (Fig. 14.3). However,
metulae remained anucleate and, thus, development
did not proceed beyond this stage. Both
aps genesencodeproteinswithgeneralfunctions
in nuclear migration and microtubule organisation
(Fischer and Timberlake 1995; Suelmann et al.
1998; Veith et al. 2005). ApsA appears to mediate
microtubule cortex interactions, which are important
for nuclear positioning. ApsB is a novel spindle
pole-associated protein which regulates the activity
of microtubule-organising centres in the cell.
Mutation of either of the two genes disturbs the
normal arrangement of cytoplasmic microtubules
(Veith et al. 2005). Whereas in apsA mutants microtubules
appear longer, in apsB mutants the number
of cytoplasmic microtubules is reduced. These fea-
turesappeartobecrucialfornuclearmigration
into metulae.
Genes involved in cell cycle regulation are other
examples earmarked for functional duality in basic
cell biology and in development. Metulae and
phialides are both uninucleate and, whereas nuclei
in metuale do not divide after two or three
phialides are produced, nuclei in phialides divide
continuously to provide all conidia with nuclei. Nuclear
division is strictly coordinated to cytokinesis.
This indicates that the cell cycle of close neighbour
cells may be regulated in an opposite manner,
i.e. arrested in metulae and adjusted to continuous
spore production in phialides. The molecular basis
for this is not yet completely understood. The
laboratory of Osmani found that two key regulators
of the A. nidulans cell cycle, NimX and NimA,
are adjusted in their activity during conidiophore
development (Ye et al. 1999; see Chap. 3, this volume).
Recently, a new cyclin, pclA, was identified
in a developmental mutant screening (Schier et al.
2001). Deletion of this gene causes abnormal conidiophores,
resembling the abaA mutants. Although
this cyclin interacts with NimX, it is not clear yet
whether the cell cylce in phialides is dependent on
this interaction (Schier and Fischer 2002). In Aspergillus
oryzae, the restriction to one nucleus per
conidium does not exist, and thus conidia normally
contain two nuclei, which migrate from the phialide
into the conidium (Ishi et al. 2005).
B. Other Ascomycetes
Whereas A. nidulans asexual spore formation has
been studied in detail over the past 30 years, our
knowledgeofthegeneticregulationofsporulation
in other mycelial ascomycetes is rather limited,
and only some isolated components have been
described. Among the better-studied species are
N. crassa and the opportunistic human pathogenic
Penicillium marneffei. The latter organism recently
gained attention, because morphogenesis appears
to be related to pathogenicity (see Chap. 1, this
volume; Boyce et al. 2005). This fungus undergoes
a dimorphic switch. It grows in a hyphal form
at 25 ◦ C whereas it proliferates yeast-like cells
through arthroconidiation at more elevated temperatures.
The hyphal form produces Penicilliumtypical,
brush-type conidiophores with four to five
metulae at the tip of the stalk, each of which bears
three to seven phialides producing the conidia.
The yeast form, by contrast, is obtained by fission