Growth, Differentiation and Sexuality

of the double septa of hyphae (Chan and Chow
1990; Cooper and McGinnis 1997; Vossler 2001).
In a systematic approach, the group of Andrianopoulos
isolated functional homologues of
A. nidulans conidiation regulators in P. marneffei,
and generally found conserved functions for
the proteins (Borneman et al. 2000, 2002). The
main regulators of conidiation, brlA and abaA, of
A. nidulans appear to be conserved (Table 14.1;
Bornemann et al. 2000; Todd et al. 2003). A stuA
defect in P. marneffei shows that StuA is needed
for metulae and phialide formation (Bornemann
et al. 2002). Although there are conserved players,
there are also variations in these processes. Contrasting
with A. nidulans rco1 mutations, a mutation
in TupA (the Tup1-type transcription factor
of P. marneffei)confersprematurebrlA-dependent
asexual development (Todd et al. 2003). GasA is
a close homologue to A. nidulans FadA and the
major Gα protein blocking conidiation in P. marneffei.
This protein seems to act through the cAMP
signalling pathway in order to inhibit expression of
brlA.TheGα protein GasC, the likely GanB homologue
of P. marneffei, negatively affects the onset of
conidiation and conidia yield but GasC is involved
in sensing neither carbon nor nitrogen depletion.
Moreover, GasA and GasC are not required for dimorphic
switching and regulation of arthroconidiation
(Zuber et al. 2002, 2003). Yeast cell production
is, however, regulated by TupA. Inactivation
of the tubA gene leads to inappropriate yeast morphogenesis
at 25 ◦ C (Todd et al. 2003). Arthrospore
production in Acremonium chrysosporium is a process
comparable to arthoconidiation in P. marneffei.
CPCR1, a winged helix transcription factor
described in A. chrysosporium, is the first regulator
known to be required for hyphal fragmentation
(Hoff et al. 2005).
In another Penicillium species, Penicillium
chrysogenum, the putative regulator WetA was
shown to be conserved in function in conidiogenesis
(Prade and Timberlake 1994). The rice
pathogen M. grisea has holoblastic, three-celled
conidia borne sympodially on an aerial conidiophore
(Shi and Leung 1995). A defect in gene ACR1
(acropetal), a homologue to A. nidulans medA,
causes indeterminate growth leading to chains of
spores arranged in a head-to-tail manner (Lau
and Hamer 1998; Nishimura et al. 2000). The Gα
protein MAGB, homologous to A. nidulans GanB,
isknowntobeanegativeregulatorofconidiation
in this species (Fang and Dean 2000). The vascular
wilt-causative Fusarium oxysporium produces
Fungal Asexual Sporulation 277
three types of asexual spores (falcate macroconidia,
ellipsoidal microconida and globose
chlamydospores). Mutants of the stuA homologue
FoSTUA lack conidiophores and produce only low
amounts of macroconidia from intercalary hyphal
philades whereas chlamydospore production is
strongly promoted and microconidiation is not
affected (Ohara and Tsuge 2004). REN1,themedA
homologue in F. oxysporium, specifically acts
in macroconidia formation. REN1 mutants lack
normal conidiophores and phialides, and form
rod-shaped, conidium-like cells directly from
hyphae by acropetal division (Ohara et al. 2004).
InthecaseofN. crassa, three different types
of asexual spores are produced (Perkins and Barry
1977; Russo and Pandit 1992; Borkovich et al. 2004).
Single-nucleate, enteroblastic ovoid microconidia
emerge within vegetative hyphae (microconidiophores)
as a lateral protuberance of a hyphal cell
which ruptures the cell wall of the phialidic mother
cell, and is released upon septum formation from
the phialide, leaving behind a hole in the hyphal cell
wall. Roundish, multi-nucleate holoblastic macroconidia
arise in long chains through apical constriction
budding at the ends of aerial hyphae. After
separation of the macroconidia from the aerial
hyphae, longer arthroconidia are formed by laying
extra crosswalls within the aerial hyphae, and splitting
the resulting hyphal sections (Springer 1993;
Fig. 14.7). The formation of micro- and macroconidia
depends on different environmental conditions
and distinct morphogenetic pathways. Mutants in
macroconidia formation may not be blocked for
microconidiation (Maheshwari 1999).
A number of genes preferentially expressed
during conidiation (con genes) have been cloned in
N. crassa and studied in expression (con-6, con-8,
con-10, con-11 and con-13) but their exact cellular
functions are not yet clear (Roberts et al. 1988;
Roberts and Yanofsky 1989; Hager and Yanofsky
1990; White and Yanofsky 1993; Corrochano et al.
1995). In other cases, functions of the genes are
known, some of these recurring from conidiation
in A. nidulans whereas others are new. The vvd
gene for the flavoprotein blue-light photoreceptor
Vivid is required for light adaptation of
conidiation-specific genes to daily changes in
light intensity, but not for circadian conidiation
(Heintzen et al. 2001; Shrode et al. 2001; Schwerdtfeger
and Linden 2003). There is a structural, lightand
clock-regulated gene eas for a spore-specific
hydrophobin (Bell-Petersen et al. 1992; Lauter et al.
1992) which has now been shown to be directly