Growth, Differentiation and Sexuality

270 R. Fischer and U. Kües
pathways are interconnected. By comparison,
deletion of gprA and gprB affected neither asexual
development nor hyphal growth, but did affect
self-fertilization (Seo et al. 2004). Functions for the
remaining six GPCRs have not yet been assigned.
In addition to seven-transmembrane receptors,
receptors for light perception have been
characterized to some extent. Two homologues of
the N. crassa WC proteins (“white collar proteins”),
acting as photoreceptors for the biological clock
(Liu et al. 2003; see Chap. 13, this volume, and
The Mycota, Vol. III, 2nd edn., Chap. 11), were
characterized in A. nidulans, although deletion of
the genes did not result in an obvious phenotype
(H. Haas, Innsbruck, Austria, personal communication).
By contrast, deletion of a phytochromeencoding
gene resulted in a slight derepression
of sexual development (cf. Chap. 13, this volume;
Blumenstein et al. 2005). A. nidulans also contains
an opsin-related gene, whose functional analysis is
under way (Schaaf and Fischer, unpublished data).
b) Heterotrimeric G-Proteins
The first indication for the involvement of Gproteins
in the regulation of asexual development
came from the analysis of the fluffy mutant flbA
(Lee and Adams 1994a; Yu et al. 1996; Fig. 14.4).
The corresponding gene encodes a regulator of
G-protein signalling (RGS protein), and negatively
controls vegetative growth signalling. Mutation
of the gene causes massive hyphal growth and
a loss of asexual and sexual spore formation. RGS
proteins contain a domain of about 130 amino
acids, which interact with the switch regions of
activated Gα subunits and subsequently act as
GTPase-activating proteins (Chidiac and Roy
2003). Hyphal growth is so strongly stimulated
in flbA mutants that hyphae even autolyze after
prolonged incubation (Fig. 14.4). In an approach
to isolate dominant mutations causing a similar
autolytic phenotype, the group of Adams isolated
the fadA (fluffy autolytic dominant A) mutant,
which turned out to encode an α subunit of
a heterotrimeric G-protein (Yu et al. 1996). The
mutation was a conversion of glycine 42 to arginine,
which supposedly causes a loss of GTPase
activity. Deletion of fadA caused a reduction of
vegetative growth and a stimulation of asexual
sporulation. Subsequently, in a screening for
extragenic suppressors of flbA signalling, sfaD
was discovered, and was found to encode the β
subunit (Rosén et al. 1999; Yu et al. 1999). Deletion
of the gene caused hyperactive sporulation and
Fig. 14.4.A–D fluffy mutants are characterized
by a cotton-like appearance on
agar plates. In A,awildtype(left colony)
and a fluffy mutant (right colony) were
inoculated on an agar plate and incubatedfor3daysat37
◦ C. B One class of
fluffy mutants undergoes lysis of the hyphae
after prolonged incubation. After
5–6 days, lysis is obvious in the middle
of the colony (arrow), and C after 8
days almost all mycelium disappeared.
D Diagram showing the regulatory interactions
of some of the fluffy genes
and the downstream components. Two
antagonistic signalling pathways appear
to regulate A. nidulans growth and development.
Growth signalling is mediated
by the FadA G-protein α subunit.
Activation of FadA by exchange of GDP
for GTP results in a proliferative phenotype
and blocks sporulation. To induce
conidiation, FlbA has to be activated,
whichinturndeactivatesthegrowth
signalling and favours the differentiation
pathway. A–C were extracted from
Fischer and Kües (2003). D was modified
after Adams et al. (1998) and supplemented
with additional data by J. Yu
(Madison, USA)