Growth, Differentiation and Sexuality

128 N.L. Glass and A. Fleissner
Fig. 7.3. A–C Stages of hyphal fusion. A The presence of
a fusion-competent hypha often results in the formation
of a peg in the receptive hypha. Peg formation is associated
with the formation of a Spitzenkörper (arrow) atthetip
of the new peg (n nuclei, S Spitzenkörper). B Contact
between fusion hyphae is associated with a switch from
polar to non-polar growth, resulting in a swelling of the
fusion hyphae at the point of contact. The Spitzenkörper
isassociatedwiththesiteofthefutureporeinbothfusion
hyphae. C Pore formation is associated with cytoplasmic
flow. A vacuole is shown traveling through the nascent
pore. Nuclei and mitochondria also pass through the
fusion pore. Septation is also often associated with hyphal
fusion events (fp fusion pore, sp septa). Hyphae stained
with FM4-64. Images adapted from Hickey et al. (2002),
reprinted with permission. Bar = 10 μm
nificantly, all of the hyphal fusion mutants so far
identified in N. crassa are also defective in germling
fusion (see below).
A number of diffusible substances that mediate
developmental processes occur in filamentous
ascomycete fungi (see also Chap. 11, Autoregulatory
signals in mycelial fungi, this volume), including
peptide pheromones (Zhang et al. 1993,
1998; Pöggeler 2000; Pöggeler and Kück 2001; Kim
et al. 2002; Kim and Borkovich 2004) and diffusible
products required for conidiation in A. nidulans
(Champe and el-Zayat 1989; Adams et al. 1998;
Tsitsigiannis et al. 2004). In the dimorphic fungusCandida
albicans, farnesol is a quorum-sensing
molecule that prevents the switch from the yeast
phase to the filamentous growth phase (Hornby
et al. 2001), while tyrosol stimulates the formation
of germ tubes (Chen et al. 2004). With regards to hyphal
fusion, in the basidiomycete species R. oryzae
the elimination of water-soluble substances surrounding
hyphae inhibited hyphal attraction and
fusion (Bhuiyan and Arai 1993), although the nature
of these molecules is unknown.
The genes for peptide pheromones that are believedtoplayaroleinmatinghavebeenidentified
from a number of filamentous ascomycete
species (Zhang et al. 1998; Shen et al. 1999; Pöggeler
2000; Bobrowicz et al. 2002; Kim et al. 2002; Turina
et al. 2003). In N. crassa, pheromonemutants
are male-sterile, but female-fertile (Kim et al.
2002). In N. crassa and Cryphonectria parasitica,
a vegetative phenotype has also been associated
with the pheromone mutants, indicating a function
of these substances beyond sexual development.
A C. parasitica pheromone mutant shows
reduced numbers of asexual fruiting bodies (pycnidia)
and spores (Zhang et al. 1993, 1998). In
N. crassa, a strain containing a mutation in mfa-1
(which encodes mating factor a-1) showsreduced
vegetative growth and aberrant sexual development
(Kim et al. 2002). In N. crassa, thegenesfor
the peptide pheromones are under the regulation
of the mating-type (mat) genes (Bobrowicz et al.
2002), which encode putative transcriptional regulators
(Glass et al. 1990; Staben and Yanofsky 1990).
Unlike the mfa-1 pheromone mutant, the mat mutants
have a wild-type growth phenotype, although
they are sterile as males and females. Significantly,
both mat A and mat a mutants are competent to
undergo hyphal fusion (Griffiths 1982).
2. Receptors
Cells recognize extracellular signaling molecules
by means of different types of receptors. One
class consists of 7-transmembrane G-protein
coupled receptors (GCPR), such as the mating
pheromone receptors in S. cerevisiae (Kurjan
1993; Dohlman and Thorner 2001; Fig. 7.4). In
N. crassa and Sordaria macrospora, the putative
pheromone receptors show amino acid identity
to the a-pheromone GCPR of S. cerevisiae and
to the lipopeptide pheromone receptors of basidiomycete
species (Pöggeler and Kück 2001;
Kim and Borkovich 2004). In N. crassa, mutation
of the putative pheromone receptor gene, pre-1,
results in female sterility, although the mutants
are morphologically normal and form female
reproductive structures; hyphal fusion in the
pre-1 mutant is normal (Kim and Borkovich
2004). Genome sequence analysis of the N. crassa
genome has revealed a large number of additional