Growth, Differentiation and Sexuality

268 R. Fischer and U. Kües
ular level (Champe et al. 1987; Champe and Simon
1992; Tsitsigiannis et al. 2004a,b).
IV. Genetics of Asexual Spore
Formation
A. Aspergillus nidulans
1. Cytology
More than 50 years ago, scientists chose A. nidulans
as a model system to study the biology of eukaryotic
cells (Pontecorvo et al. 1953). Asexual spore
formation is a phenotype which can be easily observed
with the naked eye, and therefore it represents
an attractive marker to study the role and
Fig. 14.1. Life cycle of A. nidulans (extracted from Scherer
and Fischer 1998)
nature of developmental genes (Clutterbuck 1969;
Figs. 14.1, 14.2). Asexual development starts with
a non-differentiated hyphal compartment, which
develops into a so-called foot. This cell produces
a stalk, without any septum between the two structures.
After the stalk has reached a height of about
100 μm, thetipswellsintoavesicle.Fromthis
structure, up to 70 cells, named metulae, emerge
in a budding-like process (Oliver 1972; Mims et al.
1988). Once a nucleus has entered the cell, a septum
forms at the base of the metula. Each metula
produces two to three phialides, which are also
uninucleate (Fischer and Timberlake 1995). The
phialides are the spore-producing cells and they
generate long chains of conidia, the youngest conidium
being at the bottom of the chain. In contrast to
hyphal cells, nuclear division and cell division are
strictly linked in metulae and phialides, and thus
these cells remain uninucleate (Mims et al. 1988). In
general, the arrangement of metulae and phialides
largely resembles that of pseudohyphae of S. cerevisiae,
and there is also molecular evidence that the
two morphogenetic pathways, conidiophore and
pseudohyphal development, share some conserved
components (see below; Gimeno et al. 1992).
2. Genetics
A. nidulans has been used intensively in mutagenesis
approaches, and several developmental
mutants have been isolated and studied genetically
(Clutterbuck 1969; Fig. 14.3). In the early
1980s, a transformation system was established,
and shortly thereafter the first developmental
regulator, the brlA gene, was cloned (Balance
et al. 1983; Johnstone et al. 1985). Since then,
many molecular components have been studied,
and thus asexual sporulation of A. nidulans has
become one of the best-studied morphogenetic
processes in mycelial fungi (Adams et al. 1998;
Fischer and Kües 2003). The isolation of regulators
and developmentally regulated genes facilitated
the detailed analysis of gene interactions. Known
molecular components are introduced below not
in chronological order but rather by functional
categories. In general, genes may be sorted into
three categories: genes whose deletion affects only
asexual development, those whose deletion affects
only sexual development, and a large group of
genes whose deletion affects both asexual and
sexual development. In addition, it has become
clear that both spore-producing pathways are
linked to vegetative hyphal growth.