Growth, Differentiation and Sexuality

210 U. Ugalde
position, possibly indicating a time-dependent
inactivation of the signal through a reduction step.
Another interesting feature is that zearalenone
was inhibitory at concentrations higher than 10 ng
per disk, and the unsaturation at the 1 ′ ,2 ′ position
on the undecenyl ring was associated with this
feature.
Recent investigations have revealed a role of
self-generated reactive oxygen species (ROS) by
mycelial fungi on the onset of sexual development.
Lara-Ortiz and et al. (2003) demonstrated that
a NADPH oxidase (NoxA), which is a member
of a family of enzymes ubiquitous to lower
eukaryotes, plays a relevant role in cleistothecial
development in Aspergillus nidulans. Expression
of NoxA is specifically induced in Hülle cells and
the outer layers of cleistothecia initials, and its
deletion blocks maturation of cleistothecia at an
early stage. Cleistothecia and associated Hülle cells
both produce ROS (peroxide which is probably
dismutated to H2O2) during development, and
treatment with DPI (diphenyleneiodonium sulphate),
a substrate inhibitor of NADPH oxidases,
blocks the process. The authors therefore propose
that H2O2 regulates the differentiation of ascogenous
and peridial tissues. A homologue of NoxA in
Podospora anserina (PaNox1) has also been shown
to be involved in the differentiation of fruiting
bodies (Malagnac et al. 2004), showing that ROS
act as sexual development signals across a wide
range of ascomycete fungi. Whether they are
specific regulators of this morphogenetic process,
or act as a signal within the wider context of the
ageing colony (Lalucque and Silar 2003) remains
to be clarified.
VI. Dimorphism
The transition between yeast and mycelial forms
is a common feature in some genera and has
traditionally been associated with pathogenic
species, such as Candida albicans, Histoplasma
capsulatum and Penicillium marneffei (see The
Mycota, Vol. XII), although the connection
between pathogenicity and the morphogenetic
transition is a controversial issue. In C. albicans,
cellular density determines the morphological
transition between yeast and mycelial growth
since, at high cell densities (>10 6 cells ml −1 ), the
yeast form predominates whereas low cell densities
favour the production of germ tubes (Hornby
Fig. 11.5.A,B Autoregulatory signals involved in the yeast–
mycelium transition in Candida albicans.Farnesol(A)and
farnesoic acid (A, second part) promote the yeast form and
block the formation of germ tubes. Tyrosol (B) induces and
contributes to maintain the mycelial form
et al. 2001). Two molecules which were responsible
for the maintenance of the yeast form at high
cell densities were identified simultaneously as
farnesol (3,7,11-trimethyl-2,6,10-dodecatrien-1-ol;
Fig. 11.5a; Hornby et al. 2001) and farnesoic acid
(3,7,11-trimethyl-2,6,10-dodecatrienoate, Fig. 11.5a,
second part; Oh et al. 2001). Both compounds are
produced by yeast cells, and are effective in the
micromolar concentration range.
Recent studies by Chen et al. (2004) have shown
that the mycelial form, obtained at low-density cultures
(