Growth, Differentiation and Sexuality

derived from linoleic (18:2), psiα, andthosefrom
linolenic (18:3), psiγ. The placing of the hydroxyl
groups gave rise to the terms psiB (hydroxyl
substitution in position 8), psiC (positions 5 and
8) and psiA (a lactoniester of psiC; Champe and
El-Zayat 1989; Mazur et al. 1990, 1991).
Dosage studies with psiα oxylipins indicated
that marked differences in biological activity
could be obtained between applications of psiBα
(8-hydroxylinoleic acid; Fig. 11.4a) and psiCα
(5,8-dihydroxylinoleic acid; Fig. 11.4b), which
stimulated sexual spore development to the
detriment of conidiation, and psiAα (a lactonized
ester of psiCα; Fig. 11.4c) which acted as an
antagonist, shifting the balance towards asexual
spore development.
Recent studies by Tsitsigiannis et al. (2004a,b)
have further identified two A. nidulans dioxygenases
(enzymes which catalyse the oxygenation of
unsaturated fatty acids) PpoA and PpoC, which are
required for the biosynthesis of psiBα and psiBβ
respectively (Fig. 11.4a,d). The former oxilipin was
confirmed to cause a decreased ratio of conidia to
Fig. 11.4. A–E Autoregulatory signals involved in sexual
development: A psiBα, B psiCα, C psiAα, D psiBβ, E zearalenone
Fungal Mycelial Signals 209
ascospores whereas the latter had a reversed effect,
increasing ascospore production and decreasing
conidial development. The concerted regulation
of both enzymatic activities through gene expression
was shown to be complex and dependent
on, but also modulating the expression of brlA and
nsdD, central regulator genes of conidiogenesis and
ascogenesis respectively. Moreover, detailed transcriptional
and biochemical analyses showed that
psiBα and psiBβ possibly act as antagonist signals
in regulatory feedback loops which ultimately control
lipid biosynthesis.
The above studies appear as the initial unveiling
of a sophisticated regulation system combining
an array of oxylipin derivatives which may collectively
modulate the balance between asexual and
sexual development. The formulation of such endogenous
autoregulatory signals is also known to
integrate exogenous stimuli, and one environmental
cue which affects the equilibrium between sexual
and asexual development in wild-type cultures
of Aspergillus nidulans is light, which stimulates
conidiation and represses sexual development. An
active veA gene, encoding for light sensitivity in
this organism (Yager 1992), is essential for sensitivity
to externally added linoleic acid derivatives
(Champe et al. 1987), and even affects fatty acid
content and composition (Calvo et al. 1999). Thus,
the interconnections between the light sensory system
and oxylipin signalling has been established,
although the details still remain to be elucidated.
The topic of sexual development and its regulation,
with special reference to Aspergillus nidulans,
is covered in the Chaps. 14 (Asexual sporulation in
mycelial fungi) and 16 (Fruiting body development
in ascomycetes) in this volume.
Another example of self-generated metabolites
influencing sexual development is that
observed in Fusarium graminearum by zearalenone
(Wolf and Mirocha 1973). Zearalenone
(3,4,5,6,9,10-hexahydro-14,16-dihydroxy-3-methyl-
1H-2-benzoaxacyclotetradecin-1,7(8H)-dione, Fig.
11.4e) was notorious for its oestrogenic effects
on mammals which had ingested fodder contaminated
with the fungus, but it was shown
that its principal role was to induce perithecial
formation at quantities as low as 0.1 ng per 1-cmdiameter
filter disks on a Petri dish. A study of
the structure–function relations of this molecule
showed that the ketone in the 6 ′ position of the
undecenil ring was necessary for activity. An
inactive derivative (zearalenol) found in culture
filtrates bears a hydroxyl group at the same