Growth, Differentiation and Sexuality

208 U. Ugalde
duction (Schimmel et al. 1998). Interestingly,
small γ-butyrolactone-containing molecules are
known quorum-sensing factors in gram-negative
bacteria, and also regulate cellular differentiation
in filamentous bacteria (Horinouchi and Beppu
1992).
The autoregulatory signal responsible for the
induction of conidiation in the model organism
Aspergillus nidulans was first recognised in genetic
studies which led to the isolation of a mutant defective
in the fluG gene, which only conidiated when
in contact with a wild-type colony (Lee and Adams
1994). The signalling factor which is transferred
when such extracellular complementation events
take place remains unidentified.
What are believed to be the principal systems
which signal conidiation induction may be modulated
by the action of other autoregulatory cues
which exert their action under specific ecological
circumstances. Recent studies by Calvo et al.
(1999) have shown that linoleic acid and two
oxylipin derivatives which are commonly found
in seeds (9S- and 13S-hydroperoxylinoleic acid)
had a sporogenic effect when administered on
membrane disks over Petri dishes inoculated
with 10 5 spores. The positive effects with Aspergillus
flavus, A. parasiticus and A. nidulans
were compatible with earlier reports describing
similar effects on Alternaria tomato (Hyeon 1976),
Sclerotinia fructicola (Katayama and Marumo
1978) and Neurospora crassa (Roeder et al. 1982).
In the latter, fluctuations in linoleic acid levels
at the hypha tips were related to oscillations
in the circadian rhythm of conidiation. In A.
flavus, linoleic acid and two of its hydroxylated
derivatives produced a conidiation halo around
the membrane disk, while saturated counterparts
such as oleic and palmitic acid had no effect.
Similar findings were recorded with the other
two species but, in A. nidulans strains which
had a mutated velvet (veA) gene which makes
conidiation light-dependent, the effects were weak
or negligible. Indeed, light was a determinant
factor in the efficacy of the unsaturated fatty
acid treatments in all wild-type strains (Calvo
et al. 1999). The authors proposed that seed
fatty acids may regulate fungal development by
mimicking and/or interfering with signals which
regulate fungal sporogenesis, in reference to psi
factors, which are also derived from linoleic acid
and are responsible for governing the balance
between asexual and sexual development (see
below).
The evidence accumulated in recent years supports
the view that mycelial fungi use autoregulatory
signals for the perception of adequate conditions
to initiate asexual development. These activate
signal transduction systems which also integrate
exogenous cues related to stress conditions
(namely, starvation and osmotic shock; Roncal and
Ugalde 2003). Aside from this system, autoregulators
are also involved in establishing the balance
between asexual and sexual development, as discussed
below.
V. Sexual Development
In addition to the formation of asexual spores,
which often serve as short-term dispersal propagules,
mycelial fungi also produce long-term resistance
structures capable of enduring changing
environmental conditions. In many cases, those
structures enclose spores obtained by meiotic division,
thus ensuring maximum recombination from
the genetic resources available in the colony. This
strategy is exemplified by the fruiting bodies of the
ascomycetes, the production of which appears to
be determined by a combination of environmental
cues and endogenous autoregulatory signals.
Pioneering work on autoregulatory signals
which determine the onset of sexual development
was initiated in the early 1980s by Sewell
P. Champe and collaborators, who undertook
a multidisciplinary approach. For a start, a series
of aconidial mutants of Aspergillus nidulans were
isolated, which typically showed low conidiation
levels, premature production of cleistothecia,
and the excretion of the antibiotic diorcinol
(3,3 ′ -dihydroxy-5,5 ′ -dimethyldiphenyl ether; Butnick
et al. 1984a,b). Preliminary analysis of these
mutants already led the investigators to conclude
that asexual and sexual development were
controlled by a common modulating pathway.
It was later observed that the overproduction
of a solvent-extractable chemical factor (psi
factor, standing for premature sexual induction)
was responsible for the phenotype. Even when
administered to wild-type strains, the factor
could inhibit conidiation and promote sexual
development (Champe et al. 1987). Three types of
hydroxylated derivatives of C18 unsaturated fatty
acids were identified, based on the starting fatty
acid molecule subject to hydroxylation. Those
based on oleic acid (18:1) were termed psiβ, those