Growth, Differentiation and Sexuality

284 R. Fischer and U. Kües
tion, the A. nidulans key regulators BrlA, AbaA
and WetA are well conserved among Aspergilli and
Penicilli. Surprisingly, however, none of these regulators
appear to be present in the mycelial pathogen
M. grisea, and BrlA is not found at all in mycelial
fungi and yeast outside of the Aspergilli and Penicilli.
By contrast, AbaA is also found in several other
ascomycetesaswellasinU. maydis.Theroleofahomologue
in S. cerevisiae has already been discussed
above (see Sect. IV.A.2.f).
An example of a well-conserved protein is the
transcriptional regulator MedA which is found in
species of the three fungal classes ascomycetes, basidiomycetes
and zygomycetes. Homologues in M.
grisea and F. oxysporium were shown to function
in conidiogenesis but evidently in distinct morphological
patterns (Sect. IV.B). Furthermore, the
molecular role of this protein is not well characterizedalsoinA.
nidulans (Sect. IV.A.f). Protein
PhiA needed for phialide and conidia formation
(Sect. IV.A.f) is detected only in mycelial ascomycetes
with phialidic conidogenesis, suggesting
this to be a protein with a very specific function,
possibly in blastic cell wall assembly.
Many proteins of the A. nidulans “fluffy”group,
FlbA, FlbC and FlbD, are also conserved among ascomycetes.
However, FluG, the one protein which
is likely to be involved in the generation of a signalling
compound (see Sect. IV.A.2b), is found only
in Gibberellae zeae and N. crassa,outsideoftheAspergilli
and Penicilli. Of the “fluffy” genes, only
ageneforaFlbA-likeRGSproteinhasbeenfound
in the genomes of the basidiomycetes. Interestingly,
this gene was already known in S. commune for long
years as thn (for “thin”, due to loss of formation
of aerial mycelium in the corresponding mutants),
before it was cloned and shown to be a FlbA homologue
(Fowler and Mitton 2000). Strikingly, in
confrontations with wild-type strains, thn mutants
can be induced by diffusible low-molecular weight
molecules to produce abundant aerial mycelium
(Schuren 1999). The flbA gene in the basidiomycete
C. cinereus may also be needed for the production
of aerial mycelium. This in turn could be a prerequisite
for developing oidiophores on aerial mycelium
as special aerial structures for asexual spore production.
Otherwise in C. cinereus, fromthesetof
tested A. nidulans sporulation genes, we only found
a gene for a MedA transcription factor. In conclusion,
in this fungus we will have to expect to
find many new functions once cloned, e.g. from
the available oidiation mutants, and subsequently
characterized.
Similarly to the poor conservation of genes in
basidiomycetes, only three of eleven tested A. nidulans
sporulation genes were detected in Rhizopus
oryzae (FlbA, MedA, StuA). This suggests that sporangiophore
and endospore production in the zygomycetes
(for description, see Esser 2001) may
also follow an own, independent route.
The overall results of the rather limited gene
survey presented in Table 14.1 suggest that the
regulatory cascade elaborated in conidiogenesis in
A. nidulans may work similarly in closely related
species but it is not a general theme among fungi,
neither in distantly related fungi from other classes,
nor in all fungi belonging to the same class. The
limited range of experimental data we have by now
from different ascomycetes with both similar and
different morphological pathways of spore production
supports this interpretation. However, there is
also some experimental evidence suggesting conserved
gene functions which act in different morphological
pathways of sporulation (Sect. IV.B).
Even if all genes were present in another fungus,
and if this fungus follows a morphologically
similar mechanism of spore production and if this
other species were even a close relative, they may
still react differently than in A. nidulans. Arecent
finding by the group of Yu (Madison, USA) suggests
that assumptions and constructions of developmental
pathways only in silico should be viewed
with caution. The group discovered that deletion
of several A. nidulans fluffy genes in A. fumigatus
did not cause any severe developmental phenotype
(J. Yu, personal communication). Whether
this phenomenon is restricted to the fluffy genes, or
whether it is a general complication of the understanding
of the genetic regulation of sporulation in
other fungi needs to be established.
V. Concluding Remarks
Our knowledge on the regulation of spore formation
in mycelial fungi has expanded enormously
during the past 10 years, since Navarro-Bordonaba
andAdams reviewedconidiaproductionin A. nidulans
in the first edition of this volume (The Mycota,
Vol. I, 1st edn., Chap. 20). A number of novel components
have been discovered in A. nidulans and
other fungi, and gene–function relationships have
been described for many developmental genes. The
involvement of the main eukaryotic signalling cascades
has been demonstrated but a detailed un-