SUMMARY The formation of mitotically derived spores, called conidia, is a common reproductive mode in filamentous fungi, particularly among the large fungal class Ascomycetes. Asexual sporulation strategies are nearly as varied as fungal species; however, the formation of conidiophores, specialized multicellular reproductive structures, by the filamentous fungus Aspergillus nidulans has emerged as the leading model for understanding the mechanisms that control fungal sporulation. Initiation of A. nidulans conidipohore formation can occur either as a programmed event in the life cycle in response to intrinsic signals or to environmental stresses such as nutrient deprivation. In either case, a development-specific set of transcription factors is activated and these control the expression of each other as well as genes required for conidiophore morphogenesis. Recent progress has identified many of the earliest-acting genes needed for initiating conidiophore development and shown that there are at least two antagonistic signaling pathways that control this process. One pathway is modulated by a heterotrimeric G protein that when activated stimulates growth and represses both asexual and sexual sporulation as well as production of the toxic secondary metabolite, sterigmatocystin. The second pathway apparently requires an extracellular signal to induce sporulation-specific events and to direct the inactivation of the first pathway, removing developmental repression. A working model is presented in which the regulatory interactions between these two pathways during the fungal life cycle determine whether cells grow or develop.

flbA encodes an Aspergillus nidulans RGS (regulator of G protein signaling) domain protein that is required for control of mycelial proliferation and activation of asexual sporulation. We identified a dominant mutation in a second gene, fadA, that resulted in a very similar phenotype to flbA loss‐of‐function mutants. Analysis of fadA showed that it encodes the alpha‐subunit of a heterotrimeric G protein, and the dominant phenotype resulted from conversion of glycine 42 to arginine (fadA(G42R)). This mutation is predicted to result in a loss of intrinsic GTPase activity leading to constitutive signaling, indicating that activation of this pathway leads to proliferation and blocks sporulation. By contrast, a fadA deletion and a fadA dominant‐interfering mutation (fadA(G203R)) resulted in reduced growth without impairing sporulation. In fact, the fadA(G203R) mutant was a hyperactive asexual sporulator and produced elaborate sporulation structures, called conidiophores, under environmental conditions that blocked wild‐type sporulation. Both the fadA(G203R) and the fadA deletion mutations suppressed the flbA mutant phenotype as predicted if the primary role of FlbA in sporulation is in blocking activation of FadA signaling. Because overexpression of flbA could not suppress the fadA(G42R) mutant phenotype, we propose that FlbA's role in modulating the FadA proliferation signal is dependent upon the intrinsic GTPase activity of wild‐type FadA.

Conidiation in the filamentous ascomycete Aspergillus nidulans requires activation of brlA, a well-characterized transcriptional regulator of genes that are induced specifically during asexual development. We have isolated and characterized developmental mutations in six loci, designated fluG, flbA, flbB, flbC, flbD, and flbE, that result in defective development and reduced brlA expression. These mutants grow indeterminately to produce masses of aerial hyphae resulting in the formation of cotton-like colonies with a "fluffy" morphology. The results of growth and epistasis tests involving all pairwise combinations of fluffy mutations indicate complex hierarchical relationships among these loci. We discuss these genetic interactions and propose that there are multiple mechanisms for activating brlA.

The timing of asexual fruiting body formation during Aspergillus nidulans colony development is precisely regulated so that conidiophores are typically produced 1-2 mm behind the growing edge of the colony. [Key Words: flbD; myb; conidiation; fungi; brlA; microbial development] Received December 7, 1994; revised version accepted January 20, 1995. Asexual reproduction in the filamentous fungus Aspergillus nidulans is a continual progression that begins with newly forming hyphae at the growing edge of the colony and progresses through differentiation of specialized cells 1-2 mm behind the leading edge of the mycelium and gives rise to multicellular spore-bearing structures, termed conidiophores (Navarro-Bordonaba and Adams 1994). The initiation of A. nidulans conidiophore development, in contrast to initiation of sporulation in many other microorganisms, is not normally triggered by unfavorable environmental conditions but instead occurs as a precisely timed, genetically programmed event in the life cycle in response to internal and external cues (Pastushok and Axelrod 1976;Champe et al. 1981;Adams et al. 1992). The molecular genetic events regulating onset of conidiation are not fully understood but ultimately result in the controlled activation of a transcriptional cascade leading to the expression of hundreds of essential (Clutterbuck 1969; Martinelli and Clutterbuck 1971;Stringer et al. 1991) and nonessential genes (Aramayo et al. 1989). Several lines of evidence indicate that a primary activator of sporulation-specific gene expression is BrlA, a C2H2 zinc finger nucleic acid-binding protein (Adams et al. 1988(Adams et al. , 1990Chang and Timberlake 1992). brlA mRNA is present at extremely low levels in hyphae and begins to accumulate shortly after developmental induction (Boylan et al. 1987;Han et al. 1993;Prade and Timberlake 1993). brlA is required for activation of other sporulation-specific genes including at least two developmental regulatory genes, abaA and wetA while brlA null mutants produce abnormal developmental structures that initially resemble conidiophore stalks but grow indeterminately, failing to differentiate other specialized cell types (Clutterbuck 1969;Boylan et al. 1987). The pivotal role for brlA in controlling activation of the conidiation pathway is most strongly supported by the observation that forced activation of brlA in vegetative hyphae re- expression (Wieser et al. 1994). Mutations in any one of these genes result in colonies that fail to make the programmed switch from undifferentiated vegetative hyphal growth at the edge of the colony to conidiophore development within. These mutants have diverse phenotypes, but all produce large masses of undifferentiated aerial hyphae that give the colony a "fluffy" or cottonlike appearance. Fluffy mutants resulting from loss of fluG function can be distinguished from strains with mutations in any of the other five genes in at least two ways. First, although fluG mutants are unable to activate brlA expression normally, the requirement...

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