As the mycelium is usually hidden in the substratum, the most conspicuous feature of many fungi is the conidiophore, a device for producing almost countless numbers of mitotically derived spores with the economy of space and time. Characteristically, fungi have airborne spores that serve to disseminate the fungus widely, cause diseases in epi-phytotic proportion or cause respiratory diseases as allergens. The conidiophore morphology and modes of development of conidia are an important taxonomic character in the classification of fungi. The synchronized and rapid development of relatively few cell types in Aspergilllus nidulans, initiated on schedule and at the correct locations and generating a characteristic form, presents opportunities for the study of eukaryotic development. Since different stimuli are involved in conidiation in different fungi (or in the same fungus), a coordinated signal transduction pathway integrating the perception of stimuli is hypothesized. Genetic evidence from A. nidulans shows that asexual reproduction is triggered by environmental factors and conidiophore development occurs in sequence. In response to environmental signals, the mycelium switches to a built-in conidiation pathway involving the transduction of signals to other genes, including flbB, flbC, flbD and flbE, that finally results in the activation of the central regulatory gene brlA. Three genes, brlA, abaA and wetA, are proposed to define a central regulatory pathway that controls the expression of conidiation-specific genes. These three genes are sequentially expressed and coordinate to direct conidiophore and conidia formation. Among them, brlA plays a major role as deduced by the fact that its forced expression is sufficient to direct conidiation. Genes homologous to brlA, abaA and wetA are found in other fungi, indicating that the mechanisms controlling development may be evolutionarily conserved. The complete genome sequence of A. nidulans will make possible the determination of the number of genes from the open reading frames. The genes with a role in sporulation can be analyzed using DNA microarrays and can address issues of whether the physical linear order of the genes is related to the time of their expression, the numbers of clusters of co-expressed genes and the effects on gene expression of mutation in a candidate regulatory gene.
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