Microcycle Conidiation

Although typically some amount of mycelial growth precedes the development of conid-iophore, in a type of development called microcycle conidiation, a germinating conidium totally bypasses the mycelium phase and directly develops a conidiophore (Maheshwari, 1991). For example, conidia of Neurospora crassa germinate on a nutrient agar medium. Normally the germ tubes fuse and mycelium is formed (Figure 7.2a), from which two types of conidia are produced: blastoconidia, by a process of budding (Figure 7.2b); and arthroconidia, by a process of septation (Figure 7.2c). In some strains grown in submerged-shake cultures, the conidia directly develop conidiophores (Figure 7.2d). In Aspergillus niger, microcycle conidiation can be induced under certain conditions of nutrition and temperature regime (Figure 7.3). Apparently, in this type of development, the conidiation genes are activated and expressed precociously. Microcycle conidiation could be a common feature of fungi growing in nature, allowing the formation of asexual propagules in the shortest possible time. The phenomenon of microcycle conidiation suggests that a master regulatory gene controls the expression of a large number of conidiation genes.

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