A morphological and genetic analysis of conidiophore development in Neurospora crassa

The filamentous fungus Neurospora crassa responds to nutrient deprivation and dessication by producing asexual spores, or conidia. These conidia are derived from differentiated aerial structures called conidiophores. The process of conidiation was analyzed in wild-type and morphological mutants using scanning electron microscopy (SEM) and specific fluorescent probes. The first discernible morphological step of conidiation is the transition from growth by hyphal tip elongation to growth by repeated apical budding, resulting in the formation of chains of proconidia that resemble beads on a string. The initial proconidial chains are morphologically distinct from those that form later and are capable of reverting to hyphal growth, whereas the later chains are committed to conidiation. As the proconidial chains are formed, nuclei migrate into the conidiophore, and cross-walls arise between adjoining proconidia in a series of steps that have been defined by staining with Calcofluor, a fluorescent chitin-binding probe. The chains ultimately disarticulate in several discrete stages into free, morphologically mature conidia. Different conidiation-defective mutants were shown to be blocked at distinct stages in conidiation. Our observations permit us to derive a developmental timeline of conidiation relating the occurrence of morphological changes and the stage blocked in specific mutants.