9.5 Conidiation in Neurospora crassa
N. crassa forms two types of conidium, microconidia and macroconidia. Microconidia are small uninucleate spores which are essentially fragmented hyphae. They are not well adapted to dispersal and are thought to serve primarily as ‘male gametes’ in sexual reproduction. Macroconidia are more common and more abundant, they are large multinucleate, multicellular spores produced from aerial conidiophores. Conidiation (and sexual reproduction, too) in N. crassa seems to respond more to environmental signals than to complex genetic controls like those operating in Aspergillus. Macroconidia are formed in response to nutritional limitation, desiccation, change in atmospheric CO2, and light exposure (blue light is most effective, and though light exposure is not essential, conidia develop faster and in greater numbers in illuminated cultures). In addition, a circadian rhythm provides a burst of sporulation each morning. When induced to form conidia, the Neurospora mycelium forms aerial branches which grow away from the substratum, form many lateral branches which become conidiophores and undergo apical budding to produce conidial chains.
The genetics of conidiation, studied by means of mutation and molecular analysis, reveal some parallels in terms of types of mutants obtained with N. crassa and A. nidulans, and a particular example would be the hydrophobic outer rodlet layer which is missing in the N. crassa ‘easily-wettable’ (eas) and A. nidulans rodA mutants. Despite such functional analogies, there is no underlying similarity between the genetic architectures used by these two organisms to control conidiation. Importantly, there is no evidence for regulatory genes in N. crassa which are similar to the brlA–abaA–wetA regulators of A. nidulans. Nevertheless, a large number of mutants have been isolated which have defects in particular stages of conidiation. A number of conidiation (con) genes are known which encode transcripts which become more abundant at specific stages during conidiation. At least four of these genes are expressed in all three sporulation pathways in Neurospora (macroconidia, microconidia and ascospores) but others have specific localisation to macroconidia. However, many of the con genes can be disrupted without affecting sporulation; so, despite being highly expressed during sporulation, they presumably encode redundant or non-essential functions.
Updated December 17, 2016