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 Neurospora crassa and Aspergillus nidulans, and a particular example would be the hydrophobic outer rodlet layer which is missing in the Neurospora crassa ‘easily-wettable’ (eas) and Aspergillus nidulans rodA mutants. Despite such functional analogies, there is no underlying similarity between the genetic architectures used by these two organisms to control conidiation. In Neurospora, the fluffy gene (fl) is necessary to induce conidiophore development (Bailey-Shrode & Ebbole, 2004), and the acon-2, acon-3, and fld genes are essential for conidium development. And while there may be some similarity in regulatory strategy it is important to note that the genome sequence of Neurospora possesses no homologues of the brlA–abaA–wetA regulators of Aspergillus nidulans (Galagan et al., 2003).

Nevertheless, a great many mutants have been isolated which have defects in specific stages of conidiation. Several 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 non-essential or replaceable functions.

Transcriptional profiling has been used to study relative mRNA expression during colony development (Kasuga & Glass, 2008). The relative expression of genes involved in protein synthesis and energy production was enriched in the middle section of the colony, while sections of the colony undergoing asexual development (conidiogenesis) were enriched in expression of genes involved in protein/peptide degradation and other proteins of unclassified function. When colony development in Neurospora crassa and Aspergillus niger was compared, shared temporal and spatial patterns were observed in the regulation of gene orthologues.

Interestingly, the expression of phylogenetically conserved groups of genes was enriched in the middle section of a Neurospora crassa colony whereas expression of genes unique to Pezizomycotina species and of N. crassa orphan genes was enriched at the colony periphery and in the older, conidiating sections of a fungal colony. Orphan genes (also called ‘ORFans’) are taxonomically-restricted genes or genes without detectable homologues in other lineages. So, we take this observation to indicate that activities of genes specific to Neurospora crassa are enhanced in the mature central regions of the colony which is engaged in making spores, and at the colony margin in support of the Neurospora-specific hyphal extension growth.

In other words, observations from these surveys emphasise the diversity theme of this chapter by demonstrating that at the growing edge of the mycelium where the species-specific morphology of the mycelium is first established, species-specific panels of genes and transcripts are put into action. Similarly, in the central, maturing, regions of the mycelium where the species-specific sporulation takes place, the macromolecules used for the creative modelling required by spore formation are more species-specific (more diverse) than those deployed during spore germination.

The genomic transcriptional profile of conidial germination in Neurospora crassa has also been investigated (Kasuga et al., 2005), and suggests that there are rather more common regulatory features affecting spore germination between widely different species than are conserved in other stages of conidiogenesis. Cross-species comparisons of expression profiles during spore germination of the ascomycete Neurospora crassa, the basidiomycete Ustilago maydis and the cellular slime mould Dictyostelium discoideum revealed that some of the same sets of orthologous genes were activated or deactivated under comparable stages of spore germination among these very different organisms (and see Breakspear & Momany (2007) and McCluskey & Baker (2017) for other comparisons of expression profiles between organisms).

Updated July, 2019