A plastic vegetative growth threshold governs reproductive capacity in Aspergillus nidulans

Threshold-limited ontogenic phases separating somatic growth from reproduction are a common feature of cellular life. Long recognized for flowering plants and animals, this life-history component may also be prevalent among multicellular fungi. We establish the environmental and genetic basis of developmental competence, the capacity to respond to induction of asexual development, in the model filamentous saprotroph Aspergillus nidulans. Density and pH are critical parameters for competence timing, and we identify five genes with heterochronic effects through genetic screens and candidate mutagenesis, including the conserved GTPase RasB and ambient pH sensor PalH. Inheritance of competence timing is quantitative, semi-dominant, transgressive, and extremely variable among progeny. Transcriptional profiling over competence acquisition demonstrates substantial activity in metabolic and signaling networks, highly concordant across species, and a wave of gene expression around centromeres indicative of chromatin remodeling. Competence, likely determined by species-specific endogenous hormones and metabolic capacity, governs much of biology associated with the mature fungal form - asexual and sexual reproduction, secondary metabolism, and, in some species, pathogenesis - and provides a new model for nutrient-limited life-history phases and their elaboration from unicellular origins.