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cAdaptation to a changing environment is essential for the survival and propagation of sessile organisms, such as plants or fungi. Filamentous fungi commonly respond to a worsening of their growth conditions by differentiation of asexually or sexually produced spores. The formation of these specialized cell types is, however, also triggered as part of the general life cycle by hyphal age or density. Spores typically serve for dispersal and, therefore, translocation but can also act as resting states to endure times of scarcity. Eukaryotic differentiation in response to environmental and self-derived signals is commonly mediated by threetiered mitogen-activated protein (MAP) kinase signaling cascades. Here, we report that the MAP kinase Fus3 of the black mold Aspergillus niger (AngFus3) and its upstream kinase AngSte7 control vegetative spore formation and secondary metabolism. Mutants lacking these kinases are defective in conidium induction in response to hyphal density but are fully competent in starvation-induced sporulation, indicating that conidiation in A. niger is triggered by various independent signals. In addition, the mutants exhibit an altered profile of volatile metabolites and secrete dark pigments into the growth medium, suggesting a dysregulation of the secondary metabolism. By assigning the AngFus3 MAP kinase pathway to the transduction of a potentially selfderived trigger, this work contributes to the unraveling of the intricate signaling networks controlling fungal differentiation. Moreover, our data further support earlier observations that differentiation and secondary metabolism are tightly linked in filamentous fungi.G rowth and propagation of filamentous fungi typically involve the differentiation of specialized cell types or multicellular structures, such as spores, spore-bearing hyphae, and fruiting bodies, which serve specific biological functions. Differentiation of sexual and vegetative spores, for example, allows the dispersal and, therefore, translocation of otherwise sessile fungi. In addition, spores commonly function as resting structures to endure hostile growth conditions, such as wintertime or droughts. The induction of fungal differentiation usually involves both internal and external signals. For example, conidiation in the red bread mold Neurospora crassa underlies an internal circadian rhythm but is also induced through environmental signals, including light and nutrient deprivation (1, 2). There is growing evidence that fungi also produce a wide range of chemical signals to govern the development and differentiation of individual cells or a mycelial colony within a population (3). It has, for example, long been appreciated that in sexually propagating, heterothallic fungi, cells polarize and direct their growth toward peptide pheromones secreted by the mating partner (4). Cell-to-cell signaling also occurs on the population level or within an individual colony. For example, the primarily unicellular fungus Candida albicans secretes the sesquiterpene alcohol farnesol and ...

Cell-cell fusion is crucial for the development and propagation of most eukaryotic organisms. Despite this importance, the molecular mechanisms mediating this process are only poorly understood in any biological system. In particular, the step of plasma membrane merger and the contributing proteins and physicochemical factors remain mostly unknown. Earlier studies provided first evidence of a role of membrane sterols in cell-cell fusion. By characterizing different ergosterol biosynthesis mutants of the fungus Neurospora crassa, which accumulate different ergosterol precursors, we show that the structure of the sterol ring system specifically affects plasma membrane merger during the fusion of vegetative spore germlings. Genetic analyses pinpoint this defect to an event prior to engagement of the fusion machinery. Strikingly, this effect is not observed during sexual fusion, suggesting that the specific sterol precursors do not generally block membrane merger, but rather impair subcellular processes exclusively mediating fusion of vegetative cells. On a colony-wide level, the altered structure of the sterol ring system affects a subset of differentiation processes, including vegetative sporulation and steps before and after fertilization during sexual propagation. Together these observations corroborate the notion that the accumulation of particular sterol precursors has very specific effects on defined cellular processes rather than non-specifically disturbing membrane functioning. Given the phenotypic similarities of the ergosterol biosynthesis mutants of N. crassa during vegetative fusion and of Saccharomyces cerevisiae cells undergoing mating, our data support the idea that yeast mating is evolutionary and mechanistically more closely related to vegetative than sexual fusion of filamentous fungi.

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