Signal exchange and integration during self-fusion in filamentous fungi

Growth and propagation of filamentous ascomycete fungi commonly involves vegetative cell fusion. In the red bread mold Neurospora crassa and many other ascomycete species, fusion occurs between germinating spores during colony formation and between hyphal branches in established mycelia. Both fusion processes promote the development and behavior of the fungal colony as a supra-cellular network. Germling and hyphal fusion in N. crassa rely on an unusual mode of cellular communication, in which the two fusion partners likely alternate between signal emission and reception, thereby establishing a kind of “cell dialog”. In recent years, numerous molecular factors mediating this unique cellular behavior have been identified, including several conserved signal transmission pathways, as well as proteins specific for ascomycete fungi. Analysis of their molecular interactions revealed the presence of an intricate signaling network, whose sophisticated interconnections are still unfolding. Despite this complexity, germling and hyphal fusion provide experimentally easily amenable model systems and might therefore advance as paradigms for signal transmission and cell fusion. In this article, we strive to highlight some of the recent advances in this field of research and to discuss the current working model of the “cell dialog”.