Defining individual size in the model filamentous fungus Neurospora crassa
Linda Ma, Boya Song, Thomas Curran, Nhu Phong, Emilie Dressaire, Marcus Roper

Fungal mycelia potentially violate each of the tenets that define individuality in paradigmatic organisms: the mycelium of a filamentous fungi can contain millions of genetically diverse nuclei, each capable of founding new mycelia. Moreover a single mycelium can potentially stretch over kilometers and it unlikely that distant parts of these ultra-large mycelia share resources or have the same fitness. Here we directly measure how a single mycelium of the model ascomycete Neurospora crassa is patterned into reproductive individuals, by analyzing the genetic diversity of thousands of asexual spores to determine the size of the nuclear sub-population that produces each spore. In a mycelium with a single growth direction the entire width of the mycelium forms a single population. By contrast, mycelia with expanding frontiers fragment into approximately 1000 reproducing subpopulation per square cm. Nuclear subpopulations fragment further when the mycelial network is perturbed by either chemical or desiccation stress. Our results provide a concept of fungal individuality that is directly connected to reproductive potential, and therefore to theories of how fungal individuals adapt and evolve over time. Our data show that the size of fungal individuals is a dynamic and environment-dependent property, even within apparently totally connected fungal mycelia.