ABSTRACTThe mycelium of higher fungi is portrayed as a developmentally versatile collective in which an initially dendritic pattern of branching is converted, by hyphal anastomosis, into a communication network. Spatial and functional patterns in the mycelium depend on the establishment of gradients allowing flow of protoplasmic resources and organellesviaradial or tangential routes between source and sink regions. Anastomosis between genetically different mycelia of the same or closely related species sets the stage for varying outcomes of genomic interplay between nuclear and mitochondrial populations. These outcomes range from the stable‘merger’of fully compatible basidiomycete matings, through various unstable associations culminating in genomic takeover, degeneracy or complex phenotypic emergence, to the mutual rejection characteristic of a somatic incompatibility response.