ABSTRACT
Filamentous fungi grow by hyphal tip extension, branching, and hyphal fusion (anastomoses [1]) to form a complex tridimensional hyphal network. As an individual colony grows in nature, it will often come into contact with other individuals from the same or related species. Interactions with other individuals can have a number of consequences. In certain cases, anastomoses among different individuals can lead to the formation of a vegetative heterokaryon, where genetically distinct nuclei occupy a common cytoplasm. Two possible outcomes to such a vegetative cell fusion event are possible. A vigorous heterokaryon may be established (the two involved strains are termed “compatible,” or the heterokaryon is inviable (the two strains are “incompatible”). Vegetative incompatibility (also referred to as heterokaryon or somatic incompatibility) is a common phenomenon in filamentous ascomycetes and basidiomycetes [2-4]. Vegetative compatibility between isolates is genetically determined by specific loci termed het (for heterokaryon incompatibility) or vic (vegestative incompatibility) loci [1,3,5]. Two strains are compatible and capable of forming vigorous heterokaryons if they have the same het (vic) genotype. It has been proposed that this phenomenon constitutes a non-self-recognition system that operates during vegetative growth. Although the selective mechanisms operating to maintain vegetative incompatibility are unclear, it has been proposed that preventing heterokaryon formation between unlike individuals may limit horizontal transfer of cytoplasmic infectious elements such as mycoviruses [6,7] and/or resource plundering between individuals [8].
