Fatal Attraction: Nonself Recognition and Heterokaryon Incompatibility in Filamentous Fungi

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Abstract
Filamentous fungi grow by tip extension, branching, and hyphal fusion to form a tridimensional hyphal network (12). Different individuals are also capable of undergoing hyphal fusion with each other to form a vegetative heterokaryon (Fig. 1). Heterokaryon formation in filamentous ascomycete fungi has potential benefits of functional diploidy and mitotic genetic exchange during the parasexual cycle (61). Heterokaryon formation can also be used to increase biomass for cooperation in resource exploitation (18). Although there are apparent benefits associated with heterokaryon formation, heterokaryosis by hyphal fusion is believed to be virtually excluded in nature by genetic differences at het (heterokaryon incompatibility) loci (14, 52, 55, 63). Heterokaryon incompatibility has been shown to reduce the risk of transmission of infectious cytoplasmic elements such as virus-like double-stranded RNAs (16, 19) and exploitation by aggressive genotypes (18). In some cases, DNA polymorphisms associated with het allele specificity show transspecies polymorphisms (82), indicating that these loci are subject to balancing selection. Transspecies polymorphisms are also found at loci in other organisms that confer nonself recognition, such as the S locus in plants (15) and the major histocompatibility complex loci in animal systems (36).