Fatal Attraction: Nonself Recognition and Heterokaryon Incompatibility in Filamentous Fungi.

Vegetative incompatibility is a common phenomenon in filamentous fungi, including ascomycete, basidiomycete, and zygomycete fungi (27,70,80).A subset of vegetative incompatibility reactions includes events that require hyphal fusion and heterokaryon formation, whereby genetically different nuclei coexist in a common cytoplasm.Nonself recognition leading to rejection of heterokaryon formation is referred to as "heterokaryon incompatibility."Heterokaryon incompatibility is a genetically regulated process and most often results in death of the hyphal fusion cell (Fig. 1).This review will focus on recent developments in our understanding of the molecular mechanisms of nonself recognition and downstream effectors of death during heterokaryon incompatibility.The recent release of the Neurospora crassa genome sequence has allowed the evaluation of the conservation of genes involved in heterokaryon incompatibility in filamentous fungi and their possible relationship to programmed cell death (PCD) in other multicellular eukaryotes.