Vegetative Hyphal Fusion in Filamentous Fungi

Abstract

The formation of channels between fungal hyphae by self fusion is a defining feature of filamentous fungi and results in the fungal colony being a complex interconnected network of hyphae. During the vegetative phase hyphal fusions are commonly formed during colony establishment by specialized conidial anastomosis tubes (CATs) and then later by specialized fusion hyphae in the mature colony. CAT induction, homing and fusion in Neurospora crassa provides an excellent model in which to study the process of vegetative hyphal fusion because it is simple and experimentally very amenable. Various mutants compromised in hyphal fusion have been isolated and characterized. Although the self-signalling ligand(s) involved in CAT induction and homing has/have not been identified, MAP kinase signalling is downstream of the initial ligand-receptor interaction(s), and has features in common with MAP kinase signalling during mating cell interactions in the budding yeast and during fungal infection structure (appressorium) formation. Hyphal fusion also resembles yeast cell mating and appressorium formation in other ways. Vegetative hyphal fusion between hyphae of different genotypes (nonself fusion) usually results in a form of programmed cell death which normally prevents heterokaryons from developing further. This process in N. crassa is controlled by heterokaryon incompatibility (het) loci. Understanding hyphal fusion in the model fungus, N. crassa, provides a paradigm for self-signalling mechanisms in eukaryotic microbes and might also provide a model for understanding somatic cell fusion in other eukaryotic species.