Biology and genetics of vegetative incompatibility in fungi

Filamentous fungi have the ability to undergo somatic cell fusion. When somatic cell fusion occurs between distinct natural isolates of a given species, the fusion cell is adversely affected to various extents. The adverse reaction ranges from a simple growth impairment to an acute cell death reaction. This phenomenon is known as vegetative (or heterokaryon) incompatibility (VI). VI can be envisioned as a conspecific somatic self/nonself recognition process analogous to other somatic allorecognition processes described in other phyla. The VI reaction is triggered by genetic differences between fungal individuals and is defined by precise gene-to-gene interactions. The plant pathogen Pseudomonas syringae may have acquired a het gene as a potential virulence factor to trigger the VI reaction in N. crassa and utilize the fungus as a sole nutrient source. Two types of heterokaryons have been reported in filamentous ascomycete fungi. Genes involved in VI have been identified so far in only two species: N. crassa and P. anserina. Two categories of genes involved in VI have been characterized. The first category includes het genes that encode recognition function and are polymorphic between individuals. The second category includes downstream or upstream effector genes, in which mutations suppress or attenuate phenotypes associated with VI. VI in fungi as a paradigm for allorecognition in genetically tractable simple eukaryotic species holds great promise to gain a better understanding of the general principles that govern the evolution of nonself recognition systems.