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Chapter 20 : Biology and Genetics of Vegetative Incompatibility in Fungi
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
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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.
Incompatibility assays, barrage tests, and forced heterokaryon tests. (A) Barrage test in P. anserina. Strains on the left plate are compatible and show normal contact zones; the strains on the right plate are incompatible and show a barrage reaction, which on this medium appears as a clear zone lined by pigmented lines. (B) Forced heterokaryon test in N. crassa. The left plate contains compatible forced heterokaryon; the right plate het-c/pin-c shows incompatible heterokaryon. (C) Forced heterokaryon assay in Aspergillus. The inoculate on the left corresponds to an incompatible heterokaryon that fails to establish; the colonies on the right correspond to compatible heterokaryons with different spore color markers.
The different incompatibility systems characterized at the molecular level in N. crassa and P. anserina. The black double arrows represent incompatible interactions. The gray double arrows represent interactions that contribute to the VI response. The HET domain is represented by a black box. The specificity regions in het-C and un-24 are represented by a white box. See the text for details.
Distribution of HET domain proteins in the genomes of sequenced Pezizomycota species. For each species, the number of HET domain proteins in the genome is given on the right. The phylogenetic tree was modified from Fitzpatrick et al., 2006 , to include the additional species that have been more recently sequenced.