Molecular genetics of heterokaryon incompatibility in filamentous ascomycetes

Abstract

Filamentous fungi spontaneously undergo vegetative cell fusion events within but also between individuals. These cell fusions (anastomoses) lead to cytoplasmic mixing and to the formation of vegetative heterokaryons (i.e., cells containing different nuclear types). The viability of these heterokaryons is genetically controlled by specific loci termed het loci (for heterokaryon incompatibility). Heterokaryotic cells formed between individuals of unlike het genotypes undergo a characteristic cell death reaction or else are severely inhibited in their growth. The biological significance of this phenomenon remains a puzzle. Heterokaryon incompatibility genes have been proposed to represent a vegetative self/nonself recognition system preventing heterokaryon formation between unlike individuals to limit horizontal transfer of cytoplasmic infectious elements. Molecular characterization of het genes and of genes participating in the incompatibility reaction has been achieved for two ascomycetes, Neurospora crassa and Podospora anserina. These analyses have shown that het genes are diverse in sequence and do not belong to a gene family and that at least some of them perform cellular functions in addition to their role in incompatibility. Divergence between the different allelic forms of a het gene is generally extensive, but single-amino-acid differences can be sufficient to trigger incompatibility. In some instances het gene evolution appears to be driven by positive selection, which suggests that the het genes indeed represent recognition systems. However, work on nonallelic incompatibility systems in P. anserina suggests that incompatibility might represent an accidental activation of a cellular system controlling adaptation to starvation.

FIG. 1

Schematic representation of heterokaryon incompatibility. (A) When two different fungal individuals meet, they spontaneously undergo a cell fusion event or anastomosis. (B) If the two individuals have the same het genotype, a heterokaryon is established. (C) If the two strains differ in het genotype, the heterokaryotic cells are destroyed or severely inhibited in their growth.

FIG. 2

Sequence motif common to several genes involved in nonallelic incompatibility. The amino acid position is given for each sequence: the het-6^PA allele product (6PA), the het-6^OR allele product (6OR), the P. anserina HET-E protein (HTE), and the N. crassa TOL protein (TOL). Residues that are identical in at least three of the four sequences are boxed in black; residues that are similar in at least three of the four sequences are boxed in grey.

FIG. 3

Cell death reaction occurring in a self-incompatible het-C het-E strain of P. anserina. A het-c het-e strain is shown in panel A (non-self-incompatible control), and a het-C het-E self-incompatible strain is shown in panel B. The mycelium was photographed 12 h after spore germination. Bar, 10 μm. Note the intense vacuolization (black arrowhead) and the presence of completely destroyed cells (white arrowhead).