Molecular genetics of heterokaryon incompatibility in filamentous ascomycetes

Filamentous fungi spontaneously undergo vegetative cell fusion events withi n but also between individuals. These cell fusions (anastomoses) lead to cy toplasmic mixing and to the formation of vegetative heterokaryons (i.e., ce lls containing different nuclear types). The viability of these heterokaryo ns is genetically controlled by specific loci termed het loci (for heteroka ryon incompatibility). Heterokaryotic cells formed between individuals of u nlike het genotypes undergo a characteristic cell death reaction or else ar e severely inhibited in their growth. The biological significance of this p henomenon remains a puzzle. Heterokaryon incompatibility genes have been pr oposed to represent a vegetative self/nonself recognition system preventing heterokaryon formation between unlike individuals to limit horizontal tran sfer of cytoplasmic infectious elements. Molecular characterization of het genes and of genes participating in the incompatibility reaction has been a chieved for two ascomycetes, Neurospora crassa and Podospora anserina. Thes e analyses have shown that het genes are diverse in sequence and do nor bel ong to a gene family and that at least some of them perform cellular functi ons in addition to their role in incompatibility. Divergence between the di fferent allelic forms of a her gene is generally extensive, but single-amin o-acid differences can be sufficient to trigger incompatibility. In some in stances her gene evolution appears to be driven by positive selection, whic h suggests that the het genes indeed represent recognition systems. However ; work on nonallelic incompatibility systems in P. anserina suggests that i ncompatibility might represent an accidental activation of a cellular syste m controlling adaptation to starvation.