Adaptive significance of vegetative incompatibility in Neurospora crassa

Certain features reminiscent of sexuality occur in the vegetative life cycle of some filamentous fungi such as Neurospora crassa. Hyphal fusions can occur between genetically different individuals, thereby endowing the new composite mycelium, a heterokaryon, with some of the advantages of heterozygosity usually associated with diploid organisms. In N. crassa, however, there are a number of incompatibility loci which prevent formation of heterokaryons unless the alleles at the incompatibility loci are identical in the two mycelia. The selection pressures that maintain incompatibility polymorphisms are not known. We suggest here that they are maintained because they prevent a kind of exploitation of heterokaryons by nuclei that are nonadaptive in homokaryons but that enjoy a proliferative advantage over other nuclei in heterokaryons. A mathematical model that abstracts the major features of the vegetative life cycle of Neurosopra crassa has been developed, and the action of selection in this model and various extensions of it is such as to maintain polymorphisms of vegetative incompatibility factors.