Theoretical population genetic models assuming homogeneous populations indicate that selection against transmission of a harmful cytoplasmic element is a more plausible explanation than is selection against a nuclear gene to explain the extent of incompatibility observed. These models predict only moderate levels of polymorphism. However, even genetic drift alone may produce large numbers of incompatibility types under certain conditions. Both in Podospora anserina and in Neurospora crassa low frequency transfer of mitochondrial plasmids between vegetatively incompatible partners can be demonstrated. The potential consequences of low frequency horizontal transfer of plasmids associated with the fungal senescence phenotype will be discussed.
Several observations point to an involvement of vegetative incompatibility in the sexual cycle.
First, in species like Neurospora crassa vegetative incompatibility is associated with the mating type idiomorphs. We have shown that in the absence of mating type vegetative incompatibility transmission of plasmids from the conidial paternal parent was ten times higher than in normal crosses. We have evidence that conidia that are vegetatively compatible with the maternal parent may enter the maternal tissue by somatic fusion, thus obtaining access to the maternal resources and initiating new fruiting bodies. It is concerned with natural genetic diversity, its causes, its distribution and its biological significance. More specifically, population genetics is concerned with the basic forces of evolution, that is, mutation, recombination, migration, natural selection and genetic drift, and the contribution that these forces make to the nature and rate of evolutionary change.