21st Century Guidebook to Fungi

Most fungal mycelia contain haploid nuclei. This is a characteristic of Kingdom Fungi; unlike the other major eukaryotic groups, most true fungi are haploid. Even in fungus-like organisms in the Oomycota (Kingdom Chromista) like Phytophthora infestans, the cause of potato blight, the nuclei are diploid. This difference in ploidy is an important contrast between ‘true’ and ‘non-true’ fungi. Of course, there are exceptions to every rule and some true fungi are diploid, like Candida albicans, a yeast which causes disease in humans; and rhizomorphs and fruit bodies of Armillaria mellea (a pathogen of trees that belongs to the Basidiomycota) (Peabody et al.,2000).

In this chapter compatibility and the individualistic mycelium will be our main concerns. Formation and breakdown of heterokaryons and the nature and maintenance of the dikaryon are major topics, as are the mechanisms that regulate these processes: vegetative compatibility and the incompatibility systems. We also discuss gene segregation during the mitotic division cycle, which culminates conceptually in what is known as the parasexual cycle. Finally, we consider the segregations of the cytoplasmic genetic entities, mitochondria, plasmids, viruses and prions.

Compatibility and the individualistic mycelium

Because of the difference in ploidy, the life cycles of true fungi and those of other major groups of eukaryotes differ significantly. For example, for most true fungi diploid nuclei are only produced transiently during sexual reproduction, whereas the haploid state is limited to the gametes in most animals and plants.