Fungal mycelia can increase in size by fusion, which can be mutually beneficial. However, using experimental evolution of the fungus Neurospora crassa, we earlier demonstrated that free fusion of mycelia favours cheater mutants. Those cheaters have a competitive benefit against the wild-type ancestor, but a negative effect on total spore production. Using whole-genome sequencing of evolved lines, we recently demonstrated, paradoxically, that all convergently evolved cheater lineages have similar fusion deficiencies and we demonstrated that fusion deficiency is the cause of cheating. Fusion-deficiency mutations prevent cheaters from initiating fusion, but nevertheless enable them to profit from fusion initiated by wild-type mycelia. This benefit is due to reduced contribution to somatic substrate-bound hyphal networks, but increased representation in the aerial reproductive hyphae. However, at higher frequency of the fusion mutant, the mycelial network becomes increasingly fragmented providing a relative benefit to wild-type rich patches. The frequency-dependence of fitness results in an equilibrium between cheater and wild type.
