Secondary mutations at the Bβ incompatibility locus and nuclear migration in the basidiomycete Schizophyllum commune

Genetical, cytological, and biochemical studies of the tetrapolar incompatibility system in the basidiomycete Schizophyllum commune are reviewed. Primary and secondary mutations of the Bβ incompatibility gene and their effect on the mating system are described. It is suggested that the deviant mating behaviour of the secondary mutations from that of the wild‐type alleles is due to changes in the control of the early steps of development of the dikaryon, i.e., intermycelial nuclear exchange and migration between the wild‐type strain and mutant strain. Cytological data about septal dissolution and nuclear movement are presented. It is emphasized that the structure of the hyphae involved in nuclear exchange and migration may affect intermycelial nuclear movements and that nuclear exchange and migration may be inhibited by alterations in the enzymes that normally degrade the septa. The biochemical part of the survey deals with recent observations of cell wall and septal structure and of some enzymes involved in septal dissolution. Suggestions concerning the relationship between incompatibility factors and enzymic degradation of the septa are reviewed.