Sexual reproduction in the fungi is extensively regulated by incom patibility, which determines, in the absence of any morphological differ entiation, the pattern of mating among individual strains. Control of the interactions that comprise the sexual reproductive process resides in specific genetic factors, the incompatibility factors, which occur in several distinct systems in the various groups of fungi and which exert their control in two basically different ways. On the one hand, the system may play the same role as dioecy in higher organisms by restricting or preventing inbreeding among...