With a fixed genome and in uniform culture conditions, the wild-type strain ofNectria haematococcaproceeds from a juvenile state to either of two morphologically distinct differentiated states. These differentiated states appear suddenly as small brown areas in the margin of the thalli and widen to hyphae in the vicinity through transmission of two extrachromosomal factors, each of them being specific for one of the two differentiated states.Mutagenesis indicates that nuclear genes are involved in controlling the establishment of differentiation. Mutations were obtained in at least six genetically independent loci. Some genes influence the maintenance of both differentiated states; others affect them specifically. Studies on the double mutants obtained from crosses between mutant strains reveal information on the mode of action of the genes. The relationships between these genes and the extrachromosomal factors are discussed.
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