Extrachromosomal factors affecting mating reactions in Schizophyllum commune

Summary

Extrachromosomal factors modifying mating reactions in Schizophyllum commune were found. The P factor appeared spontaneously, while the F factor appeared following nitrosoguanidine treatment. When a strain carrying P was mated with wild-type strains, it invariably developed pseudoclamps (usually typical of common-B heterokaryons), while there developed on the side of the wild-type strain the dikaryon or heterokaryon expected from the genotypes involved. In a cross Ax Bx P × Ay By, fruiting bodies from the side of the wild type gave rise only to wild-type segregants with mating types as expected from their genotypes. A selfed fruiting body from the side of the P strain gave rise to progeny which did carry the P factor, but its expression was modified. It had inconsistent and symmetrical effects in A ≠ B ≠ and A ≠ B = matings, but the original type of asymmetrical reaction in A = B ≠ matings. In the next generation the effect of P was weak and symmetrical. Strains carrying F factor were flat (morphology typical of mutation of the B factor) and were compatible with strains with which they should be incompatible according to the genotypes, e.g. Ax Bx F × Ay Bx gave a dikaryon. The factor showed non-Mendelian segregation and was extremely unstable.