Origin of Expressed Mutations in Schizophyllum commune

Abstract

PREVIOUS investigations on tetrapolar sexuality in Basidiomycetes^1–3 have shown that morphologically aberrant sectors arise in surprisingly high frequency in the common-A heterokaryons of Schizophyllum commune. These heterokaryons are established whenever mat ings are made of two strains which share common factors at one of the two complex loci governing incompatibility, the A factor¹. On further isolation these sectors are capable of growing into homokaryotic mycelia of a limited number of phenotypes, and in the majority of cases each type can be shown to be the result of a single-gene mutation⁴. No such occurrence of visibly expressed mutations has ever been observed within homokaryons. Among the hypotheses which have been proposed to account for the origin of these expressed mutations are³ : (a) increased mutagenesis within the heterokaryon at a few specific loci ; (b) increased mutagenesis at all loci followed by selection of the few specific types encountered ; (c) no increase in mutagenesis, but selection of a few specific types of low-level, spontaneous mutation.