A modifier gene involved in the expression of the dominant mating type allele inParamecium caudatum

Abstract

SUMMARYMating type inParamecium caudatum, syngen 3 is determined by a pair of alleles with simple dominance; the recessive allele restricts homozygotes to mating type V and the dominant allele permits expression of mating type VI. Clones of mating type V never show natural selfing, but most clones of mating type VI self naturally. A mutant clone of mating type VI which never selfed over a period of more than 3 years was obtained by treatment withN-methyl-N′-nitro-N-nitrosoguanidine. When this mutant clone was crossed to a wild-type stock of mating type V, all F1clones of mating type VI gave rise to selfers. From selfing of these F1of mating type VI, clones of F2were obtained. Nearly 3:1 segregation of selfer to non-selfer clones was observed among the F2clones of mating type VI. The results were consistent with the interpretation that a dominant modifier gene,Su(+mtV), controls the instability in the expression of mating type VI.