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Mating type in Paramecium 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 VT self naturally. A mutant clone of mating type VI which never selfed over a period of more than 3 years was obtained by treatment with i^-methyl-i^'-nitro-i^-nitrosoguanidine. When this mutant clone was crossed to a wild-type stock of mating type V, all F 2 clones of mating type VI gave rise to selfers. From selfing of these F 2 of mating type VI, clones of F 2 were obtained. Nearly 3:1 segregation of selfer to non-selfer clones was observed among the F 2 clones 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. available at https://www.cambridge.org/core/terms. https://doi.

Cells of Paramecium caudatum, syngen 3 usually become sexually mature about 50 fissions after conjugation. In order to study the genetic mechanisms that control fission-dependent expression of maturity, an attempt was made to obtain early mature mutants by treatment with N-methyl-N′-nitro-N-nitro-soguanidine. A new cytoplasmic marker, erythromycin resistance, was used to eliminate nonconjugant and macronuclear regeneration clones. Twenty early mature clones were obtained from five different mutagenized cultures. Three of them were genetically analyzed by crosses to wild-type stocks. The results show all three mutants to be controlled by incompletely dominant genes, i.e., the homozygotes became mature 20-25 fissions and the heterozygotes 15 fissions earlier than the wild-type clones. At least two different loci are suggested for the early maturity.

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In Paramecium caudatum, syngen 3, cells become mature (i.e. capable of undergoing conjugation) by about 50 fissions after conjugation. Matingtype-instability, another mating characteristic, begins to occur between 80 and 120 fissions after conjugation in heterozygotes for the mating type locus (Myohara & Hiwatashi, 1975). Mating-type instability also occurs in dominant homozygotes, earlier than in heterozygotes. In two different early maturing mutants the homozygotes become mature 20 fissions earlier than in wild-type clones, and mating-type instabiliiy also begins earlier than in wild-type clones. The number of fissions from maturity to the occurrence of the mating type change does not differ.