Age‐dependent intraclonal conjugation in Euplotes crassus

In Euplotes crassus each of the five different mating‐types is determined by a different allele at the mating‐type locus. There is complete dominance among these alleles so that in diploid cells only one of them is phenotypically expressed. It was observed that in heterozygous clones expressing only one mating‐type, cells arose after about 500 consecutive fissions which expressed another mating‐type. This observation led to the finding that in E. crassus , late in the life history of a clone, a stage is reached in which the dominance relationship among the mating‐type alleles is abolished. A cell heterozygous at the mating‐type locus is at that stage no longer stable with respect to mating‐type transmission, its mitotic progeny expresses either one of their two mating‐type alleles and are therefore able to conjugate with each other, as soon as different mating‐types are expressed. It is further shown that intraclonal conjugation inaugurates new life cycles so that mating‐type instability seems to serve the same function as autogamy in other ciliates, i.e., it starts new life cycles for cells which have failed to find interclonal mates. A few heterozygous clones have been found which were not able to enter the stage of mating‐type instability. It is shown that they are genetically related and that they can be interpreted as carrying a dominant mutation which does not allow them to enter the life cycle stage of mating‐type instability.