Ecological Genetics of Tetrahymena thermophila: Mating Types, i-Antigens, Multiple Alleles and Epistasis

ABSTRACT Until recently, Tetrahymena thermophila has rarely been isolated from nature. With improved sampling procedures, T. thermophila has been found in ponds in many northeastern states. The availability of resident populations makes possible both population and ecological genetic studies. All seven known mating types have been recovered; no eighth mating type has been found. Crosses among whole‐genome homozygotes derived from Pennsylvania isolates reveal a spectrum genotypes with mating type alleles resembling traditional A (IV‐ and VII‐) and B(I‐) categories. The genotypes differ significantly with respect to mating type frequency, both among themselves and from previously described genotypes. One A‐category genotype appears to lack mating type II, while one A‐category and all B‐category genotypes have low frequencies of mating type III, thus accounting for the low frequency of III in the pond. The low frequency of III in all five B‐category genotypes examined suggests that the founding allele in this region was low for III. These and other differences are discussed both in terms of mating type frequencies in the pond and in terms of the possible molecular structure of mat alleles. By contrast, numerous variants of the cell surface immobilization antigen are found in addition to the previously described i‐antigens. Variants of the known SerH alleles include those with restriction fragment length polymorphisms and temperature sensitivity as well as alleles with new antigenic specificity. Multiple alleles are present in single ponds. Genes exhibiting serially dominant epistasis over SerH genes also are found. In two instances (K and C), families of antigenically similar polypeptides are expressed in place of H i‐antigen. Molecular weight differences suggest that these paralogous i‐antigen genes evolve by gene duplication and unequal crossing over within central repeats. The existence of complex patterns of epistasis together with seasonal changes in i‐ag frequencies suggest that i‐ag play an important, but as yet unknown, ecological role related to the occurrence of frequent conjugation.