Mating Type Inheritance in Syngen 7 of TETRAHYMENA PYRIFORMIS : Intra- and Interallelic Interactions.

ATING type inheritance in a cililate follows one of two basic patterns (SONNEBORN 1947; BEALE 1954; KIMBALL 1964; NANNEY 1968). In ciliates with genic inheritance mating types are controlled directly by one or more genes. In ciliates with epigenetic inheritance an array of mating type potentialities is controlled by nuclear genes, but which mating type is actually expressed by a given cell is determined through nuclear differentiation. In the second case different phenotypes are obtained in cells with the same genotype and these phenotypes are stable and may persist throughout the life cycle. Like most ciliates, Tetrahymena pyriformis has two types of nuclei. The macronucleus or somatic nucleus is polyploid and controls the vegetative phenotype, while the diploid micronucleus or germinal nucleus serves as a reservoir of genetic material for new macronuclei. After conjugation each exconjugant develops two new macronuclei and two new micronuclei from the diploid fertilization nucleus by division. The macronuclear anlagen undergo rapid DNA synthesis, while the micronuclei remain diploid. At the first cell division after conjugation the two new macronuclei in each exconjugant are distributed to the daughter cells without division, so that all four of these cells, which yield cultures called caryonides, have the normal number of one macronucleus and one micronucleus (NANNEY 1953). At each following cell division both nuclei divide so that all the members of a caryonide clone have a macronucleus derived from the same macronuclear anlage. Since all four macronuclear anlagen from one conjugating pair originate from identical fusion nuclei they are genetically identical; but in ciliates with epigenetic mating type inheritance, different caryonides from the same pair may have different mating types. In fact, there is no correlation between the mating types of different caryonides, but mating types of subcaryonide clones are highly correlated. Thus, caryonidal or epigenetic inheritance involves nuclear diff erentiation at the time the macronuclear anlagen are formed. Another type of epigenetic inheritance has been observed in Paramecium aurelia and Paramecium caudatum ( SONNEBORN 1947). In the group B syngens of Paramecium aurelia, mating type expression follows a clonal rather than a caryonidal pattern. That is, each exconjugant usually has the same mating type as its cytoplasmic parent, and the two caryonides derived from one exconjugant are identical. At first this type of mating type inheritance appeared to depend