Genetic Analysis of Mating-type Differentiation I N Paramecium Tetraurelia

Whereas each of the two c9mplementary mating types, 0 and E, of Paramecium tetraulrelia normally shows cytoplasmic inheritance, an abnormal heredity of mating type was observed in the progeny of crosses between two stocks of different geographical origin of Paramecium tetraurelia (stock 51 and stock 32). The modified pattern of mating-type inheritance was shown to result from the interaction of the two wild-type alleles at the locus mtD (mtD51 and mtD32), leading to a new differentiated state O*, different from the normal 0 and E states observed in both stock 51 and stock 32 cells. The genetic analysis of O* clones showed that the O* phenotype involves both a new heritable cytoplasmic state and possibly a nuclear change which can be transmitted through conjugation and segregates in a Mendelian fashion. All the data can be interpreted if the assumption is made that mating-type determination is achieved only by the commitment or noncommitment t o the expression of mating-type E, and that this commitment may simply reflect the activation o r nonactivation of the locus mtD, under the influence of one or two “cytoplasmic factors” including the product of the gene mtD itself. I N metazoa, different cell types exhibit different functions specific for each tissue. As commonly defined, these terminal differences result from two SUCcessive processes: ( 1 ) determination (a limitation of devclopmental capacity) which occurs at an early stage of the development of the organism; once established determination is transmitted through cell divisions; (2) differentiation (the expression of the defined capacity). Determixition and differentiation are not restricted to metazoa. In unicellular organisms, too, some functions can be considered as differentiated functions in that they are not expressed throughout the entire cell cycle and occur only under particular physiological conditions, and also in that they correspond to functions for which the cell possesses two (or more) alternative genetic pathways, one of which is selectively expressed. This is the case for differentiation of mating type in some species of Paramecium. Although genetically competent for the expression of both mating types, called “odd” (0) and “even” ( E ) , each cell is generally determined to express only one mating type. Mating type is expressed only under starvation conditions and its determination is inherited throuch vegetative multiplication. It is determbed at each sexual event (conjugation or autogamy) where the breakdown of the “old” macronucleus and the development of a “new” one from the newly formed diploid zygote nucleus occurs. During these nuclear Genetics 87: 633-653 December, 1977