Mating Type Determination in Paramecium Aurelia , a Model of Nucleo-Cytoplasmic Interaction

Abstract

A general hypothesis of mating type detn. of P. aurelia is proposed, which can be applied equally well to the Group A and Group B systems formerly descr. It is known that each variety has 2 mating types, designated by odd and even Roman numerals, and that macronuclei control the mating types. In Group B the sister caryonides produced from a single, reorganized cell after fertilization are usually alike in mating type and have the same mating type as the parental clone, whereas in Group A odd-numbered caryonides are produced in both odd and even clones. The knowledge that in stock 51 of variety 4 there were twice as many kappa particles in mating type VII as in mating type VIII when the same no. of K genes was present in the micronuclei, suggested that the macronuclei in type VII may have twice as many many K genes as those in type VIII. This may be interpreted by a numerical difference in chromosomes of the 2 mating types, either on a polyploid or aneuploid basis. The hypothesis is developed that the macronuclear pieces represent discrete units with balanced chromosome sets and that the subnuclei are diploid in 1 mating type, tetraploid in the other mating type. These subnuclei may arise through doubling or failure of doubling of their chromosomes. In the case of occasional cells, called selfers, which produce both mating types during vegetative growth, the macronuclei may have subnuclei of both diploid and tetraploid types. Since cytoplasm plays an important role in nuclear development, it is proposed that the cytoplasmic activity is associated with a polyploidizing substance pl, which has no effect on the micronuclei. The assumptions are made that the macronucleus produces pl, that a macronucleus with 4n subnuclei produces a larger amt. of pl than one with 2n subnuclei, and that the amt. of pl in the cytoplasm of a diploid is usually not enough to effect polyploidizing of the macronuclear anlagen, whereas in a tetraploid the amt. is nearly always adequate. In Group B it is postulated that the concn. of pl in the cytoplasm is higher in 4n than in 2n animals, whereas in Group A it is independent of the level of ploidy but near the threshold concn. necessary for the induction of tetraploidy.