New approaches to the problem of macronuclear assortment

Macronuclear assortment is a vegetative (somatic) process in the ciliate protozoan genus Tetrahymena which leads to the production of “homozygous” phenotypes from sexually-produced heterozygotes. The appearance of pure types follows a model of 45 randomly distributed and replicating subunits in the polyploid macronucleus. Models for the organization of macronuclear chromatin must explain the following: (1) haploid genomes are distributed at each fission; (2) not all loci assort; (3) different loci begin assortment at different times following conjugation. Therefore, either haploid subunits distributed at fission must frequently exchange parts (by somatic recombination, gene conversion, or progressive chromosome fragmentation), or they may actually disintegrate between fissions. Some of these possibilities can be distinguished by considering the kinetics of the appearance of pure types at two or more loci simultaneously. Conditions which give optimum discrimination between hypotheses of limited recombination and complete scrambling (by disintegration of haploid units between fissions) have been developed. A method for determining the number of assorting subunits in very young macronuclei is also given.