Abstract
A method utilizing allelic exclusion has been developed to isolate mutants of Tetrahymena pyriformis, syngen 1, in which the normal pattern of expression of mutally exclusive surface antigens is altered. Cells homozygous for the recessive mutant allele R-1(r) do not express the L, H and T serotypes when grown under conditions appropriate for their expression. Rather, a new immobilization antigen, r, is expressed. Cells homozygous for the recessive mutant allele R-3(r) also express the r antigen instead of H serotypes, but are normal in their expression of T antigens. Genetic analyses show that R-1 and R-3 are not closely linked, that R-1 is linked to T by 9.3 units, and that R-3 may be loosely linked to the mt locus. Different linkage values were obtained, however, when different inbred laboratory strains were used, suggesting the possible existence of crossover modifying genes. The rates of assortment of R-1(R)/R-1(r) and R-3(R)/R-3(r) heterozygotes into pure sublines expressing either H or r serotypes are close to the values observed for the differentiation of heterozygotes at other loci. The data confirm the previous observation that genetic coupling relationships are not maintained in macronuclear phenotypes and are consistent with the hypothesis that the macronucleus contains 45 assorting subunits. The assortment of the double heterozygote R-1(R)/R-1(r), R-3(R)/R-3(r) at R(f)=0.0112 suggests that the units of assortment are not individual genetic loci or chromosome fragments, but that the units may be complete genomes.