Macronuclear Genetics of Tetrahymena I. Random Distribution of Macronuclear Gene Copies in T. PYRIFORMIS, Syngen 1

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The objective of this study was to test the idea that the macronuclear (somatic) genetic information is randomly distributed at each cell division in Tetrahymena pyriformis, syngen 1. We took advantage of a quick and reliable test for the detection of stable vegetative segregants in clones heterozygous for ts2, a heat-sensitive mutation. Clones that originated from cells inferred to contain very few (1-3) copies of the ts2+ allele in their macronuclei were selected for pedigree analysis. Experimental results were compared with the results of a computer simulation of the experiment. Our results are fully consistent with the predictions of the Allen-Nanney-Schensted model concerning the replication and distribution of functional gene copies. This model proposes that the macronucleus contains many functional copies of a given gene that are duplicated once during the cell cycle and are randomly distributed to the two-daughter nuclei. Our work has provided a more sensitive test of the assumption of randomness than was previously available. Our evidence for complete randomness suggests that 45, the effective number of macronuclear gene copies previously inferred from the segregation rate, indeed represents the actual number of copies. This conclusion, coupled with previously available evidence that the macronucleus is approximately 45-ploid, suggests that the segregating genetic units in the macronucleus are in effect haploid. This appears to remove the need to postulate inter-allelic repression to account for the phenomenon of phenotypic assortment. Our results, as well as those of others, also are inconsistent with any simple form of the master-slave hypothesis of Allen and Gibson.

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