Variability in the timing and outcome of macronuclear assortment in Tetrahymena thermophila

Rate of phenotypic assortment in Tetrahymena thermophila

During vegetative, asexual reproduction in heterozygous Tetrahymena thermophila, the macronucleus divides amitotically to produce clonal lineages that express either one or the other allele but not both. Because such phenotypic assortment has been described for every locus studied, its mechanism has important implications concerning the development and structure of the macronucleus. The primary tools to study assortment are Rf, the rate at which subclones come to express a single allele stably, and the output ratio, the ratio of assortee classes. Because Rf is related to the number of assorting units, a constant Rf for all loci suggests that all genes are maintained at the same copy number. Output ratios reflect the input ratio of assorting units, with a 1:1 output ratio implying equal numbers of alleles at the end of macronuclear development. Because different outcomes would suggest a different macronuclear structure, it is crucial that these parameters be accurately measured. Although published Rf values are similar for all loci measured, there has been no commonly accepted form of presentation and analysis. Here we examine the experimental determination of Rf. First, we use computer simulation to describe how the variability inherent in the assortment process affects experimental determination of Rf. Second, we describe a simple method of plotting assortment data that permits the uniform calculation of Rf, and we describe how to measure Rf accurately in instances when it is possible to score only the recessive allele. Using this method to produce truly comparable Rfs for all published data, we find that most, if not all, loci assort at Rfs consistent with approximately 45 assorting units, as has been asserted.

The functional units of macronuclear assortment in Tetrahymena thermophila

Macronuclei assorting simultaneously forH, Chx, Mpr, andco, and containing only one or two copies of theHDallele produced several combinations of phenotypes at the other loci, instead of only one or two such combinations. It follows that macronuclear subnuclei, if they exist at all, must frequently exchange parts. Models involving somatic recombination, transient subnuclei, or progressive chromosome fragmentation are discussed as possible explanations.