Vegetative Mutants and Clonal Senility inTetrahymena*

Laboratory and evolutionary history of Tetrahymena thermophila

An account is given of the early efforts to domesticate tetrahymenas as laboratory instruments for genetics. The rationale for developing a new organismic technology was the comparative leverage provided by a eukaryotic microorganism at a large evolutionary distance from both prokaryotic microbes and multicellular organisms. The tetrahymenine ciliates were considered more favorable materials than paramecia because of their ability to grow on simple media, though in fact their simpler nutritional needs have never been fully exploited. The first task was to sort the large set of phenotypically similar but evolutionarily and molecularly diverse ciliates referred to at the time as T. pyriformis. Then a species amenable to genetic manipulation was identified and its culture and cytogenetics were brought under control. Fortunately, the very first breeding system investigated--that in the species now called T. thermophila--has proved to be suitable for a wide range of studies. A large factor in the program's success was its use of the foundation previously established by studies on paramecia. However, serious unforeseen difficulties were encountered on the way to "domestication." These included inbreeding deterioration associated with their outbreeding life-style and germinal deterioration (mutational erosion) in the unexpressed micronuclear genome after long maintenance in vegetative culture. Cryogenic preservation was an important means of escaping these organismic limitations, and somatic (macronuclear) assortment has proved a valuable supplement to meiotic recombination.

Tetrahymena dimorpha sp.nov. (Hymenostomatida: Tetrahymenidae), a new ciliate parasite of Simuliidae (Diptera) with potential as a model for the study of ciliate morphogenesis

A new species of hymenostome ciliate, Tetrahymena dimorpha sp.nov., is described. This ciliate occurs as a parasite in the haemocoel of larval, pupal and adult Simuliidae (Diptera). In larval hosts the total number of parasites never exceeds about 240 and the infection is benign. Within larval hosts the ciliates are large and broadly oval and possess an unusually wide range of somatic kineties, from 30 to 66; moreover a variable proportion of these kineties are characteristically disorganized, being incomplete, meandering or branched. Metamorphosis of the host to the adult fly is accompanied by a dramatic increase in the number of ciliates, which reach pathogenic intensity. Adult hosts may contain up to 19000 ciliates and the flies soon die from this heavy burden. Associated with ciliate population growth during host metamorphosis is a startling morphological transformation of the ciliates themselves. In adult hosts the ciliates are smaller and pyriform in shape and the cortex is greatly modified; the total number of somatic kineties is considerably reduced and has a limited range of 19—22. Most significantly, the kineties are ordered with typical tetrahymenine precision. By application of appropriate culture conditions to ciliates isolated from any host stage, either of the distinctive morphological forms of T. dimorpha may be selectively induced in vitro . In bacterized infusions, ciliates are produced that have the general form and cortical characteristics of those found naturally in adult hosts. Sterile culture in serum-supplemented Mitsuhashi and Maramorosch insect tissue culture medium produces a population showing features characteristic of ciliates from larval hosts. Sterile culture in proteose-peptone-yeast-extract medium results in populations exhibiting concurrent dimorphism, even after cloning. The extreme nature and multiple facets of the dimorphism together with the ease of its manipulation in vitro afford opportunities for the experimental investigation of many problems, particularly those related to cell surface patterning in ciliates, and these possibilities are discussed in relation to current concepts of ciliate morphogenesis.

Variable copy number of macronuclear DNA molecules in Tetrahymena

In Tetrahymena, the DNA of the macronucleus exists as very large (100 to 4,000-kb) linear molecules that are randomly partitioned to the daughter cells during cell division. This genetic system leads directly to an assortment of alleles such that all loci become homozygous during vegetative growth. Apparently, there is a copy number control mechanism operative that adjusts the number of each macronuclear DNA molecule so that macronuclear DNA molecules (with their loci) are not lost and aneuploid death is a rare event. In comparing Southern analyses of the DNA from various species of Tetrahymena using histone H4 genes as a probe, we find different band intensities in many species. These differences in band intensities primarily reflect differences in the copy number of macronuclear DNA molecules. The variation in copy number of macronuclear DNA molecules in some species is greater than an order of magnitude. These observations are consistent with a developmental control mechanism that operates by increasing the macronuclear copy number of specific DNA molecules (and the genes located on these molecules) to provide the relatively high gene copy number required for highly expressed proteins.

Mutation affecting cell separation and macronuclear resorption during conjugation in Tetrahymena thermophila: early expression of the zygotic genotype

A new recessive conjugation lethal mutation was found in Tetrahymena thermophila which was named mra for macronuclear resorption arrest. Other events affected by the mra mutations are separation of pairs, DNA replication in the macronuclear anlagen, and resorption of one of the two micronuclei. In wild-type crosses 50% of the pairs had separated by 12 hr after mixing two mating types and had completed resorption of the old macronucleus 1-2 hr later. In contrast most mra conjugants did not separate even by 24 hr after mixing and the old relic (condensed) macronucleus was seen in over 90% of them. After addition of 10 mM calcium to the conjugation medium, the mra conjugants did separate but they still failed to complete resorption of the old macronucleus and to replicate macronuclear anlagen DNA in the exconjugants. The calcium induced separation of the mra conjugants occurred later than the separation of control pairs. During normal conjugation cell separation occurs before the first expression of known macronuclear genes and prior to processing of the macronuclear DNA. Therefore, the mra phenotype infers that separation of conjugants requires a signal which is produced by the macronuclear anlagen at an unusually early time.

Clonal lifespans cultured in chemically defined medium and conventional bacterized medium in Paramecium octoaurelia

The unicellular ciliate, paramecium, reproduces by binary fission, but can not continue to divide unlimitedly without sexual reproduction. We examined the clonal life span of Paramecium octaurelia stock 299 cultured in conventional bacterized medium (BM) and a chemically defined medium (DM). The cells that lived in BM divided 300 times. Although the cells in DM divided more slowly, some cells continued to divide more than 100 times. The mean life span of 90 cell lines cultured in BM was 151 +/- 49 fissions and that of 84 cell lines in DM was 68 +/- 28. When some older cells, which had been cultured in DM, were transferred to BM, most of them showed much longer life spans than those remaining in DM. The results showed that the life spans of cell clones were affected by the culture conditions.

Is the initiation of macronuclear DNA synthesis in Euplotes dependent on micronuclear functions?

To determine whether the micronucleus makes essential contributions during asexual reproduction, observations were made on cells of Euplotes octocarinatus from which the micronucleus had been removed with a micropipette. Most cells underwent one postenucleation division, then became arrested in macronuclear G1, slowed down in food uptake, developed macronuclear deformations, and finally died. Such cells could be rescued if a micronucleus was reimplanted before macronuclear deformations had developed. When provided with a new micronucleus, cells initiated macronuclear DNA synthesis about 12-16 h later. The data suggest that the micronucleus is involved in the control of the cell's transition from macronuclear G1 to S, and a model is proposed which postulates that in Euplotes macronuclear DNA synthesis is initiated when a micronucleus-encoded "initiator protein" has accumulated to a critical amount.

The 5S ribosomal RNA gene clusters in Tetrahymena thermophila: strain differences, chromosomal localization, and loss during micronuclear ageing

The organization of the 5S genes in the genome of Tetrahymena thermophila was examined in various strains, with germinal ageing, and the 5S gene clusters were mapped to the MIC chromosomes. When MIC or MAC DNA is cut with the restriction enzyme EcoRI, electrophoresed, blotted, and probed with a 5S rDNA probe, the banding patterns represent the clusters of the 5S rRNA genes as well as flanking regions. The use of long gels and 60 h of electrophoresis at 10 mA permitted resolution of some 30-35 5S gene clusters on fragments ranging in size from 30-2 kb (bottom of gel). The majority of the 5S gene clusters were found in both MIC and MAC genomes, a few being MIC limited and a few MAC limited. The relative copy number of 5S genes in each cluster was determined by integrating densitometric tracings made from autoradiograms. The total number of copies in the MAC was found to be 33% greater than in the MIC. When different inbred strains were examined, the majority of the 5S gene clusters were found to be conserved, with a few strain-specific clusters observed. Nine nullisomic strains missing both copies of one or more MIC chromosomes were used to map the 5S gene clusters. The clusters were distributed non-randomly to four of the five MIC chromosomes, with 17 of them localized to chromosome 1. A deletion map of chromosome 1 was constructed using various deletion strains. Some of these deletion strains included B strain clones which had been in continuous culture for 15 years. Losses of 5S gene clusters in these ageing MIC could be attributed to deletions of particular chromosomes. The chromosomal distribution of the 5S gene clusters in Tetrahymena is unlike that found for the well-studied eukaryotes, Drosophila and Xenopus.

Germinal aging in Tetrahymena thermophila

The manifestations of germinal aging in the ciliate Tetrahymena thermophila include death of the cells at conjugation and macronuclear retention in which the normal replacement of the old macronucleus by a new one fails to occur. Available data suggest that methods of routine maintenance that reduce the number of fissions may delay aging. Differences in breeding performance following maintenance for 1-5 years in axenic peptone broth vs. bacterized Cerophyl were not significant; those following maintenance at different temperatures were significant. The analysis of several hundred crosses is consistent with a random mutational basis for aging in the micronucleus and does not support the hypothesis of an age-correlated program in which the rate of deterioration increases with time. Following routine cultural maintenance for as long as 9-11 years, some lines show no deficiencies in their ability to produce viable progeny, and sublines of the same clone frequently differ significantly in their breeding performance. Moreover, breeding degeneration occurs at constant, but different, rates in different inbred strains.

Monstrous Tetrahymena with intraclonal variation in structure produced by hereditary modification of normal cells

Monstrous Tetrahymena pyriformis strain GL may be isolated after exposure of normal cells to numerous heat shocks, to flattening on agar or gelatin plates, or to viscous solutions of methyl cellouse. It is shown that in some cases the abnormalities are inherited and that this results in clones where the cells are different from each other and have various abnormalities with respect to cortical pattern, swimming and feeding behavior, and generation time. Furthermore, it is shown that these cells are produced rather than selected by the experimental treatments. Evidence is presented that growth without division is important for production of the abnormal organisms. The basis of the inheritance of the abnormalities is discussed.

Mutant with heat-sensitive capacity for phagocytosis in tetrahymena: isolation and genetic characterization

A mutant ofTetrahymenawith heat-sensitive phagocytosis was obtained using a tantalum-particle enrichment procedure. The mutant phenotype is most likely determined by a somatic (macronuclear) mutation(s). The inability of the mutant to sustain cell division and to phagocytize at 37 °C are most likely determined by the same mutation. The phenotype of the mutant is stably inherited under vegetative propagation at 30 °C. At 37 °C, the mutation affects the development of the oral apparatus, the phagocytotic organelle. This mutant has proven useful for the study of cellular functions related to phagocytosis.

Tetrahymena: effect of freezing and subsequent thawing on breeding performance

Tetrahymena pyriformis, syngen 1, frozen in 10 percent dimethyl sulfoxide, stored for 2 months, and then thawed could conjugate normally. More significantly, they were viable, in normal numbers, through two sex ual reorganizations. The strains ap parently did not sustain genetic dam age during the treatment. The tech niques offer considerable promise for the maintenance of breeding stocks in ciliated protozoans.

Genomic exclusion: a rapid means for inducing homozygous diploid lines in Tetrahymena pyriformis, syngen 1

Genomic exclusion is an abnormal form of conjugation occurring between cells with defective micronuclei and normal cells with diploid micronuclei. The progeny are heterocaryons; each cell has an old macronucleus but a new diploid micronucleus derived from one meiotic product of the normal mate. Such cells express genes found in the old macronucleus, are sexually mature, and can be specifically selected. When inbred, they give rise to lines genetically homozygous at all known loci.