Form and pattern in ciliated protozoa: analysis of a genic mutant with altered cell shape in Tetrahymena pyriformis, Syngen 1

Anterior-posterior pattern formation in ciliates

As single cells, ciliates build, duplicate, and even regenerate complex cortical patterns by largely unknown mechanisms that precisely position organelles along two cell‐wide axes: anterior–posterior and circumferential (left–right). We review our current understanding of intracellular patterning along the anterior–posterior axis in ciliates, with emphasis on how the new pattern emerges during cell division. We focus on the recent progress at the molecular level that has been driven by the discovery of genes whose mutations cause organelle positioning defects in the model ciliate Tetrahymena thermophila. These investigations have revealed a network of highly conserved kinases that are confined to either anterior or posterior domains in the cell cortex. These pattern‐regulating kinases create zones of cortical inhibition that by exclusion determine the precise placement of organelles. We discuss observations and models derived from classical microsurgical experiments in large ciliates (including Stentor) and interpret them in light of recent molecular findings in Tetrahymena. In particular, we address the involvement of intracellular gradients as vehicles for positioning organelles along the anterior‐posterior axis.

Two Antagonistic Hippo Signaling Circuits Set the Division Plane at the Medial Position in the Ciliate Tetrahymena

In a single cell, ciliates maintain a complex pattern of cortical organelles that are arranged along the anteroposterior and circumferential axes. The underlying molecular mechanisms of intracellular pattern formation in ciliates are largely unknown. Ciliates divide by tandem duplication, a process that remodels the parental cell into two daughters aligned head-to-tail. In the elo1-1 mutant of Tetrahymena thermophila, the segmentation boundary/division plane forms too close to the posterior end of the parental cell, producing a large anterior and a small posterior daughter cell, respectively. We show that ELO1 encodes a Lats/NDR kinase that marks the posterior segment of the cell cortex, where the division plane does not form in the wild-type. Elo1 acts independently of CdaI, a Hippo/Mst kinase that marks the anterior half of the parental cell, and whose loss shifts the division plane anteriorly. We propose that, in Tetrahymena, two antagonistic Hippo circuits focus the segmentation boundary/division plane at the equatorial position, by excluding divisional morphogenesis from the cortical areas that are too close to cell ends.

Mutational robustness of morphological traits in the ciliate Tetrahymena thermophila

Ciliate nuclear architecture, in particular the sequestration of a transcriptionally silent germline genome, allows for the accumulation of mutations that are "hidden" from selection during many rounds of asexual reproduction. After sexual conjugation, these mutations are expressed, potentially resulting in highly variable phenotypes. Morphological traits are widely used in ciliate taxonomy, however, the extent to which the values of these traits are robust to change in the face of mutation remains largely unknown. In this study, we examine the effects of mutations accumulated in the germline genome to test the mutational robustness of four traits commonly used in ciliate morphological taxonomy (number of somatic kineties, number of postoral kineties, macronuclear size, and cell size). We found that the number of postoral kineties is robust to mutation, confirming that it should be preferentially used in taxonomy. By contrast, we found that, as in other unicellular and multicellular species, cell and macronucleus sizes change in response to mutation. Thus, we argue that cell and macronucleus sizes, which are widely used in taxonomy, should be treated cautiously for species identification. Finally, we found evidence of correlations between cell and macronucleus sizes and fitness, suggesting possible mutational pleiotropy. This study demonstrates the importance of, and methods for, determining mutational robustness to guide morphological taxonomy in ciliates.

Linkage and Genetic Map Length in TETRAHYMENA THERMOPHILA

The genetic map of T. thermophila has been extended with tests of over 170 marker combinations. Three linkage groups have been identified, in addition to several apparent linkages which are seen only when certain strains are used. A sampling argument was developed to estimate the total genetic map length from the observed fraction of marker combinations showing linkage. If the strain-dependent linkages are actually due to structural heterozygosity, the total map length is estimated at about 1,200 cM.

Induction of blocks in nuclear divisions and overcondensation of meiotic chromosomes with cycloheximide during conjugation of Tetrahymena thermophila

During conjugation, the micronucleus of Tetrahymena thermophila undergoes five consecutive nuclear divisions: meiosis, third prezygotic division (pregamic mitosis) and two postzygotic mitoses of the synkaryon. The four products of the synkaryon differentiate into macronuclear anlagen and new micronuclei and the old macronucleus is resorbed. The protein synthesis inhibitor cycloheximide, applied during conjugation, induced several developmental blocks. Pairs shifted to the drug during early meiotic prophase (stages I-III) were arrested at prophase. Cycloheximide applied to cells at pachytene (stages IV-VI) to metaphase arrested the conjugants at the stage of modified prometaphase/metaphase with overcondensed, swollen bivalents. In contrast to other systems, in the presence of cycloheximide, separation of chromatids, decondensation of chromosomes and exit from metaphase I were inhibited in both diploid and haploid cells. Pairs shifted to the drug after metaphase I were arrested at postmeiotic interphase after completing one nuclear cycle. The same rule applied to the subsequent cycle; then cells were arrested at the stage of pronuclei, and those pairs with functional pronuclei and synkarya were arrested at the stage of two products of the first postzygotic division (pronuclei were not arrested in nuclear transfer and karyogamy). Only pairs with two products of the first postzygotic division were arrested at the same stage after the cycloheximide treatment. Pairs shifted to cycloheximide during the second postzygotic division were arrested in development of macronuclear anlagen and resorption of old macronuclei. The postmeiotic conjugants pulse-treated with cycloheximide (2 h) yielded heterokaryons retaining parental macronuclei (i.e. they exhibited macronuclear retention).

Efficient mass transformation of Tetrahymena thermophila by electroporation of conjugants

Conjugating cells of the ciliate Tetrahymena thermophila were electroporated in the presence of plasmid DNA containing a paromomycin-resistant ribosomal RNA gene (rDNA). Cells were selected with paromomycin following 12-24 hr of growth on nonselective medium. Resistant cells appeared after 2-3 days. Processing vectors containing the micronuclear rDNA and somatic vectors containing the macronuclear gene transformed the cells, with the former yielding frequencies up to 900 transformants per microgram of plasmid DNA. A ribosomal protein gene (rpL29) conferring cycloheximide resistance also transformed conjugating cells. The transformation efficiency of the plasmid containing only the rpL29 gene was increased by insertion of an rDNA replication origin and by cotransformation and preselection with an rDNA vector. These results indicate that electroporation can be used for the production of large numbers of transformed Tetrahymena.

Cytoterminology of cortical components of ciliates: somatic and oral kinetids

The terminology of cortical structures in ciliated protists is determined partly by the organizational perspective from which these organisms are viewed. A general descriptive terminology is to be preferred to a particular and specialized one since the latter approach will lead to inflation in the number of terms. For the components of the ciliate kinetid, postciliary (microtubular) ribbons, transverse (microtubular) ribbons, nematodesma and kinetodesmal fibril are preferred terms. One, two and more than two kinetosome kinetids are referred to as mono-, di- and polykinetids, whether in somatic or oral regions of the cortex. In the oral region, adoral, circumoral, and paroral are the preferred descriptive adjectives, modified by the name of the taxonomic category in which the structures are found.

A mutation affecting basal body duplication and cell shape in Paramecium

The thermosensitive mutant sm19 of Paramecium tetraurelia undergoes a progressive reduction in cell length and basal body number over successive divisions at the nonpermissive temperature of 35 degrees C. In spite of these defects, sm19 cells retain the same generation time as wild-type cells at 35 degrees C. Cytological observations at both electron and light microscopy levels reveal no other perturbation than the rarefaction of basal bodies and the rare (3%) absence of one or two microtubules in basal bodies or ciliary axonemes. The temperature-sensitive period, during the last 30 min of the cell cycle, corresponds to the phase of basal body duplication. Upon transfer back to the permissive temperature, all basal bodies are normally duplicated. The mutational defect is transiently restored by microinjection of wild-type cytoplasm or of a soluble proteic fraction from wild-type cell homogenates. Altogether, the cytological and physiological data support the conclusion that the sm19+ gene codes for a diffusible product required for the initiation of basal body duplication and would thus be the first identified gene involved in this process. Our data also indicate that in Paramecium basal body number is not coupled with control of the cell cycle, but helps determine the shape of the cell via the organization of the cytoskeleton.

Effect of the antitubulin drug nocodazole on meiosis and postmeiotic development in Tetrahymena thermophila. Induction of achiasmatic meiosis

Nocodazole (ND), a potent antitubulin drug, can be used to dissect the steps of meiosis in Tetrahymena, presumably by interfering with the assembly of microtubules. Its effects depend upon the time during conjugation at which the drug is applied. When applied prior to the elongation of the micronucleus into the characteristic 'crescent' configuration, no crescent is formed and the chromosomes of prepachytene and pachytene condense into spherical nuclei. If ND is applied after micronuclear elongation has begun, but before it is fully elongated, the chromosomes fail to synapse and appear in metaphase I as unpaired monovalents. In contrast, the metaphase I chromosomes appear as bivalents when ND is applied later, during or after the crescent has reached its maximum elongation. Still later, application of ND inhibits chromosome movements during anaphase and telophase of either meiotic division, but does not prevent separation of kinetochores. In some of the blocked restitutive nuclei an additional round of chromosome replication occurs, corresponding to the third pregamic division in normal conjugation. The hyperploid micronuclei produced by such treatment may be useful in certain genetic manipulations and in studying the regulation of nuclear DNA content.

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

New experimental designs have detected unexpectedly large variations in the time at which macronuclear assortment begins, and in the ratios of the stabilized products. Variation is detected between strains, and, within strains, between conjugating pairs. TheChxlocus gave results ranging from late assortment (40–60 fissions) to early assortment with skewed input, indicating the existence of some relation between the parameters of input ratio and time of determination.

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.

Life cycle variation and regulation of macronuclear DNA content in Tetrahymena thermophila

The mean DNA content of G2 macronuclei varies during the life cycle of the ciliate Tetrahymena thermophila. Early in the life cycle the mean is about 130 C; later it is about 94 C. In hybrids between strains A and B the decrease from 130 C to 94 C usually began after 60 fissions after conjugation. In B X B clones the decrease was complete by 50 fissions. The data suggest that there may be a genetic difference between strains A and B with respect to the onset of the decrease in DNA content. The downward regulation of the mean DNA content appears to be related to the mechanism which removes the variance in macronuclear DNA content which is added to macronuclei by unequal macronuclear division. Unequal macronuclear division regularly occurs at all stages of the life cycle, with larger macronuclei tending to divide more unequally. In the absence of regulation, unequal macronuclear division would constantly add variance to G1 macronuclei and their range would continue to increase. Analysis of the variances of G1 and G2 macronuclei suggests that at all stages of the life cycle the added variance is removed by acting upon nuclei which become too small or too large. According to this model, macronuclei with smaller amounts of DNA are regulated upward by an extra macronuclear S phase, while larger amounts are regulated downward by chromatin extrusion and the skipping of macronuclear S. The mean DNA content appears to change during the life cycle because the thresholds at which macronuclei become too small or too large are readjusted. It is postulated that these thresholds are a function of gene dosage.

Size increase and form allometry during evolution of ciliate species in the genera Colpoda and Tillina (Ciliophora: Colpodida)

With the absence of a fossil record to provide direct evidence of evolutionary relationships, many researchers have inferred by circular reasoning that body size increase and form allometry have occurred during the evolution of ciliated protozoa. This study establishes ancestor-descendant relationships among five species of the related genera Colpoda and Tillina using ultrastructural characteristics which are not apparently dependent on size and form. This independent test suggests that phylogenetic size increase and phylogenetic form allometry have occurred during the evolution of these ciliates. Interrelationships of body size increase, increase in number of cortical organelles, and form allometry are discussed in reference to the divergence of these ciliate species.

Transformation in Tetrahymena pyriformis: description of an inducible phenotype

Transformation of Tetrahymena pyriformis to a rapid-swimming (presumably dispersal) form can be induced by washing cells and suspending them in distilled H2O, Dryl's solution or 10 mM Tris. Transformation is possible with high efficiency in mass cultures of axenically grown cells within approximately 5 h at 30 C. The radically different phenotype produced during transformation is characterized by a more elongate body form, increased numbers of somatic basal bodies and cilia, a long caudal cilium and oral membranelles positioned beneath the cell surface. DNA quantities characteristic of G1, S, and G2 cells are found in these transformed ciliates, suggesting that achievement of a particular stage in the DNA-division cycle is not a prerequisite for transformation. Preliminary observations on cells belonging to syngens 2-12 indicate that they also have a capacity to form a caudal cilium, but that the amicronucleate strain GL-C does not. Possible relevance of the transformed phenotype for taxonomy of Tetrahymena is discussed.

Molecules and morphologies: the perpetuation of pattern in the ciliated protozoa

Three apparently conflicting generalizations concerning the relationships between molecules and cell structure may be derived from studies on cellular patterning in the ciliates. (A) Cells with identical genes and molecular composition may have different hereditary patterns. (B) Genes, through their prescribed molecular derivatives, constrain the modes of pattern permutation and define the states of greatest stability. (C) Cells with identical hereditary patterns may have entirely different genes and molecular compostitons. These priniciples may be reconciled through the recognition that they are characteristically applicable over different time intervals. Hereditary differences within a clone and without related molecular differences (principle A) may persist for hundreds of cell generations, but they are resolved eventually within a constant environment (principle B) as the configuration of minimal free energy is approached. On an even longer time scale, molecular substitutions have occurred for many or most components of the cell, but these have been constrained by selective pressures on an ancient design (principle C) that disallow substitutions affecting certain form-function relations which have been elevated to an adaptive peak.

Causal relations among cell cycle processes in Tetrahymena pyriformis. An analysis employing temperature-sensitive mutants

Utilization of temperature-sensitive mutants of Tetrahymena pyriformis affected in cell division or developmental pathway selection has permitted elucidation of causal dependencies interrelating micronuclear and macronuclear replication and division, oral development, and cytokinesis. In those mutants in which cell division is specifically blocked at restrictive temperatures, micronuclear division proceeds with somewhat accelerated periodicity but maintains normal coupling to predivision oral development. Macronuclear division is almost totally suppressed in an early acting mutant (mola) that prevents formation of the fission zone, and is variably affected in other mutants (such as mo3) that allow the fission zone to form but arrest constriction. However, macronuclear DNA synthesis can proceed for about four cycles in the nondividing mutant cells. A second class of mutants (psm) undergoes a switch of developmental pathway such that cells fail to enter division but instead repeatedly carry out an unusual type of oral replacement while growing in nutrient medium at the restrictive temperature. Under these circumstances no nuclei divide, yet macronuclear DNA accumulation continues. These results suggest that (a) macronuclear division is stringently affected by restriction of cell division, (b) micronuclear division and replication can continue in cells that are undergoing the type of oral development that is characteristic of division cycles, and (c) macronuclear DNA synthesis can continue in growing cells regardless of their developmental status. The observed relationships among events are consistent with the further suggestion that the cell cycle in this organism may consist of separate clusters of events. with a varying degree of coupling among clusters. A minimal model of the Tetrahymena cell cycle that takes these phenomena into account is suggested.