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

The Conjusome-A Transient Organelle Linking Genome Rearrangements in the Parental and Developing Macronuclei

The conjusome plays an important role in the conjugation events that occur in Tetrahymena thermophila. The conjusome appears in the anterior of conjugant pairs during the early stages of new macronuclei (anlagen) development. It lacks a membrane, and is composed of a network of fibrous, electron dense material, containing background cytoplasm and ribosomes. Several proteins localize to this organelle, including Pdd1p, a chromodomain protein that participates in the formation of chromatin-containing structures in developing macronuclear anlagen, and is associated with the elimination of specific germ-line sequences from developing macronuclei. Conjugants lacking the PDD1 allele in the parental macronucleus do not show Pdd1p antibody staining in conjusomes. Investigations were performed using mutant cell lines, uniparental cytogamy and drug treatment, and show that the conjusome appears to be dependent on parental macronuclei condensation, and is a transitory organelle that traffics nuclear determinants from the parental macronucleus to the developing anlagen. These data, taken together with Pdd1p knockout experiments, suggest the conjusome is involved in the epigenetic phenomena that occur during conjugation and sexual reorganization. This is likely a conserved organelle. Conjusome-like structures were also observed in another Ciliate, Stylonichia. In general, conjusomes have features that resemble germ line P-granules.

Developmental progression of Tetrahymena through the cell cycle and conjugation

The ciliate Tetrahymena thermophila can be said to undergo a variety of developmental programs. During vegetative growth, cells coordinate a variety of cell-cycle operations including macronuclear DNA synthesis and a-mitotic fission, micronuclear DNA synthesis and mitosis, cytokinesis and an elaborate program of cortical morphogenesis that replicates the cortical organelles. When starved, cells undergo oral replacement, transformation into fast-swimming dispersal forms or, when encountering cells of a complementary mating type, conjugation. Conjugation involves a 12 hour program of meiosis, mitosis, nuclear exchange and karyogamy, and two postzygotic divisions of the fertilization nucleus. This chapter reviews experimental data exploring the developmental dependencies associated with both vegetative and conjugal development.

Isolation of a Tetrahymena thermophila Strain which Induced Metaphase I Meiotic Arrest: New Pathway of Abortive Conjugation

A hypodiploid strain of Tetrahymena thermophila has been obtained that shows arrest at the stage of condensed nuclei, corresponding to metaphase I of normal conjugants and induced arrest at meiotic metaphase I (i.e. at the stage of condensed, bivalent chromosomes) in its wt partner mate. The metaphase I arrested conjugants retained their old macronuclei and most of them underwent cell fusion, instead of separation of exconjugants. The doublets were viable and cortically integrated. When the arrest inducing strain was crossed to the haploid tester strain, the haploid micronuclei were arrested in the meiotic metaphase I as the diploid ones had been; the monovalent, chromosomes were condensed, the arms of sister chromatids were not separated, and they were not segregated. Separation of the arms of sister chromatids and disjunction of bivalent chromosomes were not prerequisite for the formation of microtubular spindles in those cells that were arrested in meiotic metaphase I. After re-feeding, the doublet cells resumed cell divisions, segregating two macronuclei and micronuclei at random. One macronucleus was derived from the arrest inducing strain and the other from the tester strain. Heterokaryon strains with macronuclei derived from the parental arrest inducing strain and with the micronucleus derived from the parental wt tester strain were obtained. Surprisingly, these heterokaryons did not induce meiotic arrest. Thus, the arrest in the melotic metaphase I was induced by the micronucleus and not by the macronucleus of the arrest inducing strain.

Role of histone deacetylation in developmentally programmed DNA rearrangements in Tetrahymena thermophila

In Tetrahymena, as in other ciliates, development of the somatic macronucleus during conjugation involves extensive and reproducible rearrangements of the germ line genome, including chromosome fragmentation and excision of internal eliminated sequences (IESs). The molecular mechanisms controlling these events are poorly understood. To investigate the role that histone acetylation may play in the regulation of these processes, we treated Tetrahymena cells during conjugation with the histone deacetylase inhibitor trichostatin A (TSA). We show that TSA treatment induces developmental arrests in the early stages of conjugation but does not significantly affect the progression of conjugation once the mitotic divisions of the zygotic nucleus have occurred. Progeny produced from TSA-treated cells were examined for effects on IES excision and chromosome breakage. We found that TSA treatment caused partial inhibition of excision of five out of the six IESs analyzed but did not affect chromosome breakage at four different sites. TSA treatment greatly delayed in some cells and inhibited in most the excision events in the developing macronucleus. It also led to loss of the specialized subnuclear localization of the chromodomain protein Pdd1p that is normally associated with DNA elimination. We propose a model in which underacetylated nucleosomes mark germ line-limited sequences for excision.

Nongenic, bidirectional transcription precedes and may promote developmental DNA deletion in Tetrahymena thermophila

A large number of DNA segments are excised from the chromosomes of the somatic nucleus during development of Tetrahymena thermophila. How these germline-limited sequences are recognized and excised is still poorly understood. We have found that many of these noncoding DNAs are transcribed during nuclear development. Transcription of the germline-limited M element occurs from both DNA strands and results in heterogeneous transcripts of < 200 b to > 1 kb. Transcripts are most abundant when developing micro- and macronuclei begin their differentiation. Transcription is normally restricted to unrearranged DNA of micronuclei and/or developing nuclei, but germline-limited DNAs can induce their own transcription when placed into somatic macronuclei. Brief actinomycin D treatment of conjugating cells blocked M-element excision, providing evidence that transcription is important for efficient DNA rearrangement. We propose that transcription targets these germline-limited sequences for elimination by altering chromatin to ensure their accessibility to the excision machinery.

Development in electrofused conjugants of tetrahymena thermophila

Electric shock can create parabiotic fusions of living Tetrahymena cells. In this study, cells were mated and successful pairs were electrofused with either vegetatively growing cells or other mating pairs. In particular, we electrofused pairs from normal [diploid x diploid] matings with vegetatively dividing cells in G- or M-phase of the cell cycle. We also fused [diploid x diploid] conjugants with mating pairs involving an aneuploid partner [diploid x "star"], which typically undergo an abortive conjugal pathway termed genomic exclusion. Using such parabiotic fusions we identified and characterized two developmentally critical landmarks: 1) the "abort" signal, which is initiated in pairs with nuclear defects (this first becomes evident soon after the completion of Meiosis I or the beginning of Meiosis II); and 2) the "terminal commitment point", a developmental stage in normal [diploid x diploid] pairs after which conjugation no longer responds to a parabiotically transmitted abort signal (this correlates with the onset of the second postzygotic nuclear division). Finally we demonstrate that a conjugal-arrest-activity varies with the vegetative cell cycle, reaching its highest level of activity during M-phase and dropping just after cytokinesis.

Macronuclear development in conjugants of Tetrahymena thermophila, which were artificially separated at meiotic prophase

Conjugant pairs of Tetrahymena thermophila were mechanically separated by vigorous pipetting at the early stages of meiotic prophase. The complete sequence of conjugational nuclear events including the appearance of pronuclei, development of the new macronuclei (postzygotic development), and resorption of the old macronuclei was observed in the separated cells, without pronuclear exchange. The pronuclei in the separated cells were recognised by the presence of components of the extranuclear cytoskeleton, which were labelled with anti-tubulin and anti-fenestrin antibodies in the same way as in undisturbed conjugants. The apical region of the separated conjugants (the post-junction area), corresponding to the junction area of conjugants was labelled with anti-fenestrin antibody and maintained the properties required for the nuclear development. The results of the genetic study were consistent with a hypothesis that cytogamy (pronuclear fusion) was induced in the separated conjugants. Therefore, the lasting cell contact is not necessary for the successful completion of conjugational nuclear events.

A cytogenetic study of development in mechanically disrupted pairs of Tetrahymena thermophila

We examined the nuclear behavior of mating Tetrahymena cells that had been mechanically disrupted at various times throughout conjugation. Disruption was achieved by agitating conjugating Tetrahymena in the presence of 0.1-3 mm glass beads. Two minutes of agitation with 1 mm beads yielded optimal pair disruption (70%) with high viability (92%). Disrupting pairs between 0-4.7 h after the initiation of mating produced mostly disrupted conjugants in which development was aborted. However, as many as 20% of these early disrupted conjugants completed development even without their mating partners. After 5 h the percentage of disrupted conjugants completing development increased dramatically, reaching 80% by 6.7 h. These results support a model suggesting that events associated with nuclear exchange and fusion 5 h into conjugation trigger a commitment to completion of the postzygotic developmental program. The early conjugants that completed development following disruption suggest that development can be sustained even in the absence of a mating partner. This represents a novel method of bringing the micronuclear genome into macronuclear expression with minimal cytoplasmic exchange between partners. We discuss these results in light of a model relating cortical and nuclear signaling events that reciprocally drive conjugal development.

A mutational analysis of conjugation in Tetrahymena thermophila. 1. Phenotypes affecting early development: meiosis to nuclear selection

Conjugation in the freshwater ciliate Tetrahymena thermophila involves a developmental program that models meiosis, fertilization, and early developmental events characteristic of multicellular eukaryotes. We describe a gallery of five early-acting conjugation mutations. These mutants, cnj1-5, exhibit phenotypes in which specific steps in the conjugal pathway have been altered or eliminated. Specifically, cnj1 and cnj2 fail to condense their micronuclear chromatin prior to each of the three prezygotic nuclear divisions. This results in nuclear division failure, failure to replicate DNA, and failure to initiate postzygotic development. The cnj3 mutant appears to exhibit a defect in chromosome separation during anaphase of mitosis. cnj4 mutants successfully carry out meiosis I, yet are unable to execute the second meiotic division and abort all further development. cnj5 mutants are unable to initiate either meiosis I or meiosis II, yet proceed to execute all subsequent developmental events. These mutant phenotypes are used to draw inferences regarding developmental dependencies that exist within the conjugation program.