Genomic exclusion in Tetrahymena thermophila: A cytogenetic and cytofluorimetric study

Genomic exclusion is an aberrant form of conjugation of Tetrahymena thermophila in which the genome of a defective conjugant is excluded from the genotype of the exconjugant progeny. This paper is concerned with the cytogenetic and nucleocytoplasmic events of genomic exclusion in senescent clones A*III and C*. In crosses between A*III or C* and strain B, functional, haploid gametic nuclei are formed only in the strain B cell. In some instances one of the gametic nuclei divides prior to transfer of the migratory gametic nucleus, and both products then undergo DNA synthesis. Two alternative cytogenetic pathways are followed after transfer of the migratory nucleus. In the first, the conjugants separate without further micronuclear divisions. This pathway was most common in A*III genomic exclusion. In exconjugants the former gametic nuclei undergo both DNA synthesis and (presumably) intranuclear separation of centromeres to restore micronuclear diploidy. The old macronucleus of each exconjugant is retained without autolysis. This class of exconjugant survives and contributes genes to future sexual progeny. In the second cytogenetic pathway the gametic nuclei divide and macronuclear anlagen are formed, as in normal conjugation. This pathway was more common in C* genomic exclusion. The initial DNA content of the anlagen ranges from haploid to diploid. Following two to three rounds of DNA synthesis, further macronuclear development ceases and the anlagen appear to undergo autolysis. The old macronucleus condenses and also undergoes autolysis, as in normal conjugation. Except for rare C* exconjugants, in which macronuclear development is completed, anlagen-bearing genomic exclusion exconjugants die. Death may be caused by aneuploidy, errors in the timing or receptivity to signals for autolysis, or the inability of anlagen-bearing exconjugants to feed. Anlagenbearing conjugants are frequently abnormal with respect to the number of anlagen and micronuclei. Most of the anomalies can be explained by postulating errors in the timing of both developmental signals and nuclear divisions. Rare conjugants in which gametic nuclei divide but do not give rise to macronuclear anlagen are also observed. In these instances, the old macronuclei condense and undergo autolysis. Destruction of the old macronucleus therefore is independent of the presence of macronuclear anlagen and requires cell pairing in order to be initiated.     

Correlation between the Shortened Period of Cell Pairing during Genomic Exclusion and the Block in Posttransfer Nuclear Development in Tetrahymena thermophia.

The duration of pairing in crosses of the micronuclearly defective strain A^*V and the amicronuclear strain BI3840 of Tetrahymena thermophila is shorter than that in normal conjugation. In controls, pairing takes about 620 min at 30°C and 60 min more at 28°C. In contrast pairing of crosses of the A^*strain took 470 min at 30°C and 490 min at 28°C, and that of crosses of BI3840 strain took 440 min at 30°C. The course of nuclear development in the tester strain crossed with the A^*strain was similar to that in the control until the 3rd prezygotic division. Unilateral transfer of the pronuclei occurred later than on reciprocal transfer in control crosses; posttransfer divisions in hemikarya was completely blocked in over 90% of the cases examined. Defective cell contact in crosses of the A^*V strain and an amicronuclear BI3840 strain may be correlated with block in nuclear division and in macronuclear development, since unfertilized cells from triplet conjugation remain haploid and develop normally.     

Scheduled and unscheduled DNA synthesis during development in conjugating Tetrahymena

Autoradiography has been used to confirm and to extend previous microspectrophotometric studies (Doerder and DeBault, 1975) on the timing of DNA synthesis during conjugation in Tetrahymena thermophila. The majority of DNA synthesis occurs at the expected periods preceding gamete formation and the two postzygotic divisions and during macronuclear development. DNA in new macronuclei is endoreplicated in an extremely discontinuous fashion. Under starvation conditions, the first endoreplication (2C to 4C) occurs immediately after the second postzygotic division when both new macronuclei and new micronuclei replicate. The second endoreplication (4C to 8C) does not occur until after separation of conjugants. If mating cells are kept under prolonged starvation conditions (20-24 hr), refeeding induces a partially synchronous division, after which an unexpectedly high percentage of cells incorporate tritiated thymidine into both macro- and micronuclei. Two previously undescribed periods of DNA synthesis were observed in the micronuclei of conjugating Tetrahymena. The first occurs during the early stages of meiotic prophase, before full crescent elongation. The second takes place in an extended period corresponding to macronuclear anlagen development, before conjugants have separated. CsCl gradient analyses indicate that, in micronuclear fractions, only main band DNA is being synthesized in both of these periods. However, in macronuclear fractions from both stages, a significant fraction (approximately 20%) of the DNA being synthesized has the buoyant density of ribosomal DNA. The finding that macro- and micronuclear DNA can be synthesized simultaneously in a single cell, both during conjugation and after refeeding starved exconjugants, raises interesting questions of how macro- or micronuclear-specific histones are targeted to the appropriate nuclei.     

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).     

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.     

The transition from conjugal development to the first vegetative cell division is dependent on RAD51 expression in the ciliate Tetrahymena thermophila

Rad51p, the eukaryotic homolog of the prokaryotic recA protein, catalyzes strand exchange between single- and double-stranded DNA and is involved in both genetic recombination and double-strand break repair in the ciliate Tetrahymena thermophila. We have previously shown that disruption of the Tetrahymena RAD51 somatic macronuclear locus leads to defective germline micronuclear division and that conjugation of two somatic rad51 null strains results in an early meiotic arrest. We have constructed Tetrahymena strains that are capable of RAD51 expression from their parental macronuclei and are homozygous, rad51 nulls in their germline micronuclei. These rad51 null heterokaryons complete all of the early and middle stages of conjugation, including meiosis, haploid nuclear exchange, zygotic fusion, and the programmed chromosome fragmentations, sequence eliminations, and rDNA amplification that occur during macronuclear development. However, the rad51 null progeny fail to initiate the first vegetative cell division following conjugal development. Coincident with the developmental arrest is a disproportionate amplification of rDNA, despite the maintenance of normal total DNA content in the developing macronuclei. Fusion of arrested rad51 null exconjugants to wild-type cells is sufficient to overcome the arrest. Cells rescued by cytoplasmic fusion continue to divide, eventually recapitulating the micronuclear mitotic defects described previously for rad51 somatic nulls.     

Morphology,Conjugation, and Postconjugational Reorganization of Dileptus tirjakovae n. sp. (Ciliophora,Haptoria)

ABSTRACT. We studied the morphology, conjugation, and postconjugational reorganization of a new haptorid ciliate, Dileptus tirjakovae n. sp., using conventional methods. Dileptus tirjakovae is characterized by two abutting, globular macronuclear nodules and scattered brush kinetids. Conjugation is similar to that in congeners, that is, it is temporary, heteropolar, and the partners unite bulge‐to‐bulge with the proboscis. Some peculiarities occur in the nuclear processes: there are two synkaryon divisions producing four synkaryon derivatives, of which two become macronuclear anlagen, one becomes the micronucleus, and one degenerates. Unlike spathidiids, D. tirjakovae shows massive changes in body shape and ciliary pattern before, during, and after conjugation: early and late conjugants as well as early exconjugants resemble Spathidium, while mid‐conjugants resemble Enchelyodon. These data give support to the hypothesis that spathidiids evolved from a Dileptus‐like ancestor by reduction of the proboscis. Dileptus tirjakovae exconjugants differ from vegetative cells by their smaller size, stouter body, shorter proboscis, and by the lower number of ciliary rows, suggesting one or several postconjugation divisions. Although 83% of the exconjugants have the vegetative nuclear pattern, some strongly deviating specimens occur and might be mistaken for distinct species, especially because exconjugants are less than half as long as vegetative cells.     

冠突伪尾柱虫有性生殖期间皮膜发育的核控制

通过显微手术去小核建立多个冠突伪尾柱虫（Pseudourostyla cristata)无小细胞系,并诱导它们与有小核细胞进行接合生殖,以评估小核及其衍生的大核原基在有性生殖期间对皮膜形态发生的影响,当无小核接合体从有小核配偶获得1枚配子核后,接合双方不仅能平行地继续核器演化,而且使第1次皮膜改组能够同步进行和正常发育,说明小核在有性周期中除了生殖功能外仍保留着某些控制皮膜发育的体功能,虽然大部分接合后体的大核原基在DNA贫乏期停止发育,但少数接合后体能够超越这一时期,并启动第2次皮膜改组和顺利完成其后续的有性发育全程,表明指令发动第2次皮膜发育的信号来自DNA贫乏期后以排出一核物质团块为标志的大核原基。     

Selection of the germinal micronucleus in Paramecium caudatum: nuclear division and nuclear death

Each cell of Paramecium caudatum has a germinal micronucleus. When a bi-micronucleate state was created artificially by micronuclear transplantation, both micronuclei divided for at least 2 cell cycles after nuclear transplantation. However, this bi-micronucleate state was unstable and reduced to a uni-micronucleate state after several fissions. Although the number of micronuclei was usually 1 during the vegetative phase, 4 presumptive micronuclei differentiated after conjugation. At the first post-conjugational fission, only 1 of the 4 micronuclei divided, indicating that there is tight regulation of micronuclear number in exconjugants. Micronuclei that did not divide at the first post-conjugational fission may persist through the first and second post-conjugational cell cycles. The decision to divide appears to be separate from the decision to degenerate, as evidenced by division of a remaining micronucleus upon removal of the dividing micronucleus at the first division. Degeneration of micronuclei in exconjugants differs from that of haploid nuclei after meiosis. Nutritional state affected micronuclear degeneration. Under well-fed conditions, the micronuclei destined to degenerate lost the ability to divide earlier than after starvation treatment, suggesting that micronuclear degeneration is an "apoptotic" phenomenon, probably under the control of the new macronuclei (macronuclear anlagen).     

Nuclear Roles in the Post-Conjugant Development of the Ciliate Euplotes aediculatus II. Experimentally Induced Regeneration of Old Macronuclear Fragments1

Following conjugation in ciliates, the usual fate of the old pre-conjugant macronucleus is resorption. In some species, however, old macronuclei, or their fragments, have the ability to reform functional vegetative macronuclei when new macronuclear anlagen are defective. The present work on Euplotes shows that if anlagen are allowed to carry out their essential roles in early exconjugant development, including influence on cortical reorganization such that feeding can resume, they can then be permanently damaged by UV-microbeam irradiation and regeneration of old macronuclear fragments can occur. E. aediculatus exconjugants were anlage-irradiated at 40–60 hr of development and the irradiated cells cultured individually and fed. Squashes revealed enlargement and anteriorward migration of the persistent (posterior) macronuclear fragments. The first post-conjugant fission of such cells was delayed (times ranged 6–43 days) and did not seem to involve the damaged anlagen, which remained rudimentary, did not divide along with the cells, and were subsequently resorbed. It appeared that cell fission was supported by the fragments of the old macronuclei, which either divided or partitioned themselves between the two daughter cells. Mating tests performed on early clones derived from irradiated exconjugants revealed ample conjugation competence; intraclonal conjugation in such clones was also apparent. The absence of the immature period seen in normal exconjugants provides further evidence that the clones arose from cells with regenerated macronuclei.     

Micronuclear DNA from Paramecium tetraurelia: serotype 51 A gene has internally eliminated sequences.

A method for the isolation of micronuclear DNA from Paramecium tetraurelia has been developed. After cell lysis, a low speed centrifugation at 1,000 g is used to remove all of the unbroken cells and macronuclei and approximately two thirds of the macronuclear fragments. Next a higher speed centrifugation of 9,000 g sediments the micronuclei and frees them from small particulates and soluble constituents. Advantage is then taken of the fact that micronuclei have a lower density than do macronuclear fragments in 45%-60% Percoll. Micronuclei float to the top during centrifugation at 24,000 g, while macronuclear fragments sediment. After several cycles of centrifugation in Percoll, the micronuclei, although heavily contaminated with cytoplasmic components, are essentially free of macronuclei and macronuclear fragments. Micronuclear DNA can then be extracted from the suspension. The whole procedure is very rapid and in about an hour micronuclear and macronuclear DNA can be separated. About 2 micrograms of micronuclear DNA can be obtained from 6 x 10(7) paramecia. We find that there are internal sequences in the micronuclear A gene DNA in wild type cells which are eliminated when the micronuclei develop into macronuclei. They yield unique restriction fragments for micronuclei and macronuclei. Therefore the purity of the preparations is easily monitored by probing Southern blots of restriction enzyme-digested DNA with the cloned A gene. No differences have been found between the micronuclear A gene in wild type and the d48 mutant.     

Micronuclear DNA from Paramecium tetraurelia: Serotype 51 A Gene Has Internally Eliminated Sequences

A method for the isolation of micronuclear DNA from Paramecium tetraurelia has been developed. After cell lysis, a low speed centrifugation at 1,000 g is used to remove all of the unbroken cells and macronuclei and approximately two thirds of the macronuclear fragments. Next a higher speed centrifugation of 9,000 g sediments the micronuclei and frees them from small particulates and soluble constituents. Advantage is then taken of the fact that micronuclei have a lower density than do macronuclear fragments in 45%–60% Percoll. Micronuclei float to the top during centrifugation at 24,000 g , while macronuclear fragments sediment. After several cycles of centrifugation in Percoll, the micronuclei, although heavily contaminated with cytoplasmic components, are essentially free of macronuclei and macronuclear fragments. Micronuclear DNA can then be extracted from the suspension. The whole procedure is very rapid and in about an hour micronuclear and macronuclear DNA can be separated. About 2 μ g of micronuclear DNA can be obtained from 6 times 10⁷ paramecia. We find that there are internal sequences in the micronuclear A gene DNA in wild type cells which are eliminated when the micronuclei develop into macronuclei. They yield unique restriction fragments for micronuclei and macronuclei. Therefore the purity of the preparations is easily monitored by probing Southern blots of restriction enzyme-digested DNA with the cloned A gene. No differences have been found between the micronuclear A gene in wild type and the d48 mutant.     

Macronuclear differentiation in conjugating pairs of Tetrahymena treated with the antitubulin drug nocodazole

During Tetrahymena conjugation gamic nuclei (pronuclei) are produced, reciprocally exchanged, and fused in each mate. The synkaryon divides twice; the two anterior nuclei develop into new macronuclei while the two posterior nuclei become micronuclei. The postzygotic divisions were blocked with the antitubulin drug nocodazole (ND). Then pronuclei (gamic nuclei) developed directly into macronuclear anlagen (primordial macronuclei), inducing amicronucleate cells with two anlagen, or, rarely, cells with one anlagen and one micronucleus. ND had a similar effect on cells that passed the first postzygotic division inducing amicronucleate cells with two anlagen, while cells treated with ND at the synkarya stage produced only one large anlage. Different intracytoplasmic positioning of the nuclei treated with ND (pronuclei, synkarya and two products of the first division) shows that most of cell cytoplasm is competent for inducing macronuclear development. Only posteriorly positioned nuclei--products of the second postzygotic division--remain micronuclei. The total cell DNA content, measured cytophotometrically in control and in ND-induced amicronucleate conjugant cells with one and two anlagen, was similar in all three samples at 12 h of conjugation. Eventually, at 24 h this content was about 2 pg (8 C) per anlagen both in nonrefed control and in amicronucleate exconjugants. Therefore "large" nuclei developing in the presence of ND were true macronuclear anlagen.     

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.     

冠突伪尾柱虫(Pseudourostyla cristata)接合生殖的研究 Ⅰ.核器演化

在25℃条件下,冠突伪尾柱虫接合生殖全程历时10天左右。接合生殖过程中的核器演化包括:①数十枚老的大核逐步瓦解。电镜观察表明,老的大核是以一种类似于食物泡消化的方式被吸收的,并在此过程中伴有大量溶酶体出现。②仅8枚左右小核中的一枚参与新核器的发生。首先,位于胞口后部的一枚小核膨大并进行一次预备分裂,接着发生三次成熟分裂。每一接合体内形成一枚雄原核和一枚雌原核。雄原核互向对方迁移并与其雌原核融合成为合子核。合子核分裂两次,四枚子核之一发育为大核原基,另一枚发育为小核原基,其余两枚退化。预备分裂和前两次成熟分裂各自产生的两枚子核中,仅一枚进入下一次分裂,另一枚解体消失。在第一次成熟分裂前期,“降落伞”的形成和发展经历着复杂的结构变化,持续一小时以上。③大核原基经过长时间的发育,伴有多线染色体的形成和解体等一系列变化,方达成熟状态。成熟的大核原基以伸长断裂、分叉断裂和哑铃形缢缩三种方式进行分裂,小核原基亦随之分裂,逐步形成具60枚左右大核、8枚左右小核的正常营养体。其后,大核融合,开始配后第一次无性分裂。值得注意的是,大核原基发育到将成熟时,最初的迹象是染色质向大核原基中央集结成团,染色质团与核膜之间充满着匀质的核液。当中央染色质团伸长时,又将     

Behavior of germinal micronuclei under control of the somatic macronucleus during conjugation in Paramecium caudatum.

In conjugating pairs of Paramecium caudatum, the micronuclear events occur synchronously in both members of the pair. To find out whether micronuclear behavior is controlled by the somatic macronucleus or by the germinal micronucleus, and whether or not synchronization of micronuclear behavior is due to intercellular communication between conjugating cells, the behavior of the micronucleus was examined after removal of the macronuclei from either or both cells of a mating pair at various stages of conjugation. When macronuclei were removed from both cells of a pair, micronuclear development was arrested 1 to 1.5 hr after macronuclear removal. When the macronucleus of a micronucleate cell mating with an amicronucleate cell was removed later than 3 to 3.5 hr of conjugation, that is, an early stage of meiotic prophase of the micronucleus, micronuclear events occurred normally in the operated cell. These results suggest that most micronuclear events are under the control of the macronucleus and that the gene products provided by the macronucleus are transferable between mating cells. One such product is required for induction of micronuclear division and is provided just before metaphase of the first meiotic division of the micronucleus. This factor is effective at a lower concentration in the cytoplasm and/or is more transferable between mating cells than the factors required for other stages. This factor, which seems to be present at least until the stage of micronuclear disintegration, is able to induce repeated micronuclear division as long as it remains active. The factor can act on a micronucleus which has not passed through a meiotic prophase. Moreover, the results suggest the existence of a second factor which is provided by the macronucleus after the first meiotic division that inhibits further micronuclear division.     

Relation Between Cell Union and Nuclear Reorganization in Triplet Conjugants of Paramecium caudatum and Paramecium multimicronucleatum

SYNOPSIS Triplet conjugants of Paramecium caudatum which appeared naturally in mating mixtures and those of Paramecium multimicronucleatum which were produced by conjugation-inducing chemicals were isolated. Triplet conjugants lasting for more than 3 h were stained to examine macronuclear events. In P. caudatum , only 2 triplets among 182 (1%) contained macronuclear fragmentation in all 3 members. The most frequently occurring triplets (79%) were those producing 1 cell without and 2 cells with macronuclear fragments. There were also triplets (17%) producing 1 cell with, and 2 without macronuclear fragments, and some (3%) with 3 cells that contained no fragments. The length of persistence of the triplet was not responsible for the occurrence of macronuclear fragmentation in the 3rd cell of the triplet. In P. multimicronucleatum , the same 4 classes of triplets occurred, but the most frequently occurring class was that consisting of 3 cells (91%) with macronuclear fragments. Induction of nearly 100% of triplets with 3 such cells was possible by isolating the triplets' from a culture which was treated chemically at about 24 h after the last feeding. Treatment with chemicals in starved cultures resulted in triplets with incompletely fragmented or nonfragmented macronuclei. Further, in P. multimicronucleatum , chemicallyinduced triplets involving only holdfast pairs to which the 3rd cells were uniting often produced 3 cells with fragmented macronuclei.     

Inactivation of a macronuclear intra-S-phase checkpoint in Tetrahymena thermophila with caffeine affects the integrity of the micronuclear genome

Aphidicolin (APH), an inhibitor of DNA polymerase α, arrested cell divisions in Tetrahymena thermophila. Surprisingly, low concentrations of APH induced an increase of macronuclear DNA content and cell size in non-dividing cells. In spite of the cell size increase, most proliferation of basal bodies, ciliogenesis and development of new oral primordia were prevented by the APH treatment. The division arrest induced by APH was partly overridden by caffeine (CAF) treatment, which caused the fragmentation ("pulverization") of the chromosomes in G2 micronuclei. Somatic progeny of dividers with pulverized micronuclei (APH+CAF strains) contained aneuploid and amicronucleate cells. The amicronucleate cells, after losing their oral structures and most of their cilia, and undergoing progressive disorganization of cortical structures, assumed an irregular shape ("crinkled") and were nonviable. "Crinkled" cells were not formed after APH + CAF treatment of the amicronuclear BI3840 strain, which contains some mic-specific sequences in its macronucleus. Most of the APH +CAF strains had a typical "*"- like conjugation phenotype: they did not produce pronuclei, but received them unilaterally from their mates and retained old macronuclei. However, 4 among 100 APH+CAF clones induced arrest at meiotic metaphase I in their wt mates. It is likely that the origin of such clones was enhanced by chromosome pulverization.