Meiosis in Drosophila melanogaster

Individual bivalents or chromosomes have been identified in Drosophila melanogaster spermatocytes at metaphase I, anaphase I, metaphase II and anaphase II in electron micrographs of serial sections. Identification was based on a combination of chromosome volume analysis, bivalent topology, and kinetochore position. — Kinetochore microtubule numbers have been obtained for the identified chromosomes at all four meiotic stages. Average numbers in D. melanogaster are relatively low compared to reported numbers of other higher eukaryotes. There are no differences in kinetochore microtubule numbers within a stage despite a large (approximately tenfold) difference in chromosome volume between the largest and the smallest chromosome. A comparison between the two meiotic metaphases (metaphase I and metaphase II) reveals that metaphase I kinetochores possess twice as many microtubules as metaphase II kinetochores. — Other microtubules in addition to those that end on or penetrate the kinetochore are found in the vicinity of the kinetochore. These microtubules penetrate the chromosome rather than the kinetochore proper and are more numerous at metaphase I than at the other division stages.     

Kinetochore and microtubules in two members of Chlorophyceae,Cladophora fracta and Spirogyra majuscula

Evidence is presented for the existence of a localised kinetochore with stratified fine structure in Cladophora and in Spirogyra. In the latter, there is the possibility of two kinetochores on the longer chromosomes. There is no evidence for a diffuse kinetochore. The nucleolus persists during mitosis in Cladophora on the nucleolar organising chromosomes, the granular material being lost from it very largely during metaphase and anaphase but the fibrillar material remaining. The persistent nucleolar material at metaphase and anaphase in Spirogyra is not attached to the nucleolar organising chromosomes but accumulates around all the chromosomes and chromatids, the microtubules of the spindle at anaphase passing through and possibly attaching to this nucleolar material and possibly assisting in the movement of the chromatids which are embedded within it.     

Differential expression of a phosphoepitope at the kinetochores of moving chromosomes

A phosphorylated epitope is differentially expressed at the kinetochores of chromosomes in mitotic cells and may be involved in regulating chromosome movement and cell cycle progression. During prophase and early prometaphase, the phosphoepitope is expressed equally among all the kinetochores. In mid-prometaphase, some chromosomes show strong labeling on both kinetochores; others exhibit weak or no labeling; while in other chromosomes, one kinetochore is intensely labeled while its sister kinetochore is unlabeled. Chromosomes moving toward the metaphase plate express the phosphoepitope strongly on the leading kinetochore but weakly on the trailing kinetochore. This is the first demonstration of a biochemical difference between the two kinetochores of a single chromosome. During metaphase and anaphase, the kinetochores are unlabeled. At metaphase, a single misaligned chromosome can inhibit further progression into anaphase. Misaligned chromosomes express the phosphoepitope strongly on both kinetochores, even when all the other chromosomes of a cell are assembled at the metaphase plate and lack expression. This phosphoepitope may be involved in regulating chromosome movement to the metaphase plate during prometaphase and may be part of a cell cycle checkpoint by which the onset of anaphase is inhibited until complete metaphase alignment is achieved.     

Chromosome micromanipulation

Two types of unusual motion within the spindle have heen studied in a grasshopper (Melanoplus differentialis) spermatocyte. The first is the motion of granules placed by micromanipulation within the normally granule-free spindle. The most specific motions are poleward, approximate the speed of the chromosomes in anaphase, and occur in the area between the kinetochores and the nearer pole during both metaphase and anaphase. Exactly the same transport properties were earlier observed by Bajer inHaemanthus endosperm spindles. The absence of significant motion in the interzone between the separating chromosomes at anaphase has been unequivocally demonstrated inMelanoplus spermatocytes. Thus very specific motion of non-kinetochoric materials is probably a general spindle capability which would much restrict admissible models of mitotic force production,if the same forces move both granules and chromosomes. The second unusual motion is seen following chromosome detachment from the spindle by micromanipulation during anaphase. These tend to move toNearer pole rather than to the pole the chromosome's kinetochoresFace. The latter preference was earlier demonstrated after detachment during prometaphase or metaphase and has been confirmed without exception in the present studies. The apparent preference for motion to the nearer pole in anaphase provides the first evidence for poleward forces within each half-spindle which cannot be entirely specified by the chromosomal spindle fibers. Almost certainly these would be the usual forces responsible for chromosome motion since they act specifically at the kinetochores of detached chromosomes. This evidence requires interpretation, however because additional factors influence chromosome motion following detachment at anaphase. On thesimplest interpretation, certain current models of mitosis clearly are not satisfactory and others are favored.     

Control of spindle form and function in grasshopper spermatocytes

The influence of the mitotic organizing centers, the kinetochores and the polar organizers, in controlling the dynamic spindle form and function has been investigated in the primary spermatocytes of two grasshoppers, Arphia xanthoptera and Melanoplus differentialis. A new measure of the total birefringent material in the spindle is introduced—volume-birefringence. This measure avoids many of the problems associated with the traditional retardation measurements of spindle organization.—The number of chromosomes (and their kinetochores) in a spindle can be altered with a piezoelectric micromanipulator in three ways: 1) chromosomes can be removed permanently from the cell, 2) chromosomes can be detached from the spindle and allowed to reenter the spindle at a later time, and 3) chromosomes can be transferred from one spindle to another in cells containing two spindles. Such operations show the volume-birefringence of the spindle is proportional to the number of chromosomes in the spindle. A residual volume-birefringence is seen and attributed to the contribution of the polar organizers to spindle structure. The relative polar contribution differs in the two species. Chromosome motion and spindle elongation in anaphase are unaffected by the number of chromosomes in the spindle. The proportion of volume-birefringence associated with a kinetochore is used to estimate the number of microtubules one might expect to see if the birefringence of the spindle is of microtubular origin. These calculations predict about twice the number of microtubules per kinetochore than seen with the electron microscope. Reasons are suggested to explain this discrepancy.— It is argued that chromosome detachment releases spindle component subunits into the total subunit pool, but that these excess subunits do not influence the metaphase form nor the anaphase function of the spindle; therefore, spindle dynamics are under the direct control of the kinetochores and the polar organizing centers.     

Microinjection of mitotic cells with the 3F3/2 anti-phosphoepitope antibody delays the onset of anaphase

The transition from metaphase to anaphase is regulated by a checkpoint system that prevents chromosome segregation in anaphase until all the chromosomes have aligned at the metaphase plate. We provide evidence indicating that a kinetochore phosphoepitope plays a role in this checkpoint pathway. The 3F3/2 monoclonal antibody recognizes a kinetochore phosphoepitope in mammalian cells that is expressed on chromosomes before their congression to the metaphase plate. Once chromosomes are aligned, expression is lost and cells enter anaphase shortly thereafter. When microinjected into prophase cells, the 3F3/2 antibody caused a concentration-dependent delay in the onset of anaphase. Injected antibody inhibited the normal dephosphorylation of the 3F3/2 phosphoepitope at kinetochores. Microinjection of the antibody eliminated the asymmetric expression of the phosphoepitope normally seen on sister kinetochores of chromosomes during their movement to the metaphase plate. Chromosome movement to the metaphase plate appeared unaffected in cells injected with the antibody suggesting that asymmetric expression of the phosphoepitope on sister kinetochores is not required for chromosome congression to the metaphase plate. In antibody-injected cells, the epitope remained expressed at kinetochores throughout the prolonged metaphase, but had disappeared by the onset of anaphase. When normal cells in metaphase, lacking the epitope at kinetochores, were treated with agents that perturb microtubules, the 3F3/2 phosphoepitope quickly reappeared at kinetochores. Immunoelectron microscopy revealed that the 3F3/2 epitope is concentrated in the middle electronlucent layer of the trilaminar kinetochore structure. We propose that the 3F3/2 kinetochore phosphoepitope is involved in detecting stable kinetochore-microtubule attachment or is a signaling component of the checkpoint pathway regulating the metaphase to anaphase transition.     

Meiotic chromosome structure. Kinetochores and chromatid cores in standard and B chromosomes of Arcyptera fusca (Orthoptera) revealed by silver staining.

The behaviour of two chromosome structures in silver-stained chromosomes was analyzed through the first meiotic division in spermatocytes of the acridoid species Arcyptera fusca. Results showed that at diakinesis kinetochores and chromatid cores are individualized while they associate in bivalents of metaphase I; only kinetochores and distal core spots associate in the sex chromosome. Metaphase I is characterized by morphological and localization changes of both kinetochores and cores which define the onset of anaphase I. These changes analyzed in both autosomes and in the sex chromosome allow us to distinguish among three different substages in metaphase I spermatocytes. B chromosomes may be present as univalents, bivalents, or trivalents. Metaphase I B univalents are characterized by separated cores except at their distal ends and individualized and flat sister kinetochores. At anaphase I sister kinetochores of lagging B chromatids remain connected through a silver-stained strand. The behaviour of cores and kinetochores of B bivalents is identical with that found in the autosomal bivalents. The differences in the morphology of kinetochores of every chromosome shown by B trivalents at metaphase I may be related to the balanced forces acting on the multivalent. The results show dramatic changes in chromosome organization of bivalents during metaphase I. These changes suggest that chromatid cores are not involved in the maintenance of bivalents. Moreover, the changes in morphology of kinetochores are independent of the stage of meiosis but correlate with the kind of division (amphitelic-syntelic) that chromosomes undergo.     

Asynchronous entry into anaphase induced by okadaic acid: spindle microtubule organization and microtubule/kinetochore attachments

Summary We have found that a brief treatment of either PtK₂ cells or stamen hair cells ofTradescantia virginiana during metaphase with okadaic acid, a potent protein phosphatase inhibitor, results in asynchronous entry into anaphase. After this treatment, the interval for the separation of sister chromatids can be expanded from a few seconds to approximately 5 min. We have performed a series of immunolocalizations of cells with anti-tubulin antibodies and CREST serum, asking whether okadaic acid induces asynchronous entry into anaphase through changes in the organization of the spindle microtubules or through a loss in the attachment of spindle microtubules to the kinetochores. Our experiments clearly indicate that asynchronous entry into anaphase after phosphatase inhibitor treatment is not the result of either altered spindle microtubule organization or the long-term loss of microtubule attachment to kinetochores. The kinetochore fiber bundles for all of the separating chromosomes are normally of uniform length throughout anaphase, but after asynchronous entry into anaphase, different groups of kinetochore fiber bundles have distinctly different lengths. The reason for this difference in length is that once split apart, the daughter chromosomes begin their movement toward the spindle poles, with normal shortening of the kinetochore fiber bundle microtubules. Thus, okadaic acid treatment during metaphase does not affect anaphase chromosome movement once it has begun. Our results suggest that one or more protein phosphatases appear to play an important role during metaphase in the regulatory cascade that culminates in synchronous sister chromatid separation.     

Holocentric chromosomes of Luzula elegans are characterized by a longitudinal centromere groove, chromosome bending, and a terminal nucleolus organizer region

The structure of holocentric chromosomes was analyzed in mitotic cells of Luzula elegans. Light and scanning electron microscopy observations provided evidence for the existence of a longitudinal groove along each sister chromatid. The centromere-specific histone H3 variant, CENH3, colocalized with this groove and with microtubule attachment sites. The terminal chromosomal regions were CENH3-negative. During metaphase to anaphase transition, L. elegans chromosomes typically curved to a sickle-like shape, a process that is likely to be influenced by the pulling forces of microtubules along the holocentric axis towards the corresponding microtubule organizing regions. A single pair of 45S rDNA sites, situated distal to Arabidopsis-telomere repeats, was observed at the terminal region of one chromosome pair. We suggest that the 45S rDNA position in distal centromere-free regions could be required to ensure chromosome stability.     

Nonrandom chromosome segregation in Neocurtilla (Gryllotalpa) hexadactyla: an ultrastructural study

During meiosis I in males of the mole cricket Neocurtilla (Gryllotalpa) hexadactyla, the univalent X1 chromosome and the heteromorphic X2Y chromosome pair segregate nonrandomly; the X1 and X2 chromosomes move to the same pole in anaphase. By means of ultrastructural analysis of serial sections of cells in several stages of meiosis I, metaphase of meiosis II, and mitosis, we found that the kinetochore region of two of the three nonrandomly segregating chromosomes differ from autosomal kinetochores only during meiosis I. The distinction is most pronounced at metaphase I when massive aggregates of electron-dense substance mark the kinetochores of X1 and Y chromosomes. The lateral position of the kinetochores of X1 and Y chromosomes and the association of these chromosomes with microtubules running toward both poles are also characteristic of meiosis I and further distinguish X1 and Y from the autosomes. Nonrandomly segregating chromosomes are typically positioned within the spindle so that the kinetochoric sides of the X2Y pair and the X1 chromosome are both turned toward the same interpolar spindle axis. This spatial relationship may be a result of a linkage of X1 and Y chromosomes lying in opposite half spindles via a small bundle of microtubules that runs between their unusual kinetochores. Thus, nonrandom segregation in Neocurtilla hexadactyla involves a unique modification at the kinetochores of particular chromosomes, which presumably affects the manner in which these chromosomes are integrated within the spindle.     

MITOSIS IN THE ALGA VACUOLARIA VIRESCENS

Details of mitosis in the chloromonadophycean alga Vacuolaria virescens Cienk. have been studied with the light microscope. The chromosomes are relatively large (up to μ in length at metaphase) and so mitotic stages are readily distinguishable. Chromosomes can be recognized in interphase nuclei as fine strands of chromatin. Contraction of these chromosomes marks the beginning of mitosis and continues progressively until the transition from metaphase to anaphase. Disintegration of nucleoli is complete by late prophase and nucleolar reformation begins in telophase. Some chromosomes exhibit less densely stained regions; centromeres are also present as indicated by their differential staining and by the behavior of chromosomes at metaphase and anaphase. At anaphase progeny chromosomes move apart parallel to the division axis of the nucleus. As anaphase progresses the chromosomes fuse at the polar surface of the progeny chromosome groups. This process continues in telophase and the chromosome groups become more spherical. By the end of telophase nucleolar reformation has begun and the chromosomes have relaxed to their interphase condition.     

Bipolar spindle attachments affect redistributions of ZW10, a Drosophila centromere/kinetochore component required for accurate chromosome segregation

Previous efforts have shown that mutations in the Drosophila ZW10 gene cause massive chromosome missegregation during mitotic divisions in several tissues. Here we demonstrate that mutations in ZW10 also disrupt chromosome behavior in male meiosis I and meiosis II, indicating that ZW10 function is common to both equational and reductional divisions. Divisions are apparently normal before anaphase onset, but ZW10 mutants exhibit lagging chromosomes and irregular chromosome segregation at anaphase. Chromosome missegregation during meiosis I of these mutants is not caused by precocious separation of sister chromatids, but rather the nondisjunction of homologs. ZW10 is first visible during prometaphase, where it localizes to the kinetochores of the bivalent chromosomes (during meiosis I) or to the sister kinetochores of dyads (during meiosis II). During metaphase of both divisions, ZW10 appears to move from the kinetochores and to spread toward the poles along what appear to be kinetochore microtubules. Redistributions of ZW10 at metaphase require bipolar attachments of individual chromosomes or paired bivalents to the spindle. At the onset of anaphase I or anaphase II, ZW10 rapidly relocalizes to the kinetochore regions of the separating chromosomes. In other mutant backgrounds in which chromosomes lag during anaphase, the presence or absence of ZW10 at a particular kinetochore predicts whether or not the chromosome moves appropriately to the spindle poles. We propose that ZW10 acts as part of, or immediately downstream of, a tension-sensing mechanism that regulates chromosome separation or movement at anaphase onset.     

Observations on dicentrics in living cells

Summary In previously irradiated endosperm cells of Haemanthus katherinae studied in vitro by means of micro-cinematography, two-kinetochore chromatids and dicentric chromosomes have been observed. Breaking of such dicentric chromatids and chromosomes has been analysed. Behaviour of some of the dicentric chromosomes during anaphase deserves special attention: interlocking dicentrics cut one through another and rejoin in a few minutes. In this way from a metaphase interlocking dicentric, two sister anaphase dicentrics are formed. Interlocked dicentrics can also uncoil and not break at all. In this case no activity was observed in one kinetochore of one dicentric in later stages of anaphase (two kinetochores were active in one dicentric and only one in its sister). Analysis of chromosome movements in two-kinetochore chromatids and dicentrics is also presented.     

Analysis of Scc1-deficient cells defines a key metaphase role of vertebrate cohesin in linking sister kinetochores

Cleavage of the cohesin subunit Scc1p/Mcd1p/Rad21 permits sister chromatid separation and is considered to trigger anaphase onset. It has also been suggested that the cohesin complex is essential for chromosome condensation and for assembling fully functional kinetochores. Here, we used vertebrate cells conditionally deficient in Scc1 to probe cohesin function in mitosis. Cells lacking cohesin arrest in prometaphase, with many chromosomes failing to align at a metaphase plate and high levels of the spindle assembly checkpoint protein, BubR1, at all kinetochores. We show that the structural integrity of chromosomes is normal in the absence of Scc1. Furthermore, specific inhibition of topoisomerase II, which is required for decatenation of replicated chromosomes, can bypass the cohesin requirement for metaphase chromosome alignment and spindle checkpoint silencing. Since the kinetochore effects of Scc1 deficiency can be compensated for by topoisomerase II inhibition, we conclude that Scc1 is not absolutely required for kinetochore assembly or function, and that its principal role in allowing the onset of anaphase is the establishment of sufficient inter-sister tension to allow biorientation.     

Involvement of chromatid cohesiveness at the centromere and chromosome arms in meiotic chromosome segregation: A cytological approach

Kinetochores and chromatid cores of meiotic chromosomes of the grasshopper species Arcyptera fusca and Eyprepocnemis plorans were differentially silver stained to analyse the possible involvement of both structures in chromatid cohesiveness and meiotic chromosome segregation. Special attention was paid to the behaviour of these structures in the univalent sex chromosome, and in B univalents with different orientations during the first meiotic division. It was observed that while sister chromatid of univalents are associated at metaphase I, chromatid cores are individualised independently of their orientation. We think that cohesive proteins on the inner surface of sister chromatids, and not the chromatid cores, are involved in the chromatid cohesiveness that maintains associated sister chromatids of bivalents and univalents until anaphase I. At anaphase I sister chromatids of amphitelically oriented B univalents or spontaneous autosomal univalents separate but do not reach the poles because they remain connected at the centromere by a long strand which can be visualized by silver staining, that joins stretched sister kinetochores. This strand is normally observed between sister kinetochores of half-bivalents at metaphase II and early anaphase II. We suggest that certain centromere proteins that form the silver-stainable strand assure chromosome integrity until metaphase II. These cohesive centromere proteins would be released or modified during anaphase II to allow normal chromatid segregation. Failure of this process during the first meiotic division could lead to the lagging of amphitelically oriented univalents. Based on our results we propose a model of meiotic chromosome segregation. During mitosis the cohesive proteins located at the centromere and chromosome arms are released during the same cellular division. During meiosis those proteins must be sequentially inactivated, i.e. those situated on the inner surface of the chromatids must be eliminated during the first meiotic division while those located at the centromere must be released during the second meiotic division.by D.P. Bazett-Jones     

Light and electron microscopy of rat kangaroo cells in mitosis

Kinetochores in rat kangaroo (PtK₂) cells in prophase of mitosis are finely fibrillar, globular bodies, 5000–8000 Å in diameter. Sister kinetochores are attached to opposite lateral faces in the primary constriction of chromosomes. No microtubules (MTs) occur in prophase nuclei. During prometaphase the ball-shaped kinetochores differentiate into trilaminar plaques. An outer kinetochore layer, less electron dense than chromatin, appears first in the fibrillar matrix. The inner layer, continuous with, but more electron dense than the chromosome, is formed later. Kinetochore-spindle MT interaction is evident at the very beginning of prometaphase. As a result, kinetochore shape is very variable, but three types of kinetochores can be distinguished by fine structure analysis. A comparison of kinetochore structure and chromosome position in the mitotic spindle yielded clues regarding initial orientation and congression. At the time the nuclear envelope (NE) breaks down chromosomes near asters orient first. Chromosomes approximately equidistant from the two spindle poles amphi-orient immediately. Chromosomes closer to one pole probably achieve mono-orientation first, then amphi-orient and congress. In normal metaphase all the chromosomes lie at or near the spindle equator and kinetochores are structurally uniform. Paraxial and para-equatorial sections revealed that they are trilaminar, roughly circular plaques of 4000–6000 Å diameter. Inner and outer layers are 400 Å, and the electron translucent middle layer which separates them is 270 Å thick. From 16 to 40 MTs are anchored in the outer layer. In cold-treated cells the kinetochores are trilaminar, but in colcemid-treated cells the inner layer is lacking. Both kinetochores and their MTs are disorganized beginning in late anaphase. In telophase the inner layer persists for some time as an electron dense patch apposed to the NE, while the outer layer disintegrates.     

The role of sister chromatid cohesiveness and structure in meiotic behaviour

Sister chromatid cores, kinetochores and the connecting strand between sister kinetochores were differentially silver stained to analyse the behaviour of these structures during meiosis in normal and two spontaneous desynaptic individuals of Chorthippus jucundus (Orthoptera). In these desynaptic individuals most of the chromosomes appear as univalents and orient equationally in the first meiotic division. Despite this abnormal segregation pattern, the changes in chromosome structure follow the same timing as in normal individuals and seem to be strictly phase dependent. Chromosomes in the first prometaphase have associated sister kinetochores and sister chromatid cores that lie in the chromosome midline; we propose that this promotes the initial monopolar orientation of chromosomes. However, the requirements of tension for stable attachment to the spindle force the autosomal univalents to acquire amphitelic orientation. Sister kinetochores behave in a chromosome orientation-dependent manner and, in the first metaphase, they appear to be interconnected by a strand that can be detected by silver impregnation, as seen in the second metaphase of wild-type individuals. The disappearance of the sister kinetochore-connecting strand, needed for equational chromatid segregation, however, can only take place in the second meiotic division. This connecting strand is ultimately responsible for the inability of chromosomes to segregate sister chromatids in the first anaphase. Received: 25 March 1997; in revised form: 14 July 1997 / Accepted: 22 August 1997     

Kiss and break up—a safe passage to anaphase in mitosis and meiosis

Eukaryotic chromosomes have many challenges to overcome between DNA replication and sister chromatid segregation. If these challenges are not met, cell death or unregulated cell division (cancer) may result. During prophase, chromosomes condense, the nuclear membrane breaks down and cohesins are removed from chromosome arms. In prometaphase, initial spindle attachments are made by sister kinetochores followed by correction of erroneous attachments, centromere oscillation between spindle poles and congression towards the cell's equator. In metaphase, all chromosomes attain stable bipolar spindle attachments and align at the metaphase plate, ready for the metaphase–anaphase transition when all ties between sister chromatids are broken. This review concentrates on recent developments that have revealed the intricacies of these processes. We now know more about how the mechanisms of cohesin removal differ between prophase and the metaphase–anaphase transition, the processes for detection and correction of improper spindle-kinetochore attachments and the concept that tension between sister kinetochores is the driving factor for satisfying the spindle checkpoint. We are also beginning to gain some understanding of the mechanisms behind the co-segregation of sister chromatids at the first meiotic division. Review related to the 15th International Chromosome Conference (ICC XV), held in September 2004, Brunel University, London, UK     

Centromere structure and chromosome number in mitosis of the colourless phytoflagellate Polytoma papillatum (Chlorophyceae, Volvocales, Chlamydomonadaceae).

Centromere structure is described in mitosis of the unicellular biflagellate alga Polytoma papillatum using transmission electron microscopy. The kinetochores are five-layered elements at the poleward surface of the chromosomes. The five layers consist of three dense plates interspersed by two transparent zones. The polemost dense layer serves as the attachment site for kinetochore microtubules and the innermost dense layer is intimately associated with the chromatin. The five-layered organization of the kinetochore in the alga is unusual. In animals, three-layered kinetochores are the rule. This type has also been found in some algae, while higher plants do not possess striated kinetochores. An attempt was made to determine the chromosome number of P. papillatum. Individual chromosomes could not be recognized with confidence, since there were numerous lateral contacts between the chromosomes throughout mitosis. An alternative approach, however, was successful. Counting the kinetochores in serial sections through mitotic metaphase and anaphase plates revealed a number of 15 chromosomes.     

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria,Rhabdocoela)

At metaphase I of meiosis in spermatocytes of Mesostoma ehrenbergii ehrenbergii [2n=10] three bivalents and four univalents form. The same two chromosome pairs always form the univalents. Analysis of metaphase I, anaphase I and metaphase II configurations in fixed testis material suggested that the distribution of the four univalents is not a random process but the correct segregation of one member of each pair to each pole is actively achieved before the end of metaphase I. In live preparations of testis material univalents were observed to move between the poles of metaphase I cells, eventually reaching the correct segregation. All cells observed to enter anaphase I had the correct segregation of univalents. It is proposed that the univalent movement during metaphase I is directed towards obtaining the correct segregation of univalents before the cells enter anaphase.