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.     

Behavior of holokinetic chromosomes in the spermatogenesis of bugs]

Chromosomes were studied on the spermatogonial metaphase and on different stages of meiotic division in males of Eurydeme geblery Kol. and E. ventrale Kol. The set of male chromosomes of the above species involves six pairs of autosomes and an XY sex pair, which is commonly the case in Pentatomidae. In the spermatogenesis, division types are reductional for autosomes and equational for sex chromosomes in the first metaphase; but the situation is quite opposite in the second metaphase. The sex chromosomes of bugs undergo a "touch and go" pairing on the metaphase plate of the second spermatocyte, prior to segregating to opposite poles in the anaphase that follows. No constrictions occur along all the chromosomes at mitosis. This, combined with their behaviour at mitosis, may suggest that, likely as in other Hemiptera, the chromosomes are holokinetic. Nevertheless it is of interest to note that in meiosis the chromosomes behave as telocentric. This replacement of the holokinetic orientation by the telokinetic one in the meiosis of some organisms with diffuse centromere is supposed to depend on the phenomen called "construction of the kinetic activation".     

Meiosis in four trisomic and one double trisomic males of the newt Pleurodeles waltlii

In the newt Pleurodeles waltlii, meiosis was studied in four trisomic and one double trisomic males. Study of first prophase shows that trivalent frequencies and trivalent configurations are correlated with chromosome length; moreover, trivalent configurations indicate that long chromosomes have multiple points of initiation of synapsis whereas two points might be adequate to secure synapsis of short chromosomes. From the study of metaphase II it appears that the extra chromosomes segregate in half of the spermatocytes II. Some abnormal spermatocytes, resulting from nondisjunction of chromosomes at mitosis or at first division of meiosis, or from precocious division of chromosomes at first division of meiosis were observed. In the male double trisomic meiosis fails at anaphase of second division; this accounts for the sterility of the individual.     

Characterization of GATA/GACA-related sequences on proximal chromosome 17 of the mouse

Chromosome behaviour during meiosis in male Syrian hamsters heterozygous for one of three translocations was analysed as part of a study of the transmission of these structural changes. Synapsis was studied using preparations of synaptonemal complexes, and chiasmate associations and the results of anaphase I segregation were studied in air-dried preparations of metaphases I and II respectively. The main findings were: (i) that, at least in the two trivalent-forming translocations, there is no simple relationship between either the frequency or the extent of synapsis and chiasma formation between the chromosomes involved in the translocation; (ii) that the presence of a univalent in a substantial proportion of metaphase I cells does not necessarily lead to irregular segregation as judged by analysis of metaphase IIs; and (iii) conversely, that in translocation heterozyotes in which metaphase I contains the chromosomes involved in the translocation as a quadrivalent or as two bivalents, with no univalents or trivalents, unexpected numerical segregation can be found. The observations of meiotic chromosomes behaviour reported here show that it is not always possible to predict the effects of structural change, or to determine the basis of these effects, from an analysis of any stage of meiosis taken in isolation, or from an analysis of an apparently similar change.     

Holocentric plant meiosis: first sisters,then homologues

Meiosis is a crucial process of sexual reproduction by forming haploid gametes from diploid precursor cells. It involves 2 subsequent divisions (meiosis I and meiosis II) after one initial round of DNA replication. Homologous monocentric chromosomes are separated during the first and sister chromatids during the second meiotic division. The faithful segregation of monocentric chromosomes is realized by mono-orientation of fused sister kinetochores at metaphase I and by bi-orientation of sister kinetochores at metaphase II. Conventionally this depends on a 2-step loss of cohesion, along chromosome arms during meiosis I and at sister centromeres during meiosis II.     

Metaphase I arrest upon activation of the Mad2-dependent spindle checkpoint in mouse oocytes

BACKGROUND: The importance of mitotic spindle checkpoint control has been well established during somatic cell divisions. The metaphase-to-anaphase transition takes place only when all sister chromatids have been properly attached to the bipolar spindle and are aligned at the metaphase plate. Failure of this checkpoint may lead to unequal separation of sister chromatids. On the contrary, the existence of such a checkpoint during the first meiotic division in mammalian oocytes when homologous chromosomes are segregated has remained controversial. RESULTS: Here, we show that mouse oocytes respond to spindle damage by a transient and reversible cell cycle arrest in metaphase I with high Maturation Promoting Factor (MPF) activity. Furthermore, the mitotic checkpoint protein Mad2 is present throughout meiotic maturation and is recruited to unattached kinetochores. Overexpression of Mad2 in meiosis I leads to a cell cycle arrest in metaphase I. Expression of a dominant-negative Mad2 protein interferes with proper spindle checkpoint arrest. CONCLUSIONS: Errors in meiosis I cause missegregation of chromosomes and can result in the generation of aneuploid embryos with severe birth defects. In human oocytes, failures in spindle checkpoint control may be responsible for the generation of trisomies (e.g., Down Syndrome) due to chromosome missegregation in meiosis I. Up to now, the mechanisms ensuring correct separation of chromosomes in meiosis I remained unknown. Our study shows for the first time that a functional Mad2-dependent spindle checkpoint exists during the first meiotic division in mammalian oocytes.     

Spo13 facilitates monopolin recruitment to kinetochores and regulates maintenance of centromeric cohesion during yeast meiosis

BACKGROUND: Cells undergoing meiosis perform two consecutive divisions after a single round of DNA replication. During the first meiotic division, homologous chromosomes segregate to opposite poles. This is achieved by (1) the pairing of maternal and paternal chromosomes via recombination producing chiasmata, (2) coorientation of homologous chromosomes such that sister chromatids attach to the same spindle pole, and (3) resolution of chiasmata by proteolytic cleavage by separase of the meiotic-specific cohesin Rec8 along chromosome arms. Crucially, cohesin at centromeres is retained to allow sister centromeres to biorient at the second division. Little is known about how these meiosis I-specific events are regulated. RESULTS: Here, we show that Spo13, a centromere-associated protein produced exclusively during meiosis I, is required to prevent sister kinetochore biorientation by facilitating the recruitment of the monopolin complex to kinetochores. Spo13 is also required for the reaccumulation of securin, the persistence of centromeric cohesin during meiosis II, and the maintenance of a metaphase I arrest induced by downregulation of the APC/C activator CDC20. CONCLUSION: Spo13 is a key regulator of several meiosis I events. The presence of Spo13 at centromere-surrounding regions is consistent with the notion that it plays a direct role in both monopolin recruitment to centromeres during meiosis I and maintenance of centromeric cohesion between the meiotic divisions. Spo13 may also limit separase activity after the first division by ensuring securin reaccumulation and, in doing so, preventing precocious removal from chromatin of centromeric cohesin.     

Meiotic chromosome interactions in inbred autotetraploid rye (Secale cereale).

Electron microscopy of whole-mount surface-spread synaptonemal complex complements and conventional light microscopy of chromosomes at first metaphase of meiosis were used to compare the relative frequencies of pairing configurations at the two stages in inbred autotetraploid rye (Secale cereale L.). Statistical tests showed significantly fewer multivalents at first metaphase than expectations based on random initiation of synapsis at each telomeric site within each group of four homologues. Direct observations of synaptic behaviour of chromosomes showed that this deviation is due primarily to a preponderance of bivalents during zygotene and pachytene. It is also the result of a significant drop in multivalent frequency from meiotic prophase to metaphase I, which is attributable both to a lack of chiasmata with which to consolidate multivalents and inhibition of chiasma formation in synaptonemal complex segments of multivalents that are nonhomologous.     

Pronuclear formation in starfish eggs inseminated at different stages of meiotic maturation: correlation of sperm nuclear transformations and activity of the maternal chromatin

Changes in sperm nuclei incorporated into starfish, Asterina miniata, eggs inseminated at different stages of meiosis have been correlated with the progression of meiotic maturation. A single, uniform rate of sperm expansion characterized eggs inseminated at the completion of meiosis. In oocytes inseminated at metaphase I and II the sperm nucleus underwent an initial expansion at a rate comparable to that seen in eggs inseminated at the pronuclear stage. However, in oocytes inseminated at metaphase I, the sperm nucleus ceased expanding by meiosis II and condensed into chromosomes which persisted until the completion of meiotic maturation. Concomitant with the formation and expansion of the female pronucleus, sperm chromatin of oocytes inseminated at metaphase I enlarged and developed into male pronuclei. Condensation of the initially expanded sperm nucleus in oocytes inseminated at metaphase II was not observed. Instead, the enlarged sperm nucleus underwent a dramatic increase in expansion commensurate with that taking place with the maternal chromatin to form a female pronucleus. Fusion of the relatively large female pronucleus and a much smaller male pronucleus was observed in eggs fertilized at the completion of meiotic maturation. In oocytes inseminated at metaphase I and II, the male and female pronuclei, which were similar in size, migrated into juxtaposition, and as separate structures underwent prophase. The chromosomes in each pronucleus condensed, intermixed, and became aligned on the metaphase palate of the mitotic spindle in preparation for the first cleavage division. These observations demonstrate that the time of insemination with respect to the stage of meiotic maturation has a significant effect on sperm nuclear transformations and pronuclear morphogenesis.     

Nuclear division of the vegetative cells, conchosporangial cells and conchospores of Porphyra yezoensis (Bangiales, Rhodophyta)

To confirm the position and timing of meiosis in Porphyra yezoensis Ueda, the nuclear division of vegetative cells, conchosporangial cells and conchospores was observed. An improved staining method using modified carbol fuchsin was introduced to stain the chromosomes of Porphyra. Pit‐connections between conchosporangial cells also stained well with this method. Leptotene, zygotene, pachytene, diplotene, diakinesis, metaphase, anaphase and telophase were observed in the conchosporangial cells. During the germination of conchospores, no characteristics of meiosis I were found. No difference between the nuclear division of vegetative cells and that of conchospores was observed, and 2–3 days were needed for the first cell division both in vegetative cells and conchospores. Therefore, the cell division that occurs during conchospore germination is not meiosis I. Our results indicate that the prophase of meiosis I begins during the formation of conchosporangial branches, and metaphase I, anaphase I and telophase I take place during the maturation of conchosporangial branches. Then the three‐bivalent nucleate sporangia complete cell division to form two individual conchospores, each with one three‐univalent nucleus. The conchospores released from the sporangia are at meiotic interphase. Meiosis II occurs at the first nuclear division during conchospore germination, which is a possible explanation for the observation of mosaic thalli in mutant germlings of P. yezoensis. The mosaic thalli might also arise from gene conversion/post meiotic segregation events, comparable to those in Sordaria fimicola (Roberge ex Desm.) Ces. & De Not. and Neurospora crassa Shear & B.O. Dodge.     

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.     

Inhibition of chromosome separation in fertilized starfish eggs by kalihinol F, a topoisomerase I inhibitor obtained from a marine sponge

Kalihinol F, a naturally occurring diterpene from a marine sponge, Acanthella sp., inhibited chromosome separation in fertilized starfish (Asterina pectinifera) eggs but allows the first cleavage to occur, thereby forming unseparated metaphase chromosomes which were elongated between the two daughter cells. The chromosomes were eventually torn off in the embryonic cells. Most of the cells gradually lost the chromosomes during the cell cycle progression. The embryonic development halted at the morula stage just before the onset of blastulation. The mitotic failure occurred when kalihinol F was applied to a fertilized egg during the second meiotic process, but not after the completion of the second meiotic division. Kalihinol F inhibited topoisomerase I activity in vitro, but had no effects on activities of DNA polymerases alpha, beta, and gamma, and of topoisomerase II. These results suggest that the topoisomerase I plays an essential role in meiosis II in this species.     

Mechanisms of chromosome orientation revealed by two meiotic mutants in Drosophila melanogaster

Two disjunction defective meiotic mutants, ord and mei-S332, each of which disrupts meiosis in both male and female Drosophila melanogaster, were analyzed cytologically and genetically in the male germ-line. It was observed that sister-chromatids are frequently associated abnormally during prophase I and metaphase I in ord. Sister chromatid associations in mei-S332 are generally normal during prophase I and metaphase I. By telophase I, sister chromatids have frequently precociously separated in both mutants. During the first division sister chromatids disjoin from one another frequently in ord and rarely in mei-S332. It is argued that the simplest interpretation of the observations is that each mutant is defective in sister chromatid cohesiveness and that the defect in ord manifests itself earlier than does the defect in mei-S332. In addition, based on these mutant effects, several conclusions regarding normal meiotic processes are drawn. (1) The phenotype of these mutants support the proposition that the second meiotic metaphase (mitotic-type) position of chromosomes and their equational orientation is a consequence of the equilibrium, at the metaphase plate, of pulling forces acting at the kinetochores and directed towards the poles. (2) Chromosomes which lag during the second meiotic division tend to be lost. (3) Sister chromatid cohesiveness, or some function necessary for sister chromatid cohesiveness, is required for the normal reductional orientation of sister kinetochores during the first meiotic division. (4) The kinetochores of a half-bivalent are double at the time of chromosome orientation during the first meiotic division. Finally, functions which are required throughout meiosis in both sexes must be considered in the pathways of meiotic control.     

Karyotype and male pre-reductional meiosis of the sharpshooter Tapajosa rubromarginata (Hemiptera: Cicadellidae)

Cicadellidae in one of the best represented families in the Neotropical Region, and the tribe Proconiini comprises most of the xylem-feeding insects, including the majority of the known vectors of xylem-born phytopathogenic organisms. The cytogenetics of the Proconiini remains largely unexplored. We studied males of Tapajosa rubromarginata (Signoret) collected at El Manantial (Tucumán, Argentina) on native spontaneous vegetation where Sorghum halepense predominates. Conventional cytogenetic techniques were used in order to describe the karyotype and male meiosis of this sharpshooter. T. rubromarginata has a male karyological formula of 2n = 21 and a sex chromosome system XO:XX (male:female). The chromosomes do not have a primary constriction, being holokinetic and the meiosis is pre-reductional, showing similar behavior both for autosomes and sex chromosomes during anaphase I. For this stage, chromosomes are parallel to the acromatic spindle with kinetic activities in the telomeres. They segregate reductionally in the anaphase I, and towards the equator during the second division of the meiosis. This is the first contribution to cytogenetic aspects on proconines sharpshooters, particularly on this economic relevant Auchenorrhyncha species.     

Chromosome numbers, meiotic behavior, and pollen viability of species of Vriesea and Aechmea genera (Bromeliaceae) native to Rio Grande do Sul, Brazil

Chromosome number, meiotic behavior, and pollen viability were analyzed in 15 species of two genera, Vriesea and Aechmea, native to Rio Grande do Sul, Brazil. This study is the first cytogenetic analysis of these taxa. The chromosome numbers are all n = 25, consistent with the proposed base number of x = 25 for Bromeliaceae. All examined taxa displayed regular bivalent pairing and chromosome segregation at meiosis. Observed meiotic abnormalities include univalents in metaphase I; missing or extra chromosomes and precocious division of centromeres in metaphase II; laggards in telophase I and anaphase II/telophase II. The high pollen viability (>88%) reflects a regular meiosis.     

Trichlorfon predisposes to aneuploidy and interferes with spindle formation in in vitro maturing mouse oocytes

The pesticide trichlorfon (TCF) has been implicated in human trisomy 21, and in errors in chromosome segregation at male meiosis II in the mouse. We previously provided evidence that TCF interferes with spindle integrity and cell-cycle control during murine oogenesis. To assess the aneugenic activity of TCF in oogenesis, we presently analysed maturation, spindle assembly, and chromosome constitution in mouse oocytes maturing in vitro in the presence of 50 or 100 microg/ml TCF for 16 h or in pulse-chase experiments. TCF stimulated maturation to meiosis II at 50 microg/ml, but arrested meiosis in some oocytes at 100 microg/ml. TCF at 100 microg/ml was aneugenic causing non-disjunction of homologous chromosomes at meiosis I, a significant increase of the hyperploidy rate at metaphase II, and a significant rise in the numbers of oocytes that contained a 'diploid' set of metaphase II chromosomes (dyads). TCF elevated the rate of precocious chromatid segregation (predivision) at 50 and 100 microg/ml. Pulse-chase experiments with 100 microg/ml TCF present during the first 7 h or the last 9 h of maturation in vitro did not affect meiotic progression and induced intermediate levels of hyperploidy at metaphase II. Exposure to > or =50 microg/ml TCF throughout maturation in vitro induced severe spindle aberrations at metaphase II, and over one-third of the oocytes failed to align all chromosomes at the spindle equator (congression failure). These observations suggest that exposure to high concentrations of TCF induces non-disjunction at meiosis I of oogenesis, while lower doses may preferentially cause errors in chromosome segregation at meiosis II due to disturbances in spindle function, and chromosome congression as well as precocious separation of chromatids prior to anaphase II. The data support evidence from other studies that TCF has to be regarded as a germ cell aneugen.     

Mitosis with undivided chromosomes

Summary Cases of cell division with single chromatids are discussed in connection with a study on mitosis with undivided chromosomes made on living material of the endosperm of Haemanthus katharinae. Such divisions are known from certain abnormal mitoses in the microspores of a few plant species, and also from the second meiotic division, in which it is possible in numerous materials to study the behaviour of daughter univalents, and, in a few cases, also daughter chromosomes derived from chromosomes that were paired during the first division.The various cases of mitosis with single chromatids show a great variation with respect to the degree of scattering of the chromosomes over the spindle at metaphase. In a few cases there is practically no tendency to form a metaphase plate. In other cases the tendency to form such a plate is more or less pronounced, but also in these cases it is difficult for the chromosomes to form this arrangement. Some of them remain scattered over the spindle. After the metaphase a kind of anaphase usually follows in which the single chromatids, without division, move to the poles, often with other chromosomes lagging in intermediate positions.An approach of chromosomes to the poles may be caused by two different mechanisms in mitoses of this kind and only in a few cases is the information sufficient to show that active centromere movements occur during these anaphases.In many aspects of their behaviour on the spindle, single chromatids are similar to ordinary univalents of the first meiotic division. For this reason the movement mechanics of the chromosomes of the first meiotic division is briefly reviewed.The interpretation is expressed that the structure of the centromere region of a single chromatid shows some similarity to that of a univalent of the first meiotic division and that this may be the reason for their similar behaviour. The chromatid centromere would have a structural multiplicity with respect to its kinetic elements, corresponding to its subdivision in half-chromatids and also to the presence of two or three consecutive chromomeres in its longitudinal direction. As these kinetic elements are arranged close to one another on one side of the narrow cylinder of the centromere constriction, it is difficult for them to orient, towards both poles simultaneously. A single chromatid having a centromere of this kind will show orientation instability and change its orientation between the two unipolar orientations and various more or less bipolar orientations. The movements following these different orientations would cause the scattering of these single chromatids over the spindle. The orientation of ordinary mitotic metaphase chromosomes, consisting of two such chromatids, could often be the consequence of a process of co-orientation similar to that in meiotic bivalents.The anaphase movement of undivided chromosomes, which by active centromere movements are shifted in the polar directions without a separation of daughter components, is discussed with reference to a similar behaviour observed by Dietz in multivalents in Ostracods. These multivalents are stabilized in the equator during metaphase, in spite of the fact that they have two or three centromeres directed towards one pole and a single one towards the other. During anaphase their chromosomes do not separate but the whole configurations are shifted towards that pole towards which the majority of the centromeres are directed (this is followed by another type of movement which does not concern us in this connection). Undivided chromosomes that are oriented with more of their kinetic material towards one of the poles and less towards the other should by the same mechanisms as moved the multivalents be shifted in the equatorial direction during metaphase and in the polar direction during anaphase. The mechanism of these events is obscure. A change in the interpretation given by Dietz is suggested.This paper is dedicated to Professor Franz Schrader on the occasion of his seventieth birthday.     

Histone modifications related to chromosome silencing and elimination during male meiosis in Bengalese finch

We report here that a germline-restricted chromosome (GRC) is regularly present in males and females of the Bengalese finch (Lonchura domestica). While the GRC is euchromatic in oocytes, in spermatocytes this chromosome is cytologically seen as entirely heterochromatic and presumably inactive. The GRC is observed in the cytoplasm of secondary spermatocytes, indicating that its elimination from the nucleus occurs during the first meiotic division. By immunofluorescence on microspreads, we investigated the presence of histone H3 modifications throughout male meiosis, as well as in postmeiotic stages. We found that the GRC is highly enriched in di- and trimethylated histone H3 at lysine 9 during prophase I, in agreement with the presumed inactive state of this chromosome. At metaphase I, dimethylated histone H3 is no longer detectable on the GRC and its chromatin is more faintly stained with DAPI. The condensed GRC is underphosphorylated at serine 10 compared to the regular chromosomes during metaphase I, being phosphorylated later at this site after the first meiotic division. From these results, we proposed that trimethylation of histone H3 at lysine 9 on the GRC chromatin increases during metaphase I. This hypermethylated state at lysine 9 may preclude the phosphorylation of the adjacent serine 10 residue, providing an example of cross-talk of histone H3 modifications as described in experimental systems. The differential underphosphorylation of the GRC chromatin before elimination is interpreted as a cytologically detectable byproduct of deficient activity of Aurora B kinase, which is responsible for the phosphorylation of H3 at serine 10 during mitosis and meiosis.     

The temporal and spatial distribution of the proliferation associated Ki-67 protein during female and male meiosis

We used immunolocalization in tissue sections and cytogenetic preparations of female and male gonads to study the distribution of the proliferation marker pKi-67 during meiotic cell cycles of the house mouse, Mus musculus. During male meiosis, pKi-67 was continuously present in nuclei of all stages from the spermatogonium through spermatocytes I and II up to the earliest spermatid stage (early round spermatids) when it appeared to fade out. It was not detected in later spermatid stages or sperm. During female meiosis, pKi-67 was present in prophase I oocytes of fetal ovaries. It was absent in oocytes from newborn mice and most oocytes of primordial follicles from adults. The Ki-67 protein reappeared in oocytes of growing follicles and was continuously present up to metaphase II. Thus, pKi-67 was present in all stages of cell growth and cell division while it was absent from resting oocytes and during the main stages of spermiocytogenesis. Progression through the meiotic cell cycle was associated with extensive intranuclear relocation of pKi-67. In the zygotene and pachytene stages, most of the pKi-67 colocalized with centromeric (centric and pericentric) heterochromatin and adjacent nucleoli; the heterochromatic XY body in male pachytene, however, was free of pKi-67. At early diplotene, pKi-67 was mainly associated with nucleoli. At late diplotene, diakinesis, metaphase I and metaphase II of meiosis, pKi-67 preferentially bound to the perichromosomal layer and was almost absent from the heterochromatic centromeric regions of the chromosomes. After the second division of male meiosis, the protein reappeared at the centromeric heterochromatin and an adjacent region in the earliest spermatid stage and then faded out. The general patterns of pKi-67 distribution were comparable to those in mitotic cell cycles. With respect to the timing, it is interesting to note that relocation from the nucleolus to the perichromosomal layer takes place at the G2/M-phase transition in the mitotic cell cycle but at late diplotene of prophase I in meiosis, suggesting physiological similarity of these stages.     

Differential anaphase-I behaviour between wheat and rye univalents in triticale-wheat hybrid plants

The chromosomes of the D genome of wheat and the genome R of rye can be distinguished at meiosis by C-banding in triticale-wheat hybrid plants. All members of both genomes almost exclusively formed univalents at metaphase I. However, at anaphase I the frequencies of equationally dividing chromosomes were higher for rye than for wheat chromosomes. The differential centromere behaviour at anaphase I is ascribed to differences in the time at which wheat and rye univalents are formed during the first meiotic prophase.