Defects in cytoskeletal microtubule deployment of microsporocytes contribute to fertility loss in genic male-sterile Chinese cabbage

The microtubular cytoskeleton of male-sterile Chinese cabbage was examined to characterize cytoplasmically based defects during microsporogenesis of fertile and sterile microsporocytes. At the onset of meiosis, microtubules (MTs) in fertile microsporocytes were short and anisotropically oriented in the microsporocyte cytoplasm. As the microsporocytes entered metaphase I, the MTs constructed a bisymmetrical spindle characterized by conspicuous kinetochore fibers closely associated with chromosomes in the medial plane. During anaphase I, interzonal MTs become conspicuous between the two sets of chromosomes and the polar regions become more distant as spindle MTs are depleted, essentially disappearing at telophase I. Radially distributed MTs increased and the microsporocyte entered meiosis II, producing two spindles at angles to one another within the wall of the microsporocyte. Indicative of the completion of anaphase II is the formation of a field of aligned MTs between two non-sister nuclei, after which the cytoplasm produced centripetal furrows, meeting in the center of the cell and dividing it into four microspores at the completion of cytokinesis. In sterile microsporocytes, however, an abnormal arrangement of MTs occurred at the conclusion of anaphase II. Although two spindles formed, the angle and the boundary between the spindles were not maintained. At the onset of telophase II, the two spindles migrated to a central region and laterally fused in irregular orientations in which the decondensing chromatin of the non-sister nuclei may form separate or merged nuclei, followed by irregular cytokinesis. The result of meiosis was 41.8 % two binuclear products, and 58.2 % one diploid and one binuclear sterile products.     

Septin2 is modified by SUMOylation and required for chromosome congression in mouse oocytes

In mitosis, centrosomes nucleate microtubules that capture the sister kinetochores of each chromosome to facilitate chromosome congression. In contrast, during meiosis chromosome congression on the acentrosomal spindle is driven primarily by movement of chromosomes along laterally associated microtubule bundles. Previous studies have indicated that septin2 is required for chromosome congression and cytokinesis in mitosis, we therefore asked whether perturbation of septin2 would impair chromosome congression and cytokinesis in meiosis. We have investigated its expression, localization and function during mouse oocyte meiotic maturation. Septin2 was modified by SUMO-1 and its levels remained constant from GVBD to metaphase II stages. Septin2 was localized along the entire spindle at metaphase and at the midbody in cytokinesis. Disruption of septins function with an inhibitor and siRNA caused failure of the metaphase I /anaphase I transition and chromosome misalignment but inhibition of septins after the metaphase I stage did not affect cytokinesis. BubR1, a core component of the spindle checkpoint, was labeled on misaligned chromosomes and on chromosomes aligned at the metaphase plate in inhibitor-treated oocytes that were arrested in prometaphase I/metaphase I, suggesting activation of the spindle assembly checkpoint. Taken together, our results demonstrate that septin2 plays an important role in chromosome congression and meiotic cell cycle progression but not cytokinesis in mouse oocytes.     

Desynapsis and precocious cytokinesis in Brachiaria humidicola (Poaceae) compromise meiotic division

The forage grass species Brachiaria humidicola is native to African savannas. Owing to its good adaptation to poorly drained and infertile acid soils, it has achieved wide utilization for pastures in Brazilian farms. Among the 55 accessions of B. humidicola analysed from the Embrapa Beef Cattle collection, one (H022), presented desynapsis and an abnormal pattern of cytokinesis in the first meiotic division. Among 28 inflorescences analysed in this accession, 12 were affected by the anomaly. In affected meiocytes, the first cytokinesis occurred in metaphase I and was generally perpendicular to a wide-metaphase plate, dividing the genome into two parts with an equal or unequal number of chromosomes. The normal cytokinesis after telophase I did not occur, and the meiocytes entered metaphase II, progressing to the end of meiosis with the occurrence of the second cytokinesis. As the first cytokinesis occurred precociously, whereas the second was normal, tetrads were formed but with unbalanced chromosome numbers in microspores. Abnormal cytokinesis occurred only in those meiocytes that underwent desynapsis after diakinesis. The implications of this abnormality in the Brachiaria breeding programme are discussed.     

ms17: a meiotic mutation causing partial male sterility in a corn silage hybrid

Cytological analysis under light microscopy of the single hybrid P30R50 of silage corn revealed an abnormal pattern of microsporogenesis that affected the meiotic products. Meiosis progressed normally until diakinesis, but before migration to the metaphase plate, bivalents underwent total desynapsis and 20 univalent chromosomes were scattered in the cytoplasm. At this stage, meiocytes also exhibited a number of chromatin-like fragments scattered throughout the cell. Metaphase I was completely abnormal in the affected cells, and univalent chromosomes and fragments were distributed among several curved spindles. Anaphase I did not occur, and each chromosome or group of chromosomes originated a micronucleus. After this phase, an irregular cytokinesis occurred, and secondary meiocytes with several micronuclei were observed. Metaphase II and anaphase II also did not occur, and after the second cytokinesis, the genomes were fractionated into polyads, generating several unbalanced microspores, with various-sized nuclei. About 35% of the tetrads were abnormal in the hybrid. This spontaneous mutation had been previously reported in a USA maize line called ms17 and was found to cause male sterility.     

Meiosis in a triploid hybrid of <Emphasis Type="Italic">Gossypium</Emphasis>: high frequency of secondary bipolar spindles at metaphase II

Studies on meiosis in pollen mother cells (PMCs) of a triploid interspecific hybrid (3x = 39 chromosomes, AAD) between tetraploid Gossypium hirsutum (4n = 2x = 52,AADD) and diploid G. arboreum (2n = 2x = 26,AA) are reported. During meiotic metaphase I, 13 AA bivalents and 13 D univalents are expected in the hybrid. However, only 28% of the PMCs had this expected configuration. The rest of the PMCs had between 8 and 12 bivalents and between 12 and 17 univalents. Univalents lagged at anaphase I, and at metaphase II one or a group of univalents remained scattered in the cytoplasm and failed to assemble at a single metaphase plate. Primary bipolar spindles organized around the bivalents and multivalents. In addition to the primary spindle, several secondary and smaller bipolar spindles organized themselves around individual univalents and groups of univalents. Almost all (97%) of the PMCs showed secondary spindles. Each spindle functioned independently and despite their multiple numbers in a cell, meiosis I proceeded normally, with polyad formation. These observations strongly support the view that in plant meiocytes bilateral kinetochore symmetry is not required for establishing a bipolar spindle and that single unpaired chromosomes can initiate and stabilize the formation of a functional bipolar spindle.     

MEI-1/katanin is required for translocation of the meiosis I spindle to the oocyte cortex in C elegans

In most animals, successful segregation of female meiotic chromosomes involves sequential associations of the meiosis I and meiosis II spindles with the cell cortex so that extra chromosomes can be deposited in polar bodies. The resulting reduction in chromosome number is essential to prevent the generation of polyploid embryos after fertilization. Using time-lapse imaging of living Caenorhabditis elegans oocytes containing fluorescently labeled chromosomes or microtubules, we have characterized the movements of meiotic spindles relative to the cell cortex. Spindle assembly initiated several microns from the cortex. After formation of a bipolar structure, the meiosis I spindle translocated to the cortex. When microtubules were partially depleted, translocation of the bivalent chromosomes to the cortex was blocked without affecting cell cycle timing. In oocytes depleted of the microtubule-severing enzyme, MEI-1, spindles moved to the cortex, but association with the cortex was unstable. Unlike translocation of wild-type spindles, movement of MEI-1-depleted spindles was dependent on FZY-1/CDC20, a regulator of the metaphase/anaphase transition. We observed a microtubule and FZY-1/CDC20-dependent circular cytoplasmic streaming in wild-type and mei-1 mutant embryos during meiosis. We propose that, in mei-1 mutant oocytes, this cytoplasmic streaming is sufficient to drive the spindle into the cortex. Cytoplasmic streaming is not the normal spindle translocation mechanism because translocation occurred in the absence of cytoplasmic streaming in embryos depleted of either the orbit/CLASP homolog, CLS-2, or FZY-1. These results indicate a direct role of microtubule severing in translocation of the meiotic spindle to the cortex.     

Microtubule organization during successive microsporogenesis in Allium cepa and simultaneous cytokinesis in Nicotiana tabacum

Microtubule cytoskeleton organization during microspore mother cell (MMC) meiosis in Allium cepa L. and microsporogenesis in Nicotiana tabacum L. was examined. The MMC microtubules (MTs) were short and well dispersed in the cytoplasm of both taxa. As the MMCs of both species entered metaphase of meiosis I, the MTs constructed a spindle that facilitated the chromosomes to orient in the meridian plane. At anaphase of meiosis I, the spindle MTs differentiated into two types: one MT type became short, pulled the chromosomes toward the two poles, and was designated as centromere MTs; the second type of MT connected the two poles, and was designated as pole MTs. In A. cepa, where successive cytokinesis was observed, pole MTs assumed a tubbish shape. Some new short MTs aggregated in the meridian plane and constricted to form a phragmoplast, which developed into a cell plate, divided the cytoplasm into two parts and produced a dyad. However, in tobacco, a phragmoplast was not generated in anaphase of meiosis I and II and cytokinesis did not occur. The spindle MTs depolymerized and reorganized the radial arrangement of MTs from the nucleate surface to the periplasm during anaphase. Following telophase of meiosis II, the cytoplasm produced centripetal furrows, which met in the center of the cell and divided it into four parts, serving as a form of cytokinesis. In this process, MTs appeared to bear no relationship to cytokinesis.     

A differential phenotypic expression of a divergent spindle mutation in interspecific Brachiaria hybrids

Several mutations are known to alter the normal progression of meiosis and can be correlated with defects in microtubule distribution. The dv mutation affects the spindle organization and chromosomes do not converge into focused poles. Two Brachiaria hybrids presented the phenotypic expressions of dv mutation but exhibited many more details in the second division. Bivalents were distantly positioned and spread over a large metaphase plate and failed to converge into focused poles. Depending on the distance of chromosomes at the poles, telophase I nuclei were elongated or the chromosomes were grouped into various micronuclei of different sizes in each cell. The first cytokinesis occurred. However, when there were micronuclei, a second cytokinesis immediately took place dividing the prophase II meiocytes into three or four cells. In each meiocyte, meiosis progressed to the second division. Slightly elongated nuclei or micronuclei were recorded in telophase II. After a third cytokinesis, hexads or octads were formed. Pollen grains of different sizes were generated. One of these hybrids presented a higher frequency of abnormal cells than when previously analyzed. The fate of these hybrids as genitors or as candidates for cultivars in the Brachiaria breeding program is discussed.     

The spatial and mechanical challenges of female meiosis

Recent work shows that cytokinesis and other cellular morphogenesis events are tuned by an interplay among biochemical signals, cell shape, and cellular mechanics. In cytokinesis, this includes cross-talk between the cortical cytoskeleton and the mitotic spindle in coordination with cell cycle control, resulting in characteristic changes in cellular morphology and mechanics through metaphase and cytokinesis. The changes in cellular mechanics affect not just overall cell shape, but also mitotic spindle morphology and function. This review will address how these principles apply to oocytes undergoing the asymmetric cell divisions of meiosis I and II. The biochemical signals that regulate cell cycle timing during meiotic maturation and egg activation are crucial for temporal control of meiosis. Spatial control of the meiotic divisions is also important, ensuring that the chromosomes are segregated evenly and that meiotic division is clearly asymmetric, yielding two daughter cells - oocyte and polar body - with enormous volume differences. In contrast to mitotic cells, the oocyte does not undergo overt changes in cell shape with its progression through meiosis, but instead maintains a relatively round morphology with the exception of very localized changes at the time of polar body emission. Placement of the metaphase-I and -II spindles at the oocyte periphery is clearly important for normal polar body emission, although this is likely not the only control element. Here, consideration is given to how cellular mechanics could contribute to successful mammalian female meiosis, ultimately affecting egg quality and competence to form a healthy embryo.     

Asymmetric division of spindle microtubules and microfilaments during bovine meiosis from metaphase I to metaphase III

The kinetics of spindle and chromosomes during bovine oocyte meiosis from meiosis I to meiosis III is described. The results of this study showed that (1) oocytes began to extrude the first polar body (Pb1) at the early anaphase I stage and the Pb1 totally separated from the mother cell only when oocytes reach the MII stage; (2) the morphology of the spindle changed from barrel-shaped at the metaphase stage to cylinder-shaped at early anaphase, and then to a thin, long triangle-shaped cone at late anaphase and telophase stages; (3) chromosome morphology went from an individual visible stage at metaphase to a less defined chromatin state during anaphase and telophase stages, and then back to visible individual chromosomes at the next metaphase; (4) chromatin that connected with the floor of the cone became the polar bodies and expelled, and almost all of the microtubules (MTs) and microfilaments (MFs) composing the spindles moved towards and contributed to the polar bodies; and (5) the size of the metaphase I (MI) spindle was larger than the metaphase II (MII) and metaphase III (MIII) spindles. The MII spindle, however, is more barrel-shaped than the MI spindle. This study suggests that spindle MTs and MFs during bovine oocyte meiosis are asymmetrically divided into the polar bodies.     

{gamma}-Tubulin and microtubule organization during meiosis in the liverwort Ricciocarpus natans (Ricciaceae)

Extant liverworts are "living fossils" considered sister to all other plants and as such provide clues to the evolution of the microtubule organizing center (MTOC) in anastral cells. This report is the first on microtubule arrays and their γ-tubulin-nucleating sites during meiosis in a member of the Ricciales, a specialized, species-rich group of complex thalloid (marchantioid) liverworts. In meiotic prophase, γ-tubulin becomes concentrated at several sites adjacent to the nuclear envelope. Microtubules organized at these foci give rise to a multipolar prometaphase spindle. By metaphase I, the spindle has matured into a bipolar structure with truncated poles. In both first and second meiosis, γ-tubulin forms box-like caps at the spindle poles. γ-Tubulin moves from spindle poles to the proximal surfaces of telophase chromosomes where interzonal microtubules are nucleated. Although a phragmoplast is organized, no cell plate is deposited, and second division occurs simultaneously in the undivided sporocyte. γ-Tubulin surrounds each of the tetrad nuclei, and phragmoplasts initiated between both sister and nonsister nuclei direct simultaneous cytokinesis. The overall pattern of meiosis (unlobed polyplastidic sporocytes, nuclear envelope MTOC, multipolar spindle origin, spindles with box-like poles, and simultaneous cytokinesis) more closely resembles that of Conocephalum than other marchantiod liverworts.     

An original meiotic mutation in Paspalum regnellii

 Cytogenetic studies carried out over a period of 2 consecutive years on a native Brazilian accession of Paspalum regnellii (2n=40) revealed a meiotic mutation that has not been previously reported for any other species. Among 13 inflorescences investigated during the first collection year, three presented anomalous meiotic behavior starting from metaphase I. At the beginning of this phase, the chromosomes occupied the entire equatorial plate in a membrane-to-membrane arrangement, and the spindle fibers, which were clearly visible, did not converge towards the poles. Degeneration of spindle fibers occurred at the end of metaphase I. Chromosome segregation did not occur and the bivalents were left scattered at random in the cytoplasm. Remnants of chromosome fibers could be seen close to the centromere during this stage. The bivalents gave origin to micronuclei in telophase I, with extremely wide variations in number and size among cells. With the absence of spindle formation during meiosis II, metaphase and anaphase II were not observed. Second cytokinesis occurred in prophase II cells after the occurrence of first cytokinesis. The final product of meiosis was completely abnormal, with a predominance of polyads with microspores of different sizes that resulted in abortive pollen grains. In the affected inflorescences, all microsporocytes presented this anomaly, which caused total sterility. Received: 27 March 1997 / Revision accepted: 7 July 1997     

Disjunction of homologous chromosomes in meiosis I depends on proteolytic cleavage of the meiotic cohesin Rec8 by separin

It has been proposed but never proven that cohesion between sister chromatids distal to chiasmata is responsible for holding homologous chromosomes together while spindles attempt to pull them toward opposite poles during metaphase of meiosis I. Meanwhile, the mechanism by which disjunction of homologs is triggered at the onset of anaphase I has remained a complete mystery. In yeast, cohesion between sister chromatid arms during meiosis depends on a meiosis-specific cohesin subunit called Rec8, whose mitotic equivalent, Sccl, is cleaved at the metaphase to anaphase transition by an endopeptidase called separin. We show here that cleavage of Rec8 by separin at one of two different sites is necessary for the resolution of chiasmata and the disjunction of homologous chromosomes during meiosis.     

Involvement of Aurora A kinase during meiosis I-II transition in Xenopus oocytes

The Aurora kinase family has been involved both in vivo and in vitro in the stability of the metaphase plate and chromosome segregation. However, to date only one member of this family, the protein kinase Aurora B, has been implicated in the regulation of meiotic division in Caenorhabditis elegans. In this species, disruption of Aurora B results in the failure of polar body extrusion. To investigate whether Aurora A is also required in meiosis, we microinjected highly specific alpha-Aurora A antibodies in Xenopus oocytes. We demonstrated that microinjected oocytes fail to extrude the first polar body and are arrested with condensed chromosomes on a typical metaphase I plate, which has not performed its normal 90 degrees rotation. We additionally found that, although the failure of first polar body extrusion observed in alpha-Aurora A-microinjected oocytes is likely mediated by Eg5, the impairment of the metaphase plate rotation does not involve this kinesin-like protein. Surprisingly, although chromosomes remain condensed at a metaphase I stage in alpha-Aurora A-microinjected oocytes, the cytoplasmic cell cycle events progress normally through meiosis until metaphase II arrest. Moreover, these oocytes are able to undergo parthenogenetic activation. We conclude that Aurora A and Eg5 are involved in meiosis I to meiosis II transition in Xenopus oocytes.     

Xkid chromokinesin is required for the meiosis I to meiosis II transition in Xenopus laevis oocytes

Xkid chromokinesin is required for chromosome alignment on the metaphase plate of spindles formed in Xenopus laevis egg extracts. We have investigated the role of Xkid in Xenopus oocyte meiotic maturation, a progesterone-triggered process that reinitiates the meiotic cell cycle in oocytes arrested at the G2/M border of meiosis I. Here we show that Xkid starts to accumulate at the time of germinal vesicle breakdown and reaches its largest quantities at metaphase II in oocytes treated with progesterone. Both germinal vesicle breakdown and spindle assembly at meiosis I can occur normally in the absence of Xkid. But Xkid-depleted oocytes cannot reactivate Cdc2/cyclin B after meiosis I and, instead of proceeding to meiosis II, they enter an interphase-like state and undergo DNA replication. Expression of a Xkid mutant that lacks the DNA-binding domain allows Xkid-depleted oocytes to complete meiotic maturation. Our results show that Xkid has a role in the meiotic cell cycle that is independent from its role in metaphase chromosome alignment.     

A mutation in the spindle checkpoint arresting meiosis II in Brachiaria ruziziensis.

Cytological characterization of BRA005568 accession of Brachiaria ruziziensis (2n = 2x = 18) showed a totally unexpected high frequency of abnormal meiotic products, from triads to hexads, and also tetrads with micro nuclei or microcytes. Meiosis I had a low frequency of abnormalities, mainly related to the chiasma terminalization process. In meiosis II, however, frequency of abnormalities increased exceptionally. Early prophase II was normal with the chromosome set enclosed by the nuclear envelope. However, in late prophase II, owing to the breakdown of the nuclear envelope, the chromosomes were scattered in the cytoplasm. Some chromosomes did not reach the metaphase II plate and remained scattered. The behavior of sister cells was inconsistent. While in one cell the chromosomes were totally aligned at the metaphase II plate, in the other they could be found completely scattered, leading to an asynchronous cell division. Cells with scattered chromosomes were unable to progress in meiosis. Thus, anaphase II failed to occur and sister chromatids were not released. Cells with non-aligned chromosomes in the metaphase II plate did not receive the "go ahead" sign to initiate anaphase II. Consequently, the scattered chromosomes produced telophase II nuclei of different sizes in situ. The asynchronous behavior led to the formation of a wide range of meiotic products. Results suggest that the present accession contains a mutation affecting the spindle checkpoint that arrests the second meiotic division.     

Meiotic spindle assembly in Drosophila females: behavior of nonexchange chromosomes and the effects of mutations in the nod kinesin-like protein

Mature Drosophila oocytes are arrested in metaphase of the first meiotic division. We have examined microtubule and chromatin reorganization as the meiosis I spindle assembles on maturation using indirect immunofluorescence and laser scanning confocal microscopy. The results suggest that chromatin captures or nucleates microtubules, and that these subsequently form a highly tapered spindle in which the majority of microtubules do not terminate at the poles. Nonexchange homologs separate from each other and move toward opposite poles during spindle assembly. By the time of metaphase arrest, these chromosomes are positioned on opposite half spindles, between the metaphase plate and the spindle poles, with the large nonexchange X chromosomes always closer to the metaphase plate than the smaller nonexchange fourth chromosomes. Nonexchange homologs are therefore oriented on the spindle in the absence of a direct physical linkage, and the spindle position of these chromosomes appears to be determined by size. Loss-of-function mutations at the nod locus, which encodes a kinesin-like protein, cause meiotic loss and nondisjunction of nonexchange chromosomes, but have little or no effect on exchange chromosome segregation. In oocytes lacking functional nod protein, most of the nonexchange chromosomes are ejected from the main chromosomal mass shortly after the nuclear envelope breaks down and microtubules interact with the chromatin. In addition, the nonexchange chromosomes that are associated with spindles in nod/nod oocytes show excessive poleward migration. Based on these observations, and the structural similarity of the nod protein and kinesin, we propose that nonexchange chromosomes are maintained on the half spindle by opposing poleward and anti-poleward forces, and that the nod protein provides the anti-poleward force.     

Gamma-tubulin is required for bipolar spindle assembly and for proper kinetochore microtubule attachments during prometaphase I in Drosophila oocytes

In many animal species the meiosis I spindle in oocytes is anastral and lacks centrosomes. Previous studies of Drosophila oocytes failed to detect the native form of the germline-specific γ-tubulin (γTub37C) in meiosis I spindles, and genetic studies have yielded conflicting data regarding the role of γTub37C in the formation of bipolar spindles at meiosis I. Our examination of living and fixed oocytes carrying either a null allele or strong missense mutation in the γtub37C gene demonstrates a role for γTub37C in the positioning of the oocyte nucleus during late prophase, as well as in the formation and maintenance of bipolar spindles in Drosophila oocytes. Prometaphase I spindles in γtub37C mutant oocytes showed wide, non-tapered spindle poles and disrupted positioning. Additionally, chromosomes failed to align properly on the spindle and showed morphological defects. The kinetochores failed to properly co-orient and often lacked proper attachments to the microtubule bundles, suggesting that γTub37C is required to stabilize kinetochore microtubule attachments in anastral spindles. Although spindle bipolarity was sometimes achieved by metaphase I in both γtub37C mutants, the resulting chromosome masses displayed highly disrupted chromosome alignment. Therefore, our data conclusively demonstrate a role for γTub37C in both the formation of the anastral meiosis I spindle and in the proper attachment of kinetochore microtubules. Finally, multispectral imaging demonstrates the presences of native γTub37C along the length of wild-type meiosis I spindles.     

Distribution of gamma-tubulin differs in primary and secondary oocytes of Ephestia kuehniella (Pyralidae,Lepidoptera)

In a previous study, barrel-shaped spindles were found in metaphase I oocytes of Ephestia kuehniella (Pyralidae, Lepidoptera). Aster microtubules (MTs) were missing (Wolf, 1993: Cell Motil Cytoskeleton 24:200-204). This points to an acentriolar organization of the spindle apparatus. The present study was aimed at the question of whether gamma-tubulin, a newly detected member of the tubulin superfamily that has often been identified in microtubule-organizing centers, plays a role in the nucleation of MTs in meiotic spindles of the moth. To this end, the distribution of gamma tubulin was examined in oocytes of E. kuehniella using an antibody against gamma-tubulin in combination with indirect immunofluorescence. The antibody evenly decorated spindle MTs in metaphase I oocytes of the moth. Enhanced staining of the spindle poles was not detectable In subsequent stages of meiosis, gamma-tubulin was gradually lost from spindle MTs and was then found at the surface of the so-called elimination chromatin. Female meiosis in Lepidoptera is achiasmatic. The elimination chromatin, i.e., modified and persisting synaptonemal complexes, is believed to keep homologous chromosomes linked until the onset of anaphase I. In meiosis I of female Lepidoptera, the elimination chromatin persists at the spindle equa or between the segregating chromatin masses. It is plausible to assume that gamma-tubulin is involved in spindle organization in the absence of canonical centrosomes. In MTs of metaphase II spindles of E. kuehniella, gamma-tubulin was no longer detectable with our immunological approach. This points to a far-reaching change in spindle organization during transition from meiosis I to meiosis II. © 1996 Wiley-Liss, Inc.     

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.