Spindle In Meiosis Research Articles

The DcPS1 cooperates with OSDLa during pollen development and 2n gamete production in carnation meiosis

BackgroundDeciphering the mechanisms of meiosis has important implications for potential applications in plant breeding programmes and species evolution. However, the process of meiosis is poorly understood in carnation, which is famous for its cut flowers.ResultsWe report that Dianthus caryophyllus parallel spindle 1 (DcPS1) regulates omission of second division like a (OSDLa) during pollen development and 2n gamete production in carnation meiosis. In DcPS1 and OSDLa RNAi lines, an absence of the second meiotic division and the abnormal orientation of spindles at meiosis II might be the main reason for dyad/triad formation, resulting in unreduced gametes. We also found that carnation OSDLa interacted with DcPS1 and DcRAD51D. In the DcPS1 RNAi lines, a decrease in OSDLa and DcRAD51D expression was observed. In the OSDLa RNAi lines, a decrease in DcPS1 and DcRAD51D expression was also observed. We propose that DcPS1 regulates OSDLa expression, allowing entry into meiosis II and the proper orientation of the metaphase II spindle in meiosis II. We also propose that OSDLa regulates DcRAD51D expression, allowing for homologous recombination.ConclusionsThese results suggest a critical role for DcPS1 and OSDLa in diplogamete production during meiosis and open a new pathway for meiosis-related studies.

Kinesin-5 Eg5 is essential for spindle assembly and chromosome alignment of mouse spermatocytes

BackgroundMicrotubule organization is essential for bipolar spindle assembly and chromosome segregation, which contribute to genome stability. Kinesin-5 Eg5 is known to be a crucial regulator in centrosome separation and spindle assembly in mammalian somatic cells, however, the functions and mechanisms of Eg5 in male meiotic cell division remain largely unknown.ResultsIn this study, we have found that Eg5 proteins are expressed in mouse spermatogonia, spermatocytes and spermatids. After Eg5 inhibition by specific inhibitors Monastrol, STLC and Dimethylenastron, the meiotic spindles of dividing spermatocytes show spindle collapse and the defects in bipolar spindle formation. We demonstrate that Eg5 regulates spindle bipolarity and the maintenance of meiotic spindles in meiosis. Eg5 inhibition leads to monopolar spindles, spindle abnormalities and chromosome misalignment in cultured GC-2 spd cells. Furthermore, Eg5 inhibition results in the decrease of the spermatids and the abnormalities in mature sperms.ConclusionsOur results have revealed an important role of kinesin-5 Eg5 in male meiosis and the maintenance of male fertility. We demonstrate that Eg5 is crucial for bipolar spindle assembly and chromosome alignment in dividing spermatocytes. Our data provide insights into the functions of Eg5 in meiotic spindle assembly of dividing spermatocytes.

SCF Ensures Meiotic Chromosome Segregation Through a Resolution of Meiotic Recombination Intermediates

The SCF (Skp1-Cul1-F-box) complex contributes to a variety of cellular events including meiotic cell cycle control, but its function during meiosis is not understood well. Here we describe a novel function of SCF/Skp1 in meiotic recombination and subsequent chromosome segregation. The skp1 temperature-sensitive mutant exhibited abnormal distribution of spindle microtubules in meiosis II, which turned out to originate from abnormal bending of the spindle in meiosis I. Bent spindles were reported in mitosis of this mutant, but it remained unknown how SCF could affect spindle morphology. We found that the meiotic bent spindle in skp1 cells was due to a hypertension generated by chromosome entanglement. The spindle bending was suppressed by inhibiting double strand break (DSB) formation, indicating that the entanglement was generated by the meiotic recombination machinery. Consistently, Rhp51/Rad51-Rad22/Rad52 foci persisted until meiosis I in skp1 cells, proving accumulation of recombination intermediates. Intriguingly bent spindles were also observed in the mutant of Fbh1, an F-box protein containing the DNA helicase domain, which is involved in meiotic recombination. Genetic evidence suggested its cooperation with SCF/Skp1. Thus, SCF/Skp1 together with Fbh1 is likely to function in the resolution of meiotic recombination intermediates, thereby ensuring proper chromosome segregation.

Monastral bipolar spindles in meiosis II of male Trichosia pubescens (Sciaridae): early stages of spindle formation and chromosome orientation.

The metaphase spindle of male meiosis II in fungus gnats (Sciaridae) is one example of naturally occurring monastral bipolar spindles. To gain further insights into how the bipolar spindle is formed in the presence of only one polar center, prometaphase of male meiosis II was investigated in the sciarid Trichosia pubescens by means of anti-tubulin immunofluorescence, DAPI chromosome staining, and electron microscopy of ultrathin serial sections. The first step in spindle formation after interkinesis seems to be the organization of an astral half-spindle, probably by MTOC activity of the astral region. With the exception of the non-disjunctional X chromosome, which always lies close to the aster, the chromosomes are found to occupy various positions with respect to the astral region, revealing different orientations of their chromatid kinetochores. It was observed that some of the mal-oriented kinetochores are associated with microtubules that, due to their orientation perpendicular to the spindle axis, are unlikely to originate from the astral region. Therefore, these mal-oriented microtubules are taken as an indication of a dispersed MTOC activity near the chromosomes or at kinetochores. According to recent models of chromosome-induced spindle self-organization [e.g., Merdes et al., 1997: J. Cell Biol. 138:953-956], they could be responsible for the formation of the other (anastral) half-spindle and for amphitelic (bipolar) orientation of the chromosomes.

Microsporogenesis in a citrus interspecific tetraploid somatic hybrid and its fusion parents

Microsporogenesis was analysed in a tetraploid somatic hybrid (SH) (2n=4x=36) of Citrus and its diploid fusion parents (2n=2x=18), Valencia sweet orange (C. sinensis L. Osbeck) and Femminello lemon (C. limon L. Burm. f.). Intergenomic pairing between lemon and orange occurred in the somatic hybrid which showed multivalent chromosome associations in diakinesis, although one quadrivalent was definitely because of a reciprocal translocation present in Valencia. The behaviour of univalents was variable in the somatic hybrid and its parents. In the somatic hybrid and Valencia, the univalents preferentially formed micronuclei and polyads whereas, in Femminello, they were generally enclosed in a nucleus although distributed randomly. The somatic hybrid showed a rate of pollen stainability of 64% and germinability of 41%. The chromosomally unbalanced pollen from the tetraploid SH was presumed viable and able to fertilize because different nuclear DNA contents were found in the back-cross progeny. Moreover, meiotic nuclear restitution mechanisms, which could be mainly dependent on the abnormal orientation of the spindles in meiosis II, are described.

Cytogenetic mechanisms for triploid and haploid egg formation in the triploid loachMisgurnus anguillicaudatus

The triploid (diploid × tetraploid) loach Misgurnus anguillicaudatus lays large, triploid eggs and small, haploid eggs simultaneously. We conducted cytogenetic examination of gonadal cells to determine the mechanisms responsible for this unusual oogenesis. In immature ovaries of triploids, both triploid (3n = 75) and hexaploid (6n = 150) metaphases were detected. Oocytes at the pachytene stage were categorized into two types: small and large. The small oocytes gave about 50 chromosome elements, comprising 25 thick, densely stained bivalents and 25 thin, faintly stained univalents, that is, bivalent-univalent complexes. The large ones showed no fewer than 60 thick elements, considered to be 75 bivalents. Cytological examination of full-grown oocytes cultured in vitro revealed that both large and small oocytes undergo two conventional successive meiotic cycles: formation of a bipolar spindle in meiosis I and equal segregation of homologues, extrusion of the first polar body, and the appearance of a bipolar spindle in meiosis II. In the small, full-grown oocytes, some chromosomes were observed to detach from the spindle and remain in the surrounding cytoplasm, and then these chromosomes (presumably unpaired univalents) were eliminated from the meiotic event. We conclude that triploid eggs are formed by a quasinormal meiosis of the hexaploid oocytes of large size, which are generated by premeiotic endomitosis, while the haploid eggs are produced from bivalents after eliminating univalents in the meiosis of triploid oocytes of small size. J. Exp. Zool. 281:608–619, 1998. © 1998 Wiley-Liss, Inc.

Meiotic restitution mechanisms and 2n pollen formation in a Solanum tuberosum dihaploid and in dihaploid × wild species hybrids*

AbstractA Solatium tuberosum dihaploid that proved to be a 2n pollen producer, exhibited two types of meiotic abnormality involved in 2n pollen formation: premature cytokinesis at prophase II and parallel/tripolar spindles in meiosis II. For this reason such a dihaploid was preferentially used as a seed parent in crosses with the diploid species S. tarijense, S. sanctae‐rosae, and S. phureja. Diploid interspecific hybrids were evaluated for male fertility, for 2n pollen production and stability over three years and for meiotic nuclear restitution mechanisms. Pollen stainability ranged from 6 to 85% and 2n pollen from 5 to 35% among hybrids. The meiotic mechanism generating 2n pollen was that of spindle abnormality in meiosis II and consequent dyad/triad formation. High correlation was found between fused/parallel spindles and dyads.

The requirements for protein synthesis and degradation, and the control of destruction of cyclins A and B in the meiotic and mitotic cell cycles of the clam embryo.

Fertilization of clam oocytes initiates a series of cell divisions, of which the first three--meiosis I, meiosis II, and the first mitotic division--are highly synchronous. After fertilization, protein synthesis is required for the successful completion of every division except meiosis I. When protein synthesis is inhibited, entry into meiosis I and the maintenance of M phase for the normal duration of meiosis occur normally, but the chromosomes fail to interact correctly with the spindle in meiosis II metaphase. By contrast, inhibition of protein synthesis immediately after completion of meiosis or mitosis stops cells entering the next mitosis. We describe the behavior of cyclins A and B in relation to these "points of no return." The cyclins are synthesized continuously and are rapidly destroyed shortly before the metaphase-anaphase transition of the mitotic cell cycles, with cyclin A being degraded in advance of cyclin B. Cyclin destruction normally occurs during a 5-min window in mitosis, but in the monopolar mitosis that occurs after parthenogenetic activation of clam oocytes, or when colchicine is added to fertilized eggs about to enter first mitosis, the destruction of cyclin B is strongly delayed, whereas proteolysis of cyclin A is maintained in an activated state for the duration of metaphase arrest. Under either of these abnormal conditions, inhibition of protein synthesis causes a premature return to interphase that correlates with the time when cyclin B disappears.

Some Observations on the Cytology in Ochna sp(Ochnaceae).

Ochna's meiosis has been analysed in search of factors that could cause the abortion of fruits. Results showed low frequency of meiotic irregularities but high frequency of tetrads with a variable number of microcytes, besides converging spindles in meiosis II leading up to the formation of triads. Abnormal behaviour of the nucleolus was also observed in many microsporocytes. These factors may explain, at least in part, the high frequency of abortion in Ochna's fruit.