Premeiotic Mitosis Research Articles

Transient multifocal genomic crisis creating chromothriptic and non-chromothriptic rearrangements in prezygotic testicular germ cells

BackgroundThe co-occurrence of multiple de novo copy number variations (CNVs) is a rare phenomenon in the human genome. Recently, an “organismal CNV mutator phenotype” has been reported to result in transient genomic instability introducing multiple de novo CNVs in primary oocytes and early-stage zygotes. These findings opened a new area of human genome research.MethodsWe performed genome-wide copy number analysis for ~ 2100 individuals with various congenital defects. Furthermore, extensive molecular analyses, including synthetic long-read whole-genome sequencing and haplotype-phasing, were carried out for an individual with multiple de novo CNVs.ResultsA boy was found to have de novo rearrangements on five chromosomes. The rearrangements comprised simple duplication and inversion as well as chaotic changes, all of which affected paternally derived chromosomes. Postzygotic genomic instability was ruled out. The duplicated regions on 6q and 13q contained both diallelic and triallelic loci, indicating that the genomic rearrangements were initially created during premeiotic mitosis and subsequently modified by physiological cross-over during meiosis I. Breakpoints of the rearrangements were indicative of non-homologous end joining, replication-based errors, and/or chromothripsis. The mutagenic event was independent of specific local DNA motifs or de novo point mutations, but may be driven by spermatogenesis-specific factors.ConclusionsThese results indicate that during spermatogenesis, a transient multifocal genomic crisis can introduce several chromothriptic and non-chromothriptic changes into the genome. These findings broaden the concept of the “organismal CNV mutator phenotype”. This study provides insights into mechanisms for altering the global chromosomal architecture of human embryos.

Analysis of Repeat Induced Point (RIP) Mutations in Leptosphaeria maculans Indicates Variability in the RIP Process Between Fungal Species.

Gene duplication contributes to evolutionary potential, yet many duplications in a genome arise from the activity of "selfish" genetic elements such as transposable elements. Fungi have a number of mechanisms by which they limit the expansion of transposons, including Repeat Induced Point mutation (RIP). RIP has been best characterized in the Sordariomycete Neurospora crassa, wherein duplicated DNA regions are recognized after cell fusion, but before nuclear fusion during the sexual cycle, and then mutated. While "signatures" of RIP appear in the genome sequences of many fungi, the species most distant from N. crassa in which the process has been experimentally demonstrated to occur is the Dothideomycete Leptosphaeria maculans In the current study, we show that similar to N. crassa, nonlinked duplications can trigger RIP; however, the frequency of the generated RIP mutations is extremely low in L maculans (< 0.1%) and requires a large duplication to initiate RIP, and that multiple premeiotic mitoses are involved in the RIP process. However, a single sexual cycle leads to the generation of progeny with unique haplotypes, despite progeny pairs being generated from mitosis. We hypothesize that these different haplotypes may be the result of the deamination process occurring post karyogamy, leading to unique mutations within each of the progeny pairs. These findings indicate that the RIP process, while common to many fungi, differs between fungi and that this impacts on the fate of duplicated DNA.

Longer oral contraception history as a possible preventive factor against fetal trisomy 21 in advanced maternal age pregnancies

Down syndrome is the most common autosomal chromosomal abnormality. According to the classical interpretation, it is the result of meiotic nondisjunction. Its occurrence is more common in advanced maternal age. Despite intensive research, pathophysiology of this genetic disorder is not fully understood. According to recent studies, a different kind of mechanism may be found in the background of trisomy 21 than was previously considered. Based on the ovarian mosaicism model, the cause of trisomy 21 (or any common trisomy) is a segregation error of a chromosome in premeiotic mitosis. The cell entering meiosis will be an oocyte with preexisting trisomy, where its (so-called "secondary") nondisjunction is essential. Maturation of the trisomic oocytes appears to fall behind the disomic oocytes, resulting in their relative accumulation in the ovaries as time progresses. The ratio of trisomic/disomic cells becomes less favorable in maternal maturity. If ovulation is inhibited - although the number of oocytes will continue to decline due to apoptosis - it can be assumed that the trisomic/disomic oocyte ratio remains more favorable with the progression of age. In our summary report, presenting and updating our previous data, we would like to propose that - according to ovarian mosaicism model - long-term oral contraception in the anamnesis may be beneficial in pregnancies with advanced maternal age. Orv Hetil. 2018; 159(28): 1146-1152.

Female Meiosis: Synapsis, Recombination, and Segregation in Drosophila melanogaster.

A century of genetic studies of the meiotic process in Drosophila melanogaster females has been greatly augmented by both modern molecular biology and major advances in cytology. These approaches, and the findings they have allowed, are the subject of this review. Specifically, these efforts have revealed that meiotic pairing in Drosophila females is not an extension of somatic pairing, but rather occurs by a poorly understood process during premeiotic mitoses. This process of meiotic pairing requires the function of several components of the synaptonemal complex (SC). When fully assembled, the SC also plays a critical role in maintaining homolog synapsis and in facilitating the maturation of double-strand breaks (DSBs) into mature crossover (CO) events. Considerable progress has been made in elucidating not only the structure, function, and assembly of the SC, but also the proteins that facilitate the formation and repair of DSBs into both COs and noncrossovers (NCOs). The events that control the decision to mature a DSB as either a CO or an NCO, as well as determining which of the two CO pathways (class I or class II) might be employed, are also being characterized by genetic and genomic approaches. These advances allow a reconsideration of meiotic phenomena such as interference and the centromere effect, which were previously described only by genetic studies. In delineating the mechanisms by which the oocyte controls the number and position of COs, it becomes possible to understand the role of CO position in ensuring the proper orientation of homologs on the first meiotic spindle. Studies of bivalent orientation have occurred in the context of numerous investigations into the assembly, structure, and function of the first meiotic spindle. Additionally, studies have examined the mechanisms ensuring the segregation of chromosomes that have failed to undergo crossing over.

The role of U-type exchanges in the differentiation of karyotypes in Najas marina

Spontaneous bridge and fragment formation was observed at the first and second meiotic division, in root tip, premeiotic and pollen mitoses of both chromosomal races of Najas murina, a dioecious and annual species. The frequency of disturbed divisions ranged from 0.9 to 6.5 per cent. Since no reverse loop pairing was observed at pachytene, and the aberrations observed were incompatible with inversion polymorphism, the most probable source of the disturbances was considered to be U-type exchanges. Aberrations in mitotic divisions were also presumed to be formed by U-type reunion of sister chromatid exchanges. All Ihe fragment size classes can be explained on the basis of a breakage and reunion in the centromeric heterochromatin and in the allocyclic regions. However. some break points outside these regions were actually seen. Hence additional 'weak points' are believed to be present in the brightly fluorescent distal parts of the chromosomes. A model is presented of the rearrangements leading from karyotype B to A. Spontaneous breakage and reunion produce continuously new combinations of chromosome segments in N. marina, and hence have been of evolutionary significance in the differentiation of the two karyotypes.

A Germ Cell–Specific Gene of theARGONAUTEFamily Is Essential for the Progression of Premeiotic Mitosis and Meiosis during Sporogenesis in Rice

The rice (Oryza sativa) genome contains 18 copies of genes of the ARGONAUTE (AGO) family. Although AGO members play important roles in RNA-mediated silencing during plant development, a family member that is specifically involved in sexual reproduction has not been identified in plants. We identified the rice AGO gene MEIOSIS ARRESTED AT LEPTOTENE1 (MEL1) from the analysis of seed-sterile mutants. In the mel1 mutant, chromosome condensation was arrested at early meiotic stages and irregularly sized, multinucleated, and vacuolated pollen mother cells (PMCs) frequently appeared in developing anthers. In addition, histone H3 lysine-9 dimethylation of pericentromeres was rarely reduced and modification of the nucleolar-organizing region was altered in mel1 mutant PMCs. The mutation also affected female germ cell development. These results indicate that the germ cell-specific rice MEL1 gene regulates the cell division of premeiotic germ cells, the proper modification of meiotic chromosomes, and the faithful progression of meiosis, probably via small RNA-mediated gene silencing, but not the initiation and establishment of germ cells themselves.

Ultrastructural aspects of the intercellular bridges between female bee germ cells

The germline cells in the ovary of the female bee are interconnected by intercellular bridges kept open by cytoskeletal reinforcements in the plasmic membrane. These bridges among the germline cells display a dynamic behavior and probably act in the determination of the oocyte among the cells of the clone formed by the premeiotic mitoses, subsequently forming a pathway that enables the products synthesized by the nurse cells to reach the oocyte during its maturation. The cytoskeletal elements in the intercellular bridges of bee gonads are basically microfilaments and microtubules, but another type of filament (thick, of non-defined nature, associated with elements of the endoplasmic reticulum) is present in the bridges between the premeiotic cystocytes. This filament crosses the bridge, using microfilaments to fasten itself to the plasmic membrane. These filaments appear to control the span of the bridge. Upon completion of the proliferation phase the cystocytes take on a rosette shape, and a fusome formed by the convergence of the bridges appears at their center. The thick filaments are not present in this conformation. The differentiation of the oocyte and the nurse cells leads to a new change, in which the bridges are reoriented to convey the content of the future nurse cells to the oocyte.

Candidate-gene cloning and targeted marker enrichment of wheat chromosomal regions using RNA fingerprinting - differential display

The usefulness of the RNA fingerprinting-differential display technique in gene cloning and targeted marker enrichment in wheat is demonstrated. A small region of chromosome 5BL was targeted that contains Ph1, a chromosome-pairing regulator gene. The cultivar Chinese Spring (CS) and mutant ph1b are almost identical except for chromosome 5BL, which, in the mutant line, carries an interstitial deletion encompassing the Ph1 gene. Poly(A)+ RNA of the two lines from anthers at developmental stages ranging from pre-meiotic mitosis to anaphase II was PCR-amplified using 38 pairwise combinations of 19 primers. The 35S-labeled amplified products were size-separated on denaturing polyacrylamide-urea gels. A total of 3154 fragment bands were observed, of which 43 were present in CS but absent in the ph1b mutant. These 43 fragment bands were eluted, re-amplified, and used as probes in gel-blot DNA analyses of wheat group 5 nullisomic-tetrasomic lines and the ph1b mutant. Twenty-four of these 43 probes were single- or few-copy sequences. Eight of the 24 probes mapped to wheat group 5 and five mapped to the deletion of the ph1b mutant. Three of these five probes were further localized to the submicroscopic region containing the Ph1 gene, by using two deletion lines flanking the region. Northern-blot analysis revealed that the gene corresponding to one of these three probes expresses mainly during meiosis and is from the B genome.

Candidate-gene cloning and targeted marker enrichment of wheat chromosomal regions using RNA fingerprinting - differential display

The usefulness of the RNA fingerprinting – differential display technique in gene cloning and targeted marker enrichment in wheat is demonstrated. A small region of chromosome 5BL was targeted that contains Ph1, a chromosome-pairing regulator gene. The cultivar Chinese Spring (CS) and mutant ph1b are almost identical except for chromosome 5BL, which, in the mutant line, carries an interstitial deletion encompassing the Ph1 gene. Poly(A)+ RNA of the two lines from anthers at developmental stages ranging from pre-meiotic mitosis to anaphase II was PCR-amplified using 38 pairwise combinations of 19 primers. The 35S-labeled amplified products were size-separated on denaturing polyacrylamide-urea gels. A total of 3154 fragment bands were observed, of which 43 were present in CS but absent in the ph1b mutant. These 43 fragment bands were eluted, re-amplified, and used as probes in gel-blot DNA analyses of wheat group 5 nullisomic-tetrasomic lines and the ph1b mutant. Twenty-four of these 43 probes were single- or few-copy sequences. Eight of the 24 probes mapped to wheat group 5 and five mapped to the deletion of the ph1b mutant. Three of these five probes were further localized to the submicroscopic region containing the Ph1 gene, by using two deletion lines flanking the region. Northern-blot analysis revealed that the gene corresponding to one of these three probes expresses mainly during meiosis and is from the B genome.Key words: RNA fingerprinting – differential display, wheat, gene cloning, marker enrichment, Ph1 gene.

Drosophila polo kinase is required for cytokinesis.

A number of lines of evidence point to a predominance of cytokinesis defects in spermatogenesis in hypomorphic alleles of the Drosophila polo gene. In the pre-meiotic mitoses, cytokinesis defects result in cysts of primary spermatocytes with reduced numbers of cells that can contain multiple centrosomes. These are connected by a correspondingly reduced number of ring canals, structures formed by the stabilization of the cleavage furrow. The earliest defects during the meiotic divisions are a failure to form the correct mid-zone and mid-body structures at telophase. This is accompanied by a failure to correctly localize the Pavarotti kinesin- like protein that functions in cytokinesis, and of the septin Peanut and of actin to be incorporated into a contractile ring. In spite of these defects, cyclin B is degraded and the cells exit M phase. The resulting spermatids are frequently binuclear or tetranuclear, in which case they develop either two or four axonemes, respectively. A significant proportion of spermatids in which cytokinesis has failed may also show the segregation defects previously ascribed to polo1 mutants. We discuss these findings in respect to conserved functions for the Polo-like kinases in regulating progression through M phase, including the earliest events of cytokinesis.

Spontaneous mutations recovered as mosaics in the mouse specific-locus test.

The specific-locus test (SLT) detects new mutants among mice heterozygous for seven recessive visible markers. Spontaneous mutations can be manifested not only as singleton whole-body mutants in controls (for which we report new data), but as mosaics-either visible (manifesting mottled coat color) in the scored generation (G2) or masked, among the wild-type parental generation (G1). Masked G1 mosaics reveal themselves by producing clusters of whole-body mutants in G2. We provide evidence that most, if not all, mosaics detected in the SLT (both radiation and control progenies) result from a single-strand spontaneous mutation subsequent to the last premeiotic mitosis and before the first postmeiotic one of a parental genome-the "perigametic interval." Such events in the genomes of the G1 and Gzero results, respectively, in visible and masked 50:50 mosaics. Per cell cycle, the spontaneous mutation rate in the perigametic interval is much higher than that in pregamete mitotic divisions. A clearly different locus spectrum further supports the hypothesis of different origin, and casts further doubt on the validity of the doubling-dose risk-estimation method. Because mosaics cannot have arisen in mitotic germ cells, and are not induced by radiation exposure in the perigametic interval, they should not be included in calculations of radiation-induced germ-line mutation rates. For per-generation calculations, inclusion of mosaics yields a spontaneous frequency 1.7 times that calculated from singletons alone for mutations contributed by males; including both sexes, the multiple is 2.2.

Cytogenetic Studies of Plasmodial Sporocytes, Chromatin Disintegration, Desynapsis and Centromere Breakage in Pearl Millet.

In pearl millet (Pennisetum americanum (L.) Leeke, n=7), in the selfed progenies of a line L. S. 326-3, a mutant with plasmodial sporocyte (syncyte) formation, chromatin disintegration, desynapsis and centromere breakage was identified and its cytogenetic features were studied. Two to 24-nucleate syncytes were formed due to suppression of cytokinesis during premeiotic mitoses. In syncytes, chromatin degeneration occurred from prepachytene to telophase I stages, thereafter only masses of empty cytoplasm were found. In uninucleate PMCs, chromatin extruded in prepachytene stages in some, degenerated upto diakinesis in several, and either bivalents or univalents (ranging from 1 to 14) were formed in the rest. Centric misdivision leading to telocentric chromosomes (ranging from 2 to 14 per cell) occurred in a proportion of PMCs at anaphase I. There was high sterility of male and female gametes.F2 and test cross progenies of mutant X normal segregated respectively to 9:3:3:1 and 1:1:1:1 ratios of (i) completely normal meiotic behaviour, (ii) syncyte formation and chromatin disintegration but without desynapsis and centromere breakage, (iii) desynapsis and centromere breakage but without syncyte formation and chromatin disintegration and (iv) meiotic behaviour like the original mutant. Crosses between the recombinants (ii) and (iii) above yielded progeny with completely normal cytological behaviour. The original mutant thus was a double mutant. The genes were designated as snd (syncyte formation and chromatin disintegration) and ds cb (desynapsis and centromere breakage).

Formation of aneuploid gametes through nondisjunction during premeiotic mitoses in coix gigantea.

Mixoploid plant showing a range of aneuploid PMCs from nullisomic (2n-2, 2n=18) to tetrasomic (2n+2, 2n=22) constitutions was isolated from a population of Coix gigantea. While majority of the PMCs were diploid carrying 2n=20 chromosomes, nearly 24% PMCs in some male racemes showed typical aneuploid chromosome configurations viz., 9II in nullisomics (2n=18), 9II+1I in monosomics (2n=19), 9II+1III or 10II+1I in trisomics (2n=21) and 11II or 9II+1IV in tetrasomics (2n=22). The chromosome configurational regularity among the aneuploid PMCs indicated that 2n-1 (2n=19) and 2n+1 (2n=21) constitutions probably arose through nondisjunction during one of the premeiotic mitoses in some sporogenous cells. Although difficult to explain the origin of PMCs with 2n-2 (2n=18) and 2n+2 (2n=22) constitutions, somatic pairing followed by nondisjunction during one of the premeiotic mitoses might lead to such situations.

A new report of a chromosomal chimaera in Capsicum annuum L.

A chromosomal chimaeral plant with three distinct primary branches exhibiting diploid, mixoploid (diploid and tetraploid) and tetraploid numbers was recorded for the first time in colchicine treated plants of ×235, a local cultivar of chilli. Significant differences were observed in morphometrics of the three branches with regard to leaves, flowers, fruits and stomatal characteristics. The mixoploid branch displayed intermediate features of both diploid and tetraploid branches. Further, fertility and yield were high in chimaeral plant compared to those of the diploid and tetraploid sibs.The chimaeral nature can be attributed to the endoreduplication of some of the cells in the apical bud due to the differential activity of the colchicine. The mixoploid numbers may be formed as a result of spindle abonrmalities during premeiotic mitosis. The chimaeral plant has immense cytogenetic importance since it has the potentiality to generate polyploids and aneuploids in the progeny which in turn are powerful weapons in understanding the chromosomal architecture and cytogenetic relationships of the crop.

Nuclear compression at premeiotic mitosis. Implications for homologue pairing.

Observations and measurements of maize sporogenous cell nuclei during premeiotic mitotic division cycles in fixed, sectioned and stained material suggest that a striking compression of the nucleus occurs, especially during early prophase of late (probably the last) premeiotic division. It is reasoned such a process could promote relocations of chromosome arms relative to each other prior to meiosis and facilitate strategic meeting or positioning of pairing partners, possibly in conjunction with membrane system or other anchoring of specialized regions.

Observations on Premeiocytes of Wheat

Electron microscopic observations of wheat premeiocytes in premeiotic mitosis (PMM) and premeiotic interphase (PMI) are reported. In archesporial cells of columns at last PMM there is a significant increase in activity of endoplasmic reticulum. Characteristic states of chromatin of premeiocyte nuclei in the stages of PMI are illustrated. Changes in the primary walls of PMCs at Stage 1, PMI, precede the formation of callose in Stage 2. In general, callose deposition throughout the PMC column follows the pattern described elsewhere in Sorghum and other Gramineae. The behavior of the plasmalemma during callose synthesis resembles that described elsewhere in Cucurbita. A role for the cell endomembrane system in callose synthesis is suggested. Blebs arising from the inner membrane of the nuclear envelope, described elsewhere in endosperm cells of wheat and by us in wheat PMCs in this report, give rise to vesicles which carry nuclear material into the cell cytoplasm. Differences in this transport mechanism as it appears in the two tissue systems are described.

OBSERVATIONS ON PREMEIOCYTES OF WHEAT

Electron microscopic observations of wheat premeiocytes in premeiotic mitosis (PMM) and premeiotic interphase (PMI) are reported. In archesporial cells of columns at last PMM there is a significant increase in activity of endoplasmic reticulum. Characteristic states of chromatin of premeiocyte nuclei in the stages of PMI are illustrated. Changes in the primary walls of PMCs at Stage 1, PMI, precede the formation of callose in Stage 2. In general, callose deposition throughout the PMC column follows the pattern described elsewhere in Sorghum and other Gramineae. The behavior of the plasmalemma during callose synthesis resembles that described elsewhere in Cucurbita. A role for the cell endomembrane system in callose synthesis is suggested. Blebs arising from the inner membrane of the nuclear envelope, described elsewhere in endosperm cells of wheat and by us in wheat PMCs in this report, give rise to vesicles which carry nuclear material into the cell cytoplasm. Differences in this transport mechanism as it appears in the two tissue systems are described.

The karyotype and ultrastructural characteristics of spontaneous preimplantation mouse parthenotes

AbstractKaryotypic and light and electron microscopical analyses were made of spontaneous preimplantation mouse parthenotes from the LT/Sv inbred strain. It was found that the activated oocyte and developing embryos were diploid. We believe that diploidization is achieved by the oogonium undergoing a premeiotic mitosis without cytokinesis followed by two meiotic divisions, thus producing diploid parthenotes.The developmental events with respect to membrane specialization, such as junctional complexes, were similar to those observed in fertilized embryos.A unique feature of the developing parthenote was the failure of the mitochondria to change during the morula stage. The mitochondria retained a few irregularly oriented cristae rather than many transversely oriented ones observed in morulae developing from fertilized eggs. The significance of this observation is discussed.

Chromosome behavior at premeiotic mitosis in maize

Chromosome behavior at premeiotic mitosis in maize

Analysis of chromosome pairing and breakage in pearl millet

SUMMARYThe relationship between chromosome pairing and chromosome fragmentation has been studied in a gene controlled mutant of pearl millet (2n= 14). Premeiotic mitosis, premeiotic cell development and early prophase I are normal without any fragments, which first appear at pachytene. The extent of fragmentation varies from zero to very extreme with two discrete classes of plants, namely those with partial fragmentation and those with multiple fragmentation. A quantitative analysis of bivalent distribution and the distribution of AI bridges in desynaptic and fragmented cells show all of them to be nonrandom events. We suggest that in cells showing partial fragmentation the bridges and fragments result from U-type exchanges at pachytene. The reduced frequency of AII bridges indicates relatively low sister chromatid reunion at pachytene. In multiple fragmented plants numerous minute fragments were seen from pachytene. Despite these anomalies most PMCs complete meiosis but subsequently abort at the pollen grain stage. The mutant gene also causes disturbances in the sequence of meiotic development in the ear and in the synchronous development of PMCs within an anther. It has no effect on the tapetum or on the physiological development of the anther.