Chromosome pairing in wheat-rye ABDR hybrids depends on the microsporogenesis pattern

The character of chromosome pairing in meiocytes was studied in F1 wheat-rye Triticum aestivum L. x Secale cereale L. (ABDR, 4x = 28) hybrids with three types of chromosome behavior: reductional, equational, and equational + reductional. A high variation of the frequencies of bivalents and ring univalents was observed in meiocytes with the reductional or equational + reductional type of chromosome behavior. The type of chromosome division was found to affect the bivalent and ring univalent frequencies. Chromosome pairing occurred in 10.28% of meiocytes with the reductional chromosome behavior, 0.93% of meiocytes with the equational chromosome behavior, and 10.81% of meiocytes with the equational + reductional chromosome behavior. On average, 0.13 bivalents per cell formed in meiocytes of the hybrid population. C-banding and genomic in situ hybridization (GISH) showed that both rye and wheat chromosomes produced ring univalents. The role of the Ph genes in regulating the bivalent formation in meiocytes with different types of chromosome behavior is discussed.     

Mitotic behavior of centromeres in meiosis as the fertility restoration mechanism in wheat-rye amphihaploids

The regulation of chromosomal behavior in meiosis in partly fertile wheat-rye amphihaploids was studied using the centromere specific probes pAWRC1 and Ae. tauschii pAet6-09. Comparative analysis of the probe localization patterns in mitosis, normal meiosis in wheat Triticum aestivum L. and rye Secale cereale L., and meiosis in amphihaploids was performed. The differences in the structure of centromeres in monopolar- and bipolaroriented chromosomes were revealed. Single dense hybridization signals were observed in the diplotene and the metaphase of the first meiotic division, while hybridization signals appeared as stretched bands with diffuse structure located across the centromere region in mitosis and the second round of meiotic division. Based upon the obtained data, we used the corresponding centromere-specific probes as a tool for the analysis of chromosomal behavior in meiosis in amphihaploids. In meiocytes with three types of chromosome behavior (reductional, equational plus reductional, and equational), dense point-like hybridization signals for the pAet6-09 probe were observed for univalents with the reductional division type and stretched bands with diffuse structure for those with the equational division type. Thus, pAet6-09 probe localization patterns suggest some structural and functional specificities of centromeres in the meiosis in wheat-rye amphihaploids that reflect special regulation of chromosomal behavior during equational division. Meiocytes with true mitotic division were also observed in anthers predominantly containing meiocytes with chromosomes undergoing equational division.     

Meiotic restitution in amphihaploids in the tribe Triticeae

In haploid and diploid organisms of the plant kingdom, meiotic division of diploid cells proceeds in two consecutive stages, with DNA replicating only once. In amphihaploids (interspecific or intergeneric hybrids), where homologs are absent, the reduction of the chromosome number does not occur, meiosis is abnormal, and the plants are sterile. Gamete viability in F₁ hybrids is ensured by a single division when chromosomes are separated into sister chromatids in either the first or the second division. Such gametes ensure partial fertility of amphihaploids, thereby facilitating their survival and stabilization of the polygenome. The frequency of the formation of viable gametes varies from a few cases to 98.8% in different anthers of the hybrids. Here, studies on the cytological mechanisms and genetic control of chromosome unreduction or restitution in different amphihaploids of the tribe Triticeae are reviewed. The current notions on the control of formation of restitution nuclei based on the principles of a prolonged metaphase I and different types of meiocytes. The main terms used for systematization of restitution mechanisms are first-division restitution (FDR), single-division meiosis (SDM), and unreductional meiotic cell division (UMCD). It has been assumed that archesporial cells of wide hybrids may have two cell division programs, the meiotic and the mitoyic ones The possible approaches to the analysis of the genetic control of chromosome restitution in amphihaploids are discussed.     

Evidence That Masking of Synapsis Imperfections Counterbalances Quality Control to Promote Efficient Meiosis

Reduction in ploidy to generate haploid gametes during sexual reproduction is accomplished by the specialized cell division program of meiosis. Pairing between homologous chromosomes and assembly of the synaptonemal complex at their interface (synapsis) represent intermediate steps in the meiotic program that are essential to form crossover recombination-based linkages between homologs, which in turn enable segregation of the homologs to opposite poles at the meiosis I division. Here, we challenge the mechanisms of pairing and synapsis during C. elegans meiosis by disrupting the normal 1∶1 correspondence between homologs through karyotype manipulation. Using a combination of cytological tools, including S-phase labeling to specifically identify X chromosome territories in highly synchronous cohorts of nuclei and 3D rendering to visualize meiotic chromosome structures and organization, our analysis of trisomic (triplo-X) and polyploid meiosis provides insight into the principles governing pairing and synapsis and how the meiotic program is “wired” to maximize successful sexual reproduction. We show that chromosomes sort into homologous groups regardless of chromosome number, then preferentially achieve pairwise synapsis during a period of active chromosome mobilization. Further, comparisons of synapsis configurations in triplo-X germ cells that are proficient or defective for initiating recombination suggest a role for recombination in restricting chromosomal interactions to a pairwise state. Increased numbers of homologs prolong markers of the chromosome mobilization phase and/or boost germline apoptosis, consistent with triggering quality control mechanisms that promote resolution of synapsis problems and/or cull meiocytes containing synapsis defects. However, we also uncover evidence for the existence of mechanisms that “mask” defects, thus allowing resumption of prophase progression and survival of germ cells despite some asynapsis. We propose that coupling of saturable masking mechanisms with stringent quality controls maximizes meiotic success by making progression and survival dependent on achieving a level of synapsis sufficient for crossover formation without requiring perfect synapsis.     

Role of rye chromosome 2R from wheat-rye substitution line 2R(2D)1 ( Triticum aestivum L. cv. Saratovskaya 29- Secale cereale L. cv. Onokhoiskaya) in genetic regulation of meiotic restitution in wheat-rye polyhaploids

A study was made of the role of rye chromosome 2R from the wheat-rye substitution line 2R(2D)₁ (Triticum aestivum L. cv. Saratovskaya 29-Secale cereale L. cv. Onokhoiskaya) in genetic regulation of meiotic restitution in wheat-rye polyhaploids 2R(2D)₁ × S. cereale L. cv. Onokhoiskaya. Rye chromosome 2R proved to affect the completeness of the meiotic program, suppressing the formation of restitution gametes. This was evident from the reductional division of univalent chromosomes in AI and the occurrence of the second meiotic division. The interrelationships between the type of chromosome division in AI and the two-step character of meiosis are discussed. The structural and functional organization of the centromeric regions of chromosomes undergoing reductional division is assumed to determine the two-step character of division. Original Russian Text ? O.G. Silkova, A.I. Shchapova, V.K. Shumny, 2007, published in Genetika, 2007, Vol. 43, No. 7, pp. 971–981.     

Role of rye chromosome 2R from wheat-rye substitution line 2R(2D)1 (Triticum aestivum L. cv. Saratovskaya 29-Secale cereale L. cv. Onokhoiskaya) in genetic regulation of meiotic restitution in wheat-rye polyhaploids

A study was made of the role of rye chromosome 2R from the wheat-rye substitution line 2R(2D)1 (Triticum aestivum L. cv. Saratovskaya 29-Secale cereale L. cv. Onokhoiskaya) in genetic regulation of meiotic restitution in wheat-rye polyhaploids 2R(2D)1 x S. cereale L. cv. Onokhoiskaya. Rye chromosome 2R proved to affect the completeness of the meiotic program, suppressing the formation of restitution gametes. This was evident from the reductional division of univalent chromosomes in AI and the occurrence of the second meiotic division. The interrelationships between the type of chromosome division in AI and the two-step character of meiosis are discussed. The structural and functional organization of the centromeric regions of chromosomes undergoing reductional division is assumed to determine the two-step character of division.     

The CYCLIN-A CYCA1;2/TAM Is Required for the Meiosis I to Meiosis II Transition and Cooperates with OSD1 for the Prophase to First Meiotic Division Transition

Meiosis halves the chromosome number because its two divisions follow a single round of DNA replication. This process involves two cell transitions, the transition from prophase to the first meiotic division (meiosis I) and the unique meiosis I to meiosis II transition. We show here that the A-type cyclin CYCA1;2/TAM plays a major role in both transitions in Arabidopsis. A series of tam mutants failed to enter meiosis II and thus produced diploid spores and functional diploid gametes. These diploid gametes had a recombined genotype produced through the single meiosis I division. In addition, by combining the tam-2 mutation with AtSpo11-1 and Atrec8, we obtained plants producing diploid gametes through a mitotic-like division that were genetically identical to their parents. Thus tam alleles displayed phenotypes very similar to that of the previously described osd1 mutant. Combining tam and osd1 mutations leads to a failure in the prophase to meiosis I transition during male meiosis and to the production of tetraploid spores and gametes. This suggests that TAM and OSD1 are involved in the control of both meiotic transitions.     

Dynamics of DNA Replication during Premeiosis and Early Meiosis in Wheat

Meiosis is a specialised cell division that involves chromosome replication, two rounds of chromosome segregation and results in the formation of the gametes. Meiotic DNA replication generally precedes chromosome pairing, recombination and synapsis in sexually developing eukaryotes. In this work, replication has been studied during premeiosis and early meiosis in wheat using flow cytometry, which has allowed the quantification of the amount of DNA in wheat anther in each phase of the cell cycle during premeiosis and each stage of early meiosis. Flow cytometry has been revealed as a suitable and user-friendly tool to detect and quantify DNA replication during early meiosis in wheat. Chromosome replication was detected in wheat during premeiosis and early meiosis until the stage of pachytene, when chromosomes are associated in pairs to further recombine and correctly segregate in the gametes. In addition, the effect of the Ph1 locus, which controls chromosome pairing and affects replication in wheat, was also studied by flow cytometry. Here we showed that the Ph1 locus plays an important role on the length of meiotic DNA replication in wheat, particularly affecting the rate of replication during early meiosis in wheat.     

Meiotic mutations in rye Secale cereale L

Spontaneous meiotic mutations of winter rye Secale cereale L. (2n = 14) were revealed in inbred F2 progenies, which were obtained by self-pollination of F1 hybrids resulting from crosses of individual plants of cultivar Vyatka or weedy rye with plants of self-fertile inbred lines. The mutations cause partial or complete sterility, and are maintained in heterozygote condition. Six types of mutations were distinguished as the result of cytological analysis of meiosis and genetic analysis. (1) Plants with nonallelic asynaptic mutations sy1 and sy9 lacked bivalents in 96.8 and 67.0% metaphase I cells, respectively, formed only axial elements but not the mature synaptonemal complex (SC), and had defects in telomere clustering in early prophase I. (2) Weak asynaptic mutant sy3 showed incomplete synapsis at the start of SC degradation at diplotene and lower chiasma number; yet only 2% meiocytes lacked bivalents in MI. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19 caused nonhomologous synapsis; i.e., a varying number of univalents and occasional multivalents were observed in MI, which was preceded by switches of pairing partners and fold-back synapsis at mid-prophase I. (4) Mutation mei6 led to the formation of protrusions and minor branched structures of the SC lateral elements. (5) Allelic mutations mei8 and mei8-10 caused irregular chromatin condensation along the chromosome length in prophase I, which was accompanied by chromosome sticking and fragmentation in MI. (6) Allelic mutations mei5 and mei10 determined chromosome supercondensation, caused the disturbance of meiotic spindle assembly, arrested meiosis at various stages but did not affect formation of the pollen wall, thus arrested meiocytes got covered with the pollen wall. Analysis of double mutants revealed recessive epistatic interactions for some mutations; the epistatic group was sy9 > sy1 > sy3 > sy19. This reflects the sequence of meiotic events controlled by the corresponding genes. The expression of sy2 and sy19 proved to be modified by additional genes. Most meiotic mutations found in rye have analogs in other plants.     

Turning Meiosis into Mitosis

Apomixis, or asexual clonal reproduction through seeds, is of immense interest due to its potential application in agriculture. One key element of apomixis is apomeiosis, a deregulation of meiosis that results in a mitotic-like division. We isolated and characterised a novel gene that is directly involved in controlling entry into the second meiotic division. By combining a mutation in this gene with two others that affect key meiotic processes, we created a genotype called MiMe in which meiosis is totally replaced by mitosis. The obtained plants produce functional diploid gametes that are genetically identical to their mother. The creation of the MiMe genotype and apomeiosis phenotype is an important step towards understanding and engineering apomixis.     

The chromosomal distribution of phosphorylated histone H3 differs between plants and animals at meiosis

Plant (Secale cereale, Triticum aestivum) and animal (Eyprepocnemis plorans) meiocytes were analyzed by indirect immunostaining with an antibody recognizing histone H3 phosphorylated at serine 10, to study the relationship between H3 phosphorylation and chromosome condensation at meiosis. To investigate whether the dynamics of histone H3 phosphorylation differs between chromosomes with a different mode of segregation, we included in this study mitotic cells and also meiotic cells of individuals forming bivalents plus three different types of univalents (A chromosomes, B chromosomes and X chromosome). During the first meiotic division, the H3 phosphorylation of the entire chromosomes initiates at the transition from leptotene to zygotene in rye and wheat, whereas in E. plorans it does so at diplotene. In all species analyzed H3 phosphorylation terminates toward interkinesis. The immunosignals at first meiotic division are identical in bivalents and univalents of A and B chromosomes, irrespective of their equational or reductional segregation at anaphase I. The grasshopper X chromosome, which always segregates reductionally, also shows the same pattern. Remarkable differences were found at second meiotic division between plant and animal material. In E. plorans H3 phosphorylation occurred all along the chromosomes, whereas in plants only the pericentromeric regions showed strong immunosignals from prophase II until telophase II. In addition, no immunolabeling was detectable on single chromatids resulting from equational segregation of plant A or B chromosome univalents during the preceding anaphase I. Simultaneous immunostaining with anti-tubulin and anti-phosphorylated H3 antibodies demonstrated that the kinetochores of all chromosomes interact with microtubules, even in the absence of detectable phosphorylated H3 immunosignals. The different pattern of H3 phosphorylation in plant and animal meiocytes suggests that this evolutionarily conserved post-translational chromatin modification might be involved in different roles in both types of organisms. The possibility that in plants H3 phosphorylation is related to sister chromatid cohesion is discussed.     

Coordinating the events of the meiotic prophase

Meiosis is a specialized type of cell division leading to the production of gametes. During meiotic prophase I, homologous chromosomes interact with each other and form bivalents (pairs of homologous chromosomes). Three major meiotic processes--chromosome pairing, synapsis and recombination--are involved in the formation of bivalents. Many recent reports have uncovered complex networks of interactions between these processes. Chromosome pairing is largely dependent on the initiation and progression of recombination in fungi, mammals and plants, but not in Caenorhabditis elegans or Drosophila. Synapsis and recombination are also tightly linked. Understanding the coordination between chromosome pairing, synapsis and recombination lends insight into many poorly explained aspects of meiosis, such as the nature of chromosome homology recognition.     

Synaptonemal Complex Components Promote Centromere Pairing in Pre-meiotic Germ Cells

Mitosis and meiosis are two distinct cell division programs. During mitosis, sister chromatids separate, whereas during the first meiotic division, homologous chromosomes pair and then segregate from each other. In most organisms, germ cells do both programs sequentially, as they first amplify through mitosis, before switching to meiosis to produce haploid gametes. Here, we show that autosomal chromosomes are unpaired at their centromeres in Drosophila germline stem cells, and become paired during the following four mitosis of the differentiating daughter cell. Surprisingly, we further demonstrate that components of the central region of the synaptonemal complex are already expressed in the mitotic region of the ovaries, localize close to centromeres, and promote de novo association of centromeres. Our results thus show that meiotic proteins and meiotic organization of centromeres, which are key features to ensure reductional segregation, are laid out in amplifying germ cells, before meiosis has started.     

Features of the regulation of meiotic restitution in androgenic haploids of wheat-rye substitution lines 2R(2D)1, 2R(2D)3, and 6R(6A) (Triticum aestivum L., cultivar Saratovskaya 29/Secale cereale L., cultivar Onokhoiskaya)

Regulation of meiotic restitution in androgenic haploids generated by cultivation of isolated anthers of three wheat-rye substitution lines 2R(2D)₁, 2R(2D)₃, and 6R(6A) (Triticum aestivum L., cultivar Saratovskaya 29/Secale cereale L., cultivar Onokhoiskaya) was studied. The presence of rye chromosomes and the absence of homeologous wheat chromosomes in the haploid plant genome was shown to cause meiotic restitution, as observed in the case of androgenic haploids 6R(6A), or to inhibit it—in meiosis of haploids 2R(2D)₁ and 2R(2D)₃. In haploids of lines 2R(2D)₁ and 2R(2D)₃, the reductional type of division of univalent chromosomes was observed, leading to preferential formation of tetrads. In haploids of line 6R(6A), the equational type of division of univalents into sister chromatids, resulting in the block of the second division and formation of diads in approximately 50% of cells, was detected. These results confirm data on the effect of the genotype of line 2R(2D)₁ on the induction of reductional type division of univalents and two-phase meiosis, which were earlier obtained in studies of meiosis in polyhaploids 2R(2D)₁ × S. cereale L., cultivar Onokhoiskaya.     

“Homologies” between non-homologous chromosomes in the grasshopper Caledia captiva

Cytological studies of hybrids between three chromosomal forms of the grasshopper, Caledia captiva, have revealed a clear case of pairing and exchange between non-homologous chromosomes. The genomes of each of the three chromosomal forms are readily identifiable by their marked differences in morphology and in the pattern of C-heterochromatin distribution. The testes of inter-racial F₁ hybrid males contain both diploid and tetraploid meiocytes within the same individual. Multiple chromosome associations are a regular feature of all diploid cells. In many cases, these multiples involve two or more non-homologous chromosomes from within the same haploid genome. Such associations reveal unambiguous evidence of meiotic exchange and chiasmata. The X chromosome is frequently observed to associate with an autosome, and anaphase I cells provide evidence of X/autosome exchanges. A correlation exists between the position of the exchange event in non-homologous pairs and the location of heterochromatin. In tetraploid meiocytes, pairing is by strict homology only, giving rise to cells with 22 bivalents plus an XX bivalent or two univalent X chromosomes. Segregation patterns in tetraploid cells are entirely normal and result in the production of diploid gametes. In the male, the increased ploidy level was observed to arise following an endoreduplication process which takes place pre-meiotically in the spermatogonial cells. The finding that non-homologous chromosomes from within the same haploid genome can pair and cross over during meiosis clearly shows that some caution must be taken when interpreting multiple associations as evidence of interchange heterozygosity in hybrids.     

Meiotic pairing in hybrids between tetraploid Triticale and related species: new elements concerning the chromosome constitution of tetraploid Triticale

Summary Two F5 strains of tetraploid triticale (2n= 4x=28), obtained from 6x triticaleX2 rye progenies, were crossed with diploid and tetraploid rye, some durum and bread wheats, and various 8x and 6x triticale lines. Meiosis in the different hybrid combinations was studied. The results showed that the haploid complement of these triticales consists of seven chromosomes from rye and seven chromosomes from wheat. High frequencies of PMCs showing trivalents were observed in hybrids involving the reference genotypes of wheat and triticale. These findings proved that several chromosomes from the wheat component have chromosome segments coming from two parental wheat chromosomes. The origin of these heterogeneous chromosomes probably lies in homoeologous pairing occurring at meiosis in the 6x triticaleX2x rye hybrids from which 4x triticale lines were isolated. A comparison among different hybrids combinations indicated that the involvement of D-genome chromosomes in homoeologous pairing is quite limited. In contrast, meiotic patterns in 4x triticale X 2x rye hybrids showed a quite high pairing frequency between some R chromosomes and their A and B homoeologues.     

Molecular Mechanisms of Homologous Chromosome Pairing and Segregation in Plants

In most eukaryotic species, three basic steps of pairing, recombination and synapsis occur during prophase of meiosis I. Homologous chromosomal pairing and recombination are essential for accurate segregation of chromosomes. In contrast to the well-studied processes such as recombination and synapsis, many aspects of chromosome pairing are still obscure. Recent progress in several species indicates that the telomere bouquet formation can facilitate homologous chromosome pairing by bringing chromosome ends into close proximity, but the sole presence of telomere clustering is not sufficient for recognizing homologous pairs. On the other hand, accurate segregation of the genetic material from parent to offspring during meiosis is dependent on the segregation of homologs in the reductional meiotic division （MI） with sister kinetochores exhibiting mono-orientation from the same pole, and the segregation of sister chromatids during the equational meiotic division （MII） with kinetochores showing bi-orientation from the two poles. The underlying mechanism of orientation and segregation is still unclear. Here we focus on recent studies in plants and other species that provide insight into how chromosomes find their partners and mechanisms mediating chromosomal segregation.     

A large-scale screen in S. pombe identifies seven novel genes required for critical meiotic events

Meiosis is a specialized form of cell division by which sexually reproducing diploid organisms generate haploid gametes. During a long prophase, telomeres cluster into the bouquet configuration to aid chromosome pairing, and DNA replication is followed by high levels of recombination between homologous chromosomes (homologs). This recombination is important for the reductional segregation of homologs at the first meiotic division; without further replication, a second meiotic division yields haploid nuclei. In the fission yeast Schizosaccharomyces pombe, we have deleted 175 meiotically upregulated genes and found seven genes not previously reported to be critical for meiotic events. Three mutants (rec24, rec25, and rec27) had strongly reduced meiosis-specific DNA double-strand breakage and recombination. One mutant (tht2) was deficient in karyogamy, and two (bqt1 and bqt2) were deficient in telomere clustering, explaining their defects in recombination and segregation. The moa1 mutant was delayed in premeiotic S phase progression and nuclear divisions. Further analysis of these mutants will help elucidate the complex machinery governing the special behavior of meiotic chromosomes.     

Centromere pairing precedes meiotic chromosome pairing in plants

Meiosis is a specialized eukaryotic cell division, in which diploid cells undergo a single round of DNA replication and two rounds of nuclear division to produce haploid gametes. In most eukaryotes, the core events of meiotic prophase I are chromosomal pairing,synapsis and recombination. To ensure accurate chromosomal segregation, homologs have to identify and align along each other at the onset of meiosis. Although much progress has been made in elucidating meiotic processes, information on the mechanisms underlying chromosome pairing is limited in contrast to the meiotic recombination and synapsis events. Recent research in many organisms indicated that centromere interactions during early meiotic prophase facilitate homologous chromosome pairing, and functional centromere is a prerequisite for centromere pairing such as in maize. Here, we summarize the recent achievements of chromosome pairing research on plants and other organisms, and outline centromere interactions, nuclear chromosome orientation,and meiotic cohesin, as main determinants of chromosome pairing in early meiotic prophase.     

黑麦6R染色体在小麦背景中的减数分裂行为

减数分裂既是高等生物染色体变异的敏感期,又是变异得以顺利传递给子代的关键期。以黑麦６Ｒ染色体为例,观察其在小麦背景中的减数分裂行为,先是发现６Ｒ抑制小麦同源染色体正常配对,造成单价体数量的增加;同时注意到６Ｒ与其部分同源的小麦染色体６Ｄ几乎不能发生配对。其次是观察到单价体在减数分裂期容易产生断裂的现象,特别是首次发现单价体碎裂,对进一步深入研究异源染色体臂间易位和小片段易位的形成具有借鉴价值。