Development of the synaptonemal complex and the “recombination nodules” during meiotic prophase in the seven bivalents of the fungus Sordaria macrospora Auersw

Complete reconstruction of seven leptotene, six zygotene, three pachytene and three diplotene nuclei has permitted to follow the pairing process in the Ascomycete Sordaria macrospora. The seven bivalents in Sordaria can be identified by their length. The lateral components of the synaptonemal complexes (SC) are formed just after karyogamy but are discontinuous at early leptotene. Their ends are evenly distributed on the nuclear envelope. The homologous chromosomes alignment occurs at late leptotene before SC formation. The precise pairing starts when a distance of 200–300 nm is reached. Each bivalent has several independent central component initiation sites with preferentially pairing starting near the nuclear envelope. These sites are located in a constant position along the different bivalents in the 6 observed nuclei. The seven bivalents are not synchronous either in the process of alignment or in SC formation: the small chromosomes are paired first. At pachytene the SC is completed in each of the 7 bivalents. Six bivalents have one fixed and one randomly attached telomeres. The fixed end of the nucleolar organizer is the nucleolus anchored end. At diffuse stage and diplotene, only small stretches of the SC are preserved. The lateral components increase in length is approximately 34% between leptotene and pachytene. Their lengths remain constant during pachytene. From zygotene to diplotene the central components contain local thickenings (nodules). At late zygotene and pachytene each bivalent has 1 to 4 nodules and the location of at least one is constant. The total number of nodules remains constant from pachytene to diplotene and is equal to the mean total number of chiasmata. The observations provide additional insight into meiotic processes such as chromosome movements, initiation and development of the pairing sites during zygotene, the existence of fixed telomeres, the variations in SC length. The correspondence between nodules and chiasmata are discussed.     

Inter- and intra-genomic transfer of small chromosomal segments in wheat-rye allopolyploids

Newly synthesized wheat-rye allopolyploids, derived from Triticum aestivum Mianyang11 × S. cereale Kustro, were investigated by sequential fluorescent in situ hybridization (FISH) and genomic in situ hybridization (GISH) using rye tandem repeat pSc200 and rye genomic DNA as probes, respectively, over the first, second and third allopolyploid generations. FISH signals of pSc200 could be observed at both telomeres/subtelomeres of all 14 chromosomes of the parental rye. In the first allopolyploid generation, there were ten rye chromosomes bearing FISH signals at both telomeres/subtelomeres and four rye chromosomes bearing FISH signals at only one telomere/subtelomere. However, in the second and the third allopolyploid generations, there were 12 rye chromosomes bearing FISH signals at both telomeres/subtelomeres and 2 rye chromosomes bearing FISH signals at only one telomere/subtelomere. Rye telomeric segments were transferred to the centromeric region of wheat chromosomes in some cells and small segments derived from non-telomeric regions of rye chromosome were transferred to the telomeric region of wheat chromosomes in some other cells. These observations indicated that the rye telomeric/subtelomeric region was unstable in newly synthesized wheat-rye allopolyploids and allopolyploidization was accompanied by rapid inter/intra-genomic exchange. The inter-genomic exchange may have occurred in somatic cells.     

Synaptonemal complex and recombination nodules in rye (Secale cereale)

Meiotic prophase in rye was investigated by serial-section reconstruction of pollen mother cell nuclei. In the mid-late zygotene nucleus, all lateral elements were continuous from telomere to telomere, and 9–20 pairing initiation sites per bivalent were observed. Chromosome and bivalent interlockings detected during zygotene were resolved at early pachytene when pairing was completed. In the three pachytene nuclei, the relative synaptonemal complex (SC) lengths and arm ratios were found to be in good correlation with light microscopic data of pachytene bivalents. Spatial tracing of the bivalents showed that they occupy separate areas in the nucleus. Three types of recombination nodules were observed: large, ellipsoïdal and small nodules at early pachytene and irregularly shaped nodules mainly associated with chromatin at late pachytene. Their number and position along the bivalents correlated well with the number and distribution of chiasmata. The classification of the seven bivalents was based on arm ratio and heterochromatic knob distribution.     

The Dissection of Meiotic Chromosome Movement in Mice Using an In Vivo Electroporation Technique

During meiosis, the rapid movement of telomeres along the nuclear envelope (NE) facilitates pairing/synapsis of homologous chromosomes. In mammals, the mechanical properties of chromosome movement and the cytoskeletal structures responsible for it remain poorly understood. Here, applying an in vivo electroporation (EP) technique in live mouse testis, we achieved the quick visualization of telomere, chromosome axis and microtubule organizing center (MTOC) movements. For the first time, we defined prophase sub-stages of live spermatocytes morphologically according to ^GFP-TRF1 and ^GFP-SCP3 signals. We show that rapid telomere movement and subsequent nuclear rotation persist from leptotene/zygotene to pachytene, and then decline in diplotene stage concomitant with the liberation of SUN1 from telomeres. Further, during bouquet stage, telomeres are constrained near the MTOC, resulting in the transient suppression of telomere mobility and nuclear rotation. MTs are responsible for these movements by forming cable-like structures on the NE, and, probably, by facilitating the rail-tacking movements of telomeres on the MT cables. In contrast, actin regulates the oscillatory changes in nuclear shape. Our data provide the mechanical scheme for meiotic chromosome movement throughout prophase I in mammals.     

普通小麦联会复合体发育过程的电镜观察

以改进的去污剂微铺展技术制备普通小麦减数分裂联会复合体标本,并对联会复合体发育的全过程作了详细的电镜观察和描述。结果表明,小麦SC以多点式起始方式于偶线期开始形成;随SC的发育,新的SC形成和已有SC片断的延伸并存;此外,在同一核内不同二价体之间,染色体配对和SC形成并不同步;SC成熟于粗线期,而以破碎方式解体,消失于双线期。在偶线期还观察到由同祖配对形成的多价体,但在随后阶段中这些多价体消失。对Ph基因的可能作用机制作了分析和讨论。     

Chinese hamster telomeres are comparable in size to mouse telomeres.

We report here the results of a telomere length analysis in four male Chinese hamsters by quantitative fluorescence in situ hybridization (Q-FISH). We were able to measure telomere length of 64 (73%) of 88 Chinese hamster telomeres. We could not measure telomere length in chromosome 10 or in the short arms of chromosomes 5, 6, 7 and 8 because of the overlaps between the interstitial and terminal telomeric signals. Our analysis in the 73% of Chinese hamster telomeres indicate that their average length is approximately 38 kb. Therefore, Chinese hamster telomeres are comparable in length to mouse telomeres, but are much longer than human telomeres. Similar to previous Q-FISH studies on human and mouse chromosomes, our results indicate that individual Chinese hamster chromosomes may have specific telomere lengths, suggesting that chromosome-specific factors may be involved in telomere length regulation.     

Telomeres act autonomously in maize to organize the meiotic bouquet from a semipolarized chromosome orientation

During meiosis, chromosomes undergo large-scale reorganization to allow pairing between homologues, which is necessary for recombination and segregation. In many organisms, pairing of homologous chromosomes is accompanied, and possibly facilitated, by the bouquet, the clustering of telomeres in a small region of the nuclear periphery. Taking advantage of the cytological accessibility of meiosis in maize, we have characterized the organization of centromeres and telomeres throughout meiotic prophase. Our results demonstrate that meiotic centromeres are polarized prior to the bouquet stage, but that this polarization does not contribute to bouquet formation. By examining telocentric and ring chromosomes, we have tested the cis-acting requirements for participation in the bouquet. We find that: (a) the healed ends of broken chromosomes, which contain telomere repeats, can enter the bouquet; (b) ring chromosomes enter the bouquet, indicating that terminal position on a chromosome is not necessary for telomere sequences to localize to the bouquet; and (c) beginning at zygotene, the behavior of telomeres is dominant over any centromere-mediated chromosome behavior. The results of this study indicate that specific chromosome regions are acted upon to determine the organization of meiotic chromosomes, enabling the bouquet to form despite large-scale changes in chromosome architecture.     

Sex-specific telomere redistribution and synapsis initiation in cattle oogenesis

The process of homolog pairing is well characterised in meiosis of male mammals, but much less information is available from female meiosis. We have therefore studied telomere dynamics by FISH and synapsis formation by immunostaining of synaptonemal complex proteins (SCP3, SCP1) on ovarian sections from 15 bovine fetuses, which covered the entire female prophase I. Telomeres displayed a dispersed intranuclear distribution in oogonia and relocated to the nuclear periphery during the preleptotene stage. Tight telomere clustering (bouquet formation) coincided with synapsis initiation at the leptotene/zygotene transition. Clustering of telomeres persisted during zygotene and even into the pachytene stage in a subset of nuclei, while it was absent in diplotene/dictyotene stage nuclei. Thus, the bouquet stage in the bovine female lasts significantly longer than in the male. Further, we observed that synapsis in the female initiated both terminally and interstitially in earliest zygotene stage oocytes, which contrasts with the predominantly terminal synapsis initiation in early zygotene spermatocytes of the bovine male. Altogether, our data disclose a sex-specific difference in telomere dynamics and synapsis initiation patterns in male and female bovine germ cells that may be related to the sex-specific differences in recombination rates observed in this and other mammalian species.     

MEIOSIS WITH TELOMERIC PAIRING IN A HAPLOID OF TRITICUM MONOCOCCUM (x = 7)

In the microsporocytes of a haploid of Trilicum monococcum (x = 7), foldback and other nonhomologous pairing was observed at pachytene. At the diplotene equivalent stage of meiosis, nonhomologous chromosomes were connected by their telomeres in associations involving two to seven chromosomes. Telomeric connections were Feulgen-positive for DNA and were disjoined by metaphase I. These connections may have resulted from earlier base-pairing of repeated sequences of guanine-rich telomere overhangs of nonhomologous chromosomes. Recent molecular studies of several widely divergent organisms have shown that all telomeres of nonhomologous chromosomes in a genome are identical, and telomere structure is conserved among widely divergent eukaryotes. Chromosome distribution at anaphase I fitted theoretical expectations of random movement of each of the seven chromosomes to one or the other of the two poles as did pollen fertility (stainability) resulting from such distribution. A single bivalent in 3.78% of the metaphase I cells provided evidence for a duplication in the genome of Triticum monococcum.     

Ultrastructural studies of late meiotic prophase nuclei of spermatocytes in Ascaris suum

Post pachytene stages of meiotic prophase in males of Ascaris suum have been analyzed with the electron microscope. No synaptonemal-like polycomplexes (PCs) have been observed in the nucleoplasm or cytoplasm during the period from pachytene to diakinesis. From Serially sectioned diplotene nuclei it was found that the bivalents are located near the periphery of the nuclei, the central part of the nuclei being vacant. Each nucleus contains one nucleolus. Up to 1 m long stretches of unpaired lateral elements (LEs) are found in some of the diplotene bivalents. These LEs are morphologically similar to unpaired LEs in early zygotene nuclei. Partial 3-dimensional reconstruction of two nuclei shows that the bivalents contain some small stretches of synaptonemal complex (SC) up to 1.9 m long. Some bivalents at diakinesis show remnants of SCs. At this stage chromosomes are fibrous, condensed, attached to the nuclear envelope and mostly with a rounded profile in cross section. The synchronous development of the spermatocytes and small bivalents at diplotene in A. suum make this system a good object for the study of localization of SC remnants.     

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.     

The control of chiasma distribution in the locust,Schistocerca gregaria (Forskål)

Chiasma distribution at diplotene in Schistocerca gregaria males can be taken to indicate the positions at which crossing over occurred prior to diplotene since chiasma terminalisation is entirely absent. Analysis of chiasma frequency and position leads to a model for the mechanism controlling distribution which has three main components. — i) The bivalents vary in length between cells due to unknown factors and increase in bivalent length leads to an increase in chiasma frequency. — ii) Within bivalents chiasma initiation is sequential from telomere to centromere with the first chiasma usually forming close to the telomere. — iii) Interference operates with the same polarity and determines a distance within which crossing over is precluded.     

Two kinds of "recombination nodules" in Neurospora crassa

Two morphological types of recombination nodules, termed early and late, are recognized in Neurospora crassa. Eighty nuclei at different substages were used to determine numbers of nodules per nucleus, distribution of nodules along the nucleolus-organizing chromosome, and distribution of nodules among the two largest chromosomes. Early nodules appear at the synaptonemal complex at early zygotene and increase in number during zygotene until a dramatic reduction occurs at zygotene-pachytene transition. Thereafter early nodules are steadily eliminated until they disappear by diplotene. Late nodules are also present during zygotene. Their number doubles at the zygotene-pachytene transition and stays at this level until diplotene. The total number of nodules is rather constant through zygotene and pachytene. Distribution of bivalents with 0, 1, 2, etc. nodules follows a Poisson distribution at zygotene, but not at pachytene, where variance is less than the mean, indicating positive interference. Nodules are distributed nonrandomly along the nucleolus-organizer bivalent. The pattern differs slightly in nuclei of different origin. Nuclei with unusual synaptonemal complexes sustain normal levels of recombination by having the same amount of nodules as normal nuclei. In abnormal nuclei nodules are preferentially associated with normal segments. It is proposed that early nodules do not participate in any form of recombination but have a role in finding an appropriate site for a crossing-over event. Morphological change to the late type indicates that the site has been reached and the exchange event can be mediated by the late nodule.     

Synapsis in Robertsonian heterozygotes and homozygotes of Dichroplus pratensis (Melanoplinae, Acrididae) and its relationship with chiasma patterns

Dichroplus pratensis has a complex system of Robertsonian rearrangements with central-marginal distribution; marginal populations are standard telocentric. Standard bivalents show a proximal-distal chiasma pattern in both sexes. In Robertsonian individuals a redistribution of chiasmata occurs: proximal chiasmata are suppressed in fusion trivalents and bivalents which usually display a single distal chiasma per chromosome arm. In this paper we studied the synaptic patterns of homologous chromosomes at prophase I of different Robertsonian status in order to find a mechanistic explanation for the observed phenomenon of redistribution of chiasmata. Synaptonemal complexes of males with different karyotypes were analysed by transmission electron microscopy in surface-spread preparations. The study of zygotene and early pachytene nuclei revealed that in the former, pericentromeric regions are the last to synapse in Robertsonian trivalents and bivalents and normally remain asynaptic at pachytene in the case of trivalents, but complete pairing in bivalents. Telocentric (standard) bivalents usually show complete synapsis at pachytene, but different degrees of interstitial asynapsis during zygotene, suggesting that synapsis starts in opposite (centromeric and distal) ends. The sequential nature of synapsis in the three types of configuration is directly related to their patterns of chiasma localisation at diplotene-metaphase I, and strongly supports our previous idea that Rb fusions instantly produce a redistribution of chiasmata towards chromosome ends by reducing the early pairing regions (which pair first, remain paired longer and thus would have a higher probability of forming chiasmata) from four to two (independently of the heterozygous or homozygous status of the fusion). Pericentromeric regions would pair the last, thus chiasma formation is strongly reduced in these areas contrary to what occurs in telocentric bivalents.     

Meiotic telomere distribution and Sertoli cell nuclear architecture are altered in Atm- and Atm-p53-deficient mice

The ataxia telangiectasia mutant (ATM) protein is an intrinsic part of the cell cycle machinery that surveys genomic integrity and responses to genotoxic insult. Individuals with ataxia telangiectasia as well as Atm(-/-) mice are predisposed to cancer and are infertile due to spermatogenesis disruption during first meiotic prophase. Atm(-/-) spermatocytes frequently display aberrant synapsis and clustered telomeres (bouquet topology). Here, we used telomere fluorescent in situ hybridization and immunofluorescence (IF) staining of SCP3 and testes-specific histone H1 (H1t) to spermatocytes of Atm- and Atm-p53-deficient mice and investigated whether gonadal atrophy in Atm-null mice is associated with stalling of telomere motility in meiotic prophase. SCP3-H1t IF revealed that most Atm(-/-) p53(-/-) spermatocytes degenerated during late zygotene, while a few progressed to pachytene and diplotene and some even beyond metaphase II, as indicated by the presence of a few round spermatids. In Atm(-/-) p53(-/-) meiosis, the frequency of spermatocytes I with bouquet topology was elevated 72-fold. Bouquet spermatocytes with clustered telomeres were generally void of H1t signals, while mid-late pachytene and diplotene Atm(-/-) p53(-/-) spermatocytes displayed expression of H1t and showed telomeres dispersed over the nuclear periphery. Thus, it appears that meiotic telomere movements occur independently of ATM signaling. Atm inactivation more likely leads to accumulation of spermatocytes I with bouquet topology by slowing progression through initial stages of first meiotic prophase and an ensuing arrest and demise of spermatocytes I. Sertoli cells (SECs), which contribute to faithful spermatogenesis, in the Atm mutants were found to frequently display numerous heterochromatin and telomere clusters-a nuclear topology which resembles that of immature SECs. However, Atm(-/-) SECs exhibited a mature vimentin and cytokeratin 8 intermediate filament expression signature. Upon IF with ATM antibodies, we observed ATM signals throughout the nuclei of human and mouse SECs, spermatocytes I, and haploid round spermatids. ATM but not H1t was absent from elongating spermatid nuclei. Thus, ATM appears to be removed from spermatid nuclei prior to the occurrence of DNA nicks which emanate as a consequence of nucleoprotamine formation.     

日本沼虾染色体制备的比较研究（简报）

近年来,随着水产经济动物遗传育种研究的迅速发展．对染色体制备技术的要求越来越高。十足类甲壳动物染色体的研究开展较早,但进展却相当缓慢,自一百多年前,Carnoy首次报道了褐虾和滨蟹的染色体,迄今为止已研究过的种类仅约70多种。其主要原因在于其细胞有丝分裂中期染色体形状小,不超过4um,且数目庞大,染色体标本制备和分析都较困难,仍然仅限于染色体数目统计水平。淡水虾类染色体材料与方法的研究极为稀少,     

日本沼虾染色体制备的比较研究(简报)

近年来,随着水产经济动物遗传育种研究的迅速发展,对染色体制备技术的要求越来越高。十足类甲壳动物染色体的研究开展较早,但进展却相当缓慢,自一百多年前,Carnoy首次报道了褐虾和滨蟹的染色体,迄今为止已研究过的种类仅约70多种。其主要原因在于其细胞有丝分裂中期染色体     

The Course of meiosis and centriole behaviour during the ascus development of the ascomycete Gelasinospora calospora

Meiosis and centriole behaviour in the ascomycete Gelasinospora calospora are described. During meiotic prophase, the ascus grows from 15 to 100 . This is accompanied by two marked changes in the primary ascus nucleus: (1) The nucleus increases in size at zygotene, and the paired bivalents elongate considerably from late pachytene-early diplotene until they reach an extremely diffuse diplotene. The time of chiasma formation in relation to these stages is discussed. (2) The nucleolus also increases in size during zygotene but regresses again at the diffuse diplotene stage. The significance of the nucleolar behaviour and the diffuse diplotene stage are discussed. The haploid number of chromosomes in this species is unquestionably demonstrated to be seven. The centrioles were found to increase in size from division I to division III, within the ascus, but to decrease in size again by division IV. At division III they can be seen to be rectangular and flattened in shape, with spindle fibres attached to one side. A three dimensional reconstruction of these organelles and an account of their behaviour is provided.