Synaptonemal complexes with modified lateral elements in Sordaria humana: development of and relationship to the “recombination nodules”

Meiotic prophase in Sordaria humana has been analyzed by three-dimensional reconstructions of 3 leptotene, 2 zygotene, 10 pachytene and 3 diplotene nuclei. Several notable features emerged. The lateral components of the synaptonemal complexes (SC) are hollow tubes which show dilations of variable sizes from late leptotene to early diplotene. These bulges occur before pairing. Their number decreases as soon as the SC are completely formed, but their mean size increases. Bulges can be present in all parts of the lateral components including telomeres and nucleolar organizer region, but their distribution along bivalents is not random. The remarkably uniform width of the SC central region, normally observed in other species is not observed in S. humana. Although as a general rule the bulges rarely affect the homologous components at identical sites, they often either fill or partially cover the central region. The recombination nodules are not clearly connected with the bulges. This work provides also additional insight into the development of both SC and the nodules. At late leptotene, the homologues are aligned before SC formation. One case of interlocking has been observed at early pachytene. Nodules are present from zygotene to diplotene. They are not evenly distributed along the bivalents during pachytene. The mean number of nodules, constant from late pachytene to diplotene, is equal to the mean number of chiasmata.     

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 Relationship between Synaptinemal Complexes, Recombination Nodules and Crossing over in NEUROSPORA CRASSA Bivalents and Translocation Quadrivalents

Reconstruction of serially sectioned zygotene and pachytene nuclei has allowed the estimation of both the number and position of central component recombination nodules in the synaptinemal complexes of two chromosomally different strains of Neurospora crassa. In both strains the number of nodules is that expected if each nodule represents one crossover event (50 map units). The distribution of nodules within the arms of bivalents shows evidence of centromeric repulsion and telomeric localization. Nodules appear quite early in the zygotene before pairing of chromosomes is complete. Evidence was found of size differences in nodules, and multiple nodules were occasionally seen. Chromosome lengths and nuclear sizes increased from early zygotene to late pachytene. The three quadrivalents present in the alcoy translocation heterozygotes were readily distinguishable in reconstructions, and their cytological dimensions were in agreement with predictions from linkage map distances.     

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.     

Genetic and cytological characterization of the recombination protein RAD-51 in<Emphasis Type="Italic"> Caenorhabditis elegans</Emphasis>

We investigated the role of Caenorhabditis elegans rad-51 during meiotic prophase. We showed that rad-51 mutant worms are viable, have no defects in meiotic homology recognition and synapsis but exhibit abnormal chromosomal morphology and univalent formation at diakinesis. During meiosis RAD-51 becomes localized to distinct foci in nuclei of the transition zone of the gonad and is most abundant in nuclei at late zygotene/early pachytene. Foci then gradually disappear from chromosomes and no foci are observed in late pachytene. RAD-51 localization requires the recombination genes spo-11 and mre-11 as well as chk-2, which is necessary for homology recognition and presynaptic alignment. Mutational analysis with synapsis- and recombination-defective strains, as well as the analysis of strains bearing heterozygous translocation chromosomes, suggests that presynaptic alignment may be required for RAD-51 focus formation, whereas homologous synaptonemal complex formation is required to remove RAD-51 foci.     

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.     

Synaptonemal polycomplexes in spermatocytes of the gooseneck barnacle,Pollicipes polymerus Sowerby (Crustacea: Cirripedia)

Polycomplexes are described for the first time in spermatocytes of a cirripede crustacean, Pollicipes polymerus Sowerby. Synaptonemal complexes of regular tripartite construction are seen from zygotene to mid-pachytene. Although some of the synaptonemal complexes are disrupted at late pachytene and may degenerate at this stage, some persist and by diplotene may form polycomplexes by the bending and self-fusion of their lateral elements. These polycomplexes are still encompassed by chromosomes and consist of four dense plates and intercalated central elements and transverse fibers. Other polycomplexes with five or six dense plates, all of which are considerably wider than lateral elements of mid-pachytene synaptonemal complexes, are also seen in diplotene nuclei. These may be attached to a chromosome at only one end or may be in the nucleoplasm, free of chromosomal involvement except for fine fibrous connectives. No polycomplexes are seen in meiotic cells after diplotene and their fate is unknown. It is suggested that poly-complexes serve as sequestra for synaptonemal material which could prevent normal chromosomal disjunction.     

鲻鱼早期卵子发生的超微结构研究

用电子显微镜技术观察鲻鱼早期卵子发生进入第一次成熟分裂前期联会丝复合体期、粗线期、双线期和网状期卵原细胞和初级卵母细胞生发泡和胸质的超微结构特点。在联会丝复合体期,生发泡内同源染色体配对,联会丝复合体中央出现重组节,胞质中不同发育类型的核仁样及其相关线粒体的分布及其数量可作为划分鲻鱼早期卵子发生各个时期的依据。另外,首次观察到靠近膜细胞有一种不规则形细胞,推测是分泌成熟抑制肽细胞。     

Distribution of the Rad51 recombinase in human and mouse spermatocytes.

In vitro, the human Rad51 protein (hRad51) promotes homologous pairing and strand exchange reactions suggestive of a key role in genetic recombination. To analyse its role in this process, polyclonal antibodies raised against hRad51 were used to study the distribution of Rad51 in human and mouse spermatocytes during meiosis I. In human spermatocytes, hRad51 was found to form discrete nuclear foci from early zygotene to late pachytene. The foci always co-localized with lateral element proteins, components of the synaptonemal complex (SC). During zygotene, the largest foci were present in regions undergoing synapsis, suggesting that Rad51 is a component of early recombination nodules. Pachytene nuclei showed a greatly reduced level of Rad51 labelling, with the exceptions of any asynapsed autosomes and XY segments, which were intensely labelled. The distribution of Rad51 in mouse spermatocytes was similar to that found in human spermatocytes, except that in this case Rad51 was detectable at leptotene. From these results, we conclude that the Rad51 protein has a role in the interhomologue interactions that occur during meiotic recombination. These interactions are spatially and temporally associated with synapsis during meiotic prophase I.     

Spreading the synaptonemal complex of Neurospora crassa

A protocol was developed to spread the synaptonemal complex (SC) of the fungus Neurospora crassa. It involves direct mechanical breakage of meiotic cells before spreading. This technique makes it possible to examine the SC of the same nucleus with both light and electron microscopy. This protocol is potentially applicable for other Pyrenomycetes. The SCs were examined at zygotene, pachytene and diplotene. The central elements and the recombination nodules (RN) were well revealed by silver staining. Ten to 13 RNs were counted at pachytene. The total genomic SC length varied with the stage. This whole mount electron microscopy of the SC is particularly useful for studying chromosomal rearrangements.     

Meiosis in Coprinus. VIII. A time-course study of the fusion and division of the spindle pole body during meiosis

The time-course study of meiosis in the fungus Coprinus cinereus (C. lagopus) by electron microscopy reveals that two monoglobular spindle pole bodies (SPB's) of prekaryogamy nuclei come together during karyogamy and are fused. The fusion SPB of postkaryogamy nucleus persists through zygotene and pachytene as evidenced by the presence of axial components and synaptonemal complexes. At early diplotene, the SPB divides. The divided SPB takes on a diglobular form, which grows in size to form two daughter SPB's. These separate and move to opposite poles at metaphase I.     

Presence of abnormal synaptonemal complexes in heterothallic species of Neurospora

Synaptonemal complex abnormalities are frequent in reconstructed meiotic prophase nuclei of Neurospora crassa and Neurospora intermedia. Three kinds of synaptonemal complex anomalies were seen: lateral component splits, lateral component junctions, and multiple complexes. The anomalies apparently are formed during or after the pairing process, as they were not seen in the largely unpaired early zygotene chromosomes. Their presence at all the other substages from mid-zygotene to late pachytene indicates that they are not eliminated before the synaptonemal complex decomposes at diplotene. Abnormal synaptonemal complexes were seen in all 19 crosses of N. crassa and N. intermedia that were examined, including matings between standard laboratory strains, inversions, Spore killers, and strains collected from nature. The frequency of affected nuclei and degree of abnormality within a nucleus varied in different matings. No abnormalities were present in the homothallic species Neurospora africana and Neurospora terricola. Structural chromosome aberrations, introgression, and heterozygosity have been eliminated as causes for pairing disorder. The abnormal synaptonemal complexes seemingly do not interfere with normal ascus development and ascospore formation. The affected nuclei are not aborted during meiotic prophase, nor are they eliminated by abortion of mature asci. The abnormal meiocytes do not lead to aneuploidy, as judged by the low frequency of white ascospores in crosses between wild type strains that have many abnormalities. Thus, the abnormal synatonemal complexes do not appear to prevent chiasma formation between homologues.     

Synaptonemal Complex and Recombination Nodules in Wild-Type DROSOPHILA MELANOGASTER Females

Electron microscope serial section reconstruction analysis of all zygotene-pachytene nuclei of meiotic cells from three wild-type germaria (a subunit of the ovary containing the early meiotic stages arrayed in temporal developmental sequence) of Drosophila melanogaster females corroborates and extends earlier observations (Carpenter 1975a) on the nature and sequence of ultrastructural events occurring during the time of meiotic recombination. Emphasis has been placed on (1) the time of appearance and disappearance of the synaptonemal complex (SC) and the changes in its dimensions that accompany a cell's progression through pachytene, and (2) the appearance, disappearance, number and chromosomal locations of recombination nodules (Carpenter 1975b). For both the SC and the recombination nodule the availability of several developmental series has provided an estimate of the biological variability in the properties of these recombination-associated structures. The much more extensive data presented here substantiate the earlier hypothesis that recombination nodules occur at sites where reciprocal meiotic recombination will occur, has occurred, or is occurring. A second morphological type of recombination nodule is reported; it is suggested that the presence of the latter type of nodule may correlate with sites of gene conversion. The hypothesis that there may be two types of meiotic recombination processes is discussed.     

Immunochemical localization of contractile proteins in mammalian meiotic chromosomes

Summary Smooth muscle heavy myosin and actin have been detected in mouse and rat meiotic chromosomes, by indirect immunofluorescence performed on testis cryostat sections and isolated germ cells. Both contractile proteins are detectable in the nuclei of meiotic cells during the first prophase. The appearance and disappearance time of myosin and actin, however, is not synchronous. While actin is visible in small spots from resting to late diplotene spermatocytes, myosin appears as filaments in the primary spermatocytes from the zygotene to the early stage of diplotene. The number of myosin filaments in the pachytene spermatocytes corresponds to the number of bivalent chromosomes, whereas actin spots constantly outnumber the pairing chromosomes by two units. These immunochemical observations suggest that the two contractile proteins are associated with the synaptonemal complex (SC). Myosin seems to be associated with the central region of the SC, while actin is present in its basal knob which is in connection with the nuclear membrane. The difference in number between myosin filaments and actin spots appears to be related to the peculiar behaviour of the pairing sex chromosomes. The presence of contractile proteins in the nuclei of primary spermatocytes seems to suggest that they might play a role in the process of pairing of homologous chromosomes.     

A search for the synaptic adjustment phenomenon in maize

It has recently been suggested from several laboratories that complex synaptic configurations (required for homologous synapsis in the presence of heterozygosity for chromosome rearrangements, or resulting from multivalent formation in polyploids, or even resulting from interlocking of normal bivalents) may be formed initially, but are altered by dissolution of the central element of the synaptonemal complex followed by its reinstatement in such a way that only free bivalents, typical of normal sequence homozygotes in diploids, are usually found at late pachytene. It has been suggested that the synaptic adjustment inferred may be a process of widespread occurrence. Maize microsporocytes heterozygous for a paracentric inversion were studied at early and late pachytene and also at early diplotene. Clear remnants of loop configurations typical of homologous synapsis of the inverted region were found in a number of cells at early diplotene, and synaptic failure of the inverted region was common at both early and late pachytene. In maize microsporocytes a synaptic adjustment process comparable to that which has been reported in mammals seems to be absent.     

Immunofluorescent synaptonemal complex analysis in azoospermic men

The molecular cause of germ cell meiotic defects in azoospermic men is rarely known. During meiotic prophase I, a proteinaceous structure called the synaptonemal complex (SC) appears along the pairing axis of homologous chromosomes and meiotic recombination takes place. Newly-developed immunofluorescence techniques for SC proteins (SCP1 and SCP3) and for a DNA mismatch repair protein (MLH1) present in late recombination nodules allow simultaneous analysis of synapsis, and of meiotic recombination, during the first meiotic prophase in spermatocytes. This immunofluorescent SC analysis enables accurate meiotic prophase substaging and the identification of asynaptic pachytene spermatocytes. Spermatogenic defects were examined in azoospermic men using immunofluorescent SC and MLH1 analysis. Five males with obstructive azoospermia, 18 males with nonobstructive azoospermia and 11 control males with normal spermatogenesis were recruited for the study. In males with obstructive azoospermia, the fidelity of chromosome pairing (determined by the percentage of cells with gaps [discontinuities]/splits [unpaired chromosome regions] in the SCs, and nonexchange SCs [bivalents with 0 MLH1 foci]) was similar to those in normal males. The recombination frequencies (determined by the mean number of MLH1 foci per cell at the pachytene stage) were significantly reduced in obstructive azoospermia compared to that in controls. In men with nonobstructive azoospermia, a marked heterogeneity in spermatogenesis was found: 45% had a complete absence of meiotic cells; 5% had germ cells arrested at the zygotene stage of meiotic prophase; the rest had impaired fidelity of chromosome synapsis and significantly reduced recombination in pachytene. In addition, significantly more cells were in the leptotene and zygotene meiotic prophase stages in nonobstructive azoospermic patients, compared to controls. Defects in chromosome pairing and decreased recombination during meiotic prophase may have led to spermatogenesis arrest and contributed in part to this unexplained infertility.     

Distribution of MLH1 foci on the synaptonemal complexes of chicken oocytes

The frequency and distribution of the mismatch repair protein MLH1 was analyzed on synaptonemal complex spreads of chicken oocytes using indirect immunofluorescence. MLH1 foci appeared in late zygotene and their number remains constant throughout pachytene. The average number of foci on autosomal synaptonemal complexes (65.02 +/- 4.02) is in agreement with the number of chiasmata estimated from lampbrush chromosomes. The distribution of foci along the synaptonemal complexes is shown to be nonrandom and nonuniform in terms of the distances between them. It is concluded that MLH1 foci are good markers of crossing over in bird (chicken) meiocytes.     

Identification of two major components of the lateral elements of synaptonemal complexes of the rat

This paper describes the identification of two major components of the lateral elements of synaptonemal complexes of the rat by immunocytochemical techniques. We prepared monoclonal antibodies against synaptonemal complexes (SCs) by immunization of mice with purified SCs. One of these antibodies, II52F10, reacts with a 30 and a 33 kDa polypeptide, which are major components of purified SCs. Using this antibody, we studied the localization of its antigens light microscopically, by means of the indirect immunoperoxidase technique, as well as ultrastructurally, by means of the immunogold labeling technique. The immunolocalization was carried out on whole-mount preparations of lysed spermatocytes. The antibody reacts with paired as well as unpaired segments of zygotene, pachytene and diplotene SCs. In light microscopic preparations, the attachment plaques, particularly those of late pachytene and diplotene SCs, also appear to react strongly. In electron micrographs the lateral elements in paired as well as unpaired segments could be seen to react. No reaction was observed in the attachment plaques; however, in late pachytene and diplotene SCs the swollen terminal segments of the lateral elements did react with the antibody. Thus, we conclude that a 30 and a 33 kDa polypeptide make part of the lateral elements of synaptonemal complexes of the rat.     

Chronology and dynamics of the development of female genital organs of cattle fetuses

The chronology and dynamics of the female germ cell development, of the mitotic activity of oogonia, and of the chromosome rearrangements at prophase I of meiosis have been quantitatively estimated in 30 cow embryos and foetuses at the age of 1.5 to 9 months. The sexual differentiation of the gonads was shown in a 1.5 month old embryo. The oocytes at the stages of preleptotene chromosome condensation and decondensation occurred in the 1.5 month old embryos and their maximum number was observed in the 2-5 month old foetuses. The leptotene oocytes were found in the 2-2.5 month old foetuses. The transition to zygotene and pachytene was also recorded in the 2-2.5 month old foetuses but their maximum number was observed in the 4-6 month old foetuses; their number was reduced to single oocytes thereafter. The first diplotene oocytes appeared in the 3 month old foetuses but the active transition of the oocytes to diplotene was observed after four months of development. The formation of a layer of follicle cells takes place around the diplotene oocytes. The vast majority of degenerating germ cells are the oocytes in zygotene-pachytene and in diplotene. The population of germ cells is formed by the mitotic division of oogonia in the cow foetuses, mainly at the age of 1.5 to 4 months of development.