Ultrastructure of the synaptic autosomes and the ZW bivalent in chicken oocytes

Chromosomal axes of chicken oocytes from pre- and post-hatching chickens were analyzed with a microspreading technique for electron microscopy. At leptotene, chromosomal axes begin to be formed as discontinuous, non-polarized axial segments. During zygotene synaptonemal complex (SC) formation begins at the axial ends attached to the nuclear envelope. Polarization of axial ends is nearly simultaneous with the beginning of SC formation. The complete SC set is found at pachytene and it consists of 38 SC's and an unequal SC which has been identified as the ZW pair. This unequal SC is formed by two axes of different length. The Z and W axes represent 6.2% and 4.5% respectively of the combined length of the SC set plus the Z axis. The unpaired segment of the Z axis shortens markedly from early to mid-pachytene and becomes thicker than the lateral elements of SCs. In the paired region the Z axis forms most of the twists around a straighter W axis, suggesting some extent of non-homologous pairing between the Z and W chromosomes in this region. The existence of partial synapsis of the Z and W axes without heteropycnosis of the sex chromosomes is in marked contrast to partial synapsis in the heteropycnotic XY body of mammalian spermatocytes.     

家鸡联会复合体的亚显微结构分析

本文以表面铺展——硝酸银染色技术,对家鸡的联会复合体(Syneptonemal Complex,SC)作亚显微结构分析。根据对10个精母细胞和10个卵母细胞SC的测量结果,绘制组型图。发现雌雄家鸡的常染色体的SC组型相同。在精母细胞中,性染色体(ZZ)的行为与常染色体相似。在卵母细胞中,性染色体ZW的长度不同,长轴为Z,短轴为W,两者之间只有部分配对,形成SC。从早粗线期到晚粗线期,由同源配对调整为非同源配对。另外,在一只雌鸡中,第一次观察到,有些细胞的常染色体能正常配对,而性染色体完全不配对的现象。     

Synaptonemal complex analysis in spermatocytes of tilapia, Oreochromis niloticus (Pisces, Cichlidae).

Some adaptations of the synaptonemal complex (SC) whole-mounting technique first used in plants permitted its application to meiotic studies in tilapia, Oreochromis niloticus. Direct observation of the chromosome pairing process and bivalent structure during the meiotic prophase of this fish species by light and electron microscopy permitted the analysis of SCs in autosomes and the possible identification of sex chromosomes. The analysis of SCs in spermatocytes of O. niloticus revealed that all 22 bivalent chromosomes completely paired, except for the occurrence of a size heteromorphism in the terminal region of the largest bivalent associated with the presence of an incompletely paired segment during the synapsis process, which may be the cytological visualization of an XX/XY sex chromosome system in this species.     

Synaptic adjustment,non-homologous pairing,and non-pairing of homologous segments in sex chromosome mutants of Ephestia kuehniella (Insecta,Lepidoptera)

Microspread pachytene nuclei of wild-type and W chromosome mutants of the mealmoth Ephestia kuehniella were used to study synaptonemal complex (SC) formation. In structurally heterozygous bivalents, axial elements of considerable length differences were brought to the same length by synaptic adjustment. The adjustment length was a compromise between the mutant and the wildtype homologue length in a structural heterozygote of a W chromosome-autosome translocation, T(A; W). The translocated non-homologous W segment really participated in SC formation as could be seen from the W chromosomal heterochromatin, used as a cytogenetic marker. Pachytene pairing of the wild-type W-Z bivalent extended from about two-thirds to the full length of the W chromosome, though from cytogenetic and genetic evidence W and Z are largely — if not completely — non-homologous. Nonhomologous pairing was even more conspicuous in sex chromosome bivalents containing a deleted W chromosome, Df(W). In one of the pairing configurations the halves of the Z chromosome were synapsed to either side of the Df(W). Thus, one side was pairing with the Df(W) in reversed order. The pairing behavior of the W with homologous chromosome segments was tested by introducing supernumerary W segments via the T(A; W) translocation. Pairing between the W and the translocated homologous W segment never occurred, whereas the Z frequently synapsed with it. Even in T(A; W) homozygotes, pairing between the two translocated W segments was not regularly found while the autosomal parts of the translocation chromosomes were always completely paired. Homologous chromosomes and the ability to form an SC are not sufficient for pairing initiation. Specific loci or sequences are postulated for this function. They are either absent from the W chromosome or are present in only low concentrations.     

Sex chromosome pairing and sex chromatin bodies in W-Z translocation strains of Ephestia kuehniella (Lepidoptera).

Structure and pairing behavior of sex chromosomes in females of four T(W;Z) lines of the Mediterranean flour moth, Ephestia kuehniella, were investigated using light and electron microscopic techniques and compared with the wild type. In light microscopic preparations of pachytene oocytes of wild-type females, the WZ bivalent stands out by its heterochromatic W chromosome strand. In T(W;Z) females, the part of the Z chromosome that was translated onto the W chromosome was demonstrated as a distal segment of the neo-W chromosome, displaying a characteristic non-W chromosomal chromomere-interchromomere pattern. This segment is homologously paired with the corresponding part of a complete Z chromosome. In contrast with the single ball of heterochromatic W chromatin in highly polyploid somatic nuclei of wild-type females, the translocation causes the formation of deformed or fragmented W chromatin bodies, probably owing to opposing tendencies of the Z and W chromosomal parts of the neo-W. In electron microscopic preparations of microspread nuclei, sex chromosome bivalents were identified by the remnants of electron-dense heterochromatin tangles decorating the W chromosome axis, by the different lengths of the Z and W chromosome axes, and by incomplete pairing. No heterochromatin tangles were attached to the translocated segment of the Z chromosome at one end of the neo-W chromosome. Because of the homologous pairing between the translocation and the structurally normal Z chromosome, pairing affinity of sex chromosomes in T(W;Z) females is significantly improved. Specific differences observed among T(W;Z)1-4 translocations are probably due to the different lengths of the translocated segments.     

Pairing of ZW gonosomes and the localized recombination nodule in two Z-autosome translocations in Gallus domesticus

Electron microscopic observations of synaptonemal complexes of oocytes from chickens heterozygous for two Z-autosome translocations have been used to identify and study the pairing region of the Z and W chromosomes. The two translocations, MN t(Z;1) and t(OH 10), have breakpoints in opposite arms of the Z, and the arm having the breakpoint of MN t(Z;1) is marked by the terminal C+ band. In both translocations the short arm of the W was specifically paired with the euchromatic short arm of the Z. In MN t(Z;1) only open quadrivalents (74%) and trivalents plus W univalents (26%) were observed, whereas t(OH 10) exhibited, in addition to the prevalent quadrivalents (62%), III + I (19%) and II + II (19%) configurations. The extent of W pairing was slightly decreased in MN t(Z;1) (68.4% of the W chromosomes paired) and considerably decreased in t(OH 10) (25.3% of the W chromosomes paired). Nonhomologous synapsis occurred regularly at the quadrivalent crosspoint in MN t(Z;1) and also in bivalents from t(OH 10). The recombination nodule normally located in the terminus of the pairing region in normal ZW pairs is present in both translocations without any alteration of its frequency or its strict terminal position. Based on these data and previous observations (Rahn and Solari, 1986), it is proposed that an obligatory recombination event occurs at a locus between 0.7 microns and 0.15 microns of the paired ZW telomeres, establishing a recombinational region and a pseudoautosomal region which determine partial sex-linkage and no sex-linkage, respectively. Most of the pairing region of the ZW pair is nonhomologously paired.     

Unequal crossing over and heterochromatin exchange in the X-Y bivalents of the deer mouse,Peromyscus beatae

Differences in length of the heterochromatic short arms of the X and Y chromosomes in individuals ofPeromyscus beatae are hypothesized to result from unequal crossing over. To test this hypothesis, we examined patterns of synapsis, chiasma formation, and segregation for maleP. beatae which were either heterozygous or homozygous for the amount of short-arm sex heterochromatin. Synaptonemal complex analysis demonstrated that mitotic differences in heterochromatic shortarm lengths between the X and Y chromosomes were reflected in early pachynema as corresponding differences in axial element lengths within the pairing region of the sex bivalent. These length differences were subsequently eliminated by synaptic adjustment such that by late pachynema, the synaptonemal complex configurations of the XY bivalent of heterozygotes were not differentiable from those of homozygotes. Crossing over between the heterochromatic short arms of the XY bivalent was documented by the routine appearance of a single chiasma in this region during diakinesis/metaphase I. Sex heterochromatin heterozygotes were characterized by the presence of asymmetrical chiasma between the X and Y short arms at diakinesis/metaphase I and sex chromosomes with unequal chromatid lengths at metaphase II. These data corroborate our hypothesis on the role of unequal crossing over in the production and propagation of X and Y heterochromatin variation and suggest that, in some cases, crossing over can occur during the process of synaptic adjustment.     

Localised chiasmata due to partial pairing: A 3D reconstruction of synaptonemal complexes in male Stethophyma grossum

The possible role of localised pairing as a mechanism producing localised chiasmata in Stethophyma grossum spermatocytes has been examined ultrastructurally. Nuclei at four successive stages of meiosis from leptotene to pachytene were reconstructed from a series of ultrathin sections and the extent of synapsis as demonstrated by synaptonemal complex (SC) formation was calculated. On the basis of the relative lengths of SCs and condensed chromosomes it was reasoned that only the centromeric ends of the long and medium length bivalents paired, and only one end of these SCs was found attached to the nuclear envelope. Only the three shortest bivalents paired completely, and both their SC ends were attached to the nuclear envelope. Thus pairing was directly related to the distribution of chiasmata. The extent of pairing at different stages suggests that the shortest bivalent paired very quickly, the longer ones progressively slower, and that pairing proceeded zip-like from a point at or very close to the end attached to the nuclear envelope, since all stretches of SC were attached to the envelope, and there were never more than 11 pieces, one for each bivalent. Chromosome decondensation and axial core formation did not occur far in advance of SC formation, and synapsis appeared to be much slower in S. grossum than in other species with non-localised chiasmata, as a larger proportion of the meiotic cysts were in zygotene, compared to Stauroderus scalaris and Locusta migratoria, although this was not quantified.     

Recombination nodules in the oocytes of the chicken, Gallus domesticus

Chicken oocytes at pachytene were processed with the microspreading technique (Moses, 1977), and their synaptonemal complex (SC) complements were analyzed by electron microscopy. Ellipsoidal nodules, 140 X 120 nm in diameter, were associated with the central space of synaptonemal complexes. The average number of nodules per pachytene oocyte was 57.5. The number of nodules per bivalent showed a clear linear relationship with SC length, except for the microchromosomes, which showed a single obligatory nodule. The distribution of nodules along the 10 longest SCs was nonrandom, with low frequencies in the vicinity of kinetochores and high frequencies near the telomeres. The microchromosomes showed a single nodule whose average location was 1.21 micron from the kinetochore. In the ZW pair there was a single nodule whose average location was 0.31 micron from the paired telomeres and not more than 0.65 micron from them. The total number of nodules per cell and the number of nodules in each of the five major bivalents showed good agreement with the total number of chiasmata and the number of chiasmata of the major bivalents of roosters. Thus, these nodules share the characteristics of recombination nodules described in other organisms. The single, obligatory, strictly localized recombination nodule found in the pairing end of the ZW pair strongly suggests that recombination between the Z and W chromosomes in the female chicken is a regular process that may be similar to the obligatory recombination between the pairing ends of the human X and Y chromosomes that was recently described in studies using DNA probes.     

An asymmetric chromosome pair undergoes synaptic adjustment and crossover redistribution during Caenorhabditis elegans meiosis: implications for sex chromosome evolution

Heteromorphic sex chromosomes, such as the X/Y pair in mammals, differ in size and DNA sequence yet function as homologs during meiosis; this bivalent asymmetry presents special challenges for meiotic completion. In Caenorhabditis elegans males carrying mnT12, an X;IV fusion chromosome, mnT12 and IV form an asymmetric bivalent: chromosome IV sequences are capable of pairing and synapsis, while the contiguous X portion of mnT12 lacks a homologous pairing partner. Here, we investigate the meiotic behavior of this asymmetric neo-X/Y chromosome pair in C. elegans. Through immunolocalization of the axis component HIM-3, we demonstrate that the unpaired X axis has a distinct, coiled morphology while synapsed axes are linear and extended. By showing that loci at the fusion-proximal end of IV become unpaired while remaining synapsed as pachytene progresses, we directly demonstrate the occurrence of synaptic adjustment in this organism. We further demonstrate that meiotic crossover distribution is markedly altered in males with the asymmetric mnT12/+ bivalent relative to controls, resulting in greatly reduced crossover formation near the X;IV fusion point and elevated crossovers at the distal end of the bivalent. In effect, the distal end of the bivalent acts as a neo-pseudoautosomal region in these males. We discuss implications of these findings for mechanisms that ensure crossover formation during meiosis. Furthermore, we propose that redistribution of crossovers triggered by bivalent asymmetry may be an important driving force in sex chromosome evolution.     

Equalization of Z and W axes in chicken and quail oocytes.

The different morphological types of ZW pairs have been classified in three main types according to the relative extension of the free segment of the Z axis: 1, "long asynaptic segment;" 2, "medium asynaptic segment;" and 3, "equalized." Pre- and post-pairing types have also been defined. Frequencies of each type were determined at day 20 and day 21 of incubation, and one and three days after hatching. The changing frequencies and the morphological transitions observed show a definite sequence of ZW types that can be used as a timetable for pachytene substaging. Measurements made on each ZW type show that the Z axis of the chicken shortens from 20.6 microns to 13.1 microns. This shortening occurs both in the free segment (at a higher rate) and in the paired segment (at a lower rate). The synaptonemal complex becomes elongated while adjustment occurs. The equalized Z axis makes many twists around the W axis. However, a segment 1 micron long from the synaptic terminus is free from twists and is assumed to be the homologously paired region. The ZW pair of the quail shows a similar behavior but equalization of the Z and W axes ends earlier and forms a straighter synaptonemal complex as compared with the chicken. In both species a recombination nodule is strictly localized near the synaptic terminus. In the ZW pair of the quail the average location of this nodule is 0.14 microns from the synaptic terminus. The meiotic behavior of ZW pairs in birds may be conserved.     

Axial shortening during pachynema unrelated to nonhomologous synapsis

The pachytene behavior of chromosomes participating in quadrivalent formation in male mice heterozygous for T(X;4)7Rl or T(X;4)8Rl was analyzed in electron micrographs of microspread spermatocytes. In each population of nuclei from the translocation heterozygotes, the longest 4X axes were approximately the proportional length expected from the respective contributions of the 4 and the X estimated from breakpoint positions in mitotic chromosomes. However, the 4X axis of these translocation quadrivalents undergoes extensive shortening. In both R7 and R8 the shortest 4X axis observed in the population of nuclei was approximately the length of the normal 4 axis. This equalization of axial lengths suggests that there may be an interchromosomal interaction between synapsed chromosomes. In R8, axial shortening of the 4X occurs as pachynema progresses. In both translocations, shortening is accompanied by twisting of the 4X around the 4. Both axial shortening and twists are characteristics exhibited by chromosomal axes of unequal length as part of the meiotic phenomenon described as "synaptic adjustment" (Moses, 1977). Synaptic adjustment involves, in addition, nonhomologous synapsis, which is delayed until the latter part of pachynema. However, axial shortening in R7 and R8 is not accompanied by nonhomologous synapsis. In R7, nonhomologous synapsis does not occur; in R8, it is confined to quadrivalents in which the 4X axis is near its maximum length (i.e., early). This behavior suggests that axial shortening and nonhomologous synapsis during the progression of pachynema (previously considered collectively under the term "synaptic adjustment") are not necessarily coupled events.     

Behaviour of the ZW sex bivalent in the snake Bothrops jararaca

The behavior of the ZW sex bivalent was investigated in female meiosis of the poisonous snake Bothrops jararaca. The Z is euchromatic and synapses end to end with the W. The W chromosome shows a heterochromatic segment distally in the short arm. Pairing occurs between the long arm of the W and the slightly longer arm of the mediocentric Z. A sex vesicle, similar to the one found in the XY placental mammals, does not occur in snakes. The Z and W chromosomes segregate reductionally in the first meiotic division and equationally in the second.This work is dedicated to the memory of my father Lino Pires de Camargo     

A Novel ZZ/ZW Sex Chromosome System for the Genus Leporinus (Pisces, Anostomidae, Characiformes)

A wide range of sex chromosome mechanisms, including simple and multiple chromosome systems is characteristic of fishes. The Leporinus genus represent a good model to study sex chromosome mechanisms, because an unambiguous ZZ/ZW sex chromosome system was previously described for seven species, while the remaining studied species of the genus do not show differentiated sex chromosomes. The occurrence of sex chromosomes in Leporinus trifasciatus and Leporinus sp2 from the Araguaia river, Amazon basin, Brazil, was here investigated. ZZ/ZW sex chromosomes were detected for both species. The Z and W chromosome morphology of L. trifasciatus is the same as described for other species of the genus Leporinus. However, the Z and W chromosomes of L. sp2 were quite different in their morphology and banding pattern suggesting that the ZW system of this species have originated independently from the ZW system previously described for other Leporinus.     

Meiosis in gray voles of the subgenus microtus (rodentia, arvicolinae) and in their hybrids

The results of light and electron microscopic (EM) studies of meiosis in Microtus arvalis males of the karyoform "arvalis" (2n = 46, NFa = 80), in hybrids between the chromosomal forms arvalis and obscurus (2n = 46, NFa = 68), in M. rossiaemeridionalis voles (2n = 54, NFa = 54), and in a hybrid between the species M. rossiaemeridionalis and M. kermanensis (2n = 54, NFa = 54) are presented. SC (synaptonemal complex) karyotypes of the parental forms and the hybrids were constructed on the basis of measurements of the length ofautosomal SCs revealed by the EM analysis in spermatocytes at the stage of middle pachytene. The SC karyotypes of M. arvalis and the hybrids female obscurus x male arvalis consist of 22 synaptonemal complexes of autosomal bivalents and the axial elements of the synaptonemal complexes of the sex chromosomes X and Y. The SC karyotypes of M. rossiaemeridionalis and the hybrid M. rossiaemeridionalis x M. kermanensis consist of 26 synaptonemal complexes of autosomal bivalents and a sex bivalent; they differ only in the length of the Y chromosome axis (Y chromosome in the hybrid was inherited from M. kermanensis). Asynaptic configurations of the autosomal SCs were not observed in the hybrids. The SC axial elements of the X and Y chromosomes in the parental forms and in the hybrids were located close to each other throughout pachytene, but they did not form a synaptic region. The normal synapsis in sterile hybrids (M. rossiaemeridionalis x M. kermanensis) and the behavior of the sex chromosomes in meiosis in fertile and sterile hybrids are discussed in the context of specific features of meiosis and reproductive isolation.     

Synaptonemal complex antigen location and conservation

The axial cores of chromosomes in the meiotic prophase nuclei of most sexually reproducing organisms play a pivotal role in the arrangement of chromatin, in the synapsis of homologous chromosomes, in the process of genetic recombination, and in the disjunction of chromosomes. We report an immunogold analysis of the axial cores and the synaptonemal complexes (SC) using two mouse monoclonal antibodies raised against isolated rat SCs. In Western blots of purified SCs, antibody II52F10 recognizes a 30- and a 33-kD peptide (Heyting, C., P. B. Moens, W. van Raamsdonk, A. J. J. Dietrich, A. C. G. Vink, and E. J. W. Redeker, 1987, Eur. J. Cell Biol., 43: 148-154). In spreads of rat spermatocyte nuclei it produces gold grains over the cores of autosomal and sex chromosomes. The cores label lightly during the chromosome pairing stage (zygotene) of early meiotic prophase and they become more intensely labeled when they are parallel aligned as the lateral elements of the SC during pachytene (55 grains/micron SC). Statistical analysis of electronically recorded gold grain positions shows that the two means of the bimodal gold grain distribution coincide with the centers of the lateral elements. At diplotene, when the cores separate, the antigen is still detected along the length of the core and the enlarged ends are heavily labeled. Shadow-cast SC preparations show that recombination nodules are not labeled. The continued presence suggests that the antigens serve a continuing function in the cores, such as chromatin binding, and/or structural integrity. Antibody III15B8, which does not recognize the 30- and 33-kD peptides, produces gold grains predominantly between the lateral elements. The grain distribution is bimodal with the mean of each peak just inside the pairing face of the lateral element. The antigen is present where and while the cores of the homologous chromosomes are paired. From the location and the timing, it is assumed that the antigen recognized by III15B8 functions in chromosome pairing at meiotic prophase. The two anti-rat SC antibodies label rat and mouse SCs but not rabbit or dog SCs. A positive control using human CREST (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) anti-centromere serum gives equivalent labeling of SC centromeres in the rat, mouse, rabbit, and dog. It is concluded that the SC antigens recognized by II52F10 and III15B8 are not widely conserved. The two antibodies do not bind to cellular or nuclear components of somatic cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synaptonemal complexes in insects

The synaptonemal complex (SC) is the key nuclear element formed in meiotic prophase I to join 2 homologous chromosomes at the pachytene bivalent. It is a highly conserved structure that is universally present in eukaryotes. The SC is presented as a tripartite protein structure, which consists of 2 lateral elements and a central region. In insects, the central region is particularly distinct and highly ordered. This made it possible to describe the fine structure of the central region and propose a model of its architecture. Chromatid DNA is arranged in chromatin loops extending radially from the SC. The loops appear to consist of a basic chromatin fiber with a diameter of 20–30 nm. In many insect species, synaptonemal polycomplexes occur in postpachytene cells. They represent one of the possible ways of SC degradation. Another process, which occurs beyond pachytene, is the formation of proteinaceous chromatid axis, the silver-stained chromatid core. Based on results in insect models, the chromatid cores have been related to the structure and formation of the SC. Research on insect models significantly contributed to understanding individual steps of the SC formation and temporal sequence of chromosome pairing. These include the formation of lateral elements of the SC, pairing initiation, interlocking of chromosomes, and synapsis of homologous chromosomes. Attention is also given to non-homologous pairing, including synaptic adjustment, correction of pairing, and pairing of sex chromosomes. In the next section, chiasmatic and achiasmatic modes of meiosis are compared with respect to the SC formation. In the chiasmatic mode, the SCs display recombination nodules that are believed to mediate the process of recombination. These nodules were discovered in insects, and indirect evidence for their role comes from insects. Two different examples of achiasmatic meiosis, occurring in the heterogametic sex of several insect orders, are given: one involves the SC formation, whereas in the other, SCs are absent. Finally, the potential of SC karyotyping for analysis of the insect genome is discussed.