The distribution of synaptonemal complex material in metaphase I bivalents of Locusta and Chloealtis (Orthoptera: Acrididae)

At metaphase I synaptonemal complex (SC) material is located in a continuous but irregularly shaped bundle between sister chromatids. Only at the site of a chiasma is it present between homologous chromosomes. When the chromosomes pull apart at anaphase I the SC material becomes rearranged into poly-SCs which dissociate from the chromosomes. The observations agree with previous reports that modified SCs may function in meiotic chromosome disjunction.     

ZHP-3 acts at crossovers to couple meiotic recombination with synaptonemal complex disassembly and bivalent formation in C. elegans

Crossover recombination and the formation of chiasmata normally ensure the proper segregation of homologous chromosomes during the first meiotic division. zhp-3, the Caenorhabditis elegans ortholog of the budding yeast ZIP3 gene, is required for crossover recombination. We show that ZHP-3 protein localization is highly dynamic. At a key transition point in meiotic prophase, the protein shifts from along the length of the synaptonemal complex (SC) to an asymmetric localization on the SC and eventually becomes restricted to foci that mark crossover recombination events. A zhp-3::gfp transgene partially complements a null mutation and reveals a separation of function; although the fusion protein can promote nearly wild-type levels of recombination, aneuploidy among the progeny is high, indicating defects in meiotic chromosome segregation. The structure of bivalents is perturbed in this mutant, suggesting that the chromosome segregation defect results from an inability to properly remodel chromosomes in response to crossovers. smo-1 mutants exhibit phenotypes similar to zhp-3::gfp mutants at higher temperatures, and smo-1; zhp-3::gfp double mutants exhibit more severe meiotic defects than either single mutant, consistent with a role for SUMO in the process of SC disassembly and bivalent differentiation. We propose that coordination of crossover recombination with SC disassembly and bivalent formation reflects a conserved role of Zip3/ZHP-3 in coupling recombination with SC morphogenesis.     

From early homologue recognition to synaptonemal complex formation

This review focuses on various aspects of chromosome homology searching and their relationship to meiotic and vegetative pairing and to the silencing of unpaired copies of genes. Chromosome recognition and pairing is a prominent characteristic of meiosis; however, for some organisms, this association (complete or partial) is also a normal part of nuclear organization. The multiple mechanisms suggested to contribute to homologous pairing are analyzed. Recognition of DNA/DNA homology also plays an important role in detecting DNA segments that are present in inappropriate number of copies before and during meiosis. In this context, the mechanisms of methylation induced premeiotically, repeat-induced point mutation, meiotic silencing by unpaired DNA, and meiotic sex chromosome inactivation will be discussed. Homologue juxtaposition during meiotic prophase can be divided into three mechanistically distinct steps, namely, recognition, presynaptic alignment, and synapsis by the synaptonemal complex (SC). In most organisms, these three steps are distinguished by their dependence on DNA double-strand breaks (DSBs). The coupling of SC initiation to (and downstream effects of) DSB formation and the exceptions to this dependency are discussed. Finally, this review addresses the specific factors that appear to promote chromosome movement at various stages of meiotic prophase, most particularly at the bouquet stage, and on their significance for homologue pairing and/or achieving a final pachytene configuration.The synaptonemal complex - 50 years     

DNA repeated sequences may be involved in synaptonemal complex formation

Synaptonemal complexes (SCs) are intranuclear structures that facilitate the reversible lateral synapsis of homologous chromosomes in the course of meiosis. It is still unclear which DNA nucleotide sequences are responsible for the attachment of chromatin to SC lateral elements. Considering the features of the dispersed repeated sequences (RSs), it is possible to assume that they participate in the structure and functional organization of the meiotic chromosomes. Using numerical analysis, we have investigated the relationship between the RS and the distribution of meiotic recombination events in mouse chromosome 1. Using in situ hybridization on spread mouse spermatocytes, we have examined the arrangement of different types of RSs relative to SCs. Hybridization signals of B1(Alu), B2, and minisatellite probes were localized predominantly in SCs regions. Based on the results, we proposed a model of meiotic chromosome organization. According to the model, RSs participate in the attachment of chromatin loops to SCs.     

Cytogenetics of the hybrids of three members of the grasshopper genus Vandiemenella (Orthoptera: Eumastacidae: Morabinae)

Certain members of the grasshopper genus Vandiemenella (earlier referred to as the viatica group) form a group of chromosomally distinct taxa with contiguous distributions through south-eastern Australia. Meiosis of hybrids between three of the members that occur on Kangaroo Island, South Australia, was studied. The three members were the taxonomically undescribed form P24(XY) and two chromosomal races of V. viatica. Hybrids were either collected from narrow overlap zones (viatica ₁₉-viatica ₁₇, viatica ₁₉-P24 (XY), P24(XY)-vitica ₁₇) or were reared from grasshoppers collected from areas close to the overlap zones. — Chromosomal heterozygotes were found in all three overlap zones, but were frequent only in the overlap zone of P24(XY) and viatica ₁₇. Male chromosomal heterozygotes of P24(XY) and viatica ₁₉ and of viatica ₁₇ and viatica ₁₉ had abnormalities of meiosis, such as asynapsis (or desynapsis), which were generally associated with the chromosomes that distinguish the taxa. These abnormalities would be expected to lead to a reduction of fertility and so may contribute to reproductive isolation. Analysis of embryos from female heterozygotes of P24(XY) and viatica ₁₇ indicated that similar abnormalities of meiosis occurred in these females. Very few meiotic abnormalities, however, were seen in the corresponding male hybrids. The structurally different X chromosomes of female hybrids of P24(XY) and viatica ₁₇ were recovered in a distorted ratio in their embryos. The distortion was only slight and would not be expected to be of importance in the evolution of the group, particularly since the distortion favoured the original rather than the derived type of chromosome.     

Defective pairing and synaptonemal complex formation in a Sordaria mutant (spo44) with a translocated segment of the nucleolar organizer

The recessive meiotic mutant spo44 of Sordaria macrospora, with 90% ascospore abortion, exhibits striking effects on recombination (67% decrease), irregular segregation of the almost unpaired homologues, and a decrease in chiasma frequency in the few cases where bivalents are formed. Three-dimensional reconstructions of ten prophase nuclei indicate that pairing, as judged by the absence of fully formed synaptonemal complexes (SC), is not achieved although lateral elements (LE) assemble. The pairing failure is attributable to defects in the alignment of homologous chromosomes. The leptotene alignment seen in the wild type before SC formation was not observed in the spo44 nuclei. Dense material, considered to be precursor of SC central elements, was found scattered among the LE in two nuclei. The behaviour of spo44 substantiates the hypothesis that chromosome matching and SC formation are separable events. — The total length of the LE in the mutant is the same as in the wild type, but due to variable numbers and length of the individual LE, homologues cannot be lined up. Light microscopic observations indicate that the irregular length and number of LE is due to extensive chromosome breakage. The wild-type function corresponding to spo44 is required for both LE integrity and chromosome matching. Reconstructions of heterozygous nuclei reveal the presence of a supernumerary nucleolar organizer in one arm of chromosome 7. It is suggested that rDNA has been inserted into a gene whose function is involved in pairing or into a controlling sequence that interacts with the pairing process.     

Chiasma formation: chromatin/axis interplay and the role(s) of the synaptonemal complex

Meiotic recombination proceeds in biochemical complexes that are physically associated with underlying chromosome structural axes. In this study, we discuss the organizational basis for these axes, the timing and nature of recombinosome/axis organization with respect to the prophase program of DNA and to structural changes, and the possible significance of axis organization. Furthermore, we discuss implications and extensions of our recently proposed mechanical model for chiasma formation. Finally, we give a broader consideration to past and present models for the role of the synaptonemal complex.     

Meiosis in mice without a synaptonemal complex

The synaptonemal complex (SC) promotes fusion of the homologous chromosomes (synapsis) and crossover recombination events during meiosis. The SC displays an extensive structural conservation between species; however, a few organisms lack SC and execute meiotic process in a SC-independent manner. To clarify the SC function in mammals, we have generated a mutant mouse strain (Sycp1(-/-)Sycp3(-/-), here called SC-null) in which all known SC proteins have been displaced from meiotic chromosomes. While transmission electron microscopy failed to identify any remnants of the SC in SC-null spermatocytes, neither formation of the cohesion axes nor attachment of the chromosomes to the nuclear membrane was perturbed. Furthermore, the meiotic chromosomes in SC-null meiocytes achieved pre-synaptic pairing, underwent early homologous recombination events and sustained a residual crossover formation. In contrast, in SC-null meiocytes synapsis and MLH1-MLH3-dependent crossovers maturation were abolished, whereas the structural integrity of chromosomes was drastically impaired. The variable consequences that SC inactivation has on the meiotic process in different organisms, together with the absence of SC in some unrelated species, imply that the SC could have originated independently in different taxonomic groups.     

Dependence on genic balance for synaptonemal complex formation in Drosophila melanogaster

Electron microscopic examination of gonads of Drosophila melanogaster with different genotypes, including a metafemale 3X;2A and an intersex XXY;3A have revealed that the formation of synaptonemal complexes is controlled by the genic balance, i.e., the ratio of X chromosomes to autosomes. The Y chromosome is not involved in the genetic control of the formation of precursors of the central element of synaptonemal complexes in males, nor does it disturb their formation in XXY females. Hyperploidy for sections 1-3A and 18A-20 of the X chromosome does not lead to the appearance of synaptonemal complexes in males and does not interfere with their formation in females. Females hyperploid for extensive regions of the X chromosome (sections 1-11A, 11A-20, and 8C-20) are fertile and show apparently normal formation of synaptonemal complexes. Hyperploidy for sections 8C-11A of the X results in a sharp decrease in the viability of females, in abnormal differentiation of ovary cells, and in the lack of synaptonemal complexes. These data suggest a possible important role for the sections 8C-11A in the genic balance controlling the formation of synaptonemal complexes in D. melanogaster. The lack of synaptonemal complexes in hypoploid females may be the result of abnormal cell differentiation in gonads.     

The nature and extent of synaptonemal complex formation in haploid barley

Normal synaptonemal complexes have been found in haploid barley meiotic prophase at stages equivalent to pachytene in diploids. Reconstructions of serially sectioned nuclei have shown that up to 60% of the haploid chromosomes may pair in either intra- or interchromosomal associations. The extent and nature of the synaptonemal complex formation suggest that the chromosome pairing is non-homologous. From the virtual absence of chiasmata in metaphase I stages of the haploids it is inferred that crossing over requires a more precise DNA alignment than is provided by synaptonemal complex formation alone.     

The effect of colchicine on synaptonemal complex formation in Allium ursinum

Interference of colchicine with meiotic chromosome pairing in the wild garlic, Allium ursinum, was studied using a whole-mount spreading technique for synaptonemal complexes. Colchicine was found to cause (i) pairing suppression (arrest of leptotene) and (ii) deficient pairing initiation at zygotene in connection with morphologically anomalous, malfunctioning pairing initiation sites. Both of these phenomena could be responsible for the reduction of chiasma frequency by colchicine previously reported in the literature.     

In vivo analysis of synaptonemal complex formation during yeast meiosis

During meiotic prophase a synaptonemal complex (SC) forms between each pair of homologous chromosomes and is believed to be involved in regulating recombination. Studies on SCs usually destroy nuclear architecture, making it impossible to examine the relationship of these structures to the rest of the nucleus. In Saccharomyces cerevisiae the meiosis-specific Zip1 protein is found throughout the entire length of each SC. To analyze the formation and structure of SCs in living cells, a functional ZIP1::GFP fusion was constructed and introduced into yeast. The ZIP1::GFP fusion produced fluorescent SCs and rescued the spore lethality phenotype of zip1 mutants. Optical sectioning and fluorescence deconvolution light microscopy revealed that, at zygotene, SC assembly was initiated at foci that appeared uniformly distributed throughout the nuclear volume. At early pachytene, the full-length SCs were more likely to be localized to the nuclear periphery while at later stages the SCs appeared to redistribute throughout the nuclear volume. These results suggest that SCs undergo dramatic rearrangements during meiotic prophase and that pachytene can be divided into two morphologically distinct substages: pachytene A, when SCs are perinuclear, and pachytene B, when SCs are uniformly distributed throughout the nucleus. ZIP1::GFP also facilitated the enrichment of fluorescent SC and the identification of meiosis-specific proteins by MALDI-TOF mass spectroscopy.     

An electron microscopic study of synaptonemal complex formation at zygotene in rye

A spreading technique was used to allow ultrastructural analysis of seventeen zygotene nuclei of rye (Secale cereale). Twenty pachytene nuclei were also examined. Lateral element lengths of the haploid complements decreased from 742 m at the beginning of zygotene to 451 m at the end of zygotene. Variation in pachytene synaptonemal complex lengths was also noted. Zygotene synaptonemal complex formation in rye is characterised by: (1) existence of a bouquet, with telomeric pairing initiation earliest; (2) multiple sites of initiation in each bivalent (maximum of 76 synaptonemal complex segments seen in one nucleus); (3) the potential number of pairing initiation sites may be higher (the average spacing of 4.42 m would allow approximately 160 sites per nucleus); (4) new pairing initiations occur almost until the end of zygotene; (5) initiation of new synaptonemal complexes and extension of existing synaptonemal complexes occur simultaneously. A simple zipping up of a few initiation sites is not the case in rye. Pairing in different bivalents of a nucleus is not completely synchronised, and the NOR in particular is often late to pair. Interlocking of lateral elements and synaptonemal complexes may lead to delayed completion of pairing in portions of bivalents, but interlocks are ultimately resolved. This resolution may involve breakage and rejoining of lateral elements.     

Development of the synaptonemal complex and polycomplex formation in three species of grasshoppers

This paper describes the development of the synaptonemal complex in three species of grasshopper: Chorthippus bicolor, Oedipoda coerulescens and Paracinema bicolor. In all three cases the development seems similar. A typical synaptonemal complex is observed during pachytene. Diplotene bivalents show a low density material associated with the chromatin and during first metaphase the beginnings of polycomplex formations are seen. Well organized polycomplexes can be recognized from first anaphase to early spermatids. The elements of the polycomplexes, as well as elements of the synaptonemal complex, show themselves to be positive after preferential staining for ribonucleoproteins. Polycomplexes observed after spreading and positive staining present similar characteristics to those observed after sectioning.This paper is dedicated to the memory of W. Bernhard for his contribution to the knowledge of the cell nucleus     

Temporal comparison of recombination and synaptonemal complex formation during meiosis in S. cerevisiae

In synchronous cultures of S. cerevisiae undergoing meiosis, an early event in the meiotic recombination pathway, site-specific double strand breaks (DSBs), occurs early in prophase, in some instances well before tripartite synaptonemal complex (SC) begins to form. This observation, together with previous results, supports the view that events involving DSBs are required for SC formation. We discuss the possibility that the mitotic pathway for recombinational repair of DSBs served as the primordial mechanism for connecting homologous chromosomes during the evolution of meiosis. DSBs disappear during the period when tripartite SC structure is forming and elongating (zygotene); presumably, they are converted to another type of recombination intermediate. Neither DSBs nor mature recombinant molecules are present when SCs are full length (pachytene). Mature reciprocally recombinant molecules arise at the end of or just after pachytene. We suggest that the SC might coordinate recombinant maturation with other events of meiosis.     

Germ cell differentiation and synaptonemal complex formation are disrupted in CPEB knockout mice.

CPEB is a sequence-specific RNA binding protein that regulates translation during vertebrate oocyte maturation. Adult female CPEB knockout mice contained vestigial ovaries that were devoid of oocytes; ovaries from mid-gestation embryos contained oocytes that were arrested at the pachytene stage. Male CPEB null mice also contained germ cells arrested at pachytene. The germ cells from the knockout mice harbored fragmented chromatin, suggesting a possible defect in homologous chromosome adhesion or synapsis. Two CPE-containing synaptonemal complex protein mRNAs, which interact with CPEB in vitro and in vivo, contained shortened poly(A) tails and mostly failed to sediment with polysomes in the null mice. Synaptonemal complexes were not detected in these animals. CPEB therefore controls germ cell differentiation by regulating the formation of the synaptonemal complex.     

Wolbachia infection in the Loxoblemmus complex (Orthoptera: Gryllidae) in Korea

The Wolbachia bacterium is one of the most prevalent intracellular symbionts of invertebrates, particularly insects. This bacterium induces four distinct reproductive anomalies such as cytoplasmic incompatibility, feminization, male killing, and parthenogenesis of its hosts. Here we report that three closely related cricket species, Loxoblemmus doenitzi, L. campestris, and L. equestris can become infected with Wolbachia. Based on the 16s rRNA sequences, all three species were single infections. However, Wolbachia infecting L. campestris showed diverse Wolbachia surface protein gene sequences resembling multiple infections. In addition, all Wolbachia strains in the three host species harbored the Wolbachia specific bacteriophage.     

Hypotheses about speciation by chromosomal rearrangements in the Steropleurus martorelli complex (Tettigoniodea,Orthoptera)

Four chromosomal races ranging from 2n♂ = 29 to 2n♂ = 23 have been found in the taxonomic species S. martorelli. It is assumed that this species is not a natural entity but a species complex. Cytogenetic analysis suggests that continental race I and coastal races II, III and IV have evolved in different ways from a common ancestor called protomartorelli. The coastal races have followed a chromosome-number reduction process, arising from race II by the occurrence of two successive chromosomal fusions. The model of speciation of these coastal forms is based on chromosomal rearrangements, similar to the stasipatric model described by White, although no zones of hybridization have been found in nature.     

Morphology of the axial structures in the neo-XY sex bivalent of Pycnogaster cucullata (Orthoptera) by silver impregnation

A simple method, using silver impregnation after 2 x SSC pretreatment allowed us to demonstrate axial structures (cores) in the metaphase I bivalents of the neo-XY race of Pycnogaster cucullata under bright-field microscopy. Axial structures can also be shown in DNA-depleted metaphase I bivalents, suggesting that DNA is not essential to demonstrate these elements. Specifically differentiated regions, which coincide with kinetochores, secondary constrictions (including the NOR), and chiasmata were also found. These regions have a characteristic morphology and therefore may be utilized for cytogenetic analysis. The simultaneous visualization of all these regions allowed us to establish their spatial relationships and hence the basic structural organization of the neo-XY sex bivalent in this species.