Pairing Initiation Sites Research Articles

A Role for SUMO in Meiotic Chromosome Synapsis

During meiotic prophase, homologous chromosomes engage in a complex series of interactions that ensure their proper segregation at meiosis I. A central player in these interactions is the synaptonemal complex (SC), a proteinaceous structure elaborated along the lengths of paired homologs. In mutants that fail to make SC, crossing over is decreased, and chromosomes frequently fail to recombine; consequently, many meiotic products are inviable because of aneuploidy. Here, we have investigated the role of the small ubiquitin-like protein modifier (SUMO) in SC formation during meiosis in budding yeast. We show that SUMO localizes specifically to synapsed regions of meiotic chromosomes and that this localization depends on Zip1, a major building block of the SC. A non-null allele of the UBC9 gene, which encodes the SUMO-conjugating enzyme, impairs Zip1 polymerization along chromosomes. The Ubc9 protein localizes to meiotic chromosomes, coincident with SUMO staining. In the zip1 mutant, SUMO localizes to discrete foci on chromosomes. These foci coincide with axial associations, where proteins involved in synapsis initiation are located. Our data suggest a model in which SUMO modification of chromosomal proteins promotes polymerization of Zip1 along chromosomes. The ubc9 mutant phenotype provides the first evidence for a cause-and-effect relationship between sumoylation and synapsis.

Meiosis in primary trisomics of rye: considerations for models of chromosome pairing.

Meiotic chromosome pairing of primary trisomics of rye was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light-microscopic observations of metaphase I cells. Despite the large-sized chromosomes of rye, prophase I trivalent frequencies were close to the two thirds expected on a simple model with two terminal independent pairing initiation sites per trisome (set of three homologous chromosomes). Direct observations mostly reveal one pairing partner switch (PPS) in prophase I trivalents, which confirms this supposition. There were no significant differences between the number of trivalent and bivalent plus univalent configurations observed at prophase and metaphase I; therefore, synapsed segments form chiasmata. In all of the trisomics, the three homologues showed variations not only in the number of telomeric C-bands but also in the amount of heterochromatin of these bands, which allowed identification of chromosomes or chromosome arms associated in most metaphase I configurations. In trisomics for chromosomes 2, 3 and 5, some metaphase I chromosome configuration frequencies did not fit those expected under the assumption of random chromosome association among all partners, suggesting the existence of preferences for pairing between two given chromosome arms of the trisome. No preferential associations either at metaphase I or pachytene were observed in the trisomics for chromosome 6. The fit between theoretical pairing models and the experimental data is also discussed.

The synaptonemal complex--the chaperone of crossing over.

During meiosis homologous chromosomes pair and exchange homologous chromosome segments. The synaptonemal complex (SC) forms between paired chromosomes. The role of the SC in the process of reciprocal exchange of flanking markers is a matter of debate. I propose a dual pathway for reciprocal exchange of flanking markers (REFM). In the first, SC-independent, path, two 'half-nodules' and an independent REFM protein combine to form a functional recombination nodule (RN). The RN binds to paired chromosomes and accomplishes reciprocal exchange of flanking markers. In the other, SC-dependent, pathway 'half-nodules' occur at pairing initiation sites. 'Half-nodules' move along the SC as it forms. Assisted by an SC-bound REFM protein, 'half-nodules' combine to form functional RNs. I propose that different organisms rely to different extents on the two pathways, and hence rely to different extents on the SC.

Chiasma localization, heterochromatin and synaptonemal complexes in the grasshopper Pyrgomorpha conica.

Surface-spread synaptonemal complexes and chiasma distributions in spermatocytes with different C-banding patterns and chiasma distributions in oocytes were analyzed in the grasshopper Pyrgomorpha conica. Male meiosis was characterized by a proximal/distal chiasma localization and complete pairing of homologous chromosomes at pachytene. However, there were indications of a relationship between the frequency and location of pairing initiation sites and chiasma distribution. The presence of a proximal supernumerary segment in a medium-sized chromosome does not increase the mean cell chiasma frequency of carrier individuals compared with those lacking it but may modify chiasma distribution in at least some carrier bivalents. This effect could be related to heterosynapsis in the region near the segment. Mean cell chiasma frequency was significantly lower in females than in males. Females also showed altered chiasma distributions compared with males, with fewer proximal chiasmata and more interstitial and distal chiasmata.

Meiosis in haploid rye: extensive synapsis and low chiasma frequency

Extensive synaptonemal complex formation was found at prophase I in whole mount spread preparations of a spontaneous haploid rye, Secale cereale, with values of up to 87.8 per cent of the chromosome complement synapsed. Pairing-partner switches were frequent, giving rise to multiple associations in which all or most of the chromosomes were involved. However, the distribution of synaptonemal complex stretches suggests that synapsis does not occur at random. The frequency of multivalents and the mean frequency of bonded arms at metaphase I were 0.03 and 0.39, respectively. Associations between chromosome arms without heterochromatin were more frequent than between the remaining arms. The observation of recombinant chromosomes for telomeric C-bands at anaphase I indicates that metaphase I bonds are true chiasmata. The correspondence between the location of pairing initiation sites and chiasmata indicates that early synapsis could be confined to homologous regions.

Meiosis in allopolyploid Crepis capillaris. II. Autotetraploids

Meiotic chromosome pairing of autotetraploid Crepis capillaris was analysed by electron microscopy of surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I chromosome configurations. Prophase I quadrivalent frequencies are high in all three tetrasomes. (A, D, and C) and partially dependent on chromosome size. At metaphase I quadrivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for multivalent elimination during prophase I in this system, and the reduction in multivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high frequencies of prophase I quadrivalents far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per tetrasome, suggesting that multiple pairing initiation occurs. Direct observations reveal relatively high frequencies of pairing partner switches (PPSs) at prophase I, which confirms this suggestion. The numbers of PPSs per tetrasome show a good fit to the Poisson distribution, and their positional distribution along chromosomes is random and nonlocalized. These observations favour a model of pairing initiation based on a large number of evenly distributed autonomous pairing sites each with a uniform and low probability of generating a PPS.

Meiosis in autopolyploid Crepis capillaris

Meiotic chromosome pairing of triploid and trisomic Crepis capillaris was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I. This system allows identification and separate analysis of each chromosome of the C. capillaris genome. Prophase I trivalent frequencies are very high in all three trisomes and only slightly dependent on chromosome size. At metaphase I, on the other hand, trivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for trivalent elimination during prophase I in this system, and the reduction in trivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high prophase I trivalent frequencies far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per trisome, suggesting that multiple pairing initiation occurs. Direct observations reveal high frequencies of pairing partner switches (PPSs) in prophase I trisomes, which confirms this supposition. The numbers of PPSs per trisome shows a better fit to the Poisson than to the binomial distribution and their positional distribution along trisomes is random and non-localized. All these observations favour a model of pairing initiation in trisomes based on a large number of evenly distributed autonomous pairing sites each with a uniform and low probability of generating a PPS.

Metaphase I pairing of deficient chromosomes and genetic mapping of deficiency breakpoints in common wheat

Metaphase I pairing of deficient chromosomes was analyzed in a set of 'Chinese Spring' (CS) wheat (Triticum aestivum L. em. Thell.) plants with varying lengths of deficiencies in the long arm of chromosome 4A (6, 8, 11, 17, 23, 34, 36, 39, and 50% missing), the long arm of chromosome 5B (49% missing), and the long arm of chromosome 2B (33% missing). Pairing in homologous chromosomes between deficient and complete arms was greatly reduced even by small differences in arm length. In deficiency homozygotes and in an isochromosome derived from a deficient 4AL arm, pairing of the two deficient arms was high and approached that of two complete arms. In plants where deficient and complete arms competed for pairing partners, pairing was exclusively between arms of the same length. These results suggest that in wheat, pairing initiation sites are distributed throughout at least the distal halves of the arms and that the alignment of telomeres may be critical for pairing success. Genetic mapping of the deficiency breakpoints was confounded by misdivision of unpaired chromosomes and abnormal transmission rates. Genetic distances between centromeres and breakpoints appeared to be proportional to metaphase I pairing frequencies.Key words: bread wheat, deficiency, chromosome pairing competition, mapping, telomere, pairing initiation.

Synapsis and chiasma formation in a ditelo-substituted haploid of rye

Synapsis and chiasma formation were analyzed in a ditelo-substituted haploid of rye (Secale cereale), in which chromosome 1R was replaced by its telosomes. The study was made by comparing synaptonemal complex formation at early meiotic prophase I, chromosome associations at metaphase I, and recombinant chromosomes at anaphase I and prophase II. For the analysis of synaptonemal complexes, 41 nuclei at stages ranging from leptotene to early diplotene were selected. In the leptotene and early zygotene nuclei, numerous alignments of axial cores involving the same or different chromosomes were observed. Pairing initiation sites occurred at both interstitial and distal segments. Throughout zygotene the extent of pairing gradually increases, with values up to 84.8% at a stage that is morphologically comparable with late pachytene. Pairing-partner exchanges were frequently observed in zygotene nuclei, giving rise to multiple associations encompassing all or most of the chromosomes. In cells at metaphase I multivalents were very rare (2%), indicating elimination of most pairing-partner exchanges. In cells at metaphase I, anaphase I, and prophase II chromosome length, centromere position, and C-banding pattern enable the identification of the chromosomes 1RS, 1RL, 4R, 5R, and 6R, and the distinction of three metacentric chromosomes (RM1, RM2, and RM3). Metaphase I bonds were found to be nonrandomly distributed. Associations between the arms 4RL, 5RL, 6RL, and RM1L, all of them without telomeric C-bands, were more frequent than between the remaining arms. The bonds were mainly located at the distal parts of the chromosomes. The frequency of recombinant chromosomes at anaphase I or prophase II suggests that metaphase I bonds were true chiasmata.Key words: Secale cereale, haploid, synaptonemal complex, chiasmate bonds, C-banding.

The effect of heterochromatin on synapsis of the sex chromosomes of <i>Peromyscus </i>(Rodentia, Cricetidae)

The pairing behavior of the sex chromosomes in male and female individuals representing seven species of Peromyscus was analyzed by electron microscopy of silver-stained zygotene and pachytene configurations. Six species possess submetacentric or metacentric X chromosomes with heterochromatic short arms. Sex-chromosome pairing in these species is initiated during early pachynema at an interstitial position on the X and Y axes. Homologous synapsis then progresses in a unidirectional fashion towards the telomeres of the X short arm and the corresponding arm of the heterochromatic Y chromosome. The distinctive pattern of synaptic initiation allowed a late-synapsing bivalent in fetal oocytes to be tentatively identified as that of the X chromosomes. In contrast to the other species, Peromyscus megalops possesses an acrocentric X chromosome and a very small Y chromosome. Sex-chromosome pairing in this species is initiated at the proximal telomeric region during late zygonema, and then proceeds interstitially towards the distal end of the Y chromosome. These observations suggest that the presence of X short-arm heterochromatin and corresponding Y heterochromatin interferes with late-zygotene alignment of the pairing initiation sites, thereby delaying XY synaptic initiation until early pachynema. The pairing initiation sites are conserved in the vicinity of the X and Y centromeres in Peromyscus, and consequently the addition of heterochromatin during sex-chromosome evolution essentially displaces these sites to an interstitial position.

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.

Synaptonemal complex spreading in Allium cepa and A. fistulosum

The general features and fine structure of homologous chromosome alignment and pairing have been investigated in two species of Allium (A. fistulosum and A. cepa), which have similar karyotypes but very different patterns of chiasma distribution. Although there is no support for the occurrence of a general pre-meiotic alignment of homologous chromosomes, both species show some alignment of homologues as an immediate prelude to synaptonemal complex (SC) formation. In both species pairing usually commences at sub-terminal sites and is succeeded by numerous separate intercalary initiations of pairing in interstitial and distal regions and then in proximal regions. The last parts to pair, in both species, are pericentromeric and telomeric regions. There is, therefore, no evident relationship between the sequence of pairing and chiasma distribution in these species. Regularly alternating convergences and divergences of aligned axial cores (ACs), termed multiple association sites, are frequently observed. It is proposed that these represent potential pairing initiation sites and from observations on their spatial distribution it is argued that they may be evenly distributed through most of the genome. Small spherical or ellipsoid nodules are found at association sites and between closely aligned ACs which persist in the SC segments present during zygotene, but most of them disappear abruptly at the end of zygotene. These are termed zygotene nodules (ZN) and it is proposed that they are involved in matching corresponding sites on homologous chromosomes as well as possibly having a recombinational role. Their composition, structure, mode of action and relationship to pachytene recombination nodules are at present unknown.

Synaptonemal complex spreading in Allium ursinum: pericentric asynapsis and axial thickenings.

In Allium ursinum meiotic pairing of homologues is always incomplete; a proximal region on each bivalent remains regularly unsynapsed even in late pachytene. The spatial correlation of the unsynapsed region with the kinetochore suggests that the kinetochore itself exerts an inhibitory effect on synapsis in its vicinity. This can be interpreted as the cytological basis of the 'centromere effect' on recombination in this species. Moreover, the high incidence of a pericentric inversion loop in a heterozygous chromosome pair shows that proximal pairing initiation is possible and that its failure cannot be responsible for pericentric asynapsis. The formation of the inversion loop is complicated by the need for two independent pairing initiation sites because synapsis cannot proceed across the pericentric region. It is proposed that the meiotic bouquet polarization helps in establishing the presynaptic alignment of the homologous sites within the inverted regions and hence to achieve a high rate of inversion loop formation. Thickenings of the axial/lateral elements are not distributed equally along the synaptonemal complex. They are underrepresented in unpaired axes but strikingly abundant at the borders with synapsed regions, suggesting their origin in the pairing forks during the process of synapsis. They are virtually always present at nucleolus-organizing regions and often they appear at corresponding sites on opposite lateral elements. Besides the thickenings several other kinds of axial deformities are present in unpaired axes.

Synaptonemal complex spreading in Allium. II. Tetraploid A. vineale

In tetraploid Allium vineale four homologous axes are closely aligned in unsynapsed regions at early zygotene. This alignment is brought about by intercalary and terminal associations. The intercalary association sites are possibly targets of forces for long distance attraction of homologues and potential pairing initiation sites. The terminal associations are mediated by dense spherules and result possibly from the attachment of the telomeres to the nuclear envelope. In pachytene the alignment is abolished and the four axes are synapsed two by two, resulting in bivalents or, owing to partner switches in the synapsed axes, in quadrivalents. From the number of partner switches per configuration the number of pairing initiation sites is estimated. Homologous alignment and synapsis are discussed, comparing them with the conditions in a triploid species, which were described in a previous paper.Key words: Allium vineale, synaptonemal complex, polyploids, chromosome pairing, meiosis.

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 μm at the beginning of zygotene to 451 μm 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 μm 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.

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, ellipsoidal 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.

Mechanisms of Chromosome Synapsis at Meiotic Prophase

Publisher Summary Chromosome distribution at mitosis and meiosis in most eukaryotes is mediated by kinetochores (centromeres) and by the microtubules and spindle pole bodies of the spindle apparatus. This chapter describes the mechanisms of chromosomes synapsis at meiotic prophase. It discusses two general observations. First, the problems of coalignment prior to synaptonemal complex formation, sequential pairing of given homologs, and the existence of constant pairing initiation sites can all be resolved through fine-structure analysis of complete nuclei in the proper stage of meiotic prophase. Second, there exists considerable variation in the characteristics of components of the synaptonemal complex and in the architecture of modified complexes. These may be variations on an ancestral theme, but the notion that mitosis, and subsequently meiosis, evolved independently in several major biological groups should be considered.