Chromosome ends and the nuclear envelope at premeiotic interphase in the male grasshopper Brachystola magna by 3-D,E.M. reconstruction

During premeiotic interphase in the male grasshopper Brachystola magna the nucleus is divided into two nuclear envelope bound compartments, one containing the X chromosome and one the autosomes. — The autosomal compartment is characterized by an invaginated nuclear envelope with nuclear pores distributed throughout the envelope. In a polarized region of the cell the pericentric heterochromatic chromocenters are associated with the inner membrane of the envelope invaginations. In this species the chromosomes are telocentric (acrocentric?) and the pericentric heterochromatin marks the proximal chromosome ends. It is concluded that the chromosome ends are attached to the nuclear envelope at premeiotic interphase. — Comparisons are made between the present observations on chromosome arrangements and the nuclear envelope at premeiotic interphase to earlier observations at early meiotic prophase in the same species (Church, 1976). It is concluded that a rearrangement of both the proximal chromosome ends and the nuclear envelope occurs as cells enter meiotic prophase.     

Centromere and telomere redistribution precedes homologue pairing and terminal synapsis initiation during prophase I of cattle spermatogenesis

Alterations in nuclear topology associated with meiotic chromosome pairing were studied in premeiotic cells and spermatocytes I of adult bovine males. To this end, we performed FISH with chromosome, pericentromeric satellite-DNA and telomere-specific probes in combination with immunostaining of synaptonemal complex proteins (SCP3, SCP1) on testis tissue sections. Nuclei of premeiotic cells (spermatogonia) exhibited a scattered telomere distribution while pericentromeres formed a few intranuclear clusters. We observed that the chromosome pairing process in cattle prophase I is preceded by repositioning of centromeres and telomeres to the nuclear periphery during preleptotene. Clustering of chromosome ends (bouquet formation) was observed during the transition from leptonema to zygonema and coincided with pairing of a sub-centromeric marker of bovine chromosomes 7. Dissolution of bouquet topology during zygonema left perinuclear telomeres scattered over the nuclear periphery at pachynema. SCP3 staining in frozen tissue sections revealed the appearance of this axial element protein in intranuclear aggregates during preleptotene, followed by extensive axial element formation during leptotene. Synapsis as revealed by SCP1 staining initiated peripherally at earliest zygotene, at this stage nuclei still contained numerous SCP3 clusters. Our observations reveal prominent non-homologous satellite-DNA associations in spermatogonia and indicate the conservation of topological features of the meiotic chromosome pairing process among mammals. The comparison of telomere dynamics in mouse and cattle prophase I suggests that a larger number of chromosomes prolongs the duration of the bouquet stage.     

Chromosomes and DNA of Mus

Using C-banded preparations of Mus dunni it is possible to study the behavior of constitutive heterochromatin in early stages of meiotic prophase. The X and the Y chromosomes, both of which contain a large amount of heterochromatin, lie apart in leptotene but move toward each other during zygotene. They then form the sex vesicle at late zygotene. In autosomes zygotene pairing appears to start from the telomeric ends. The centromere of the Y chromosome associates end-to-end with the terminal end of the long arm of the X chromosome. The autosomal heterochromatic short arms show forked morphology in certain bivalents at pachytene, suggesting probable incomplete synapsis.     

The effect of the wheat Ph1 locus on chromatin organisation and meiotic chromosome pairing analysed by genome painting

The Ph1 locus in wheat influences homo(eo)logous chromosome pairing. We have analysed its effect on the behaviour and morphology of two 5RL rye telosomes in a wheat background, by genomic in situ hybridisation (GISH), using rye genomic DNA as a probe. Our main objective was to study the effect of different alleles of the Ph1 locus on the morphology and behaviour of the rye telosomes in interphase nuclei of tapetal cells and in pollen mother cells at early stages of meiosis. The telosomes, easily detectable at all stages, showed a brightly fluorescing chromomere in the distal region and a constriction in the proximal part. These diagnostic markers enabled us to define the centromere and telomere regions of the rye telosomes. In the presence of functional copies of Ph1, the rye telosomes associated at pre-leptotene, disjoined and reorganised their shape at leptotene, and became fully homologously paired at zygotene – pachytene. In plants without functional alleles (ph1bph1b), the rye telosomes displayed an aberrant morphology, their premeiotic associations were clearly disturbed and their pairing during zygotene and pachytene was reduced and irregular. The Ph1 locus also influenced the behaviour of rye telosomes in the interphase nuclei of tapetal cells: in Ph1Ph1 plants, the rye telosomes occupied distinct, parallel-oriented domains, whereas in tapetal nuclei of ph1bph1b plants they were intermingled with wheat chromosomes and showed a heavily distorted morphology. The results shed new light on the effect of Ph1, and suggest that this locus is involved in chromosome condensation and/or scaffold organisation. Our explanation might account for various apparently contradictory and pleiotropic effects of this locus on both premeiotic associations of homologues, the regulation of meiotic homo(eo)logous chromosome pairing and synapsis, the resolution of bivalent interlockings and centromere behaviour. Received: 27 April 1998; in revised form: 5 August 1998 / Accepted: 11 August 1998     

Evidence for the association of homologous chromosomes during premeiotic stages inImpatiens andSalvia

Chromocenters in the last premeiotic interphase nuclei ofImpatiens balsamina (2n=14) andSalvia nemorosa (2n=14) were found to be associated in groups of two's, some times so closely that only the haploid number was seen. InImpatiens balsamina, the chromosomes during the mitotic prophase in sporogenous cells and the chromocenters in the root tip cells were observed to be homologously aligned. It is concluded on the basis of their morphology that the homologous chromocenters remain associated in pairs at least during the last premeiotic interphase and loosely aligned in other tissues of the plants studied.This work was supported by a German Academic Exchange Service (DAAD) Scholarship awarded toK. P. S. Chauhan, who is thankful to Prof. Dr.E. Knapp for kindly providing the necessary facilities to carry out this work at his Institut.     

Organisation of complex nuclear domains in somatic mouse cells.

The number and associations of heterochromatin chromocenters, nucleoli, centromeres and telomeres were studied in the nucleus of different somatic cells of Mus domesticus. Fibroblasts of the cell line 3T3, kidney cells (primary culture), and bone marrow cells were used. The above mentioned nuclear and chromosome markers were identified by DAPI/actinomycin D, indirect immunofluorescence with anti-centromere antibodies, silver impregnation for nucleolar proteins and fluorescence in situ hybridisation (FISH) with telomeric probes. The quantitative analysis of the nuclei showed that the pericentromeric heterochromatin is organised in about 18 chromocenters per nucleus in the 3T3 cells, and about seven in kidney and bone marrow cells, having generally a peripheral distribution in the nucleus of all the studied cells. Several aggregated centromeres were participating in each of the chromocenters, about four centromeres per 3T3 cell and about six centromeres per kidney and bone marrow cells. Some of the chromocenters were also in close association with nucleoli. The number of telomeric labels per nucleus was as expected for each chromosome set (2n = 68-70 and 2n = 40). About half of the telomeric signals were loosely aggregated within the heterochromatic blocks while the rest were distributed in the nucleus as unrelated units not bound with chromocenters. The three cell types have complex nuclear territories formed by different chromosomal domains: the pericentromeric heterochromatin, centromeres, proximal telomeres and nucleoli. With the exception of some bone marrow cells, we have not found a nuclear polarisation of the analysed chromosomal markers compatible with the Rabl configuration. However, Rabl anaphasic polarisation allows the contact of centromeric regions making possible that centromeric associations arise. If in addition, associative elements such as constitutive heterochromatin or nucleoli are close to the centromeric regions, like in Mus domesticus chromosomes, then the associations might be consolidated and persist until the interphase. These associations may be the origin of the nuclear domains described here for Mus domesticus somatic cells.     

Localization of chromosomal DNA sequences homologous to ribosomal gene type I insertion DNA in Drosophila melanogaster

Summary Chromosomal sites which have DNA homology to the 1 kb (kilobase pair) BamHI restrictable fragment of the 5 kb type I insertion present in many ribosomal genes in Drosophila melanogaster, were identified by using in situ hybridization and autoradiography. XX and XY complements of polytene chromosomes showed the nucleolus and chromocenter to be heavily labeled. Of the light label over euchromatic regions, the 102C band of chromosome 4 labeled particularly intensely. In mitotic XX and XY complements, the NORs (nucleolus organizer regions) of both sex chromosomes labeled as did the centromeric heterochromatin of autosomes. Label also appeared less frequently over telomeric and euchromatic regions.     

Three stages of meiotic homologous chromosome pairing in wheat: cognition, alignment and synapsis

 Chromosome painting enabled the study of homologous chromosome behaviour prior to and during meiosis. Total genomic DNA from rye, used as a probe for in situ hybridization, identified the rye chromosome arm in a wheat-rye translocation line (T5AS·5RL) at meiotic prophase and the preceding interphase. Accurate staging of the development of the meiocytes was attained by parallel studies of chromatin morphology, nucleolar behaviour and synaptonemal complex formation in electron microscopy thin sections and silver-stained surface spreads. Three stages of pairing were identified for the large cereal genomes that are organized in a Rabl configuration: first, cognition occurs during the long interphase before leptotene, bringing the homologous chromosome domains into close proximity and possibly starting at the centromere; second, homologous chromosome segments align at late leptotene; and third, zygotene synapsis initiates near the telomere, although it was also observed to occur near the centromere. A pairing model is proposed for wheat, with a genome size of 17000 Mbp, that shows prallels to and notable differences from yeast and mammalian models of meiosis. Received: 25 January 1997 / Revision accepted: 14 July 1997     

The fine structure of meiotic chromosome polarization and pairing in Locusta migratoria spermatocytes

At the leptotene stage of meiotic prophase in Locusta spermatocytes (2n=22 telocentric autosomes + X-chromosome), each chromosome forms an axial core. The 44 ends of the autosomal cores are all attached to the nuclear membrane in a small region opposite the two pairs of centrioles of the juxtanuclear mitochondrial mass. At later stages of meiotic prophase, the cores of homologous chromosomes synapse into synaptinemal complexes. Synapsis is initiated near the nuclear membrane, in the centromeric and the non-centromeric ends of the chromosomes. Homologous cores have their attachment points close together and some cores are co-aligned prior to synapsis. At subsequent stages of zygotene, the number of synaptinemal complexes at the membrane increases, while the number of unpaired axial cores diminishes. At pachytene, all 11 bivalents are attached to the membrane at both ends, so that there are 22 synaptinemal complexes at the membrane near the centrioles. Because each bivalent makes a complete loop, the configuration of the classic Bouquet stage is produced. The X-chromosome has a poorly defined single core at pachytene which also attaches to the nuclear membrane. These observations are based on consecutive serial sections (50 to 100) through the centriolar zone of the spermatocytes. Labeling experiments demonstrated that tritiated thymidine was incorporated in the chromatin of young spermatocytes prior to the formation of the axial cores at leptotene. It is concluded that premeiotic DNA synthesis is completed well in advance of pairing of homologous chromosomes, as marked by the formation of synaptinemal complexes.     

Centromere Pairing in Early Meiotic Prophase Requires Active Centromeres and Precedes Installation of the Synaptonemal Complex in Maize

Pairing of homologous chromosomes in meiosis is critical for their segregation to daughter cells. In most eukaryotes, clustering of telomeres precedes and facilitates chromosome pairing. In several species, centromeres also form pairwise associations, known as coupling, before the onset of pairing. We found that, in maize (Zea mays), centromere association begins at the leptotene stage and occurs earlier than the formation of the telomere bouquet. We established that centromere pairing requires centromere activity and the sole presence of centromeric repeats is not sufficient for pairing. In several species, homologs of the ZIP1 protein, which forms the central element of the synaptonemal complex in budding yeast (Saccharomyces cerevisiae), play essential roles in centromere coupling. However, we found that the maize ZIP1 homolog ZYP1 installs in the centromeric regions of chromosomes after centromeres form associations. Instead, we found that maize STRUCTURAL MAINTENANCE OF CHROMOSOMES6 homolog forms a central element of the synaptonemal complex, which is required for centromere associations. These data shed light on the poorly understood mechanism of centromere interactions and suggest that this mechanism may vary somewhat in different species.     

Premeiotic and early meiotic stages in the pollen mother cells of Eremurus and in human embryonic oocytes

Summary The premeiotic and meiotic stages are described in the pollen mother cells of the liliaceous plant Eremurus. In human oocytes from embryonic ovaries, the premeiotic and early meiotic stages up to dictyotene have been identified on morphological grounds. In Eremurus, in which each stage can be independently verified by the sequence of buds situated in a spiral, there is no indication of somatic pairing of homologous chromosomes, nor is there any sign of a premeiotic contraction of the chromosomes. The interphase following this mitosis is, in turn, succeeded by leptotene in which the DNA synthesis occurs, as determined by using autoradiography. This stage is followed by a distance pairing stage in which the homologous chromosomes lie parallel to each other at a distance. In typical zygotene, segments of chromosomes are paired intimately, others are unpaired, and the points at which pairing begins and ends are clearly visible. Each bivalent shows several pairing blocks. Pairing is completed at pachytene; diplotene which is characterized by the separation of the chromosomes follows. In middiplotene the chromosomes collect together in the so-called second contraction stage.The same meiotic prophase stages that occur in Eremurus, including the distance pairing stage, are found in the embryonic human oocytes. In the last premeiotic interphase, the chromosomes appear as condensed prochromosomes which unravel directly to form the leptotene chromosomes. In the oocytes, too, DNA synthesis seems to take place in leptotene.This is publication No. 1947 from the Genetics Laboratory, University of Wisconsin. It has been aided by the following grants from the National Institutes of Health (Washington): GM 15 422 and HD 03 084-03. For excellent photographic help we are grateful to Mr. Walter Kugler, Jr.     

Delayed termination of nuclear histone doubling after premeiotic DNA synthesis inTriturus vulgaris male meiosis

It has been shown by means of double wavelength cytophotometry of DNA (Feulgen reaction) and histone (fast green, pH 8.2) inTriturus vulgaris spermatocytes that the doubling of DNA content in nuclei terminates at the end of preleptotene to beginning of leptotene whereas the doubling of histone content begun at premeiotic interphase is delayed and proceeds till the end of leptotene to beginning of zygotene. As a result preleptotene spermatocytes contain approximately 4C DNA and only 3C histone. Histone content in leptotene amounts to 93% of 4C, and in zygotene, pachytene and metaphase I both DNA and histone contents equal 4C. Thus, the temporal pattern of nucleo-histone doubling in meiotic chromosomes ofT. vulgaris differs from the synchronous DNA and histone doubling in mitotic chromosomes of all previously studied species. The delay of histone doubling inT. vulgaris meiocytes is less pronounced than in the previously studied insectsAcheta domestica andPyrrhocoris apterus where the histone content amounts to 3C in leptotene—zygotene and the equal histone/DNA ratio is restored only in pachytene.—Responsibilities for this phenomenon and its biolgoical sinnificance are discussed in connection with recent hypotheses concerning mechanisms of homologous chromosome pairing.     

OBSERVATIONS ON SYNAPTONEMAL COMPLEXES IN PREMEIOTIC INTERPHASE OF WHEAT

Synaptonemal complexes (SCs) were found in stage 3, premeiotic (S phase) pollen mother cell (PMC) nuclei of wheat which were labeled with ³H-thymidine. Three nucleoli are present in PMC nuclei at the beginning of stage 3, premeiotic interphase (S3). During S3, nucleoli move toward the nuclear envelope and fuse to form one nucleolus near the end of the stage. PMC nuclei labeled with ³H-thymidine were serially sectioned to show that more than one nucleolus was present and that SCs were also present in these DNA synthetic nuclei. Entire S3 PMC nuclei were serially sectioned to show the presence of SCs and all three nucleoli. Entire leptotene nuclei were also serially sectioned and segments of SCs were found. It is concluded that the association of homologous chromosomes in S3 of wheat is an early step in SC formation which proceeds through leptotene and is completed in zygotene and pachytene. Thus there is evidence that the continuum of chromosome pairing in wheat starts much earlier than was once thought.     

Three-dimensional organization of interphase fibroblast nuclei in two closely related shrew species (<Emphasis Type="Italic">Sorex granarius</Emphasis> and <Emphasis Type="Italic">Sorex araneus</Emphasis>) differing in the structures of their chromosome termini

A FISH with a probe for telomeric and rDNA repeats and immunofluorescence with ANA CREST and antibodies to nucleolae protein B23 were used to study the three-dimensional (3D) organization of fibroblast interphase nuclei in two shrew twin species, Sorex granarius and Sorex araneus, of the Cordon race. Karyotypes of these species are composed of nearly identical chromosomal arms and differ in the number of their metacentrics and the structures of their terminal chromosome regions. In the short arms of S. granarius, 32 of the acrocentrics have large telomeres that contain an average of 218 kb telomere repeats, which alternate with ribosomal repeats. These regions also contain active nucleolar organizing regions (NORs). In contrast, in active NORs in S. araneus are localized at the terminal regions of 8 chromosomal arms (Zhdanova et al., 2005; 2007b). Here, we show that associations of chromosomes by telomeres and the contact of a part of the telomere clusters with the inner nuclear membrane and nucleolus characterize the interphase nuclei of both Sorex granarius and Sorex araneus. We also reveal the partial colocalization of telomere and ribosomal clusters and the spatial proximity of centomeric and telomeric regions in the interphase nuclei of S. granarius. It appears that only ribosomal clusters containing a sufficient number of active ribosomal genes exhibit a connection with the nucleolus. Nucleolus disassembly during the fibroblastís transition to mitosis and the role of the B23 protein in this process have been studied.     

Fluorescent staining of L cell centromeres and chromocenters with 1-dimethylaminonaphthalene-5-sulfonyl chloride and G-bandings

Fluorescent staining patterns of L cell chromosomes with 1-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl chloride) were studied. Ordinary air-dried L cell metaphase chromosomes exhibited relatively uniform and bright yellowish green fluorescence by dansyl-staining under the fluorescence microscope. However, after the chromosome preparations were treated with 10 mM NaCl for 24 h at 4 °C, which produced distinctive G-bands with Giemsa-staining, the centromeric regions and several interstitial regions of some particular chromosomes were clearly fluorescent but other regions showed only dull fluorescence. After the treatment of chromosome slides with cupric sulfite reagent, which converts sulfhydryls and disulfides to thiosulfates chromosomes showed clear G-bands which were indistinguishable from those after 10 mM NaCl treatment. By dansyl-staining, however, the cupric sulfite-treated chromosomes exhibited very faint fluorescence on their contour alone, and neither centromeric regions nor some interstitial regions of marker chromosomes had distinctly bright fluorescence.Although Giemsa-staining disclosed dark chromocenters in approx. 75% of interphase nuclei irrespective of pretreatments, dansyl-staining revealed bright chromocenters in approx. 60% of interphase nuclei in control slides, in about 40% of nuclei in 10 mM NaCl-treated slides, and in only about 30% of nuclei in cupric sulfite-treated preparations.These observations indicated that in the air-dried chromosome preparations, the distribution of protein over the metaphase chromosome is relatively uniform along its length, and that G-bands in the chromosome and Giemsa-staining of chromocenters in interphase nuclei are not significantly affected by apparent loss of protein from the preparations. It was also suggested that particular protein may be associated with the centromeric regions of L cell chromosomes. Some technical details of dansyl fluorochroming and the significance of the observations were discussed.     

Homolog pairing and sister chromatid cohesion in heterochromatin in <Emphasis Type="Italic">Drosophila</Emphasis> male meiosis I

Drosophila males undergo meiosis without recombination or chiasmata but homologous chromosomes pair and disjoin regularly. The X–Y pair utilizes a specific repeated sequence within the heterochromatic ribosomal DNA blocks as a pairing site. No pairing sites have yet been identified for the autosomes. To search for such sites, we utilized probes targeting specific heterochromatic regions to assay heterochromatin pairing sequences and behavior in meiosis by fluorescence in situ hybridization (FISH). We found that the small fourth chromosome pairs at heterochromatic region 61 and associates with the X chromosome throughout prophase I. Homolog pairing of the fourth chromosome is disrupted when the homolog conjunction complex is perturbed by mutations in SNM or MNM. On the other hand, six tested heterochromatic regions of the major autosomes proved to be largely unpaired after early prophase I, suggesting that stable homolog pairing sites do not exist in heterochromatin of the major autosomes. Furthermore, FISH analysis revealed two distinct patterns of sister chromatid cohesion in heterochromatin: regions with stable cohesion and regions lacking cohesion. This suggests that meiotic sister chromatid cohesion is incomplete within heterochromatin and may occur at specific preferential sites.     

3D organization of interphase fibroblast nuclei in two closely related shrew species, Sorex granarius and S. araneus, differed by the structure of chromosome termini

To study 3D organization of fibroblast interphase nuclei in two sibling shrew species, Sorex araneus from Cordon race and S. granarius, FISH with probe to telomeric and rDNA repeats, and immunofluorescence with ANA CREST and antibodies to nucleolus protein B23 were used. Karyotypes of studied species are composed of near identical chromosomal arms and differ by the number of metacentrics and the structure of terminal chromosome regions. The large telomeres containing on the average 218 kbp of telomere repeats characterize the short arms in all of 32 S. granarius acrocentrics. Telomere repeats in them alternate with nbosomal repeats. These regions also contain active NORs. In contrast, active NORs in S. araneus are localized at the terminal regions of 8 chromosomal arms (Zhdanova et al., 2005, 2007b). We have shown that telomere associations of chromosomes and contacts of a part of telomere clusters with inner nuclear membrane and nucleolus characterize interphase nuclei of both S. granarius and S. araneus. Moreover, the partial colocalization of telomere and ribosomal clusters, and spatial nearness of centomeric and telomeric regions were revealed in the interphase nuclei of S. granarius. Evidently, only those ribosomal clusters that contain a number of active ribosomal genes display connection with nucleolus. The stripping of nucleolus materials during transition of fibroblasts to mitosis and the role of B23 protein in this process has been studied.     

Acridine-orange differential fluorescence of fast- and slow-reassociating chromosomal DNA after in situ DNA denaturation and reassociation

A cytological technique based on heat denaturation of in situ chromosomal DNA followed by differential reassociation and staining with acridine orange was developed. Mouse nuclei and chromosomes in fixed cytological preparations show a red-orange fluorescence after thermal DNA denaturation (2–4 minutes at 100° C), and fluoresce green if denaturation is followed by a total DNA reassociation (two minutes or more at 65–66°C). — A reassociation time between a few and 60–90 seconds demonstrates the centromeric heterochromatin of chromosomes (which sometimes aggregate in the form of clusters) and the interphase chromocenters in green, the chromosomal arms fluorescing red-orange. Under the same conditions, the Y chromosome presents a pale green or yellow-green fluorescence along its chromatids, but its centromeric region fluoresces weakly. — The interpretation is suggested that the fast-reassociating chromosomal DNA (as detected by AO in centromeric heterochromatin and interphase chromocenters), represents repetitive DNA.     

Preleptotene chromosome contraction in Lilium longiflorum “Croft”

A period of chromosome contraction between premeiotic interphase and leptotene was regularly observed in three samples of Lilium longiflorum Croft. Extensive preleptotene chromosome contraction was also observed in L. longiflorum Ace and in the Lilium hybrid Enchantment. Although the stage resembles late mitotic prophase, microsporocytes never develop to metaphase, but despiralize to leptotene, and regular alignment, pairing and chiasma formation follow. As preleptotene chromosome contraction is discovered in an increasing number of organisms it becomes less likely that it represents a true reversion to mitosis. However its absence in many organisms and its extreme variability in others do not support the concept of preleptotene chromosome contraction as a regular meiotic stage. It is suggested that the line of demarcation between mitosis and meiosis is often imprecise, and meiocytes may fluctuate to some extent between these states before a final transition to meiosis is made. The occurrence and extent of this fluctuation may possibly be related to some externally produced substances required for the orderly development of meiotic prophase.     

Intranuclear fibrillar material in cereal pollen mother cells

Ultrastructural studies of cereal anthers found intranuclear bundles of microfilaments in pollen mother cells (PMCs) but not elsewhere. The ultrastructure, distribution, and behaviour of this fibrillar material (FM) are described. FM was seen in all 19 genotypes studied comprising Aegilops, Triticum, Secale, Hordeum and Avena species, which together included haploid, diploid and allo-and autopolyploid, and natural and synthetic polyploid examples. Detailed studies in diploid S. cereale, and hexaploid T. aestivum and Triticale showed that FM was present in PMC nuclei during premeiotic interphase, leptotene and zygotene but not at pachytene and later meiotic stages. Moreover, it was most abundant at late premeiotic interphase in T. aestivum, and at leptotene in S. cereale and Triticale, when it occurred in up to 100% of sampled PMC nuclei in an anther. Although FM and synaptonemal complex (SC) occurred together in some PMC nuclei at later stages, FM was present long before SC, and reached its peak of abundance before SC did. Bundles of FM often formed links at their ends between either two masses of chromatin, or more rarely, between chromatin and the nuclear membrane. Individual bundles of FM varied in length but showed roughly similar ranges of lengths and widths in these three species. They were up to about 0.2 m in diameter and about 3 m in length, equivalent to about 20% of the maximum diameter of the nuclei containing them. Reconstructions of PMC nuclei indicated that FM was never associated with centromeres but was sometimes, and perhaps usually, associated with telomeric or sub-telomeric chromosome segments.The function of FM is unknown but its possible role is discussed in relation to (1) previously described intranuclear inclusions in meiocytes and (2) the cytogenetics and developmental behaviour of meiotic nuclei in the wheat comparium. As FM was a constant and characteristic structural component of PMC nuclei, its presence is probably of functional significance to the meiotic process. If so, it may function before, and over greater distances, than SC in establishing or maintaining the coorientation of chromosomes prerequisite for normal chromosome pairing. As FM was most abundant at stages when major chromosome movements occur, yet its distribution was non-centromeric, it is suggested that it may function in the attachment and movement of telomeres at the nuclear membrane formed after premeiotic mitosis. The possibility that a bundle of FM normally links corresponding sites on two homologues is considered.