Myths and mechanisms of meiosis

Meiotic Crossing Over in Maize Knob Heterochromatin

There is ample evidence that crossing over is suppressed in heterochromatin associated with centromeres and nucleolus organizers (NORs). This characteristic has been attributed to all heterochromatin, but the generalization may not be justified. To investigate the relationship of crossing over to heterochromatin that is not associated with centromeres or NORs, we used a combination of fluorescence in situ hybridization of the maize 180-bp knob repeat to show the locations of knob heterochromatin and fluorescent immunolocalization of MLH1 protein and AFD1 protein to show the locations of MLH1 foci on maize synaptonemal complexes (SCs, pachytene chromosomes). MLH1 foci correspond to the location of recombination nodules (RNs) that mark sites of crossing over. We found that MLH1 foci occur at similar frequencies per unit length of SC in interstitial knobs and in the 1 µm segments of SC in euchromatin immediately to either side of interstitial knobs. These results indicate not only that crossing over occurs within knob heterochromatin, but also that crossing over is not suppressed in the context of SC length in maize knobs. However, because there is more DNA per unit length of SC in knobs compared to euchromatin, crossing over is suppressed (but not eliminated) in knobs in the context of DNA length compared to adjacent euchromatin.

Pairing and recombination features during meiosis in Cebus paraguayanus (Primates: Platyrrhini)

BACKGROUND

Among neotropical Primates, the Cai monkey Cebus paraguayanus (CPA) presents long, conserved chromosome syntenies with the human karyotype (HSA) as well as numerous C+ blocks in different chromosome pairs.In this study, immunofluorescence (IF) against two proteins of the Synaptonemal Complex (SC), namely REC8 and SYCP1, two recombination protein markers (RPA and MLH1), and one protein involved in the pachytene checkpoint machinery (BRCA1) was performed in CPA spermatocytes in order to analyze chromosome meiotic behavior in detail.

RESULTS

Although in the vast majority of pachytene cells all autosomes were paired and synapsed, in a small number of nuclei the heterochromatic C-positive terminal region of bivalent 11 remained unpaired. The analysis of 75 CPA cells at pachytene revealed a mean of 43.22 MLH1 foci per nucleus and 1.07 MLH1 foci in each CPA bivalent 11, always positioned in the region homologous to HSA chromosome 21.

CONCLUSION

Our results suggest that C blocks undergo delayed pairing and synapsis, although they do not interfere with the general progress of pairing and synapsis.

C-band fusion and behaviour of the involved chromosomes during meiotic prophase I of the male domestic pig

Behaviour of acrocentric chromosome synaptonemal complexes (SCs) and constitutive heterochromatin (C-band) during meiotic prophase I of the male domestic pig was studied using surface spreading and silver staining techniques. At late zygotene C-band regions of complete or incomplete SCs of the acrocentric chromosomes are represented by enlarged structures. At the same time these structures commenced fusion, which is accomplished at pachytene. Using kinetochores and NORs as markers, the pachytene metacentric, acrocentric, and nucleolar SCs can be distinguished. In 96.7% of the pachytene nuclei, either all or some of the acrocentric SCs are associated at their centromeric regions, forming one or two large globular or shapeless masses that represent chromocenters. The SCs are assorted at random in all types of association except in single association of all the SCs. Order of associated SCs varies from one association to another. The C-band region of SC, the portion inside the chromocenter core, is differentiated as thicker and darker structures from the chromocenter mass. A silver-positive core-like structure in meiotic chromosome C-bands, probably a vector for meiotic C-band ectopic pairing, provides faithful representations of C-band behaviour during meiosis.

Unusually high amount of inactive ribosomal DNA in the grasshopper Stauroderus scalaris

Fluorescence in-situ hybridization (FISH) was employed to determine the chromosomal location of the ribosomal DNA cistrons in spermatocytes of two populations of the grasshopper Stauroderus scalaris. The results showed that paracentromeric C-bands, which in this species constitute about 50% of the total chromatin, contain substantial amounts of rDNA in all chromosomes. However, silver impregnation showed the presence of a single active nucleolus organizing region (NOR) in chromosome 3 of primary spermatocytes, indicating an extremely high amount of silent rDNA across the whole genome of this species in the two geographically distant populations analysed. The significance of such an unusual phenomenon is discussed.

Effect of the pairing gene Ph1 and premeiotic colchicine treatment on intra- and interchromosome pairing of isochromosomes in common wheat

The analysis of the pattern of isochromosome pairing allows one to distinguish factors affecting presynaptic alignment of homologous chromosomes from those affecting synapsis and crossing-over. Because the two homologous arms in an isochromosome are invariably associated by a common centromere, the suppression of pairing between these arms (intrachromosome pairing) would indicate that synaptic or postsynaptic events were impaired. In contrast, the suppression of pairing between an isochromosome and its homologous chromosome (interchromosome pairing), without affecting intrachromosome pairing, would suggest that homologous presynaptic alignment was impaired. We used such an isochromosome system to determine which of the processes associated with chromosome pairing was affected by the Ph1 gene of common wheat-the main gene that restricts pairing to homologues. Ph1 reduced the frequency of interchromosome pairing without affecting intrachromosome pairing. In contrast, intrachromosome pairing was strongly reduced in the absence of the synaptic gene Syn-B1. Premeiotic colchicine treatment, which drastically decreased pairing of conventional chromosomes, reduced interchromosome but not intrachromosome pairing. The results support the hypothesis that premeiotic alignment is a necessary stage for the regularity of meiotic pairing and that Ph1 relaxes this alignment. We suggest that Ph1 acts on premeiotic alignment of homologues and homeologues as a means of ensuring diploid-like meiotic behavior in polyploid wheat.

Telomeres cluster de novo before the initiation of synapsis: a three-dimensional spatial analysis of telomere positions before and during meiotic prophase

We have analyzed the progressive changes in the spatial distribution of telomeres during meiosis using three-dimensional, high resolution fluorescence microscopy. Fixed meiotic cells of maize (Zea mays L.) were subjected to in situ hybridization under conditions that preserved chromosome structure, allowing identification of stage-dependent changes in telomere arrangements. We found that nuclei at the last somatic prophase before meiosis exhibit a nonrandom, polarized chromosome organization resulting in a loose grouping of telomeres. Quantitative measurements on the spatial arrangements of telomeres revealed that, as cells passed through premeiotic interphase and into leptotene, there was an increase in the frequency of large telomere-to-telomere distances and a decrease in the bias toward peripheral localization of telomeres. By leptotene, there was no obvious evidence of telomere grouping, and the large, singular nucleolus was internally located, nearly concentric with the nucleus. At the end of leptotene, telomeres clustered de novo at the nuclear periphery, coincident with a displacement of the nucleolus to one side. The telomere cluster persisted throughout zygotene and into early pachytene. The nucleolus was adjacent to the cluster at zygotene. At the pachytene stage, telomeres rearranged again by dispersing throughout the nuclear periphery. The stage-dependent changes in telomere arrangements are suggestive of specific, active telomere-associated motility processes with meiotic functions. Thus, the formation of the cluster itself is an early event in the nuclear reorganizations associated with meiosis and may reflect a control point in the initiation of synapsis or crossing over.

Presynaptic association of Rad51 protein with selected sites in meiotic chromatin

Eukaryotic homologs of Escherichia coli Rec-A protein have been shown to form nucleoprotein filaments with single-stranded DNA that recognize homologous sequences in duplex DNA. Several recent reports in four widely diverse species have demonstrated the association of RecA homologs with meiotic prophase chromatin. The current immunocytological study on mouse spermatocytes and oocytes shows that a eukaryotic homolog, Rad5l, associates with a subset of chromatin sites as early as premeiotic S phase, hours before either the appearance of precursors of synaptonemal complexes or the initiation of synapsis. When homologous chromosomes do begin to pair, the Rad5l-associated sequences are sites of initial contact between homologues and of localized DNA synthesis. Distribution of Rad5l foci on the chromatin of fully synapsed bivalents at early pachynema corresponds to an R-band pattern of mitotic chromosomes. R-bands are known to be preferred sites of both synaptic initiation and recombination. The time course of appearance of Rad51 association with chromatin, its distribution, and its interaction with other Rad5l-associated sequences suggests that it plays an important role preselection of sequences and synaptic initiation.

Chiasma distribution in the first bivalent of mice carrying a double insertion of homogeneously-staining regions in homo- and heterozygous states

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously-staining regions (HSRs) was carried out. In a previous study is was shown that the region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal one of Is(HSR;1D)2Icg is unpaired during early pachytene and heterosynapsed at midpachytene. No synaptic disturbances were revealed in homozygotes in this study. Chiasmata number per first bivalent in heterozygous (1.87) and homozygous (1.88) males was shown to be higher than in normal ones (1.61). In normal males a single chiasma is located in the medial part of chromosome 1. In heterozygotes this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing a single chiasma and an increase in the frequency of bivalents bearing double chiasmata located mostly at subcentromeric and subtelomeric regions of the chromosome. In homozygous males the frequency of double chiasmata is also increased, and even triple chiasmata become possible because of the increase in the physical length of the bivalents. Thus insertion of heterochromatic regions, which are inert with respect to recombination, leads to an increase in the length of the genetic map of the chromosome because of relaxation of interference restrictions.

The evolution of heteromorphic sex chromosomes

The facts and ideas which have been discussed lead to the following synthesis and model. 1. Heteromorphic sex chromosomes evolved from a pair of homomorphic chromosomes which had an allelic difference at the sex-determining locus. 2. The first step in the evolution of sex-chromosome heteromorphism involved either a conformational or a structural difference between the homologues. A structural difference could have arisen through a rearrangement such as an inversion or a translocation. A conformational difference could have occurred if the sex-determining locus was located in a chromosomal domain which behaved as a single control unit and involved a substantial segment of the chromosome. It is assumed that any conformational difference present in somatic cells would have been maintained in meiotic prophase. 3. Lack of conformational or structural homology between the sex chromosomes led to meiotic pairing failure. Since pairing failure reduced fertility, mechanisms preventing it had a selective advantage. Meiotic inactivation (heterochromatinization) of the differential region of the X chromosome in species with heterogametic males and euchromatinization of the W in species with heterogametic females are such mechanisms, and through them the pairing problems are avoided. 4. Structural and conformational differences between the sex chromosomes in the heterogametic sex reduced recombination. In heterogametic males recombination was reduced still further by the heterochromatinization of the X chromosome, which evolved in response to selection against meiotic pairing failure. 5. Suppression of recombination resulted in an increase in the mutation rate and an increased rate of fixation of deleterious mutations in the recombination-free chromosome regions. Functional degeneration of the genetically isolated regions of the Y and W was the result. In XY males this often led to further meiotic inactivation of the differential region of the X chromosome, and in this way an evolutionary positive-feedback loop may have been established. 6. Structural degeneration (loss of material) followed functional degeneration of Y or W chromosomes either because the functionally degenerate genes had deleterious effects which made their loss a selective advantage, or because shorter chromosomes were selectively neutral and became fixed by chance. 7. The evolutionary routes to sex-chromosome heteromorphism in groups with female heterogamety are more limited than in those with male heterogamety. Oocytes are usually large and long-lived, and are likely to need the products of X- or Z-linked genes. Meiotic inactivation of these chromosomes is therefore unlikely. In the oocytes of ZW females, meiotic pairing failure is avoided through euchromatinization of the W rather than heterochromatinization of the Z chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency and distribution of chiasmata in Syrian hamster spermatocytes studied by the BrdU antibody technique

The frequency and distribution of chiasmata and the nature of terminal "associations" was re-examined in Syrian hamster spermatocytes using the 5-bromo-2'-deoxyuridine (BrdU) antibody technique (BAT) for differential chromatid labelling. Differential chromatid substitution was achieved by BrdU incorporation at the penultimate pre-meiotic S-phase followed by one of three different staining protocols: (i) fluorescence plus Giemsa (FPG), (ii) acridine orange staining or (iii) BAT. For analysis of chiasmata frequency and localization in the diplotene/diakinesis stages the resolution of FPG and acridine orange staining was comparable to that of BAT. In metaphase II chromosomes BAT was more informative than FPG and acridine orange staining and revealed small, terminal crossover exchanges. This finding proves that many terminal associations of meiotic chromosomes actually represent chiasmata at the end of the first meiotic division. Some crossover exchanges were localized in the constitutive heterochromatin of autosomes. Using BAT we also detected crossover exchanges in close vicinity to each other. This observation is reminiscent of the fact that crossing over interference means a reduction in frequency and does not imply total inhibition.

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 of Peromyscus beatae are hypothesized to result from unequal crossing over. To test this hypothesis, we examined patterns of synapsis, chiasma formation, and segregation for male P. beatae which were either heterozygous or homozygous for the amount of short-arm sex heterochromatin. Synaptonemal complex analysis demonstrated that mitotic differences in heterochromatic short-arm 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.

Electron microscopic observations on the synaptonemal complex of spermatocytes of the giant panda (Ailluropoda melanoleuca)

Surface-spread and silver-stained preparations of spermatocytes from a giant panda were observed by electron microscopy for synaptonemal complex karyotyping. Ten pachytene spermatocyte nuclei were selected for length quantitation of SC. The mean relative lengths and centromeric indices of each SC agreed closely with those of the mitotic chromosomes. The pairing between lateral elements of autosomal chromosomes starts at early zygotene and leads progressively along their length to complete pairing at pachytene. The whole Y is paired with 1/3 length of X at mid-pachytene. The morphology of X and Y chromosome axes and the nonhomologous pairing of X and Y is discussed.

Formation of parallel four-stranded complexes by guanine-rich motifs in DNA and its implications for meiosis

We have discovered that single-stranded DNA containing short guanine-rich motifs will self-associate at physiological salt concentrations to make four-stranded structures in which the strands run in parallel fashion. We believe these complexes are held together by guanines bonded to each other by Hoogsteen pairing. Such guanine-rich sequences occur in immunoglobulin switch regions, in gene promoters, and in chromosomal telomeres. We speculate that this self-recognition of guanine-rich motifs of DNA serves to bring together, and to zipper up in register, the four homologous chromatids during meiosis.

Meiotic pairing constraints and the activity of sex chromosomes

The state of activity and condensation of the sex chromosomes in gametocytes is frequently different from that found in somatic cells. For example, whereas the X chromosomes of XY males are euchromatic and active in somatic cells, they are usually condensed and inactive at the onset of meiosis; in the somatic cells of female mammals, one X chromosome is heterochromatic and inactive, but both X chromosomes are euchromatic and active early in meiosis. In species in which the female is the heterogametic sex (ZZ males and ZW females), the W chromosome, which is often seen as a condensed chromatin body in somatic cells, becomes euchromatic in early oocytes. We describe an hypothesis which can explain these changes in the activity and condensation of sex chromosomes in gametocytes. It is based on the fact that normal chromosome pairing seems to be essential for the survival of sex cells; chromosomal anomalies resulting in incomplete pairing during meiosis usually result in gametogenic loss. We argue that the changes seen in the sex chromosomes reflect the need to avoid pairing failure during meiosis. Pairing normally requires structural and conformational homology of the two chromosomes, but when the regions is avoided when these regions become heterochromatinized. This hypothesis provides an explanation for the changes found in gametocytes both in species with male heterogamety and those with female heterogamety. It also suggests possible reasons for the frequent origin of large supernumerary chromosomes from sex chromosomes, and for the reported lack of dosage compensation in species with female heterogamety.

The behavior and morphology of the X and Y chromosomes during prophase I in the Sitka deer mouse (Peromyscus sitkensis)

Surface-spread, silver-stained primary spermatocytes from individuals of the Sitka deer mouse (Peromyscus sitkensis) were analyzed by electron microscopy. Pairing of the X and Y chromosomes is initiated at early pachynema and is complete by mid pachynema. The pattern of sex chromosome pairing is unique in that it is initiated at an interstitial position, with subsequent synapsis proceeding in a unidirectional fashion towards the telomeres of the homologous segments. One-third the length of the X and two-thirds the length of the Y are involved in the synaptonemal complex of the sex bivalent. Various morphological complexities develop in the heteropycnotic (unpaired) segments as pachynema progresses, but desynapsis is not initiated until diplonema. Analysis of C-banded diakinetic nuclei indicated that sex chromosome pairing involves the heterochromatic short arm of the X and the long arm of the heterochromatic Y. An interstitial chiasma between the X and Y was observed in the majority of the diakinetic nuclei. The observation of a substantial pairing region and chiasma formation between the sex chromosomes of these deer mice is interpreted as indicating homology between the short arm of the X and the long arm of the Y.

A model for effective pairing and recombination at meiosis based on early replicating sites (R-bands) along chromosomes

A model for meiotic pairing is proposed in which early replicating sites (R-band equivalent) along chromosomes are envisaged as sites for synaptic initiation. Only within such sites will "effective" pairing for recombination be established. Pairing in later replicating (G- and C-band equivalent) regions will be "ineffective" and will not provide for the stringent requirements of the crossover process. Exchange events might be predetermined at S-phase, and possibly at junctions between early and later replicating sequences, these being seen as vulnerable sites for breakage. Temporal shifts in replication from early to late S, are postulated to produce localized pairing disruption and lowering of crossover values as regions of chromatin shift from being effectively to ineffectively paired.

Meiotic chromosome pairing in the normal human female

The synaptonemal complexes of oocytes from 16-22 week human fetuses were spread using detergent and silver-stained for examination by light microscopy. Zygotene chromosome synapsis generally begins at the telomeres, without obvious prealignment, and proceeds towards the centromeres. Synapsis is not synchronous and longer bivalents may sometimes be completely paired before shorter ones. At pachytene, when pairing is usually complete, some regions presumed to correspond to the heterochromatic blocks of chromosomes 1.9 and 16 may remain unpaired. Residual univalents are uncommon, and little interlocking is evident at this stage. Desynapsis indicating the beginning of diplotene frequently begins at the telomeres, although there is a general relaxation of pairing throughout the bivalents which become increasingly diffuse as diplotene proceeds. The total synaptonemal complex complement length at pachytene in the female is 519 micron, which is about twice that found in the human male. The implications of these results for genetic mapping are discussed.

The pattern of pairing that is effective for crossing over in complex B-A chromosome rearrangements in maize II

The problem of meiotic homologue pairing is approached by comparing chiasma frequencies in rearranged chromosome segments that differ substantially in relative length and intrachromosomal location. Results are consistent with affirmative answers to some questions previously raised: (1) whether there may be an underlying direct relationship between frequency of pairing and length of segment, (2) whether pairing commonly can be initiated independently in intercalary regions, and (3) whether there also can be a role for extension of pairing in adjoining regions for the establishment of pairing in intercalary regions, which requires pairing partner change. In addition, results here suggest that there may be: (1) greater capacity for establishment of pairing of more distal compared to proximal regions in a way that may also be dependent on their lengths, at least when these are relatively short, and additionally in a way which cannot be attributed to special properties of telomeres, (2) nearly random distribution of pairing of any two genetically long intercalary region representatives where three are present, without regard to the matching of the remainder of the chromosomes involved, and (3) a strong tendency for change of pairing partner in long distal segments when these are present in triplicate. Although sharp heterogeneities of pairing capacity were not found, it is suggested that they may exist with spacing too close for easy detection with the resolving power available.

The morphological sequence of XY pairing in the Norway rat Rattus norvegicus

The sequence of XY pairing at meiotic prophase in the Norway rat, Rattus norvegicus, has been studied in spread preparations of spermatocytes obtained from pubertal males. As in most mammals, sex chromosome pairing is delayed in relation to that of the autosomes. At one stage in pachytene, the Y is fully paired in synaptonemal complex association with about one-third of the X. Observation in spread preparations at pachytene and diplotene and in air-dried metaphase I preparations indicates that the long arm of the Y pairs with the short arm of the X. Pairing of the Y with both ends of the X is seen in about 4% of pachytene spermatocytes. The possibility that XY pairing in the rat may be nonhomologous (Ashley 1983) is considered, and the view is expressed that the XY synaptonemal complex may be incomplete in fine structural detail, thus not providing for the effective pairing required in true reciprocal recombination. The same mechanism that excludes crossing over from heterochromatic regions of autosomes may also operate to minimize or prevent crossing over in the sex pair of mammals.

A general treatment of chromosome synapsis in even-numbered polyploids

Relationships between frequencies of fully paired pachytene configurations are examined for even-numbered polyploids, up to decaploids and beyond. Sequential association is assumed at any given number of equidistant synaptic sites showing uniform propensities for partner exchange. Only with two sites per chromosome, can each configuration represent an equal proportion of the nucleus. In the limit as the number of sites tends to infinity and the intervals between them tend to zero, pachytene multivalent frequency is seen to be a function of bivalent frequency, the minimum number of changes in pairing partner and ploidy. Each type of multivalent may represent the entire nucleus only in the lowest ploidy which can support it, thereafter becoming rare. Sets of homologous multivalents are never likely to be common and large multivalents are expected to be scarce, unless representing maximal configurations. Only bivalents can prevail at every even-numbered ploidy level. At constant bivalent frequency, the transition from discrete to continuous pairing favours the largest type of multivalent. These deductions provide a basis for investigations of the mechanics of chromosome synapsis and may lead to a comprehensive theory of genome analysis.

The mechanism of meiotic homologue pairing

Homologous chromosome pairing involves the moving together of matching chromosomes or chromosome segments across substantial distances within a nucleus. Although the time in the life cycle of initial association of homologues varies among organisms, it may well be that similar underlying mechanisms for its occurrence prevail throughout sexually reproducing eukaryotes. The means by which pairing its accomplished is in no case understood. In the apparent absence of a long range specific force of attraction, simple partial models have been proposed which relay for the most part upon interactions of chromosome ends (telomeres) with specialized portions of the nuclear envelope. While such interactions, as well as the persistence of chromosome orientation established by mitotic anaphase poleward movement of centromere regions, may provide in many cases for closer than random positioning of some parts of homologues, the distances remaining to be traversed are still long range in physical-chemical terms. Also, the specific pairing observed in some kinds of rearranged segments is not facilitated by such circumstances, even if synapsis is initiated at available homologous telomere pairs and proceeds to completion by a "zip-up" process. A unified, more complex model is considered which is designed to accommodate the various relevant findings. It invokes the interaction of intranuclear structures with intercalary and/or terminal chromosomal pairing sites, e.g. filamentous structures which specifically bind to these, and a contractile system involving proteins such as actin and myosin to draw homologues together.

C heterochromatin variation in couples with recurrent early abortions

The possible influence of the high polymorphic C heterochromatic regions of human chromosomes 1, 9, 16, and Y on meiotic chromosome segregation was investigated. Faulty chromosome segregation may be the result of either an abnormal quantity of C heterochromatin on the homologues, or disequilibrium between the homologues. The aim of our study was to determine whether either a variation in the amounts of total C heterochromatin or differences in the amounts of C heterochromatin between homologues could lead to faulty chromosome segregation. The study was performed on C banded metaphases obtained from peripheral lymphocyte cultures of 15 couples with recurrent early abortions and 15 control couples, all Caucasians. Analysis of variance was first performed on separate metaphases to measure intra-individual, inter-individual, and between population variation in a hierarchical model. Since the significant intra-individual differences covered the other parameters we performed, secondly, a one way analysis of variance on the mean values of metaphases per person in order to measure the inter-individual and between population variation. The results did not show a relationship between C heterochromatin lengths and occurrence of recurrent abortions.

A re-examination of chiasma terminalization and chiasma frequency in male mice

The distribution and frequency of chiasmata have been analyzed in male BALB/c mice. Bivalents were classified in terms of the number of interstitial chiasmata (CH) and terminal associations (TA) present as follows: 1CH, 2CH, 1TA, 2TA, 1CH X 1TA, 1CH X 2TA, 2CH X 1TA, 2CH X 2TA and 0. We provide evidence that the TA frequently dissociates during 1st meiotic prophase. Consequently six of the observed bivalents may be derived from three basic bivalent types (namely 2CH X 2TA, 1CH X 2TA and 2TA) by dissociation of the TA according to the following schemas: (1) 2CH X 2TA leads to 2CH X 1TA leads to 2CH, (2) 1CH X 2TA leads to 1CH X 1TA leads to 1CH, and (3) 2TA leads to 1TA leads to 0. We also provide evidence that interstitial chiasmata do not move, which implies that a TA can not be formed by chiasma terminalization. The chiasma frequency estimated by assuming that terminal associations do not result from terminalized chiasmata is 17.2+/-2.4 compared to a value of 25.4+/-2.2 calculated on the assumption of chiasma terminalization.