Synaptonemal complex formation in hybrids of Lolium temulentum x Lolium perenne (L.)

Comparisons were made between two kinds of tetraploids derived from the hybrid Lolium temulentum x L. perenne. One hybrid behaves like an autotetraploid with multivalents at first metaphase of meiosis in pollen mother cells. The other behaves like an allotetraploid, in which pairing at first metaphase is restricted to bivalents comprised of strictly homologous chromosomes. The diploidisation of the latter form is controlled by determinants located on both the normal, A chromosomes and on supernumary B chromosomes. Reconstruction of synaptonemal complexes and their elements, from serial sections through pollen mother cell nuclei examined under the electron microscope, reveals that at zygotene pairing in both forms results in multivalent formation involving non-homologous as well as homologous chromosomes. The mechanism responsible for the diploidisation is, therefore, not based on a restriction of pairing at early meiosis to homologous chromosomes but on a correction or transformation of the multivalent chromosome associations to bivalents subsequent to zygotene. The transformation is not completed until late pachytene. In the multivalent-forming tetraploid a maximum of four chromosomes are associated at first metaphase. Yet configurations of a higher valency are found at zygotene. There is, therefore, a partial transformation of multivalents even in this autotetraploid form which restricts configurations at metaphase I to homologous and homoeologous chromosomes only. In both hybrids some homologous bivalents are not the product of resolution of multivalents but result from two-by-two pairing from the beginning of zygotene.     

Chromosome pairing in a Lolium temulentum X Lolium perenne diploid hybrid with a low chiasma frequency

Summary Despite an average difference of about 50% in DNA amount, homoeologous chromosomes pair effectively at first metaphase in the diploid interspecific hybrid between Lolium temulentum and Lolium perenne. However, in the presence of accessory B chromosomes and diploidising genes pairing at metaphase I is severely reduced. Reconstruction of serial electron micrographs through pollen mother cell nuclei show that synaptonemal complexes are formed at pachytene between not only homoeologous but also non-homologous chromosome segments resulting in multivalent formation. These associations are largely ineffective in terms of chiasma formation and degenerate at late pachytene. It is highly probable that the pairing determinants exercise their control on chromosome pairing largely by prohibiting the siting of crossovers in homoeologously paired chromosome segments.     

Meiotic prophase in anthers of asynaptic wheat

The light microscope showed that zygotene and pachytene were completely suppressed in pollen mother cells of an asynaptic mutant of Triticum durum; the chromosomes passed through a normal chromomeral leptotene condition and remained unpaired throughout prophase. The electron microscope confirmed the absence of synaptinemal complexes, as would be expected with no pairing. Prominent opaque axial cores were present in the chromatin from the onset of leptotene up to an indeterminate stage during prophase condensation. At an early time during condensation 150 Å particles appeared between chromatin masses. Coincident to the disappearance of axial cores from the chromatin, polycomplexes consisting of linearly associated core fragments arrayed in single layer sheets appeared between chromatin masses. The aligned fragments were invariably spaced about 625 Å from centre to centre; this is approximately half the distance between centres of the lateral elements (axial cores) of the synaptinemal complex of pachytene of synaptic sister seedlings. There was no central element between the associated fragments. The significance of these observations is discussed, as is also the essential difference between asynapsis and desynapsia.     

Spiral cores of synaptonemal complex lateral elements at the diplotene stage in rye include the ASY1 protein

After completing their functioning, synaptonemal complexes (SCs) degrade during the diplotene stage. In the pollen mother cells of rye Secale cereale L., this occurs through the formation of gaps in lateral elements of the SCs and the shortening of fragments of SCs until their complete disappearance. However, when contrasting SCs with silver nitrate solution at a pH 3.5–4.5, these gaps appear to be filled with threads associated with SC lateral elements. As the diplotene stage proceeds and gradual degradation of SC fragments continues, these threads turn into submicroscopic spirals. In this study, we found that the threads and spirals associated with degrading synaptonemal complexes are stained by antibodies to the ASY1 protein of Arabidopsis thaliana lateral elements and thus are degradation products of the lateral elements of SCs.     

Synaptonemal complex formation in hybrids of Lolium temulentum × Lolium perenne (L.)

Chromosome pairing and synaptonemal complex formation at zygotene and pachytene are described from serial section reconstructions of pollen mother cell nuclei in a triploid hybrid containing two haploid sets of Lolium perenne chromosomes, one of L. temulentum and two acces-sory B chromosomes. At pachytene the homologous L. perenne chromosomes form complete and continuous synaptonemal complexes while the L. temulentum chromosomes show extensive nonhomologous pairing both within and between themselves. At zygotene however, homoeologous pairing in the form of a trivalent and very little non-homologous pairing is observed. Evidently, there exists a mechanism that eliminates homoeologous association during zygotene to ensure strict bivalent formation between homologous chromosomes at pachytene. In Lolium this mechanism is under the influence of the B chromosomes and bears close similarity with that in allohexaploid wheat controlled by the Ph locus.     

Deformed lateral elements in synaptonemal complexes of Phaedranassa viridiflora

An electron microscopical study was made of an intermittent structural deformity in the lateral elements of synaptonemal complexes in pollen mother cells during zygotene of diploid and triploid forms of Phaedranassa viridiflora. The deformed regions, which extended linearly for distances up to 0.6 m in otherwise normal elements, are interpreted as hollow-centred bulges in a structure comprised of protein fibres apparently arranged in the form of a tightly wound helix. It is suggested that the bulging may arise if contraction of the lateral element is out of step with that in the chromosome, during prophase contraction at zygotene/ pachytene, as may perhaps follow under certain environmental conditions in organisms where there is some degree of genetical unbalance. There was evidence which suggested that local precocious duplication may sometimes have occurred in the chromosome axes proximal to the deformities in the lateral elements.     

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.     

The meiotic prophase in Bombyx mori females analyzed by three-dimensional reconstructions of synaptonemal complexes

Serial sectioning followed by three dimensional reconstruction of lateral components of the synaptonemal complex have been used to follow chromosome pairing during the prophase of the achiasmatic meiotic division in the silkworm, Bombyx mori. During leptotene and early zygotene, the lateral components become attached to the nuclear envelope at a specific region, thus forming a chromosome bouquet. The attachment of lateral components to the nuclear envelope precedes the completion of the components between their attachment points. Synapsis and synaptonemal complex formation start during the period of lateral component organization in the individual nucleus. Telomeric movements on the nuclear envelope occur at two stages of the prophase: the chromosome pairing appears to be initiated by an association of unpaired ends of homologous chromosomes, the nature of this primary attraction and recognition being unknown. Secondly, the paired chromosomes become dispersed in the nucleus by shifting of attachment sites of completed synaptonemal complexes at the end of zygotene. This movement is possibly related to a membrane flow occurring during this stage. Membrane material is synthesized at the region of synaptonemal complex attachment. Later, the excess membrane material is shifted to the opposite pole where it protrudes into the lumen of the nuclei thus forming vacuoles. — Two previously undescribed features of chromosome pairing were revealed. In late zygotene, chromosome pairing and synaptonemal complex formation were frequently observed to be delayed or even prevented over a short distance by interlocking of two bivalents, both being attached to the nuclear envelope. Such interlocking of bivalents was not found in pachytene. Secondly, one nucleus was found in which two homologous chromosomes were totally unpaired while the remaining 27 bivalents were completed or in a progressed state of pairing. The lateral components of the two unpaired chromosomes had the same length and were located several microns apart, thus eliminating the possibility of a permanent association of homologous chromosomes before the onset of meiosis in Bombyx mori females. — During pachytene, one of the 8 cells belonging to the syncytial cell cluster characteristic of oogenesis continues the meiotic prophase whereas the remaining 7 cells, the nurse cells, enter a different developmental sequence, finally resulting in their degeneration. The synaptonemal complex of the oocyte develops into a sausage-like structure after pachytene by a deposition of dense material onto the lateral components, thus filling out most of the central region. The diameter of this modified synaptonemal complex reaches at least 300 nm, as compaired to a pachytene width of approximately 130 nm. Also, the length of synaptonemal complexes increases from 212 at zygotene/pachytene to at least 300 at the modified pachytene stage. In nurse cells, synaptonemal complexes are shed from the bivalents shortly after pachytene simultaneously with a condensation of the chromatin. These free synaptonemal complex fragments associate and form various aggregates, either more or less normal looking polycomplexes or various complex figures formed by reorganized synaptonemal complex subunits. Later stages have not been included in the present investigation.     

Ultrastructure of meiotic pairing in B chromosomes of Crepis capillaris

The meiotic pairing behaviour of four B isochromosomes of Crepis capillaris was studied by synaptonemal complex (SC) surface spreading of pollen mother cells. The four B chromosomes form a tightly associated group, separate from the standard chromosomes, throughout zygotene and pachytene. All four B chromosomes are also folded around their axis of symmetry, the centromere, and the eight homologous arms are closely aligned from the earliest prophase I stages. A high frequency of multivalent pairing of the four B chromosomes is observed at pachytene, in excess of 90%, mirroring the situation observed at metaphase I but exceeding the frequency expected (76.2%) on the assumption of random pairing among the eight B isochromosome arms with a single distal pairing initiation site per arm. The higher than expected frequency of multivalents is due to the occurrence of multiple pairing initiations along the B isochromosome arms, resulting in high frequencies of pairing partner switches. Pairing of the standard chromosome set is frequently incomplete in the presence of four B chromosomes, and abnormalities of SC structure such as thickening and splitting of axes and lateral elements are also frequently seen. Similarly, B chromosomes show partial pairing failure, the extent of which is correlated with pairing failure in the standard chromosome set. The B chromosomes themselves also show abnormalities of SC structure. Both standard and B chromosomes show non-homologous foldback pairing of regions that have failed to pair homologously.by D. Schweizer     

Synaptonemal complex formation in hybrids of Lolium temulentum × Lolium perenne (L.)

The chromosomes of Lolium temulentum are longer and contain on average 50% more nuclear DNA than the chromosomes of L. perenne. In the hybrid, despite the difference in length and DNA content, pairing between the homoeologous chromosomes at pachytene is effective and the chiasma frequency at first metaphase in pollen mother cells is high, about 1.6 per bivalent, comparable to that in the L. perenne parent. Electron microscopic observations from reconstructed nuclei at pachytene show that synaptonemal complex (SC) formation in 40% of bivalents is perfect, complete and continuous from telomere to telomere. In others, SCs extend from telomere to telomere but incorporate lateral component loops in interstitial chromosome segments. Even in these bivalents, however, pairing is effective in the sense of chiasma formation. The capacity to form perfect SCs is achieved by an adjustment of chromosome length differences both before and during synapsis. Perfect pairing and SC formation is commoner within the larger bivalents of the complement. At zygotene, in contrast to pachytene, pairing is not confined to homoeologous chromosomes. On the contrary there is illegitimate pairing between non-homologous chromsomes resulting in multivalent formation. There must, therefore, be a mechanism operative between zygotene and pachytene that corrects and modifies associations in such a way as to restrict the pairing to bivalents comprised of strictly homoeologous chromosomes. Such a correction bears comparison with that known to apply in allopolyploids. In the hybrid and in the L. perenne parent also, certain specific nucleolar organisers are inactivated at meiosis.     

The effect of colchicine on meiosis in Lilium speciosum cv. “Rosemede”

Chromosome pairing and chiasma frequency were studied in meiocytes at diakinesis of Lilium speciosum cv. Rosemede fixed up to 21 days after the start of either continuous or 3 day pulse colchicine treatment. The two treatments gave similar results. In pulse treated pollen mother cells (PMCs) the mean chiasma frequency per cell fell from 26.4 in controls to 8.5 after fourteen days while the mean number of univalents per cell increased from 0.05 to 17.58. There was a negative correlation between mean chiasma frequency per bivalent and per PMC in colchicine treated buds; univalents were preferentially induced in bivalents with one chiasma, and preferentially excluded in bivalents with 4 chiasmata. Some chiasmata were redistributed to surviving bivalents despite the concurrent reduction in chiasma frequency per meiocyte. — Colchicine sensitivity began in premeiotic interphase and extended to mid or late zygotene in PMCs; ongoing synapsis was unaffected. However, susceptibility to univalency was asynchronous between bivalents occurring at zygotene in short chromosomes but at late premeiotic interphase in the longest chromosomes. The number of chiasmata per bivalent could be altered by colchicine without inducing univalents, but the ultimate effect was to reduce the number of chiasmata per bivalent (or per chromosome arm) directly to zero. The major factors determining the order and extent of reduced pairing and chiasma number were total chromosome length and arm length. Pairing and chiasma formation in embryo sac mother cells were less sensitive to colchicine than in PMCs, but their behavior was otherwise similar.     

Nuclear behaviour during ascus formation in Saccharomyces rosei (Guilliermond) Lodder et Kreger-van Rij

Stained cells of Saccharomyces rosei prepared from 4 to 10-day-old cultures were studied under the light microscope. Mitotic and meiotic divisions involving a ring-like structure as well as preceding and subsequent stages were observed. Cells presenting supernumerary mitoses in a varying number were frequent. These mitoses, having terminated their multiplication activity, suspended the process shortly before its conclusion and, in a development which was identical at all, assumed a curious arrangement forming a mitoses-ring. Meiosis-buds were detected. These especial buds, where karyogamy and meiosis took place, resulted from the development of the mitoses-ring, whose mitoses upon resuming their activity moved toward the cell wall giving rise to the appearance of these appendices. Each one of these buds received the corresponding pair of daughter nuclei, diploidization occurring subsequently. Meiosis was usually processed in a single bud (effective-meiosis-bud) and all four meiotic nuclei migrated to the mother cell, and gave rise to a tetra-nucleate spore or binucleate spores if two were formed.Other modalities of sporulation were observed. These may result either from the association of two cells, in which one assumed the function of meiosis-bud (false-meiosis-bud), or from a cell association in which this function was performed for several linearly arranged cells forming a protuberance.Conjugation between mother cell and an attached bud, or between independent cells, was not observed.     

Cyto-morphological studies op some species and hybrids in the Eu-Sorghums

Summary Important morphological features such as plant height, leaf size and number of leaves, shape of the panicle and sessile spikelets, staminate condition of the pedicellate florets, nature of lemma, colour of the stigmatic surface and seeds etc., were studied in 8 Sorghum species and 10 F₁ hybrids between them. Based on the data, interrelationship amongst the species are discussed.Pachytene pairing was complete and apparently normal, followed by regular meiosis at later stages resulting in high pollen stainability and good seed setting in all the parental species except the male sterile Kafir. Studies on the pairing properties of the differentially stained regions showed that synapsis starts from the proximal to the distal end and separation of the split chromosomes starts from the distal to the proximal.The interspecific hybrids studied are classified into four types based on pachytene pairing and pollen sterility. 1. normal pairing accompanied by high pollen fertility, 2. normal pairing accompanied by partial pollen sterility. 3. irregularities in the pachytene pairing followed by partial pollen sterility and 4. irregularities in the pachytene pairing accompanied by normal pollen fertility. Suitable explanations are advanced to explain the meiotic aberrations noted in some of the hybrids under study.Cytogenetical mechanisms underlying species differentiation in the Eu-Sorghum species are discussed.     

Light and electron microscope studies of chromosome pairing in relation to chiasma localisation in Stethophyma grossum (Orthoptera: Acrididae)

The relationship of chromosome pairing to chiasma localisation in the grasshopper Stethophyma grossum was investigated by a combined light and electron microscope study. Observations on the extent of synaptonemal complex formation in spermatocytes suggest that pairing is complete in all chromosome regions and that localised chiasmata do not therefore follow from localised pairing of homologues. This study also revealed an unprecedented variant of synaptonemal complex structure. Each bivalent was found to contain an asymmetrical region within which one lateral element was enormously enlarged while the other lateral element retained normal dimensions. This asymmetrical region is probably confined to one end of each bivalent and seems to extend constantly about 4 from the nuclear membrane attachment site. The possible significance of this variant of synaptonemal complex structure is briefly discussed.     

Chromosome pairing and potential for intergeneric recombination in some hypotetraploid somatic hybrids of Lycopersicon esculentum (+) Solanum tuberosum.

Three somatic hybrids resulting from protoplast fusions of a diploid kanamycin-resistant line of tomato (Lycopersicon esculentum) and a dihaploid hygromycin-resistant transformant of a monohaploid potato (Solanum tuberosum) line were used for a cytogenetic study on chromosome pairing and meiotic recombination. Chromosome counts in root-tip meristem cells revealed two hypotetraploids with chromosome complements of 2n = 46 and one with 2n = 47. Electron microscope analyses of synaptonemal complex spreads of hypotonically burst protoplasts at mid prophase I showed abundant exchanges of pairing partners in multivalents involving as many as eight chromosomes. In the cells at late pachytene recombination nodules were found in multivalents on both sides of pairing partner exchanges, indicating recombination at both homologous and homoeologous sites. Light microscope observations of pollen mother cells at late diakinesis and metaphase I also revealed multivalents, though their occurrence in low frequencies betrays the reduction of multivalent number and complexity. Precocious separation of half bivalents at metaphase I and lagging of univalents at anaphase I were observed frequently. Bridges, which may result from an apparent inversion loop found in the synaptonemal complexes of a mid prophase I nucleus, were also quite common at anaphase I, though the expected accompanying fragments could be detected in only a few cells. Most striking were the high frequencies of first division restitution in preparations at metaphase II/anaphase II, giving rise to unreduced gametes. In spite of the expected high numbers of balanced haploid and diploid gametes, male fertility, as revealed by pollen staining, was found to be negligible.     

Chromosome-specific control of chiasma formation in Crepis capillaris

An experimental population of Crepis capillaris (2n=6) displays frequent chromosome-specific univalence affecting all three chromosome pairs of the complement independently in different plants. The frequency of univalence in the population varies from 0% in some plants to about 40% of pollen mother cells in other plants. Most commonly, affected pollen mother cells contain just one pair of univalents and wherever the frequency of cells containing univalents exceeds 10%, a chromosome-specific effect almost invariably appears. Univalence affecting the A, C and D chromosome pairs is about equally frequent in the population. The mean cell chiasma frequencies of affected plants are generally lower than those of normal plants from the same population, despite suggestions of compensating increases in the chiasma frequencies of unaffected chromosomes pairs in the presence of specific univalence of a third pair. Breeding tests have been carried out, and observations made on pachytene stages which demonstrate that the specific univalence is due to recessive genes causing desynapsis following apparently normal pairing of homologues.     

Chromosome mosaics and the recovery of the original strain from octoploidHordeum murinum

Summary Plants with chromosome numbers (2n=18–54) lower than that of the normal 2n=56 were found in the C₁ progeny of colchicine induced octoploid form ofHordeum murinum. Pollen mother cells in three octoploid plants had reduced chromosome complements at meiosis. Some of these plants had the original chromosome number of 2n=28. A number of these cells had regular bivalent pairing at first metaphase. Evidence indicated independent as well as nonindependent assortment of chromosomes at somatic reduction, the latter type resulting in ancestral type pollen mother cells (14^II) and gametes respectively. Since normal tetraploid plants were recovered in the progeny (C₂) of chromosome mosaics, ancestral type fully balanced egg cells must have also been formed. It is believed that the recovered tetraploids originated presumably from the fusion of double reduced gametes. Break-down diploids occurring occasionally among progenies of autoploids may originate from the fusion of such gametes.With 7 Figures in the TextContribution No. 107 from the Genetics and Plant Breeding Research Institute, Research Branch, Canada Department of Agriculture, Ottawa, Ontario, Canada.     

Synaptonemal complex spreading in plants: Technical aspects and preliminary observations on the synaptonemal complex inPaeonia andOrnithogalum

A modified method for obtaining surface spread synaptonemal complexes (SCs) from pollen mother cells has been developed. Silver-stained SC-preparation of one monocotyledonous species,Ornithogalum flavovirens, and one dicotyledonous plant,Paeonia tenuifolia, were analysed by light and electron microscopy. The SCs in both species frequently broke into roughly equally sized SC pieces with staggered or blunt breakpoints. The telomeric ends of the SCs normally were lacking attachment organelles and, therefore, were hardly distinguishable from blunt breakpoints. Interstitially, shorter stretches of SCs often exhibited unpaired lateral elements. This phenomenon is discussed with regard to segmental incomplete homology and as it relates to the normal sequence of SC morphological changes during the course of meiotic prophase.     

Cytological and Statistical Analysis of Secondary Association in the Genus Phaseolus

Four cultivars of Phaseolus coccineus L., five cultivars of P. vulgaris L. and one collectionof P. vulgaris var. aborigineus (Burk.) Baudet were examined cytologically for the presenceof secondary association of bivalents (bivalent pairing) during metaphase 1 of meiosis in pollen mother cells. Statistical methods were presented for evaluating the deviation of the observed degree of association to that expected at random. The degree of secondary pairing wasfound to be highly significant. Transmission electron microscope techniques were modified to enable the viewing of squashed but intact pollen mother cells which showed that a physicalconnection could occur between bivalents secondarily associated.     

Meiosis in triploid Lolium. I. Synaptonemal complex formation and chromosome configurations at metaphase I in aneuploid autotriploid L. multiflorum.

A spreading technique for synaptonemal complexes (SCs) was applied to pollen mother cells of two aneuploid genotypes of autotriploid Lolium multiflorum (2n = 3x + 1 = 22). In the earliest nuclei analyzed the axial elements are in six groups of 3 and one group of 4. Most groups have formed multivalents with from one to five pairing partner exchanges, but there are also groups that have formed bivalents and univalents. Some axial elements have formed triple associations, in one case for the length of the trivalent. Unsynapsed axial elements remain aligned with their homologous SCs into pachytene, but this alignment is abolished as these axes pair heterologously among themselves until the entire axial element complement is synapsed. At metaphase I most chromosomes are associated as trivalents and quadrivalents.