Multivalent Formation Research Articles

Synaptonemal complex spreading in Allium cepa and Allium fistulosum. III. The F1 hybrid

Synaptonemal complexes (SCs) were analysed, by surface spreading, in the F1 hybrid between Allium cepa and Allium fistulosum. These closely related species have a similar karyomorphology, but the A. cepa genome has 27% more DNA than A. fistulosum. At metaphase I, in the hybrid, heteromorphic bivalents were observed, and the pollen mother cell chiasma frequency in the F1 hybrid was reduced when compared with the parents. Synapsis was incomplete and disturbed to some extent in all the prophase I nuclei observed. The reduced level of synapsis at late zygotene – mid pachytene in the hybrid (mean percent homoeologous synapsis is 60%) corresponds to the percent reduction in chiasma frequency. It is suggested that failure of synapsis is the main cause of reduction of chiasma frequency in the F1 hybrid. The chiasma distribution in the hybrid is most similar to that of the A. cepa parent. Although some proximal chiasmata were observed (10% of total), none of the bivalents observed had only proximally localised chiasmata as found in the A. fistulosum parent. In the F1 hybrid, synapsis of the centromeric regions of the bivalents was invariably disturbed throughout prophase I. It is suggested that there are major DNA differences in these regions preventing regular synapsis or progression of synapsis and possibly proximal chiasma formation. Nonhomoeologous synapsis occurred within axes, giving foldback SCs, and between axes, resulting in multivalent formation. It is proposed that disturbance of synapsis is due to DNA differences. There is no indication of any rearrangement or adjustment of synapsis during prophase I either to give perfectly synapsed bivalents or to resolve multivalents into bivalents.Key words: synaptonemal complex, Allium, incomplete pairing, chiasma, nonhomologous pairing, DNA content.

Cytogenetics of decaploid Agropyron elongatum (Elytrigia elongata) (2n = 70). I. Frequency of decavalent formation

The multivalents that appeared in the decaploid strain of Agropyron elongatum (2n = 10x = 70), a relative of wheat, ranged from trivalent to decavalent. Few univalents occurred. The metaphase I chromosome association in 12 cells where all configurations could clearly be identified averaged 0.42 ring X + 0.17 chain X + 0.42 ring VIII + 0.17 branched VIII + 0.25 chain VIII + 0.17 chain VII + 1.17 ring VI + 0.33 branched VI + 0.5 chain VI + 1.67 ring IV + 0.42 branched IV + 0.58 chain IV + 0.08 branched III + 0.17 chain III + 12.58 ring II + 3.75 open II + 0.25 I. The occurrence of decavalents, up to two in one cell, and of a cell with five multivalents, each of which involved more than five chromosomes, and many multivalents of ring shape indicated that the strain is autodecaploid.The chromosome associations of each cell can be interpreted as seven groups of 10 homologous chromosomes. The high frequency of bivalents indicated a tendency toward reduced multivalent formation, for which an explanation is suggested.Key words: Agropyron elongatum, meiotic configuration, decaploid, multivalent.

Cytological analysis of tetraploid hybrids between sweet potato and diploid Ipomoea trifida (H. B. K.) Don.

Tetraploid F1 hybrids between Ipomoea batatas, sweet potato (2n = 6x = ca. 90), and diploid (2n = 2x = 30) I. trifida (H. B. K.) Don. showed various degrees of fertility reduction. The present study aimed to clarify its causes by cytological analysis of meiotic chromosome behavior in the diploid and sweet potato parents and their tetraploid hybrids. The diploid parents showed exclusively 15 bivalents, and the sweet potato parents exhibited almost perfect chromosome pairing along with predominant multivalent formation. Their hybrids (2n = 4x= 57-63) formed 2.6-5.0 quadrivalents per cell, supporting the autotetraploid nature. The meiotic aberratios of the hybrids were characterized by the formation of univalents, micronuclei, and abnormal sporads (monad, dyad, triad, and polyad). The causes underlying these aberrations were attributed in part to the multivalent formation, and in part to a disturbance in the spindle function. Three hybrids showing serious meiotic aberrations were very low in fertility. The utilization of the sweet potato-diploid I. trifida hybrids for sweet potato improvement is described and, further, the role of interploidy hybridization in the study of the sweet potato evolution is discussed.

Multivalent formation and pairing behavior of germ line limited chromosomes in male meiosis of Acricotopus lucidus (Diptera, Chironomidae)

The pairing behavior of the germ line limited chromosomes of Acricotopus lucidus was investigated in male meiosis using G-banding. Each of the nine different types of limited chromosomes can be absent or can be present in metaphase I with two or four homologous chromosomes, one type even with up to 10. Usually the homologues form bivalents, but frequently quadrivalents and also hexavalents consisting of the same chromosome type were observed. In some cases multivalents composed of different limited chromosomes occurred. This resulted probably from pairing and crossing-over between hom(oe)ologous segments in otherwise nonhomologous chromosomes. The observations are discussed in relation to origin and diversity of the germ line limited chromosomes.Key words: germ line limited chromosomes, multivalent formation, male meiosis, Acricotopus lucidus.

Cytogenetic analyses of intersectional hybrids between Parthenium argentatum and Parthenium confertum

Induced tetraploid guayule (Parthenium argentatum Gray, a rubber-producing and woody species of section Parthenicaeta) was crossed to P. confertum var. lyratum Gray, a herbaceous species of section Argyrocheta. Hybrids were obtained when guayule was used as female. Like their diploid progenitors, induced tetraploids were sexual and self-incompatible. At meiosis, they averaged 0.30 I, 29.63 II, 0.12 III, and 3.02 IV. Parthenium confertum had 69 chromosomes and was apomictic. At meiosis, it averaged 1.00 I, 32.98 II, and 0.51 IV. Hybrids received 36 chromosomes from guayule and 34 chromosomes from P. confertum. At meiosis, they averaged 0.52 I, 33.44 II, 0.20 III, and 0.50 IV. F1 plants behaved like an amphidiploid in which homologous chromosomes of guayule genome paired with each other and homologous chromosomes of P. confertum genome also paired preferentially. Chromosome association in the hybrids indicated that P. confertum parent may be a tetraploid with suppressed multivalent formation. Although hybrids were partially fertile and produced backcross progeny, the transfer of desirable traits (rapid growth and herbaceous growth habit) from P. confertum into guayule will depend on elimination of preferential pairing in the hybrids.Key words: guayule, natural rubber, meiosis, chromosome pairing.

Elimination of multivalents during meiotic prophase in Scilla autumnalis. II. Tetraploid

The pairing behaviour of chromosomes during meiotic prophase in two Scilla autumnalis tetraploids was investigated by observing synaptonemal complexes from serial sections and whole-mount surface spreads. In one of the tetraploids with the genomic constitution AAB7B7 pairing at first metaphase of meiosis is restricted to homologous chromosomes and only bivalents appear. In the second tetraploid, AB7B7B7, some multivalents are found at first metaphase, but most of the configurations are bivalent. The bivalents, however, result from chiasma formation between homoeologous as well as homologous chromosomes. Both tetraploids show extensive multivalent formation at zygotene. The transformation of these multivalents to bivalents is described and interpreted in the two tetraploid forms.Key words: Scilla autumnalis, synaptonemal complex, multivalents, elimination, polyploids.

Synaptonemal complex formation among supernumerary B chromosomes: an electron microscopic study on spermatocytes of Chinese raccoon dogs

The diploid chromosome number of the Chinese raccoon dog varies from 54 (no B chromosomes) to 58 (4 B chromosomes). The B chromosomes are totally heterochromatic. An electron microscopic study was made of the synaptonemal complexes (SC) in spermatocytes of these animals. The SC karyotype consists of 27 regular chromosome pairs (autosomes and the sex chromosomes) plus the B chromosomes. The Bs pair effectively with one another at pachytene, but the SC axes of the B chromosomes are much denser than those of the A chromosomes. Depending on the number of Bs, both bivalents and multivalents have been observed. When three B chromosomes are present in a cell, parallel alignment of all three SCs can be seen. Formation of multivalents indicates high homology among these supernumerary heterochromatic chromosomes. Fusiform bulges are found along unpaired regions of all chromosomes which are particularly pronounced in diplotene.

The effect of substitution of chromosome 5S1 of Aegilops longissima for its wheat homoeologues on spike morphology and on several quantitative traits

Each pair of homoeologous group 5 chromosomes of common wheat (Triticum aestivum) cv. Chinese Spring was replaced by the homoeologous pair 5S1 of Aegilops longissima. The resulting substitution lines were selected by the isozyme markers SKDH or TPI-2 as well as by the presence of 21 bivalents at meiosis. In lines lacking 5B, the alien chromosome could not compensate for the missing Ph1 allele, as was evident by multivalent formation at meiosis and by reduced fertility. In the absence of 5A, chromosome 5S1 not only failed to compensate for the occurrence of spike speltoidy but enhanced its expression. Chromosome 5S1 had a moderate dosage effect on reducing plant height and promoting earliness. When added to the genome of common wheat, this chromosome induced a considerable increase in grain weight, but it could not compensate for the absence of a pair of any of its homoeologues. Other yield components were negatively affected in both addition and substitution lines, as well as in other aneuploids. Grain protein percentage of the different lines was negatively related to their yield per spike. This rendered difficult the evaluation of genes for grain protein percentage. The implication of the obtained results on the use of alien genes for wheat improvement is discussed.Key words: substitution lines, wheat, Triticum aestivum, Aegilops longissima, alien chromosomes, quantitative traits.

The control of chiasma formation in colchicine treated meiocytes of Senecio squalidus.

Colchicine induced multivalent formation is used to examine the effects of multivalent formation on chiasma frequency in Senecio squalidus. The increases observed are interpreted as the results of increases in numbers of pairing segments or decreases of chiasma interference following pairing partner exchange.

Cytogenetic analysis of structural rearrangements in three varieties of common wheat, Triticum aestivum

The winter wheat varieties 'Starke' and 'Cappelle Desprez' and the spring wheat 'Chinese Spring' were analysed for structural chromosome rearrangements that resulted in the formation of multivalents in F1 hybrids. The analyses were carried out using hybrids involving euploids, monosomic and ditelosomic stocks, and double-monotelodisomic constructs. The study confirmed that 'Cappelle Desprez' differs from 'Chinese Spring' in a reciprocal translocation between chromosomes 5B and 7B (Riley et al. 1967); a translocation involving chromosomes 3B and 3D could not be verified. Furthermore, the analysis showed that 'Starke' differs from 'Chinese Spring' in a reciprocal translocation between chromosomes 7A and 7D. Both translocations have a coefficient of multivalent realisation of about 0.84. Further multivalents in euploid 'Starke', in euploid and some aneuploid stocks of 'Cappelle Desprez', and in euploid as well as various types of aneuploid hybrids between all three varieties could nearly all be explained hypothesizing that chromosome 2B of both 'Starke' and 'Cappelle Desprez' is a duplication-deficiency chromosome. In the hypothesis a part of the long arm of 2B is missing and replaced by a duplicated part of the long arm of chromosome 2D. The multivalents of this rearrangement showed an average coefficient of realisation of about 0.09.

Cytology of triploid and pentaploid hybrids of Elymus trachycaulus × Pseudoroegneria spicata and P. spicata ssp. inermis

Emasculated clones of tetraploid and octaploid Elymus trachycaulus crossed with Pseudoroegneria spicata yielded 69 triploids and 13 pentaploid hybrids. The triploid hybrids were morphologically intermediate between the parental species but the pentaploid hybrids resembled E. trachycaulus more closely than P. spicata. A very high degree of plant size variability (very weak to very vigorous) was observed among the 50 triploids that survived to maturity. The variability among the pentaploids, though apparent, was not high. Mean chromosome associations among the selected eight triploids varied from low multivalent formation at meiosis (7.60 I + 6.18 II + 0.24 III + 0.08 IV) to high multivalent-forming meiosis (7.28 I + 5.66 II + 0.32 III + 0.36 IV). The bivalent configurations in these triploids were attributed to homoeologous pairing between the S1 genome of P. spicata and S genomes of E. trachycaulus and the multivalents indicated intergenomal and intragenomal pairings. Mean chromosome associations of 5.22 I + 11.94 II + 1.97 III in 78.7% and 7 I + 14 II in 21.3% of the 150 cells were distributed among the three pentaploid hybrids. The low frequency of trivalents and the absence of multivalents higher than trivalent configurations suggest that homoeologous pairing was substantially reduced and there was no intergenomal and intragenomal pairing in these 2n = 35 x hybrids. Fourteen triploids and three pentaploid hybrids were produced from 4x and 8x forms of E. trachycaulus and P. spicata spp. inermis crosses. The triploids were intermediate between the parents but the pentaploids were more similar to E. trachycaulus. Mean chromosome associations in the triploids (6.020 I + 5.315 II + 0.554 III + 0.235 IV + 0.250 V + 0.048 VI + 0.03 VII) and in the pentaploids (3.50 I + 10.18 II + 3.54 III + 0.13 IV) indicated a very high degree of intergenomal and intragenomal pairing in the triploids; nevertheless, it was greatly reduced in the pentaploids. The Pseudoroegneria spicata ssp. inermis genome may have gene(s) that affect a high degree of intergenomal and intragenomal pairing in triploid hybrids with E. trachycaulus. However, in pentaploid hybrids (SS HH S2) the S and H genomes of E. trachycaulus in double dose seem to have restored its regular meiotic bahaviour, at least partially, and reduced homoeologous pairing as well as intergenomal and intragenomal pairing. Key words: Elymus trachycaulus, Pseudoroegneria spicata, regular meiotic behaviour, double dose, intergenomal pairing, intragenomal pairing.

Cytogenetic studies of aneuploids in rapeseed. II. Morphology, fertility, and cytology of self-pollinated progenies of monosomic plants of Brassica napus L.

Cytogenic studies were conducted on self-pollinated progenies from two monosomic lines of Brassica napus to ascertain the mode of transmission of the monosomic chromosome. An approximately 1:1 ratio of disomic to monosomic plants was obtained. Only one nullisomic plant was found among a total of 71 plants analyzed. The 1:1 ratio and the anaphase I disjunction (19–18) suggest that the gametes (n) were twice as competitive or functional as their counterpart (n − 1). Since quadrivalents were observed in the disomic plants and only trivalents in the monosomics, the monosomic chromosomes were believed to be the ones that were involved in the formation of multivalents. The percentage of multivalent association in monosomic plants was almost double that in disomic plants. In the disomic plants, an average of 11.96% of pollen mother cells had one to two quadri-valents. The average frequency of trivalents in the monosomics was 23.63%. It was suggested that pairing of these homologs was genetically controlled with a dosage effect. The absence of one chromosome increased the frequency of multivalents. Key words: monosomic, Brassica, rapeseed, aneuploid.

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

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.

Synaptonemal complex formation in hybrids of Lolium temulentum x 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.

Induced colchiploidy in Nicandra physaloides.

Using colchicine (0.2%) as the polyploidizing agent colchiploid Nicandra physaloides was raised. The induced autoploids were evaluated on such parameters, as morphology, cytology, pollen and plant sterility. The colchicine was given either by soaking the seeds in the solution or by wetting cotton plugs with the solution placed on apical meristems. Treated plants initially showed impaired growth which later improved and ultimately showed gigantism. The pollen grains and stomata, however, were consistently bigger in size.Despite the presence of multiple homologues, the autotetraploids tended to avoid multivalent formation and produced exclusively bivalents which appeared to help normal disjunction of chromosomes at anaphase. Yet, abnormalities appeared due to the mutagenic effect of the chemical; the consequences of these being in-creased pollen sterility which lowered plant fertility. From the seeds of C1, C2 plants could be generated but majority of them showed reversion to the diploid state. The gigas characters of C1 also tended to lessen in C2 generation. Con-comitantly a general improvement in pollen and plant fertility was noticed in this generation.

Autosyndetic pairing in Gibasis (Commelinaceae) hybrids revealed by C-banding

Two closely related species of Gibasis, G. karwinskyana and G. consobrina, and their F1 hybrids were studied cytologically at the diploid and tetraploid level. Despite similarity in their basic karyotype, pairing was extremely limited in the diploid hybrid and almost exclusively autosyndetic in the tetraploid, except for multivalent formation due to interchange heterozygosity. The analysis was considerably facilitated by the use of C-banding techniques at meiosis, by which the chromosomes of each species could be readily identified. In the parents, quadrivalents were formed between homologous but non-identical chromosomes, which also formed autosyndetic bivalents in the hybrids. Meiotic pairing in the hybrids was unaffected by polytypy for C-bands among different populations of the parental species.

Induced autotetraploidy in Chrysanthemum coronarium Linn.

Cytogenetical studies were carried out in colchicine induced autotetraploids of Chrysanthemum coronarium. Autotetraploids are found to be inferior and do not show any increase in size of its parts. Meiosis is characterised by multiple associations ranging from octavalent to univalents. There is moderate frequency of multivalent formation. Though there is tendency for normal distribution at A-I and high pollen fertility, viable achenes are not produced.

Effects of seed-irradiation on meiosis in M1 plants of Lactuca and related plants

Meiosis was studied in control and seed-irradiated materials of Lactuca sativa, cultivars Tom, Cos and Webbs, L. serriola from the wild, and Cichorium intybus. In meiosis of the M1 plants from irradiated seed, observations of univalent and multivalent formation, bridge formation at Anaphase I and II, micronucleus formation and persistence, and pollen fertility, were carried out, and compared with controls. Quantitative estimations of bridge formation were made in L. sativa Tom and Cos and in L. serriola, at anaphase I only. It was found that the frequency of anaphase I bridges and reduction of pollen fertility in the M1 and also M2 generation, were different for each variety or species; these could be sequenced radiation-resistant to sensitive in the order Tom, Cos and L. serriola.

New basic chromosome numbers and cytology for some scrophulariacious weeds of Bihar (India).

New chromosome numbers and detailed cytology for seven weeds (Scrophulariaceae) from Bihar are reported here. First counts are reported for five species belonging to five different genera Centranthera tranquobarica n=15, Dopatrium junceum n=14, Limnophila indica n=16, Lindernia viscosa n=10 and Suterad issecta n=12 and new numbers are also reported for two species of one genus Lindernia i.e. Lindernia crustacea n=14 and Lindernia multiflora n=7. Meiosis in general was non-synchronized. And anomalies like univalent and multivalent formation, secondary association of chromosomes etc, have been observed in some pollen mother cells (PMCs) at metaphase 1. In most of the weeds pollen grains were found to be of variable size.

Synaptonemal complexes as indicators of induced structural change in chromosomes after irradiation of spermatogonia

The incidence of multivalent formation has been compared in primary spermatocytes at pachytene, using synaptonemal complex preparations and at diakinesis/metaphase, using air-dried preparations, from testes of Syrian hamsters several weeks after exposure to an acute dose of X-rays. The pachytene preparations revealed nearly twice the number of multivalents found at diakinesis/metaphase I, and showed the ability to reveal types of structural alteration undetectable in conventional air-dried techniques.