Cytogenetic studies on <Emphasis Type="Italic">Metasequoia glyptostroboides</Emphasis>, a living fossil species

The chromosome morphology and meiotic pairing behavior in the pollen mother cells (PMCs) of Metasequoia glyptostroboides were investigated. The results showed that: (1) The chromosome number of the PMCs was 2n=22. (2) The PMCs developed in the successive manner, and the nucleoids in the dynamic development were similar to those of the other gymnosperms. (3) At prophase, most of the chromosomes were unable to be identified distinctively because the chromosomes were long and tangled together. The chromosome segments were paired non-synchronously. At pachytene, the interstitial or terminal regions of some bivalents did not form synapsis and the paired chromosomes showed difference in sizes, indicating that there were structure differences between the homologous chromosomes. (4) At diakinesis, the ring bivalents showed complicated configurations due to the differences in location and number of chiasmata. In addition, there were cross-linked bivalents. (5) At metaphase I, the chromosome configuration of each cell was 8.2II ⁰ + 1.1II + 1.3II ⁺ + 0.8I. Most of the chromosomes were ring bivalents, but some were cross-linked bivalents, rod bivalents, or univalents. (6) 15\% PMCs at anaphase I and 22\% PMCs at anaphase II presented chromosome bridges, chromosome fragments, micronuclei, and lagging chromosomes. Twenty seven percent microspores finally moved into one to three micronuclei. Twenty five percent pollens were abortive. The results indicated that the observed individual of M. glyptostroboideswas probably a parpcentric inversion heterozygote, and there were structural and behavioral differences between the homologous chromosomes. The chromosomal aberration of M. glyptostroboidesmay play an important role in the evolution of this relict species, which is known as a living fossil. Further evidence is needed to test whether the differences between homologous chromosomes were due to hybridization.     

Cytological studies on meiosis and male gametophyte development in autotetraploid cucumber

With improved staining and chromosome preparation techniques, meiosis of pollen mother cells (PMCs) and male gametophyte development in autotetraploid cucumber (Cucumis sativus L.) was studied to understand the correlation between chromosomes behaviour and fertility. Various chromosome configurations, e.g. multivalent, quadrivalents, trivalents, bivalents and univalents were observed in most PMCs at metaphase I. Lagging chromosomes were frequently observed at anaphase in both meiotic divisions. In addition, chromosomes segregations were not synchronous and equal in some PMCs during anaphase II and telophase II. Dyads, triads, tetrads with micronuclei and polyads were observed at tetrad stage, and the frequencies of normal tetrad with four microcytes were only 55.4 %. The frequency of abnormal behaviour in each stage of meiosis was counted, and the average value was 37.2 %. The normal meiotic process could be accomplished to form the microspore tetrads via simultaneous cytokinesis. Most microspores could develop into fertile gametophytes with 2 cells and 3 germ pores through the following stages: single-nucleus early stage, single-nucleus late stage and 2-celled stage. The frequency of abnormalities was low during the process of male gametophyte development. The germination rate of pollen grains was 46.9 %. These results suggested that abnormal meiosis in PMCs was the reason for low pollen fertility in the autotetraploid cucumber.     

东乡野生稻与栽培稻正反交种间杂种F1的雄配子发生与发育

该文以东乡野生稻与栽培稻(美国光壳稻P1003)的正反交种间杂种F₁(正交为光壳稻P1003×东乡野生稻; 反交为东乡野生稻×光壳稻P1003)为试材, 研究其各个减数分裂时期的染色体行为特征、染色体交换重组及雄配子发育特点。结果表明: 正反交杂种F₁花粉母细胞细胞核减数分裂的同步性较高, 细胞质为连续型胞质分裂类型。在细胞核分裂的过程中, 核仁在前期I到中期I逐渐消失。染色体在前期I到中期I逐渐收缩, 变得更加清晰可见。在终变期和中期I, 90.54%以上的花粉母细胞能形成12个二价体(含少数棒状二价体和8字型二价体), 部分花粉母细胞(正交9.46%, 反交7.65%)出现少量的单价体、三价体和四价体。后期I观察到1.27%–1.35%的花粉母细胞含有1至数条落后染色体。最终有92.6%–94.8%的小孢子能发育成形态正常、染色能力强的成熟花粉粒。另外, 在正交杂种F₁的粗线期中存在2个核仁, 而反交杂种F₁及其双亲的粗线期只有1个核仁。这些研究结果可为作物品种改良、种质资源创新以及种间亲缘关系研究提供细胞学证据。     

Asynchronous meiotic rhythm as the cause of selective chromosome elimination in an interspecific <Emphasis Type="Italic">Brachiaria</Emphasis> hybrid

This paper reports the occurrence of chromosome elimination during microsporogenesis in an interspecific hybrid between a sexual diploid accession (SEX) of Brachiaria ruziziensis (2n=2x=18) and an apomictic tetraploid accession (APO) of B. brizantha (2n=4x=36). Meiosis was very abnormal in the triploid hybrid (2n=3x=27); we observed a distinct asynchrony from metaphase I to the end of meiosis. The APO and the SEX genomes did not show the same meiotic rhythm. When the former, with nine bivalents, was in metaphase I, the nine SEX univalents were not yet aligned; when the latter reached the plate, the APO genome was already in anaphase. In subsequent stages, the APO genome had reached the poles while the SEX was undergoing sister-chromatid segregation. As the SEX genome always remained temporally behind, it gave rise to one extra-nucleus in each pole. In the second division, the behavior was the same but anaphase II did not occur for the SEX genome, and only one extra-nucleus was observed in each cell in telophase II. Chromosome elimination for the SEX genome ranged from partial to total. The importance of these findings with respect to Brachiaria breeding programmes is discussed.     

Cytogenetics of Crotalaria VI. Chiasma frequency and position,and univalent behaviour in a (partially) asynaptic mutants of C. juncea

In Crotalaria juncea (n=8) a plant exhibiting partial asynapsis was isolated in the M₁ of a combined treatment of 50 kR gamma rays +0.2% EMS. The majority (48.14%) of PMCs at diplotene, diakinesis and metaphase I had 16 univalents. The bivalents in the asynaptic mutant were always rod-shaped with one terminal chiasma. In comparison, controls had on the average 7.08 ring bivalents. The asynapsis is genetically controlled, monofactorially recessive, and it is concluded that chromosome pairing is interrupted at a very early stage. There is a possible correlation between the number of bivalents and the arrangement of the univalents at metaphase I. When there were less than four bivalents, the univalents tended to be polar, and when there were more than four, the univalents were more equatorial in arrangement. The arrangement of univalents was random and apparently not influenced by the bivalents, when their number (4) was exactly half the zygotic number.     

Diploidisation ofLotus corniculatus L. (Fabaceae) by elimination of multivalents

Lotus corniculatus L. (Fabaceae) is a natural tetraploid of probably hybrid origin, which regularly forms bivalents at metaphase I of meiosis. Whole-mount surface-spreading of synaptonemal complexes (SCs) under the electron microscope reveals that diploidisation of this spccies is achieved not by exclusive pairing of homologues during meiotic prophase, but by the elimination of multivalents in favour of bivalents before metaphase I. Observations show that 43% of multivalents are eliminated between zygotene and pachytene, presumably by dissolution and reassembly of SCs between homologous chromosomes. A further 63% are eliminated between pachytene and diakinesis, with a commensurate increase in the number of univalents. Elimination ensures few multivalents reach first metaphase and effectively diploidises this tetraploid.     

Differential meiotic behaviour of diploid and tetraploid cells in a partially asynaptic mutant

A partially asynaptic individual of the grasshopperEuchorthippus pulvinatus (2n = 16 + X) was analysed at meiosis, using a Giemsa C-banding technique. Long chromosomes formed univalents less frequently than did medium and short ones. Homologues which succeeded in forming bivalents showed reduced chiasma frequency, the long chromosomes being affected by most. Changes in chiasma distribution were also observed. The presence of univalents at metaphase I seems to affect the function of the spindle, since most cells at the second division were unreduced. Cytokinesis was also subsequently suppressed in a great number of these products of restitution, resulting in the formation of diploid and tetraploid spermatids. Fifteen tetraploid metaphase I cells were also found in which pairing level and chiasma frequency were almost twice the average value in normal diploid individuals. The nature of this mutant is discussed.     

Premiers stades de l'oogenèse de Rhabdophaga saliciperda (Cecidomyiidae,Diptera)

The females of Rhabdophaga saliciperda have in their somatic cells 8 chromosomes and the males 6. The type of sex determination is therefore: X₁X₁X₂X₂—♀; X₁X₂—♂. The cells of the germinal line have 46 chromosomes, but a variation of their number was observed. In the oogonia and spermatogonia the number of heterochromatic chromosomes may exceed the number of E chromosomes, i.e. 8. In the beginning of the growth stage of the oocytes an incorporation of somatic cells was observed. The nuclei of these somatic cells persist in the cytoplasm of the oocytes until the maturation divisions. The possibility of their participation in the reconstruction of the nucleus of the mature egg is envisaged. The metaphase of the I segmentation division has a complex character. During prophase of the first meiotic division the E chromosomes form 4 bunches of 6–8 chromosomes each. Some univalents may also be present. The 8 S chromosomes form 4 regular bivalents. The 4 groups of E chromosomes persist until metaphase I. During metaphase I a phenomenon of expulsion of the majority of E chromosomes from the metaphase spindle was observed. The 4 bivalents remain in the equatorial plain of the spindle with some E Chromosomes. After this expulsion 2 groups of chromosomes are formed. In connection with them 2 spindles develop. An irregular distribution of E chromosomes follows without their division. The bivalents are probably separated in regular manner. These 2 spindles correspond to the I maturation division. The II maturation division was not observed because of lack of respective stages.     

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria,Rhabdocoela)

At metaphase I of meiosis in spermatocytes of Mesostoma ehrenbergii ehrenbergii [2n=10] three bivalents and four univalents form. The same two chromosome pairs always form the univalents. Analysis of metaphase I, anaphase I and metaphase II configurations in fixed testis material suggested that the distribution of the four univalents is not a random process but the correct segregation of one member of each pair to each pole is actively achieved before the end of metaphase I. In live preparations of testis material univalents were observed to move between the poles of metaphase I cells, eventually reaching the correct segregation. All cells observed to enter anaphase I had the correct segregation of univalents. It is proposed that the univalent movement during metaphase I is directed towards obtaining the correct segregation of univalents before the cells enter anaphase.     

Meiosis,SC-formation,and karyotype structure in diploidPaeonia tenuifolia and tetraploidP. officinalis

The meiosis of the diploidPaeonia tenuifola and the allotetraploidP. officinalis was studied after conventional methanol/acetic acid-fixation and synaptonemal complex (SC) spreading. Meiosis inP. tenuifolia (2n = 10) is normal with five bivalents in metaphase I, and the SCs in pachytene show regular features. InP. officinalis (2n = 4x = 20) univalents, bivalents and multivalents are found in metaphase I. The SCs reveal several abnormalities: a high number of unpaired lateral elements, partner exchanges between three and four lateral elements, loops and lateral element thickenings. These characteristics are compared with the situations found in other polyploid and hybrid species. It is noteworthy that the abnormalities in meiosis ofP. officinalis are not reflected in its somatic karyotype. Its features were analysed after silver staining and fluorescent staining with chromomycin and compared with those ofP. tenuifolia. Synaptonemal Complex Spreading in Plants2; for part1 see Pl. Syst. Evol.154, 129–136 (1986).     

Low glucosinolate Brassica juncea breeding line revealed to be nullisomic.

The low glucosinolate Brassica juncea breeding line 1058 was derived from a BC1F3 plant of an interspecific cross between high glucosinolate Indian B. juncea (genome AABB, 2n = 36) line 60143 and B. rapa (genome AA, 2n = 20) canola strain CZY. Line 60143 had 2n = 36 chromosomes (18 bivalents at metaphase I) and strain CZY had 2n = 20 chromosomes (10 bivalents). Line 1058 was nullisomic, with 2n - 2 = 34 chromosomes, with 17 bivalents formed at metaphase I and an even chromosomal segregation of 17:17 at anaphase I. In F1 hybrid plants of the cross 1058 x CZY, 98.3% of the pollen mother cells had 10 bivalents and seven univalents. This is evidence that plants of line 1058 are nullisomic, missing one pair of B-genome chromosomes.     

Segregation of the amphitelically attached univalent X chromosome in the spittlebug <Emphasis Type="Italic">Philaenus spumarius</Emphasis>

In meiosis I, homologous chromosomes combine to form bivalents, which align on the metaphase plate. Homologous chromosomes then separate in anaphase I. Univalent sex chromosomes, on the other hand, are unable to segregate in the same way as homologous chromosomes of bivalents due to their lack of a homologous pairing partner in meiosis I. Here, we studied univalent segregation in a Hemipteran insect: the spittlebug Philaenus spumarius. We determined the chromosome number and sex determination mechanism in our population of P. spumarius and showed that, in male meiosis I, there is a univalent X chromosome. We discovered that the univalent X chromosome in primary spermatocytes forms an amphitelic attachment to the spindle and aligns on the metaphase plate with the autosomes. Interestingly, the X chromosome remains at spindle midzone long after the autosomes have separated. In late anaphase I, the X chromosome initiates movement towards one spindle pole. This movement appears to be correlated with a loss of microtubule connections between the kinetochore of one chromatid and its associated spindle pole.     

Studies on male and female meiosis in Indian Allium

Male meiosis in autotetraploid Allium tuberosum (4×=32) is fairly regular, keeping in view its cytological status, with 81 percent of the chromosomes associated in quadrivalents and trivalents. About 5% of the cells have 32 univalents. Anaphase segregation is slightly irregular. While 48% of the pollen mitoses show 16 chromosomes, 87% of the mature pollen is viable as indicated by carmine or iodine staining. — Megaspore mother cells have 64 chromosomes associated in 32 bivalents at metaphase I. Anaphase segregation is normal. In three out of 56 cells studied multivalents, bivalents and univalents are observed as in male meiosis. — It is concluded that the species reproduces by pseudogamous parthenogenesis made possible by meiotic modification. This modification is almost perfect and almost completely specific for female meiosis. Slight effects are observed in male meiosis.     

Cytogenetic studies on natural intergeneric hybridization inAster alliances

Intergeneric hybrids ofAster ageratoides subsp.ovatus (2n=36)×Kalimeris pinnatifida (2n=18) were produced artificially. The chromosomes of the hybrid were found to be 2n=27, and to consist of 9 large chromosomes and 18 small chromosomes. In meiosis of the PMCs of the hybrid, a chromosome configuration of 9_II+9_I was regularly observed. While all the univalents were large, and all the bivalents were comparatively small. The large and small chromosomes ofA. ageratoides subsp.ovatus were found to be rather distant in homology, and the small chromosomes of the subspecies and the chromosomes ofK. pinnatifida were found to have a high degree of homology. The tetraploidovatus was concluded to be an amphidiploid, composed of the large chromosomes ofAster and the small chromosomes ofKalimeris.     

Introgression of chromosomes of Festuca arundinacea var. glaucescens into Lolium multiflorum revealed by genomic in situ hybridisation (GISH)

An F₁ hybrid (n=4x=28) between the tetraploid species Festuca arundinacea var. glaucescens (GGG′G′) and a synthetic tetraploid Lolium multiflorum (LmLmLmLm) was backcrossed to diploid L. multiflorum to produce triploid (2n=3x=21) BC₁ hybrids (LmLmG). At metaphase I of meiosis the triploids had a preponderance of ring bivalents and univalents with some linear and frying-pan trivalents. Genomic in situ hybridisation (GISH) differentiated the Festuca chromosomes from Lolium and revealed that the bivalents were exclusively between Lolium homologues, while the univalents were Festuca. Despite the limited amount of homoeologous chiasmata pairing in the triploids, some recombinant chromosomes were recovered in the second backcross when the hybrids were further crossed to diploid L. multiflorum. The progeny from the second backcross was predominantly diploid. Genotypes with recombinant chromosomes and chromosome additions involving an extra Festuca chromosome were identified using GISH. Changes in plant phenotype were related to the presence of Festuca chromatin. Received: 20 September 2000 / Accepted: 05 January 2001     

Production and characterization of intertribal somatic hybrids of Raphanus sativus and Brassica rapa with dye and medicinal plant Isatis indigotica

Intertribal somatic hybrids of Raphanus sativus (2n = 18, RR) and Brassica rapa spp. chinensis (2n = 20, AA) with the dye and medicinal plant Isatis indigotica (2n = 14, I I) were firstly obtained by polyethylene glycol-induced symmetric fusions of mesophyll protoplasts. One mature hybrid with R. sativus established in field had intermediate morphology but was totally sterile. It had the expected chromosome number (2n = 32, RRI I) and parental chromosomes were distinguished by genomic in situ hybridization (GISH) analysis, and these chromosomes were paired as 16 bivalents in pollen mother cells (PMCs) at diakinesis and mainly segregated equally as 16:16 at anaphase I (A I), but the meiotic disturbance in second division was obvious. Five mature hybrids with B. rapa established in field were morphologically intermediate but showed some differences in phenotypic traits and fertility, two were partially fertile. Cytological and GISH investigations revealed that these hybrids had 2n = 48 with AAIIII complement and their PMCs showed normal pairing of 24 bivalents and mainly equal segregation 24:24, but meiotic abnormalities of lagging chromosomes and micronuclei appeared frequently during second divisions. AFLP analysis showed that all of these hybrids had mainly the DNA banding pattern from the addition of two parents plus some alterations. Some hybrids should be used for the genetic improvement of crops and the dye and medicinal plant.     

Meiotic segregation of chromosomes in Drosophila melanogaster oocytes

A new technique was developed for a light microscopic analysis of meiosis in Drosophila oocytes. — When the nuclear envelope breaks down the bivalents, till then compressed into a karyosome, separate in early prometaphase. The homologues remain associated by chiasmata except for the fourth chromosomes which are no longer associated. Non-homologous chromosomes regularly segregating from each other in genetic experiments are also unconnected after karyosome disintegration but during metaphase I the fourth chromosomes and the heterologous pairs coorient on the same arc of the spindle and move precociously towards opposite poles. Nondisjunction and other irregularities are not infrequent in oocytes having an uneven number of achiasmatic elements. The fourth chromosomes and the Xs or the large autosomes, when lacking chiasmata, may be involved in non-homologous segregation. In c3G homozygotes all chromosomes appear as univalents in prometaphase. Segregation is variable but the observations suggest the polar distribution of equal numbers of chromosomes in variable combinations irrespective of the size. — Coorientation of univalents may be accounted for if the centromeres, whether homologous or non-homologous, are associated in pairs during early meiotic prophase, and that in the karyosome these pairing relationships are preserved until spindle organization at the onset of prometaphase.     

Co-segregation of sex chromosomes in the male black widow spider <Emphasis Type="Italic">Latrodectus mactans</Emphasis> (Araneae,Theridiidae)

During meiosis I, homologous chromosomes join together to form bivalents. Through trial and error, bivalents achieve stable bipolar orientations (attachments) on the spindle that eventually allow the segregation of homologous chromosomes to opposite poles. Bipolar orientations are stable through tension generated by poleward forces to opposite poles. Unipolar orientations lack tension and are stereotypically not stable. The behavior of sex chromosomes during meiosis I in the male black widow spider Latrodectus mactans (Araneae, Theridiidae) challenges the principles governing such a scenario. We found that male L. mactans has two distinct X chromosomes, X₁ and X₂. The X chromosomes join together to form a connection that is present in prometaphase I but is lost during metaphase I, before the autosomes disjoin at anaphase I. We found that both X chromosomes form stable unipolar orientations to the same pole that assure their co-segregation at anaphase I. Using micromanipulation, immunofluorescence microscopy, and electron microscopy, we studied this unusual chromosome behavior to explain how it may fit the current dogma of chromosome distribution during cell division.     

Meiotic studies in Acanonicus hahni (Stål) (Coreidae,Heteroptera) I. Behaviour of univalents in desynaptic individuals

Male meiosis was studied in a population of Acanonicus hahni (Stål), and nine of the sixteen individuals analyzed showed desynapsis. The frequency of univalents varied from one to seven percent in eight of them, while in the ninth the percentage of cells with univalents was higher (12%). The univalents auto-orientate at metaphase I in the center of the ring formed by autosomal bivalents and divide equationally at anaphase I; at metaphase II they show touch-and-go pairing, and lie in the center of the ring of autosomes.A desynaptic origin of the univalents is proposed, and the arrangement of the chromosomes in the first and second metaphase plate in the normal and desynaptic individuals is compared and discussed. The meiotic characteristics of these desynaptic individuals are also compared with those described in other insects with holocentric and monocentric chromosomes. It is suggested that any achiasmatic chromosome, whether a univalent, m or sex chromosome, will induce the formation of a ring and with some or all of them lying in its centre.     

An apomictic polyhaploid obtained from a pearl millet x Pennisetum squamulatum apomictic interspecific hybrid

Summary In a research program to transfer apomixis from Pennisetum squamulatum Fresen to pearl millet, P. americanum L. Leeke, a polyhaploid plant (2n=21) was discovered in the uniform open-pollinated progeny of an apomictic interspecific hybrid (2n = 41) between pearl millet and P. squamulatum. The polyhaploid was shorter, less vigorous and was smaller morphologically than its maternal parent. It probably originated by parthenogenetic development of a reduced gametophyte in the apomictic interspecific hybrid. The most common metaphase I chromosome association in the polyhaploid was 4 bivalents plus 13 univalents. Irregular chromosome distribution, tripolar spindles, bridges and fragments were observed at anaphase I and telophase I. The polyhaploid was male-sterile and partially female- fertile having multiple aposporous embryo sacs in 95% of the ovules. Seed set was 3% when open-pollinated and 33% when pollinated with pearl millet pollen. Low seed set was due to competition among multiple embryos developing in the same ovule. Seventeen progeny from seed produced under open-pollination on the polyhaploid each had 2n=21 chromosomes and were morphologically uniform and identical to the female parent. The expression of obligate apomixis in the polyhaploid conditioned by the P. squamulatum genome between the simplex and duplex condition indicates that apomictic reproduction is possible in nonpolyploid plants.