Meiotic chromosome pairing behaviour of natural tetraploids and induced autotetraploids of Actinidia chinensis

Key message

Non-preferential chromosome pairing was identified in tetraploid Actinidia chinensis and a higher mean multivalent frequency in pollen mother cells was found in colchine-induced tetraploids of A. chinensis compared with naturally occurring tetraploids.

Abstract

Diploid and tetraploid Actinidia chinensis are used for the development of kiwifruit cultivars. Diploid germplasm can be exploited in a tetraploid breeding programme via unreduced (2n) gametes and chemical-induced chromosome doubling of diploid cultivars and selections. Meiotic chromosome behaviour in diploid A. chinensis ‘Hort16A’ and colchicine-induced tetraploids from ‘Hort16A’ was analysed and compared with that in a diploid male and tetraploid males of A. chinensis raised from seeds sourced from the wild in China. Both naturally occurring and induced tetraploids formed multivalents, but colchicine-induced tetraploids showed a higher mean multivalent frequency in the pollen mother cells. Lagging chromosomes at anaphase I and II were observed at low frequencies in the colchicine-induced tetraploids. To investigate whether preferential or non-preferential chromosome pairing occurs in tetraploid A. chinensis, the inheritance of microsatellite alleles was analysed in the tetraploid progeny of crosses between A. chinensis (4x) and A. arguta (4x). The frequencies of inherited microsatellite allelic combinations in the hybrids suggested that non-preferential chromosome pairing had occurred in the tetraploid A. chinensis parent.     

Further insights on chromosomal pairing of autopolyploids: a triploid and tetraploids of rye

Chromosomal pairing of one triploid and three tetraploid plants of rye, Secale cereale, was analyzed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I cells. Prophase I is characterized by: (i) the weak alignment showed by the three or four unsynapsed or partially homologous synapsed axes; (ii) the low number ber of pairing partner switches (PPSs) displayed by both trivalents and quadrivalents; and (iii) the existence of complex multivalents in which up to 13 chromosomes in the triploid and 22 chromosomes in the tetraploids were involved. However, only few heterologous chromosomal associations were maintained at metaphase I. The results obtained are discussed under the assumptions of the random end pairing model with some modifications.     

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.     

Meiosis in haploid barley — An interpretation of non-homologous chromosome associations

Detailed meiotic studies were conducted on ten haploid plants representing six different genotypes of barley (Hordeum vulgare, 2n=14). At pachytene stages the non-homologous chromosomes were observed to pair as intimately as homologous chromosomes in many cells. Foldback pairing, involving single chromosomes, and multivalent associations were common. At diplotene, up to 4 chiasmatalike structures were observed in paired chromosomes but it is not likely that they resulted from crossing over. At diakinesis the bivalent frequency mean was from 1 to 1.3 per cell whereas by metaphase I the paired associations were rare with a single rod bivalent being observed in 3 to 5% of the cells. The frequencies of various types of secondary associations at metaphase were also recorded. — The origin and significance of bivalents and secondary associations in haploids is reviewed and discussed. Caution is urged in the interpretation that low levels of chromosome pairing in haploids is evidence of homology. It is concluded that very little chromosome duplication is likely to be found within the haploid set of barley chromosomes and that the basic chromosome number is seven.     

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.     

The genetics of meiotic chromosome pairing in Lolium temulentum x Lolium perenne tetraploids

Summary The degree of preferential pairing of homologous chromosomes was estimated in a series of tetraploid hybrids of Lolium temulentum x Lolium perenne by means of cytological and genetic analyses. The correlations between the frequency of bivalents at first metaphase of meiosis in the hybrid tetraploids and the degree of preferential pairing calculated from the segregation pattern of isozyme alleles in a test cross was extremely high. The results showed clearly that suppression of heterogenetic pairing in these Lolium tetraploids is achieved by a genetic system involving the A chromosomes as well as the B chromosome system which has been known for some time. Certain similarities with the genetic system controlling pairing in polyploid wheats are discussed.     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on chromosome pairing at meiosis was investigated in the species hybrid Lolium temulentum x L. perenne at both the diploid and tetraploid level. The presence of B chromosomes drastically reduced association of homoeologous chromosomes in both the diploids and tetraploids. This was evident from the high frequency of univalents recorded in PMC's of diploid hybrids with B's and from the predominantly bivalent association of homologous chromosomes in tetraploids of this type. In the absence of B's homoeologous pairing was extensive giving a high frequency of bivalents in the diploids and multivalents as well as bivalents and univalents in the tetraploids.     

A mixed polyploid model for linkage analysis in outcrossing tetraploids using a pseudo-test backcross design.

Based on how chromosomes pair at meiosis, the nature of polyploids can be described by bivalent polyploids, multivalent polyploids, and mixed polyploids. In bivalent polyploids, only two chromosomes pair, during which two more similar chromosomes have a higher pairing probability (preferential pairing) than two less similar chromosomes, whereas in multivalent polyploids more than two chromosomes pair at a time, which results in double reduction. Preferential chromosome pairings and double reduction affect the frequencies of gamete formation and, therefore, linkage analysis of polymorphic markers in bivalent and multivalent polyploids, respectively. For mixed polyploids, in which both bivalent and multivalent formations occur simultaneously, linkage analysis is affected by both preferential pairings and double reduction. In this study, we develop a hierarchical maximum likelihood model for discerning gamete genotypes derived from different pairing mechanisms and different formation modes. The first-stage model in the hierarchy is formulated to characterize the relative frequencies of bivalent and multivalent pairing configurations in terms of the preferential pairing factor. The second-stage model is derived to rule out identical gamete genotypes into their different formation modes with relative probabilities determined by the recombination fraction. The first-stage pairing mechanism and second-stage formation mode are integrated to provide the simultaneous maximum likelihood estimates of the preferential pairing factor, the frequency of double reduction, and the recombination fraction, by implementing the EM algorithm. We performed extensive simulation studies to demonstrate the statistical properties of our hierarchical model for linkage analysis in tetraploids. The implications of our model for polyploid linkage mapping are discussed.     

Intraspecific hybridization and its bearing on chromosome evolution inVicia narbonensis (Fabaceae)

Nine accessions ofVicia narbonensis, considered to be the wild progenitor of faba bean (Vicia faba), were investigated to ascertain the nature and extent of intraspecific karyotypic polymorphism. The chromosome complements resolved into four distinct types (A, B, C, D), and the meiotic data of F₁ hybrids (A × B, B × C, A × C) revealed that alteration in chromosome morphology is the result of segmental interchanges. The interchange complexes indicate that the parents differ from each other by 1 to 2 interchanges. It is also evident that karyotype B, and not A as previously reported, is the normal karyotype of the species, and A and C are single homozygotes for unequal interchange. The comparative karyomorphology of the parents and the hybrids, and of two interchange heterozygotes of four chromosomes each in F₁ hybrids of A × C shows that the chromosomes involved in the single interchange homozygotes (A, C) are not common and the breaks in both interchanges occurred in short and long arms of the involved chromosomes. Identification of the interchanged chromosomes in the complements and the frequency of ring and chain quadrivalents in the heterozygotes enabled location of the breakpoints. The present results provide probably the first example indicating that interchange homozygosity (A) is not only firmly established but also has enabled the species to spread further by adapting to a wide range of habitats. — The genetic relationships between A and D are very different. All seven chromosome pairs in D could be distinguished from A, and for that matter, B and C as well. From the meiotic pairing properties it is also amply clear that genome D is well differentiated from A and possibly B, and C, and deserves special status.     

Cytology and origin of a tetraploid Trichosanthes palmata Roxb.

Cytology of male plants of a tetraploid Trichosanthes palmata Roxb. has been studied. The plants showed 44 chromosomes in root tip cells. Chromosomal associations during diakinesis and early meiotic metaphase were studied. Chromosome number and morphology, high multivalent frequency, meiotic irregularities, general morphological characters and larger size of the stomata and epidermal cells compared to the diploid relative suggest that this collection is of possible autopolyploid origin.Although the plants are dioecious, cytologically distinet sex chromosomes were not detected neither in the diploids reported earlier, nor in the tetraploids described in the present study.     

Partial diploidization of meiosis in autotetraploid Arabidopsis thaliana

Meiosis was analyzed cytogenetically in autotetraploids of Arabidopsis, including both established lines and newly generated autotetraploid plants. Fluorescent in situ hybridization with 5S and 45S rDNA probes was used to identify the different chromosomes at metaphase I of meiosis. Multivalents were observed frequently in all the lines analyzed, but there were significant differences in multivalent frequency not only between the newly generated tetraploids and the established lines but also among the different established lines. The new tetraploids showed high multivalent frequencies, exceeding the theoretical 66.66% predicted by the simple random-end pairing model, in some cases significantly, thus indicating that Arabidopsis autotetraploids have more than two autonomous pairing sites per chromosome, despite their small sizes. The established lines showed fewer multivalents than the new autotetraploids did, but the extent of this reduction was strongly line and chromosome dependent. One line in particular showed a large reduction in multivalents and a concomitant increase in bivalents, while the other lines showed lesser reductions in multivalents. The reduction in multivalents was not uniformly distributed across chromosomes. The smaller chromosomes, especially chromosomes 2 and 4, showed the most marked reductions while the largest chromosome (1) showed virtually no reduction compared to the new tetraploids. It is concluded that the established autotetraploid lines have undergone a partial diploidization of meiosis, but not necessarily genetical diploidization, since their creation. Possible mechanisms for the resulting change in meiotic chromosome behavior are discussed.     

Non-homologous chromosome pairing and crossover formation in haploid rice meiosis

While many studies have provided significant insight into homolog pairing during meiosis, information on non-homologous pairing is much less abundant. In the present study, fluorescence in situ hybridization (FISH) was used to investigate non-homologous pairing in haploid rice during meiosis. At pachytene, non-homologous chromosomes paired and formed synaptonemal complexes. FISH analysis data indicated that chromosome pairing could be grouped into three major types: (1) single chromosome paired fold-back as the univalent structure, (2) two non-homologous chromosomes paired as the bivalent structure, and (3) three or more non-homologous chromosomes paired as the multivalent structure. In the survey of 70 cells, 65 contained univalents, 45 contained bivalents, and 49 contained multivalent. Moreover, chromosomes 9 and 10 as well as chromosomes 11 and 12 formed non-homologous bivalents at a higher frequency than the other chromosomes. However, chiasma was always detected in the bivalent only between chromosomes 11 and 12 at diakinesis or metaphase I, indicating the pairing between these two chromosomes leads non-homologous recombination during meiosis. The synaptonemal complex formation between non-homologs was further proved by immunodetection of RCE8, PAIR2, and ZEP1. Especially, ZEP1 only loaded onto the paired chromosomes other than the un-paired chromosomes at pachytene in haploid.     

Species relationships in the avenae

In order to assess the genome homologies of a number of diploid and tetraploid species of Avena, two meiotic characters — mean chiasmata per cell and frequency of types of pairing configurations — have been studied in the species and in a number of diploid, triploid and tetraploid hybrids. The results indicate extensive structural differentiation of the genome of A. longiglumis from that which is common to the other diploids A. strigosa, A. brevis, A. hirtula, A. glabrata and A. wiestii. Structural differentiation is found also between the genomes of the three tetraploids A. vaviloviana, A. abyssinica and A. barbata. Chromosome pairing in triploid hybrids indicates the similarity of the genome in the A. strigosa group to one of those in the tetraploids and a partial but significant affinity with the other. These data, though derived from a very limited range of genotypes, lead to the conclusions that (a) structural differentiation of chromosomes may be common in the genus and important in its evolution, and (b) that current ideas on evolution of the polyploid species through simple allopolyploidy are unlikely to be true. The polyploids probably have a more complex origin in which autopolyploids or near autopolyploids and structural change of chromosomes have played a part.     

Nondisjunction, disturbances in spindle structure, and characteristics of chromosome alignment in maturing oocytes of mice heterozygous for Robertsonian translocations

To correlate the chromosomal constitution of meiotic cells with possible disturbances in spindle function and the etiology of nondisjunction, we examined the spindle apparatus and chromosome behavior in maturing oocytes and analyzed the chromosomal constitution of metaphase II-arrested oocytes of CD/Cremona mice, which are heterozygous for a large number of Robertsonian translocation chromosomes (18 heterobrachial metacentrics in addition to two acrocentric chromosomes 19 and two X chromosomes). Spreading of oocytes during prometaphase 1 revealed that nearly all oocytes of the heterozygotes contained one large ring multivalent, apart from the bivalents of the two acrocentric chromosomes 19 and the X chromosomes, indicating that proper pairing and crossing-over between the homologous chromosome arms of all heterobrachial chromosomes took place during prophase. A large proportion of in vitro-matured oocytes arrested in metaphase II exhibited numerical chromosome aberrations (26.5% hyperploids, 40.8% hypoploids, and 6.1% diploids). In addition, some of the oocytes with euploid chromosome numbers (26.5% of the total examined) appeared to be nullisomic for one chromosome and disomic for another chromosome, so that aneuploidy levels may even be higher than expected on the basis of chromosome counts alone. Although oocytes of the complex heterozygous mice seemed able initially to form a bipolar spindle during first prometaphase, metaphase I spindles were frequently asymmetrical. Chromosomes in the multivalent did not align properly at the equator, centromeres of neighboring chromosomes in the multivalent remained maloriented, and pronounced lagging of chromosomes was observed at telophase I in oocytes obtained from the Robertsonian translocation heterozygotes. Therefore, disturbance in spindle structure and chromosome behavior appear to correlate with the chromosomal constitution in these oocytes and, ultimately, with failures in proper chromosome separation. In particular, reorientation appears to be a rare event, and malorientation of chromosomes may remain uncorrected throughout prometaphase, as we could not find many typical metaphase I stages in heterozygotes. This, in turn, could be the basis for malsegregation at anaphase and may ultimately induce a high rate of nondisjunction and aneuploidy in the oocytes of CD/Cremona mice, leading to total sterility in heterozygous females.     

A bivalent polyploid model for linkage analysis in outcrossing tetraploids

Polyploids can be classified as either allopolyploids or autopolyploids based on their presumed origins. From a perspective of linkage analysis, however, the nature of polyploids can be better described as bivalent polyploids, in which two chromosomes pair at meiosis, multivalent polyploids, in which more than two chromosomes pair, and general polyploids, in which bivalent and multivalent formations occur simultaneously. In this paper, we develop a statistical method for linkage analysis of polymorphic markers in bivalent polyploids. This method takes into account a unique cytological pairing mechanism for the formation of diploid gametes in tetraploids-preferential bivalent pairings at meiosis during which two homologous chromosomes pair with a higher probability than two homoeologous chromosomes. The higher frequency of homologous over homoeologous pairing, defined as the preferential pairing factor, affects the segregation patterns and linkage analysis of different genes on the same chromosome. A maximum likelihood method implemented with the EM algorithm is proposed to simultaneously estimate linkage and parental linkage phases over a pair of markers from any possible marker cross type between two outbred bivalent tetraploid parents demonstrating preferential bivalent pairings. Simulation studies display that the method can be well used to estimate the recombination fraction between different marker types and the preferential pairing factor typical of bivalent tetraploids. The implications of this method for current genome projects in polyploid species are discussed.     

美味猕猴桃原生质体再生植株细胞遗传学研究 Ⅲ .母株减数分裂前期Ⅰ染色体配对的光镜观察

首次报道在光镜下观察美味猕猴桃 (品种 :No.2 6原生质体植株的母株 )花粉母细胞( PMC)染色体在减数分裂前期的配对 ,发现其配对和凝缩有明显不同步性。不同细胞间染色体配对形式变化较大 ,一般以二价联会为主 ,其次由其它多种配对方式 (包括有复合配对、重复配对、着丝点或端粒处联合和多价联会 )形成多价体 ,还有少数未配对或发生内配对 (偶见 )的单价体和几条二价体之间的次级配对。粗线期观察到少数染色体有缺失 (或重复 )、倒位、易位和疏松配对等结构性改变。表明该植株是一个复杂的区段异源六位体 ,少数染色体在结构上累积有变异。还认为该植株是研究减数分裂染色体配对和联会机制的好材料。     

The fate of multivalents during meiotic prophase in the hybrid Gibasis consobrina x G. karwinskyana Rafin. (Commelinaceae)

The chromosomes of the two closely related diploid species, Gibasis consobrina and G. karwinskyana (Commelinaceae; 2n=2x=10), are morphologically alike, yet form few chiasmate associations at metaphase I in the f₁ hybrid. During meiotic prophase, however, synaptonemal complexes join the majority of the chromosomes of the complement in complex multiple pairing configurations. The F₁ hybrid between different tetraploid genotypes of the same two species similarly forms multivalents during meiotic prophase, which are subsequently eliminated in favour of strictly homologous bivalents before metaphase I. One quadrivalent comprising interchange chromosomes inherited from one of the parents, usually persists to first metaphase. Evidently the resolution of multivalents to bivalents at first metaphase, which accounts for diploidisation, is not attributable to the elimination of multivalents per se, but of multivalents comprising chromosomes of limited homology.     

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     

Estimation of the efficiency of seed irradiation by thermal neutrons for inducing chromosomal aberrations in M<Subscript>1</Subscript> of cotton <Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.

Exposure of cotton seeds to thermal neutrons at doses of 15, 25, and 35 Gy was shown to induce many biomorphologically abnormal plants, including sterile and chimeric ones. Most of these phenotypic changes were shown to result from novel genomic, chromosomal, and desynaptic mutations. The presence of these mutations in the karyotype of M₁ plants often decreased meiotic index and pollen fertility. In translocation forms, the decrease in pollen fertility was caused by the prevalence of quadrivalents in form of rings and chains with adjacent segregation of chromosomes from the translocation complexes. Based on the shapes and sizes of multivalent associations, we performed preliminary localization of translocation breakpoints. A specific feature of the effect of thermal neuron irradiation in M₁ was induction of numerous unique chromosomal aberrations, consisting in the appearance in the same plant of several types of mutations (genomic and chromosomal), interchange complexes in the same nucleus, and multiple interchanges involving three nonhomologous chromosomes.     

Chromosome behaviour and fertility of the raw and evolved synthetic tetraploids of pearl millet,Pennisetum typhoides stapf et hubb.

The effect of colchicine-induced chromosome doubling on phenotypic characters of Pearl millet,Pennisetum typhoides, is described and the mutagenic effect of colchicine preliminarily indicated.Meiosis in the raw tetraploids was characterized by a high frequency of multivalents, mostly quadrivalents. The evolutionary implication of the occurrence of multivalents higher than quadrivalents is discussed.When the synthetic autototraploids were carried through C₆, a shift from a multivalent to a bivalent type of association was noticed, and some of the quadrivalents formed in the C₅ and C₆ generations appeared to be loosely associated. It is suggested that structural divergence of the chromosomes combined with genetic factors may jointly be the cause. Evolutionary and breeding consequences are discussed. With the acquisition of a predominantly bivalent type of meiotic association, seed setting in the tetraploids improved. The relative roles of meiotic abnormalities and genetic factors in bringing about seed sterility in the tetraploids are considered.Cytological instability resulted in aneuploids and diploids in the progeny. Factors which may predispose a tetraploid taxon to cytological instability are discussed. It has been suggested that the secondarily diploid nature of Pearl millet may contribute to the cytological instability of its tetraploids.Preliminary observations on three trisomics and one monosomic, isolated in this study for the first time, are recorded. It was interesting to note that a monosomic plant of Pearl millet could survive up to maturity. This observation would further suggest that Pearl millet is not a true diploid but a secondarily balanced species with its phyletically basic number less thanx=7, as earlier inferred by the author from several evidences published elsewhere.