Chromosome pairing and chiasma formation in allohexaploid wheat, Triticum aestivum analyzed by spreading of meiotic nuclei

Meiosis in Polyploids and Implications for Genetic Mapping: A Review

Plant cytogenetic studies have provided essential knowledge on chromosome behavior during meiosis, contributing to our understanding of this complex process. In this review, we describe in detail the meiotic process in auto- and allopolyploids from the onset of prophase I through pairing, recombination, and bivalent formation, highlighting recent findings on the genetic control and mode of action of specific proteins that lead to diploid-like meiosis behavior in polyploid species. During the meiosis of newly formed polyploids, related chromosomes (homologous in autopolyploids; homologous and homoeologous in allopolyploids) can combine in complex structures called multivalents. These structures occur when multiple chromosomes simultaneously pair, synapse, and recombine. We discuss the effectiveness of crossover frequency in preventing multivalent formation and favoring regular meiosis. Homoeologous recombination in particular can generate new gene (locus) combinations and phenotypes, but it may destabilize the karyotype and lead to aberrant meiotic behavior, reducing fertility. In crop species, understanding the factors that control pairing and recombination has the potential to provide plant breeders with resources to make fuller use of available chromosome variations in number and structure. We focused on wheat and oilseed rape, since there is an abundance of elucidating studies on this subject, including the molecular characterization of the Ph1 (wheat) and PrBn (oilseed rape) loci, which are known to play a crucial role in regulating meiosis. Finally, we exploited the consequences of chromosome pairing and recombination for genetic map construction in polyploids, highlighting two case studies of complex genomes: (i) modern sugarcane, which has a man-made genome harboring two subgenomes with some recombinant chromosomes; and (ii) hexaploid sweet potato, a naturally occurring polyploid. The recent inclusion of allelic dosage information has improved linkage estimation in polyploids, allowing multilocus genetic maps to be constructed.

Pericentromeric chromatin reorganisation follows the initiation of recombination and coincides with early events of synapsis in cereals

The reciprocal exchange of genetic information between homologous chromosomes during meiotic recombination is essential to secure balanced chromosome segregation and to promote genetic diversity. The chromosomal position and frequency of reciprocal genetic exchange shapes the efficiency of breeding programmes and influences crop improvement under a changing climate. In large genome cereals, such as wheat and barley, crossovers are consistently restricted to subtelomeric chromosomal regions, thus preventing favourable allele combinations being formed within a considerable proportion of the genome, including interstitial and pericentromeric chromatin. Understanding the key elements driving crossover designation is therefore essential to broaden the regions available for crossovers. Here, we followed early meiotic chromatin dynamism in cereals through the visualisation of a homologous barley chromosome arm pair stably transferred into the wheat genetic background. By capturing the dynamics of a single chromosome arm at the same time as detecting the undergoing events of meiotic recombination and synapsis, we showed that subtelomeric chromatin of homologues synchronously transitions to an open chromatin structure during recombination initiation. By contrast, pericentromeric and interstitial regions preserved their closed chromatin organisation and become unpackaged only later, concomitant with initiation of recombinatorial repair and the initial assembly of the synaptonemal complex. Our results raise the possibility that the closed pericentromeric chromatin structure in cereals may influence the fate decision during recombination initiation, as well as the spatial development of synapsis, and may also explain the suppression of crossover events in the proximity of the centromeres.

Diter von Wettstein and The Meiotic Program of Pairing and Recombination

Recombination and pairing are prominent features of meiosis where they play an important role in increasing genetic diversity. In most organisms recombination also plays mechanical roles in mediating pairing of homologous chromosomes during prophase and in ensuring regular segregation of homologous pairs at the first meiotic division. The laboratory directed by D. von Wettstein identified six key steps in the meiotic process: (1) Recombination mediated processes occur in physical and functional linkage with the synaptonemal complex (SC), a highly conserved, meiosis-specific structure that links homologous axes along their lengths. (2) The pairing process involves formation and resolution of chromosomal entanglements/interlockings. (3) The SC normally forms specifically between homologous chromosomes, but in unusual situations can form between nonhomologous chromosomes or regions resulting in two-phase SC formation. (4) In hexaploid common wheat, extensive multivalents form with multiple, pairing partner shifts, indicating homology recognition and SC formation among homoeologs as well as homologs. (5) Linkage between recombination and the SC is revealed by crossover-correlated nodules localized in the SC central region. (6) Modified SCs sometimes play a direct role in homolog segregation, providing the required connection between homologs in absence of crossovers/chiasmata.

Chromosome Pairing in Polyploid Grasses

Polyploids are species in which three or more sets of chromosomes coexist. Polyploidy frequently occurs in plants and plays a major role in their evolution. Based on their origin, polyploid species can be divided into two groups: autopolyploids and allopolyploids. The autopolyploids arise by multiplication of the chromosome sets from a single species, whereas allopolyploids emerge from the hybridization between distinct species followed or preceded by whole genome duplication, leading to the combination of divergent genomes. Having a polyploid constitution offers some fitness advantages, which could become evolutionarily successful. Nevertheless, polyploid species must develop mechanism(s) that control proper segregation of genetic material during meiosis, and hence, genome stability. Otherwise, the coexistence of more than two copies of the same or similar chromosome sets may lead to multivalent formation during the first meiotic division and subsequent production of aneuploid gametes. In this review, we aim to discuss the pathways leading to the formation of polyploids, the occurrence of polyploidy in the grass family (Poaceae), and mechanisms controlling chromosome associations during meiosis, with special emphasis on wheat.

A Co-Expression Network in Hexaploid Wheat Reveals Mostly Balanced Expression and Lack of Significant Gene Loss of Homeologous Meiotic Genes Upon Polyploidization

Polyploidization has played an important role in plant evolution. However, upon polyploidization, the process of meiosis must adapt to ensure the proper segregation of increased numbers of chromosomes to produce balanced gametes. It has been suggested that meiotic gene (MG) duplicates return to a single copy following whole genome duplication to stabilize the polyploid genome. Therefore, upon the polyploidization of wheat, a hexaploid species with three related (homeologous) genomes, the stabilization process may have involved rapid changes in content and expression of MGs on homeologous chromosomes (homeologs). To examine this hypothesis, sets of candidate MGs were identified in wheat using co-expression network analysis and orthology informed approaches. In total, 130 RNA-Seq samples from a range of tissues including wheat meiotic anthers were used to define co-expressed modules of genes. Three modules were significantly correlated with meiotic tissue samples but not with other tissue types. These modules were enriched for GO terms related to cell cycle, DNA replication, and chromatin modification and contained orthologs of known MGs. Overall, 74.4% of genes within these meiosis-related modules had three homeologous copies which was similar to other tissue-related modules. Amongst wheat MGs identified by orthology, rather than co-expression, the majority (93.7%) were either retained in hexaploid wheat at the same number of copies (78.4%) or increased in copy number (15.3%) compared to ancestral wheat species. Furthermore, genes within meiosis-related modules showed more balanced expression levels between homeologs than genes in non-meiosis-related modules. Taken together, our results do not support extensive gene loss nor changes in homeolog expression of MGs upon wheat polyploidization. The construction of the MG co-expression network allowed identification of hub genes and provided key targets for future studies.

CENH3 morphogenesis reveals dynamic centromere associations during synaptonemal complex formation and the progression through male meiosis in hexaploid wheat

During meiosis, centromeres in some species undergo a series of associations, but the processes and progression to homologous pairing is still a matter of debate. Here, we aimed to correlate meiotic centromere dynamics and early telomere behaviour to the progression of synaptonemal complex (SC) construction in hexaploid wheat (2n = 42) by triple immunolabelling of CENH3 protein marking functional centromeres, and SC proteins ASY1 (unpaired lateral elements) and ZYP1 (central elements in synapsed chromosomes). We show that single or multiple centromere associations formed in meiotic interphase undergo a progressive polarization (clustering) at the nuclear periphery in early leptotene, leading to formation of the telomere bouquet. Critically, immunolabelling shows the dynamics of these presynaptic centromere associations and a structural reorganization of the centromeric chromatin coinciding with key events of synapsis initiation from the subtelomeric regions. As short stretches of subtelomeric synapsis emerged at early zygotene, centromere clusters lost their strong polarization, gradually resolving as individual centromeres indicated by more than 21 CENH3 foci associated with unpaired lateral elements. Only following this centromere depolarization were homologous chromosome arms connected, as observed by the alignment and fusion of interstitial ZYP1 loci elongating at zygotene so synapsis at centromeres is a continuation of the interstitial synapsis. Our results thus reveal that centromere associations are a component of the timing and progression of chromosome synapsis, and the gradual release of the individual centromeres from the clusters correlates with the elongation of interstitial synapsis between the corresponding homologues.

Meiosis in autopolyploid and allopolyploid Arabidopsis

All newly formed polyploids face a challenge in meiotic chromosome segregation due to the presence of an additional set of chromosomes. Nevertheless, naturally occurring auto and allopolyploids are common and generally show high fertility, showing that evolution can find solutions. Exactly how meiosis is adapted in these cases, however, remains a mystery. The rise of Arabidopsis as a model genus for polyploid and meiosis research has seen several new studies begin to shed light on this long standing question.

The challenge of evolving stable polyploidy: could an increase in "crossover interference distance" play a central role?

Whole genome duplication is a prominent feature of many highly evolved organisms, especially plants. When duplications occur within species, they yield genomes comprising multiple identical or very similar copies of each chromosome (“autopolyploids”). Such genomes face special challenges during meiosis, the specialized cellular program that underlies gamete formation for sexual reproduction. Comparisons between newly formed (neo)-autotetraploids and fully evolved autotetraploids suggest that these challenges are solved by specific restrictions on the positions of crossover recombination events and, thus, the positions of chiasmata, which govern the segregation of homologs at the first meiotic division. We propose that a critical feature in the evolution of these more effective chiasma patterns is an increase in the effective distance of meiotic crossover interference, which plays a central role in crossover positioning. We discuss the findings in several organisms, including the recent identification of relevant genes in Arabidopsis arenosa, that support this hypothesis.

Contribution of Structural Chromosome Mutants to the Study of Meiosis in Plants

Dissection of the molecular mechanisms underlying the transition through the complex events of the meiotic process requires the use of gene mutants or RNAi-mediated gene silencing. A considerable number of meiotic mutants have been isolated in plant species such as Arabidopsis thaliana, maize or rice. However, structural chromosome mutants are also important for the identification of the role developed by different chromosome domains in the meiotic process. This review summarizes the contribution of studies carried out in plants using structural chromosome variations. Meiotic events concerning the search of the homologous partner, the control of number and distribution of chiasmata, the mechanism of pairing correction, and chromosome segregation are considered.

Forcing the shift of the crossover site to proximal regions in wheat chromosomes

Terminal deletions obligate the first crossover to be formed in more proximal positions. This increases the recombination rate in intercalary intervals but not in the proximity of the centromere. Crossovers are not uniformly distributed along chromosomes in wheat. They take place preferentially in distal positions. The effect of the chromosomal architecture on crossover positioning has been analyzed from the chiasmate bonds at metaphase I formed by the truncated arms of 51 terminal deletion lines of eight wheat chromosomes. Chromosome 4A and the B genome chromosomes, in their standard or truncated conformation, and their arms, were identified by C-banding. Chromosomes studied show a similar chiasma distribution. Reduction of the size of the truncated arms is accompanied by a gradual decrease of the chiasma frequency in chromosome arms 1BL, 3BS, 3BL, 4BL, 5BS, 5BL, 6BL, 7BS, 7BL and 4AL. In chromosome arm 1BS, most chiasmata are concentrated in the distal half of the satellite and, in 4AS, in the distal 24 %. The arms 2BS, 2BL and 6BS do not show a simple decreasing gradient of the recombination rate, the chiasma frequency increases in subdistal intervals compared to more distal regions. Although terminal deletions usually induce an increase of chiasma frequency in intercalary regions, the level of intact chromosome arms is maintained in only a few deletion lines. Truncated arms containing only the 20 % proximal of the intact arm do not form chiasmata. The relationships of chiasma positioning with chromatin structure and genome organization is discussed.

Meiosis and its deviations in polyploid plants

Meiosis is a fundamental process in all sexual organisms that ensures fertility and genome stability and creates genetic diversity. For each of these outcomes, the exclusive formation of crossovers between homologous chromosomes is needed. This is more difficult to achieve in polyploid species which have more than 2 sets of chromosomes able to recombine. In this review, we describe how meiosis and meiotic recombination 'deviate' in polyploid plants compared to diploids, and give an overview of current knowledge on how they are regulated. See also the sister article focusing on animals by Stenberg and Saura in this themed issue.

Dynamics of rye chromosome 1R regions with high or low crossover frequency in homology search and synapsis development

In many organisms, homologous pairing and synapsis depend on the meiotic recombination machinery that repairs double-strand DNA breaks (DSBs) produced at the onset of meiosis. The culmination of recombination via crossover gives rise to chiasmata, which locate distally in many plant species such as rye, Secale cereale. Although, synapsis initiates close to the chromosome ends, a direct effect of regions with high crossover frequency on partner identification and synapsis initiation has not been demonstrated. Here, we analyze the dynamics of distal and proximal regions of a rye chromosome introgressed into wheat to define their role on meiotic homology search and synapsis. We have used lines with a pair of two-armed chromosome 1R of rye, or a pair of telocentrics of its long arm (1RL), which were homozygous for the standard 1RL structure, homozygous for an inversion of 1RL that changes chiasma location from distal to proximal, or heterozygous for the inversion. Physical mapping of recombination produced in the ditelocentric heterozygote (1RL/1RL_inv) showed that 70% of crossovers in the arm were confined to a terminal segment representing 10% of the 1RL length. The dynamics of the arms 1RL and 1RL_inv during zygotene demonstrates that crossover-rich regions are more active in recognizing the homologous partner and developing synapsis than crossover-poor regions. When the crossover-rich regions are positioned in the vicinity of chromosome ends, their association is facilitated by telomere clustering; when they are positioned centrally in one of the two-armed chromosomes and distally in the homolog, their association is probably derived from chromosome elongation. On the other hand, chromosome movements that disassemble the bouquet may facilitate chromosome pairing correction by dissolution of improper chromosome associations. Taken together, these data support that repair of DSBs via crossover is essential in both the search of the homologous partner and consolidation of homologous synapsis.

Preliminary characterisation of two early meiotic wheat proteins after identification through 2D gel electrophoresis proteomics

Various genetic-based approaches including mutant population screens, microarray analyses, cloning and transgenesis have broadened our knowledge of gene function during meiosis in plants. Nonetheless, these genetic tools are not without inherent limitations. One alternative approach to studying plant meiosis, especially in polyploids such as Triticum aestivum L. (bread wheat), is proteomics. However, protein-based approaches using proteomics have seldom been described, with only two attempts at studying early plant meiosis reported. Here, we report the investigation of early bread wheat meiosis using proteomics. Five differentially expressed protein spots were identified using 2D gel electrophoresis (2DGE) on protein extracts from four pooled stages of meiosis and three genotypes (Chinese Spring wild-type, ph1b and ph2a wheat mutant lines). Tandem mass spectrometry (MS/MS) identification of peptides from these protein spots led to the isolation and characterisation of the full-length clones of a wheat Speckle-type POZ protein, an SF21-like protein and HSP70, and a partial coding sequence of a hexose transporter. Significantly, the putative functions of the Speckle-type POZ protein and HSP70 were confirmed using in vitro DNA binding assays. Through the use of a 2DGE proteomics approach, we show that proteomics is a viable alternative to genetic-based approaches when studying meiosis in wheat. More significantly, we report a potential role for a Speckle-type POZ protein and a HSP70 in chromosome pairing during the early stages of meiosis in bread wheat.

Finding the correct partner: the meiotic courtship

Homologous chromosomes are usually separated at the entrance of meiosis; how they become paired is one of the outstanding mysteries of the meiotic process. Reduction of spacing between homologues makes possible the occurrence of chromosomal interactions leading to homology detection and the formation of bivalents. In many organisms, telomere-led chromosome movements are generated that bring homologues together. Additional movements produced by chromatin conformational changes at early meiosis may also facilitate homologous contacts. Organisms used in the study of meiosis show a surprising variety of strategies for homology detection. In dipterans, homologous chromosomes remain paired throughout most of development. Pairing seems to arise as a balance between promoter and suppressor pairing genes. Some fungi, plants and animals, use mechanisms based on recombinational interactions. Other mechanisms leading to homology search are recombination-independent and require specialized pairing sites. In the worm Caenorhabditis elegans, each chromosome carries a pairing center consisting of a chromosome-specific DNA-protein complex, and in the fission yeast Schizosaccharomyces pombe, the sme2 locus encodes a meiosis-specific non-coding RNA that mediates on homologous recognition. In addition, mismatch correction plays a relevant role, especially in polyploids, which evolved genetic systems that suppress pairing between non-homologous related (homoeologus) chromosomes.

Improving the chances of finding the right partner

Recognition and pairing of homologous chromosomes are absolutely required for successful segregation during meiosis. We still have no model, however, that adequately explains the mechanism of these processes in a quantitative way. The fact that homologue pairing takes similar times across several orders of magnitude in genome size rules out simple, linear homology searching mechanisms. Although homology searching must ultimately depend on DNA sequence comparisons, a number of more specific mechanisms have been described in different organisms, including telomere clustering, centromere association and interaction of specific pairing sequences. These mechanisms can be interpreted as limiting the required search and thus improving the efficiency of pairing.

Meiotic genes and proteins in cereals

We review the current status of our understanding and knowledge of the genes and proteins controlling meiosis in five major cereals, rye, wheat, barley, rice and maize. For each crop, we describe the genetic and genomic infrastructure available to investigators, before considering the inventory of genes and proteins that have roles to play in this process. Emphasis is given throughout as to how translational genomic and proteomic approaches have enabled us to circumvent some of the intractable features of this important group of plants.

Nuclear architecture and chromosome dynamics in the search of the pairing partner in meiosis in plants

The formation of haploid gametes in organisms with sexual reproduction requires regular bivalent chromosome pairing in meiosis. In many species, homologous chromosomes occupy separate territories at the onset of meiosis. To be paired at metaphase I, they need to be brought into a close proximity for interactions that include homology recognition and the establishment of some form of bonds. How homologues find each other is one of the least understood meiotic events. Plant species with large or medium sized genomes, such as wheat or maize, are excellent materials for the cytological analysis of chromosome dynamics at early meiosis, but genes that control meiosis have been identified mainly in small genome species such as Arabidopsis thaliana. This review is focused on the contribution studies on plants are providing to the knowledge of the initial steps of the meiotic process.

Fine structure mapping of a gene-rich region of wheat carrying Ph1, a suppressor of crossing over between homoeologous chromosomes

The wheat gene-rich region (GRR) 5L0.5 contains many important genes, including Ph1, the principal regulator of chromosome pairing. Comparative marker analysis identified 32 genes for the GRR controlling important agronomic traits. Detailed characterization of this region was accomplished by first physically localizing 213 wheat group 5L-specific markers, using group 5 nulli-tetrasomics, three Ph1 gene deletion/insertion mutants, and nine terminal deletion lines with their breakpoints around the 5L0.5 region. The Ph1 gene was localized to a much smaller region within the GRR (Ph1 gene region). Of the 61 markers that mapped in the four subregions of the GRR, 9 mapped in the Ph1 gene region. High stringency sequence comparison (e < 1 x10(-25)) of 157 group 5L-specific wheat ESTs identified orthologs for 80% sequences in rice and 71% in Arabidopsis. Rice orthologs were present on all rice chromosomes, although most (34%) were on rice chromosome 9 (R9). No single collinear region was identified in Arabidopsis even for a smaller region, such as the Ph1 gene region. Seven of the nine Ph1 gene region markers mapped within a 450-kb region on R9 with the same gene order. Detailed domain/motif analysis of the 91 putative genes present in the 450-kb region identified 26 candidates for the Ph1 gene, including genes involved in chromatin reorganization, microtubule attachment, acetyltransferases, methyltransferases, DNA binding, and meiosis/anther specific proteins. Five of these genes shared common domains/motifs with the meiosis specific genes Zip1, Scp1, Cor1, RAD50, RAD51, and RAD57. Wheat and Arabidopsis homologs for these rice genes were identified.

To check or not to check? The application of meiotic studies to plant breeding

Understanding the barriers that prevent pairing and recombination of the chromosomes from two parental species is important for crop improvement strategies. It had been generally thought that plants do not possess checkpoint mechanisms during meiosis. However, recent data may question this assumption and suggest that exploitation of such mechanisms could be crucial to breeding.

Terminal regions of wheat chromosomes select their pairing partners in meiosis

Many plant species, including important crops like wheat, are polyploids that carry more than two sets of genetically related chromosomes capable of meiotic pairing. To safeguard a diploid-like behavior at meiosis, many polyploids evolved genetic loci that suppress incorrect pairing and recombination of homeologues. The Ph1 locus in wheat was proposed to ensure homologous pairing by controlling the specificity of centromere associations that precede chromosome pairing. Using wheat chromosomes that carry rye centromeres, we show that the centromere associations in early meiosis are not based on homology and that the Ph1 locus has no effect on such associations. Although centromeres indeed undergo a switch from nonhomologous to homologous associations in meiosis, this process is driven by the terminally initiated synapsis. The centromere has no effect on metaphase I chiasmate chromosome associations: homologs with identical or different centromeres, in the presence and absence of Ph1, pair the same. A FISH analysis of the behavior of centromeres and distal chromomeres in telocentric and bi-armed chromosomes demonstrates that it is not the centromeric, but rather the subtelomeric, regions that are involved in the correct partner recognition and selection.

Understanding the cytological diploidization mechanism of polyploid wild wheats

The allohexaploid Aegilops species (2n = 6x = 42), Ae. neglecta 6x (UUXtXtNN), Ae. juvenalis (DcDcXcXcUU), and Ae. vavilovii (DcDcXcXcSsSs) regularly form bivalents at metaphase I. However, in Ae. crassa 6x (DcDcXcXcDD) 0.27 quadrivalents per cell were observed probably as a consequence of the partial homology displayed by the D and Dc genomes. Likewise, the synthetic amphiploid Ae. ventricosa-Secale cereale (DDNNRR) is fertile and displays a diploid-like behavior at metaphase I, despite its recent origin. The pattern of synapsis at late zygotene and pachytene in the natural and artificial allohexaploids was analyzed by whole-mount surface-spreading of synaptonemal complexes under an electron microscope. It revealed that chromosomes were mostly associated as bivalents in all cases, the mean of multivalents per nucleus ranging from 0.17 (Ae. neglecta 6x) to 1.03 (Ae. crassa 6x) in the natural species and 1.05 in the Ae. ventricosa-S. cereale amphiploid. It can be concluded that the mechanism controlling bivalent formation in these species and also in the synthetic amphiploid acts mainly at zygotene by restricting synapsis to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in the homoeologous associations. These observations are discussed in relation to the origin and evolution of the mechanism of diploidization in the allopolyploid species of the Poaceae family.

Chromosomes form into seven groups in hexaploid and tetraploid wheat as a prelude to meiosis

Hexaploid wheat possesses 42 chromosomes derived from its three ancestral genomes. The 21 pairs of chromosomes can be further divided into seven groups of six chromosomes (one chromosome pair being derived from each of the three ancestral genomes), based on the similarity of their gene order. Previous studies have revealed that, during anther development, the chromosomes associate in 21 pairs via their centromeres. The present study reveals that, as a prelude to meiosis, these 21 chromosome pairs in hexaploid (and tetraploid) wheat associate via the centromeres into seven groups as the telomeres begin to cluster. This results in the association of multiple chromosomes, which then need to be resolved as meiosis progresses. The formation of the seven chromosome clusters now explains the occasional occurrence of remnants of multiple associations, which have been reported at later stages of meiosis in hexaploid (and tetraploid) wheat. Importantly, the chromosomes have the opportunity to be resorted via these multiple interactions. As meiosis progresses, such interactions are resolved through the action of loci such as Ph1, leaving chromosomes as homologous pairs.

Meiosis in allopolyploids -- the importance of 'Teflon' chromosomes

Polyploids possess two or more sets of related chromosomes as a result of either the doubling of chromosomes following sexual hybridization within the same species (autopolyploidy), or between closely related species containing related but not completely homologous (homoeologous) genomes (allopolyploidy). For allopolyploids to produce viable gametes and be fertile, they must behave as diploids during meiosis, so that only identical chromosomes (homologues) pair. A solution to this problem is an enhanced ability to resolve incorrect pairing, which in turn promotes correct pairing. This gives nonhomologous chromosomes an almost 'Teflon'-like status, so that only the correct pairs 'stick'.

The synaptic behaviour of the wild forms of Triticum turgidum and T. timopheevii

Different wild allopolyploid species of Triticeae show extensive bivalent formation at zygotene while a considerable number of multivalents is present in cultivated polyploid wheats. To study the chromosome behaviour at early meiotic stages in wild forms of tetraploid wheats Triticum turgidum and T timopheevii (2n = 4x = 28) we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late zygotene and at pachytene of wild accessions of these species. The mean number of synaptonemal complex (SC) bivalents at mid-zygotene ranged from 12.22 to 13.14 among the accessions studied indicating a strong restriction of synapsis initiation to homologous chromosomes. The mean of bivalents increased at pachytene because of the transformation of multivalents into bivalents. Ring bivalents observed at metaphase I support that SC bivalents were formed by homologous chromosomes. The average values of SC bivalents at mid-zygotene in the wild forms are much higher than the average values observed in the cultivated tetraploid wheats but similar to that of a mutant line of T turgidum with a duplication that includes Ph1, the major homoeologous pairing suppressor locus. These results suggest that the efficiency of the mechanism operating in the homologous recognition for synapsis is higher in wild wheat populations than in cultivated varieties. Apparently, a relatively detrimental modification of the pairing regulating genetic system accompanied the domestication of the wild wheat forms.

The single-end invasion: an asymmetric intermediate at the double-strand break to double-holliday junction transition of meiotic recombination

We identify a novel meiotic recombination intermediate, the single-end invasion (SEI), which occurs during the transition from double-strand breaks (DSBs) to double-Holliday junction (dHJs). SEIs are products of strand exchange between one DSB end and its homolog. The structural asymmetry of SEIs indicates that the two ends of a DSB interact with the homolog in temporal succession, via structurally (and thus biochemically) distinct processes. SEIs arise surprisingly late in prophase, concomitant with synaptonemal complex (SC) formation. These and other data imply that SEIs are preceded by nascent DSB-partner intermediates, which then undergo selective differentiation into crossover and noncrossover types, with SC formation and strand exchange as downstream consequences. Late occurrence of strand exchange provides opportunity to reverse recombinational fate even after homologs are coaligned and/or synapsed. This feature can explain crossover suppression between homeologous and structurally heterozygous chromosomes.

The Ph1 locus is needed to ensure specific somatic and meiotic centromere association

The correct pairing and segregation of chromosomes during meiosis is essential for genetic stability and subsequent fertility. This is more difficult to achieve in polyploid species, such as wheat, because they possess more than one diploid set of similar chromosomes. In wheat, the Ph1 locus ensures correct homologue pairing and recombination. Although clustering of telomeres into a bouquet early in meiosis has been suggested to facilitate homologue pairing, centromeres associate in pairs in polyploid cereals early during floral development. We can now extend this observation to root development. Here we show that the Ph1 locus acts both meiotically and somatically by reducing non-homologous centromere associations. This has the effect of promoting true homologous association when centromeres are induced to associate. In fact, non-homologously associated centromeres separate at the beginning of meiosis in the presence, but not the absence, of Ph1. This permits the correction of homologue association during the telomere-bouquet stage in meiosis. We conclude that the Ph1 locus is not responsible for the induction of centromere association, but rather for its specificity.

Identification of barley genome segments introgressed into wheat using PCR markers

Barley has several important traits that might be used in the genetic improvement of wheat. For this report, we have produced wheat-barley recombinants involving barley chromosomes 4 (4H) and 7 (5H). Wheat-barley disomic addition lines were crossed with 'Chinese Spring' wheat carrying the ph1b mutation to promote homoeologous pairing. Selection was performed using polymerase chain reaction (PCR) markers to identify lines with the barley chromosome in the ph1b background. These lines were self pollinated, and recombinants were identified using sequence-tagged-site (STS) primer sets that allowed differentiation between barley and wheat chromosomes. Several recombinant lines were isolated that involved different STS-PCR markers. Recombination was confirmed by allowing the lines to self pollinate and rescreening the progeny via STS-PCR. Progeny testing confirmed 9 recombinants involving barley chromosome 4 (4H) and 11 recombinants involving barley chromosome 7 (5H). Some recombinants were observed cytologically to eliminate the possibility of broken chromosomes. Since transmission of the recombinant chromosomes was lower than expected and since seed set was reduced in recombinant lines, the utility of producing recombinants with this method is uncertain.Key words: introgression, sequence-tagged-site, recombination.

Organization of highly repeated sequences in surface-spread pachytene chromosomes of rye

A method of preparing two-dimensional surface spreads of plant synaptonemal complexes (SCs) associated with fluorescence in situ hybridization (FISH) has been applied to analyze the location and organization of five different highly repeated DNA sequences in rye. Our observations indicate that, depending on the type of sequence, the chromatin displays different types of organization. Telomeric sequences were seen tightly associated with the SC while other repetitive DNA sequences were found to form loops that are associated with SCs only at their bases. On the contrary, the FISH signal of a centromeric satellite had a granular appearance, reflecting that the hybridization occurs only with parts of the chromatin loops.Key words: fluorescence in situ hybridization, meiosis, repetitive DNA, rye, synaptonemal complex.

CEREAL CHROMOSOME STRUCTURE, EVOLUTION, AND PAIRING

▪ Abstract  The determination of the order of genes along cereal chromosomes indicates that the cereals can be described as a single genetic system. Such a framework provides an opportunity to combine data generated from the studies on different cereals, enables chromosome evolution to be traced, and sheds light on key structures involved in cereal chromosome pairing. Centromeric and telomeric regions have been highlighted as important in these processes.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Relationship between pachytene synapsis, metaphase I associations, and transmission of 2B and 4B chromosomes in rye

2B rye plants selected for high (H) or low (L) B transmission rate were studied at pachytene and metaphase I of meiosis to determine the relationship between synapsis, bivalents at metaphase I, and B transmission rate. The results show that the 2 B chromosomes (Bs) form bivalents at pachytene in both the H and L lines, whereas the frequency of bivalents at metaphase I is much higher in the H than in the L line. This demonstrates that B transmission is mainly related to the proper association of Bs at metaphase I, as well as that synapsis of the 2 Bs in the L line is normal, but the bivalent is not consolidated by a chiasma in most cases. Crosses were made between 2B plants of the H and L lines in all combinations (H × H, H × L, L × H, and L × L) to obtain 4B plants. Similarly, bivalent formation at pachytene and metaphase I was studied. The results show that 4B plants of the H × H and L × L classes differ significantly at pachytene and metaphase I since the former forms more bivalents. The heterozygous 4 Bs of the H × L and L × H classes show intermediate values. The relation H × H > H × L > L × H > L × L was consistently found for the variables transmission rate, bivalents at pachytene, bivalents at metaphase I, and B mean chiasma frequency. A maternal effect was also found. Our data suggest that there are two separate mechanisms acting upon synapsis and chiasma formation in H and L B chromosomes: (i) there is variable efficiency of the control of synapsis at early stages of meiosis; and (ii) there is variable efficiency of the control of the number of chiasmata.

Key words: B chromosomes, synaptonemal complex, Secale cereale.

The effects of oral administration with 17 alpha-methyltestosterone on chromosomal synapsis in Oreochromis niloticus (Pisces, Cichlidae)

The pattern of chromosomal synapsis after treatment with 17 alpha-methyltestosterone (MT), a testosterone analogue routinely used for the reversal of phenotypic sex in aquaculture, was investigated using the Nile tilapia (Oreochromis niloticus) as a model teleost species. Progeny-tested, monosex diploid (2n = 44) individuals were orally administered with diets containing 50 mg/kg MT for 30 days after first feeding (XX(MT) neomales and XY(MT) males) and compared to controls (XY males). The formation and structure of the synaptonemal complex (SC) and the nature of chromosomal synapsis were investigated in control and treated groups by computer-assisted image analysis of transmission electron microscope (TEM) microphotographs taken from SC spreads. Nuclei at the pachytene stage were first observed in XX(MT) neomales, indicating an earlier commitment of genetically female spermatocytes to enter the first meiotic prophase. Administration of MT did not result in obvious SC lesions, breakage, asynapsis or formation of multivalents in genotypic females (XX(MT) neomales). Administration of MT resulted in a significant increase in the SC lengths in XY(MT) males, although it did not significantly alter the pattern of synapsis (SC structure and number and morphology of bivalents) in comparison to XY controls. The significance of the effects and the putative mode(s) of action of MT on chromosomal synapsis in teleosts is discussed.

Differences in the synaptic pattern in two autotetraploid cultivars of rye with different quadrivalent frequencies at metaphase I

Synaptic behaviour of the two tetraploids rye cultivars Gigantón (G) and Tetrapico (T) displaying significant differences in their quadrivalent frequencies at metaphase I was analyzed by electron microscopy in surface-spread prophase I nuclei. A different behaviour was observed between the two cultivars; the synaptonemal complex (SC) quadrivalents frequency being significantly higher in G than in T at prophase I. Moreover, the G SC quadrivalents had more synaptic partner exchanges (SPEs) and their location was more distal than the T SC quadrivalents. However, inverse findings were found at metaphase I, the quadrivalent frequency was higher in T than in G. The role that different factors, mainly the number and location of the SPEs and the frequency and distribution of chiasmata, could play in the evolution from prophase I to metaphase I in both cultivars is discussed.Key words: autotetraploid rye, synaptonemal complex, spreading.

MEIOTIC CHROMOSOME ORGANIZATION AND SEGREGATION IN PLANTS

During meiosis, homologous chromosomes are brought together to be recombined and segregated into separate haploid gametes. This requires two cell divisions, an elaborate prophase with five substages, and specialized mechanisms that regulate the association of sister chromatids. This review focuses on plant chromosomes and chromosome-associated structures, such as recombination nodules and kinetochores, that ensure accurate meiotic chromosome segregation.

A method for fluorescence in situ hybridization against synaptonemal complex-associated chromatin of plant meiocytes

An improved method of preparing two-dimensional surface spreads of plant synaptonemal complexes (SCs) associated with fluorescent in situ hybridization is described. This technique produces clear preparations of SCs and, in addition, consistently reveals the organization and location of different repetitive DNA sequences in plant meiotic prophase chromosomes.

Chromosome pairing in the allotetraploid Aegilops biuncialis and a triploid intergeneric hybrid

Chromosome pairing behaviour of the natural allotetraploid Aegilops biuncialis (genome UUMM) and a triploid hybrid Ae. biuncialis × Secale cereale (genome UMR) was analyzed by electron microscopy in surface-spread prophase I nuclei. Synaptonemal-complex analysis at zygotene and pachytene revealed that synapsis in the allotetraploid was mostly between homologous chromosomes, although a few quadrivalents were also formed. Only homologous bivalents were observed at metaphase I. In contrast, homoeologous and heterologous chromosome associations were common at prophase I and metaphase I of the triploid hybrid. It is concluded that the mechanism controlling bivalent formation in Ae. biuncialis acts mainly at zygotene by restricting pairing to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in the homoeologous associations. In the hybrid the mechanism fails at both stages. Key words : Aegilops biuncialis, allotetraploid, intergeneric hybrid, pairing control, synaptonemal complex.

Characterization of deletions in common wheat induced by an Aegilops cylindrica chromosome: detection of multiple chromosome rearrangements

An Aegilops cylindrica chromosome induces terminal deletions of chromosomes in wheat as identified by C-banding. We are constructing high-density physical maps of wheat chromosomes and have detected additional chromosome rearrangements. Among 63 lines with chromosomal subarm deletions in group 7 chromosomes, 7 lines (11.1%) were shown to harbor additional chromosome rearrangements. Two other lines were also omitted from the physical mapping because of the nature of the breakpoint calculations. The presence or absence of chromosome-specific restriction fragment length polymorphism (RFLP) or random amplified polymorphic DNA (RAPD) markers indicated that additional interstitial deletions are present in 3 lines (4.8%) with deletions in the short chromosome arms and in 4 lines (6.3%) with deletions in the long chromosome arms. We also used chromosome pairing analysis of F1 plants of deletion lines with double ditelosomic lines of 'Chinese Spring' wheat to detect small terminal deletions. The deletion of the most distal 1% of chromosome arm 7AL was associated with a pairing reduction of 60%.

Synaptic abnormalities in spread nuclei of Secale. II. Secale vavilovii

Secale vavilovii PMCs have more univalents and a lower frequency of bound arms at metaphase I than other diploid Secale species. The spreading technique applied at prophase I showed that the nuclei were able to complete synapsis at pachytene. However, 25% of the nuclei analyzed, which had more than 90% of their total length paired, showed two abnormalities: long fold-back loops, which were located mainly on the nucleolar organizer bivalent, and pairing-partner switches, probably involving all the chromosome complement. These synaptic abnormalities are unusual in diploid species and give rise to a high frequency of nonhomologous pairing regions and, therefore, could produce desynapsis, which could explain the data obtained from metaphase I. The possible origin of the unusual synaptic abnormalities of S. vavilovii is discussed.Key words: S. vavilovii, synaptonemal complex, spreading, synaptic abnormalities.

Synaptic abnormalities in spread nuclei of Secale. I. Inbred lines

The meiotic behaviour at metaphase I of two inbred lines of Secale cereale showed a decrease in the frequency of bound arms and the presence of univalents. The study of synaptonemal complex formation revealed that these inbred lines had asynapsis and many nuclei did not complete the pairing process. Synaptic abnormalities such as fold-back loops, short pairing partner switches, interstitial nonhomologous pairing regions, and asynchrony between bivalents were associated and correlated with interlocking. It is suggested that the asynapsis is due to a failure in the interlocking repair. All these abnormalities could affect the formation and maintenance of chiasmata and so could explain the metaphase I behaviour of these inbred lines and the differences between them.

Synaptic patterns of rye B chromosomes. IV. The B isochromosomes

Pairing of B isochromosomes in rye was studied by surface spreading of synaptonemal complexes in pollen mother cells from plants containing one small iso-B (iso-BS), one large iso-B (iso-BL), two iso-BL, one iso-BS plus one standard B (iso-BS+stB), one iso-BS plus two standard Bs (iso-BS + 2 stB) and one iso-BL plus two standard Bs (iso-BL + 2 stB). The main characteristics of iso-Bs were as follows. (1) Their location in the surface-spread nuclei was peripheral. (2) Their period of pairing relative to that of the A-set was delayed. (3) The commonest pachytene configuration was a symmetrical hairpin loop, although they could also undergo nonhomologous synapsis. (4) Synapsis could start at the chromosome ends, at the putative centromeric region or at both regions more or less simultaneously. (5) The frequency of iso-BL bivalents was only 46 per cent rather than the 66 per cent predicted by the random-end-pairing model. Interchromosomal pairing was frequent in these configurations. (6) In pairing competition situations involving three and four doses of the short arm of the standard B (iso-BS+stB, iso-BS + 2 stB), a large predominance of self-synapsed iso-BS univalents (63 per cent and 80 per cent respectively) was found. On the other hand, there was no apparent pairing preference among four doses of the long arm of the standard B in the iso-BL + 2 stB combination. Similarities and differences between the synaptic behaviours of iso-Bs of Crepis capillaris and Secale cereale are discussed.

Synaptic patterns of rye B chromosomes. III. The deficient B

Chromosomal pairing of a deficient B chromosome in rye has been studied by surface spreading of synaptonemal complexes in pollen mother cells from plants containing one deficient B (dB), one deficient B plus one standard B (dBstB) and one deficient B plus two standard Bs (dB2stB). Single dBs are often located peripherally in surface spread nuclei, and they undergo extensive non-homologous synapsis at pachytene. In the dBstB bivalents, homologous and non-homologous synapses occur simultaneously throughout pachytene without axial equalization. Competition for pairing between the dB and the two stBs results in a large predominance (80%) of stB bivalents. This finding is explained by the identical origin of the standard Bs and/or by the minor homology between the stB and the dB chromosomes.

Molecular perspectives of chromosome pairing at meiosis

Ideas about the mechanisms that regulate chromosome pairing, recombination, and segregation during meiosis have gained in molecular detail over the last few years. The purpose of this article is to survey briefly the shifts in paradigms and experiments that have generated new perspectives. It has never been very clear what it is that brings together the homologous chromosomes at meiotic prophase. For a while it appeared that the synaptonemal complex might be the nuclear organelle responsible for synapsis, but the supporting evidence has not been entirely convincing. Whatever the mechanism, it has always been assumed that homologous synapsis creates the opportunity for homologous DNA sequences to initiate recombination. At present, alternative ideas are developing. Attractive is the concept that double strand DNA repair mechanisms, that find and use the undamaged homologue for repair, have evolved into a meiotic mechanism for the recognition and pairing of homologous sequences. Subsequent intimate synapsis of homologous chromosomes in the context of the synaptonemal complex may serve later functions in the regulation of interference and segregation at first anaphase. A number of areas that are being tested at present and some that may be investigated in the future are discussed at the end of the review.

Synaptic patterns of rye B chromosomes. II. The effect of the standard B chromosomes on the pairing of the A set

In order to elucidate the possible effects of rye B chromosomes (Bs) on synapsis and metaphase-I associations of the A set, a comparative study between pachytene and metaphase-I-cells of rye plants carrying different numbers of Bs (0-8) has been carried out. The number of Bs was found to be positively correlated with the frequency of synaptic irregularities of the A set, i.e. multivalents and foldback pairing, and with the frequency of pachytene interlockings. It is proposed that interlockings are the origin of these irregularities because both appeared in close proximity in many nuclei. Examples of A-B pairing are described. The frequency of synaptic abnormalities seems to be unrelated to the mean of A chromosome-bound arms at metaphase I.