Evidence for the coincident initiation of homolog pairing and synapsis during the telomere-clustering (bouquet) stage of meiotic prophase

Regulation of heterochromatin organization in plants

Heterochromatin is a nuclear area that contains highly condensed and transcriptionally inactive chromatin. Alterations in the organization of heterochromatin are correlated with changes in gene expression and genome stability, which affect various aspects of plant life. Thus, studies of the molecular mechanisms that regulate heterochromatin organization are important for understanding the regulation of plant physiology. Microscopically, heterochromatin can be characterized as chromocenters that are intensely stained with DNA-binding fluorescent dyes. Arabidopsis thaliana exhibits distinctive chromocenters in interphase nuclei, and genetic studies combined with cytological analyses have identified a number of factors that are involved in heterochromatin assembly and organization. In this review, I will summarize the factors involved in the regulation of heterochromatin organization in plants.

Fine-scale contemporary recombination variation and its fitness consequences in adaptively diverging stickleback fish

Despite deep evolutionary conservation, recombination rates vary greatly across the genome and among individuals, sexes and populations. Yet the impact of this variation on adaptively diverging populations is not well understood. Here we characterized fine-scale recombination landscapes in an adaptively divergent pair of marine and freshwater populations of threespine stickleback from River Tyne, Scotland. Through whole-genome sequencing of large nuclear families, we identified the genomic locations of almost 50,000 crossovers and built recombination maps for marine, freshwater and hybrid individuals at a resolution of 3.8 kb. We used these maps to quantify the factors driving variation in recombination rates. We found strong heterochiasmy between sexes but also differences in recombination rates among ecotypes. Hybrids showed evidence of significant recombination suppression in overall map length and in individual loci. Recombination rates were lower not only within individual marine-freshwater-adaptive loci, but also between loci on the same chromosome, suggesting selection on linked gene 'cassettes'. Through temporal sampling along a natural hybrid zone, we found that recombinants showed traits associated with reduced fitness. Our results support predictions that divergence in cis-acting recombination modifiers, whose functions are disrupted in hybrids, may play an important role in maintaining differences among adaptively diverging populations.

Bread wheat satellitome: a complex scenario in a huge genome

In bread wheat (Triticum aestivum L.), chromosome associations during meiosis are extremely regulated and initiate at the telomeres and subtelomeres, which are enriched in satellite DNA (satDNA). We present the study and characterization of the bread wheat satellitome to shed light on the molecular organization of wheat subtelomeres. Our results revealed that the 2.53% of bread wheat genome is composed by satDNA and subtelomeres are particularly enriched in such DNA sequences. Thirty-four satellite DNA (21 for the first time in this work) have been identified, analyzed and cytogenetically validated. Many of the satDNAs were specifically found at particular subtelomeric chromosome regions revealing the asymmetry in subtelomere organisation among the wheat subgenomes, which might play a role in proper homologous recognition and pairing during meiosis. An integrated physical map of the wheat satellitome was also constructed. To the best of our knowledge, our results show that the combination of both cytogenetics and genome research allowed the first comprehensive analysis of the wheat satellitome, shedding light on the complex wheat genome organization, especially on the polymorphic nature of subtelomeres and their putative implication in chromosome recognition and pairing during meiosis.

Meiotic instability and irregular chromosome pairing underpin heat-induced infertility in bread wheat carrying the Rht-B1b or Rht-D1b Green Revolution genes

Mutations in the Rht-B1a and Rht-D1a genes of wheat (Triticum aestivum; resulting in Rht-B1b and Rht-D1b alleles) cause gibberellin-insensitive dwarfism and are one of the most important elements of increased yield introduced during the 'Green Revolution'. We measured the effects of a short period of heat imposed during the early reproductive stage on near-isogenic lines carrying Rht-B1b or Rht-D1b alleles, with respect to the wild-type (WT). The temperature shift caused a significant fertility loss within the ears of Rht-B1b and Rht-D1b wheats, greater than that observed for the WT. Defects in chromosome synapsis, reduced homologous recombination and a high frequency of chromosome mis-segregation were associated with reduced fertility. The transcription of TaGA3ox gene involved in the final stage of gibberellic acid (GA) biosynthesis was activated and ultra-performance liquid chromatography-tandem mass spectrometry identified GA1 as the dominant bioactive GA in developing ears, but levels were unaffected by the elevated temperature. Rht-B1b and Rht-D1b mutants were inclined to meiotic errors under optimal temperatures and showed a higher susceptibility to heat than their tall counterparts. Identification and introduction of new dwarfing alleles into modern breeding programmes is invaluable in the development of climate-resilient wheat varieties.

The Complete Sequence and Comparative Analysis of Ape Sex Chromosomes

Apes possess two sex chromosomes-the male-specific Y and the X shared by males and females. The Y chromosome is crucial for male reproduction, with deletions linked to infertility. The X chromosome carries genes vital for reproduction and cognition. Variation in mating patterns and brain function among great apes suggests corresponding differences in their sex chromosome structure and evolution. However, due to their highly repetitive nature and incomplete reference assemblies, ape sex chromosomes have been challenging to study. Here, using the state-of-the-art experimental and computational methods developed for the telomere-to-telomere (T2T) human genome, we produced gapless, complete assemblies of the X and Y chromosomes for five great apes (chimpanzee, bonobo, gorilla, Bornean and Sumatran orangutans) and a lesser ape, the siamang gibbon. These assemblies completely resolved ampliconic, palindromic, and satellite sequences, including the entire centromeres, allowing us to untangle the intricacies of ape sex chromosome evolution. We found that, compared to the X, ape Y chromosomes vary greatly in size and have low alignability and high levels of structural rearrangements. This divergence on the Y arises from the accumulation of lineage-specific ampliconic regions and palindromes (which are shared more broadly among species on the X) and from the abundance of transposable elements and satellites (which have a lower representation on the X). Our analysis of Y chromosome genes revealed lineage-specific expansions of multi-copy gene families and signatures of purifying selection. In summary, the Y exhibits dynamic evolution, while the X is more stable. Finally, mapping short-read sequencing data from >100 great ape individuals revealed the patterns of diversity and selection on their sex chromosomes, demonstrating the utility of these reference assemblies for studies of great ape evolution. These complete sex chromosome assemblies are expected to further inform conservation genetics of nonhuman apes, all of which are endangered species.

Three-dimensional chromatin architecture in plants - General features and novelties

Research on the three-dimensional (3D) structure of the genome and its distribution within the nuclear space has made a big leap in the last two decades. Work in the animal field has led to significant advances in our general understanding on eukaryotic genome organization. This did not only bring along insights into how the 3D genome interacts with the epigenetic landscape and the transcriptional machinery but also how 3D genome architecture is relevant for fundamental developmental processes, such as cell differentiation. In parallel, the 3D organization of plant genomes have been extensively studied, which resulted in both congruent and novel findings, contributing to a more complete view on how eukaryotic genomes are organized in multiple dimensions. Plant genomes are remarkably diverse in size, composition, and ploidy. Furthermore, as intrinsically sessile organisms without the possibility to relocate to more favorable environments, plants have evolved an elaborate epigenetic repertoire to rapidly respond to environmental challenges. The diversity in genome organization and the complex epigenetic programs make plants ideal study subjects to acquire a better understanding on universal features and inherent constraints of genome organization. Furthermore, considering a wide range of species allows us to study the evolutionary crosstalk between the various levels of genome architecture. In this article, we aim at summarizing important findings on 3D genome architecture obtained in various plant species. These findings cover many aspects of 3D genome organization on a wide range of levels, from gene loops to topologically associated domains and to global 3D chromosome configurations. We present an overview on plant 3D genome organizational features that resemble those in animals and highlight facets that have only been observed in plants to date.

Telomere and subtelomere high polymorphism might contribute to the specificity of homologous recognition and pairing during meiosis in barley in the context of breeding

Barley (Hordeum vulgare) is one of the most popular cereal crops globally. Although it is a diploid species, (2n = 2x = 14) the study of its genome organization is necessary in the framework of plant breeding since barley is often used in crosses with other cereals like wheat to provide them with advantageous characters. We already have an extensive knowledge on different stages of the meiosis, the cell division to generate the gametes in species with sexual reproduction, such as the formation of the synaptonemal complex, recombination, and chromosome segregation. But meiosis really starts with the identification of homologous chromosomes and pairing initiation, and it is still unclear how chromosomes exactly choose a partner to appropriately pair for additional recombination and segregation. In this work we present an exhaustive molecular analysis of both telomeres and subtelomeres of barley chromosome arms 2H-L, 3H-L and 5H-L. As expected, the analysis of multiple features, including transposable elements, repeats, GC content, predicted CpG islands, recombination hotspots, G4 quadruplexes, genes and targeted sequence motifs for key DNA-binding proteins, revealed a high degree of variability both in telomeres and subtelomeres. The molecular basis for the specificity of homologous recognition and pairing occurring in the early chromosomal interactions at the start of meiosis in barley may be provided by these polymorphisms. A more relevant role of telomeres and most distal part of subtelomeres is suggested.

Meiosis at three loci in autotetraploids: Probabilities of gamete modes and genotypes without and with preferential cross-over formation

A long-standing goal in the field of polyploid biology has been the derivation of mathematical models of gamete mode formation. These models form the basis of statistical inference and evolutionary theory. Here, we present 3-locus models of gamete mode formation in autotetraploids without and with preferential cross-over formation. The three loci are assumed to occur on one arm of the same chromosome. For preferential cross-over formation, one of the three loci affects the tendency for sets of sister chromatids to pair and therefore affects rates of recombination. The models are derived such that the process of double reduction is a function of rates of synaptic partner switches and recombination, as opposed to being independent of these processes. We assume potentially one synaptic partner switch per meiosis. We also assume the coefficient of coincidence is one, such that cross-over events are independent, given a set of cross-over rates. Illustrative cases are examined demonstrating differences in the gamete mode probabilities without and with preferential cross-over formation. Lastly, we explore the accuracy of maximum likelihood estimates of the probability of synaptic partner switches and preferential cross-over formation when the locus controlling preference is at a proximal, middle, or distal location on the chromosome arm. All Supplementary Information is available at https://github.com/ckgriswold/3-locus-autotetraploid-meiosis .

Toward cis-regulation in soybean: a 3D genome scope

In eukaryotic cells, 3D genome plays an important role in the regulation of gene spatiotemporal expression, which is essential for the biological and developmental processes in a life cycle. In the past decade, the development of high-throughput technologies greatly enhances our ability to map the 3D genome organization, identifies multiple 3D genome structures, and investigates the functional role of 3D genome organization in gene regulation, which facilitates our understandings of cis-regulatory landscape and biological development. Comparing with the comprehensive analyses of 3D genome in mammals and model plants, the progress in soybean is much less. Future development and application of tools to precisely manipulate 3D genome structure at different levels will significantly strengthen the functional genome study and molecular breeding in soybean. Here, we review the recent progresses in 3D genome study and discuss future directions, which may help to improve soybean 3D functional genome study and molecular breeding.

An F-box protein ACOZ1 functions in crossover formation by ensuring proper chromosome compaction during maize meiosis

Meiosis is an essential reproductive process to create new genetic variation. During early meiosis, higher order chromosome organization creates a platform for meiotic processes to ensure the accuracy of recombination and chromosome segregation. However, little is known about the regulatory mechanisms underlying dynamic chromosome organization in plant meiosis. Here, we describe abnormal chromosome organization in zygotene1 (ACOZ1), which encodes a canonical F-box protein in maize. In acoz1 mutant meiocytes, chromosomes maintain a leptotene-like state and never compact to a zygotene-like configuration. Telomere bouquet formation and homologous pairing are also distorted and installation of synaptonemal complex ZYP1 protein is slightly defective. Loading of early recombination proteins RAD51 and DMC1 is unaffected, indicating that ACOZ1 is not required for double strand break formation or repair. However, crossover formation is severely disturbed. The ACOZ1 protein localizes on the boundary of chromatin, rather directly to chromosomes. Furthermore, we identified that ACOZ1 interacts with SKP1 through its C-terminus, revealing that it acts as a subunit of the SCF E3 ubiquitin/SUMO ligase complex. Overall, our results suggest that ACOZ1 functions independently from the core meiotic recombination pathway to influence crossover formation by controlling chromosome compaction during maize meiosis.

Homologous chromosome associations in domains before meiosis could facilitate chromosome recognition and pairing in wheat

The increasing human population demands an increase in crop yields that must be implemented through breeding programmes to ensure a more efficient and sustainable production of agro-food products. In the framework of breeding, genetic crosses are developed between cultivated species such as wheat and their relative species that are used as genetic donors to transfer desirable agronomic traits into the crop. Unfortunately, interspecific associations between chromosomes from the donor species and the cultivar are rare during meiosis, the process to produce gametes in organisms with sexual reproduction, hampering the transfer of genetic variability into wheat. In addition, little is known about how homologous (equivalent) chromosomes initiate interaction and recognition within the cell nucleus to enter meiosis. In this context, we aim to get insight into wheat chromatin structure, particularly the distribution of homologous chromosomes within the cell nucleus and their putative interactions in premeiotic stages to facilitate chromosome associations and recombination at the beginning of meiosis. Cytogenetics allows the study of both the structure and the behaviour of chromosomes during meiosis and is key in plant breeding. In this study we visualized an extra pair of barley homologous chromosomes in a wheat genetic background to study the spatial distribution, arrangements and interactions occurring exclusively between this pair of homologous chromosomes during premeiosis using fluorescence in situ hybridization (FISH). Our results suggest that homologous chromosomes can initiate interactions in premeiotic stages that could facilitate the processes of specific chromosome recognition and association occurring at the onset of meiosis.

ZmPRD1 is essential for double-strand break formation, but is not required for bipolar spindle assembly during maize meiosis

Homologs of PUTATIVE RECOMBINATION INITIATION DEFECT 1 (PRD1) are known to be essential for meiotic double-strand break (DSB) formation in mouse (Mus musculus), Arabidopsis, and rice (Oryza sativa). Recent research has shown that rice PRD1 also plays an unanticipated role in meiotic bipolar spindle assembly, revealing that PRD1 has multiple functions in plant meiosis. In this study, we characterize the meiotic function of PRD1 in maize (Zea mays; ZmPRD1). Our results show that Zmprd1 mutant plants display normal vegetative growth but have complete male and female sterility. Meiotic DSB formation is fully abolished in mutant meiocytes, leading to failure in homologous pairing, synapsis, and recombination. ZmPRD1 exhibits a different pattern of chromosome localization compared to its rice homologs. The ZmPRD1 protein interacts with several DSB-forming proteins, but does not directly interact with the kinetochore proteins REC8 and SGO1. Possibly as a result of this, there are no significant abnormalities of bipolar spindle assembly in Zmprd1 meiocytes. Overall, our results demonstrate that ZmPRD1 is essential for DSB formation and homologous recombination in maize meiosis. However, the recently-identified function of PRD1 in bipolar spindle assembly during rice meiosis is not conserved in maize.

The blooming of an old story on the bouquet

As an evolutionarily conserved process, the bouquet stage during meiosis was discovered over a century ago, and active research on this important stage continues. Since the discovery of the first bouquet-related protein Taz1p in 1998, several bouquet formation-related proteins have been identified in various eukaryotes. These proteins are involved in the interaction between telomeres and the inner nuclear membrane (INM), and once these interactions are disrupted, meiotic progression is arrested, leading to infertility. Recent studies have provided significant insights into the relationships and interactions among bouquet formation-related proteins. In this review, we summarize the components involved in telomere-INM interactions and focus on their roles in bouquet formation and telomere homeostasis maintenance. In addition, we examined bouquet-related proteins in different species from an evolutionary viewpoint, highlighting the potential interactions among them.

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.

Sequence analysis of wheat subtelomeres reveals a high polymorphism among homoeologous chromosomes

Bread wheat, Triticum aestivum L., is one of the most important crops in the world. Understanding its genome organization (allohexaploid; AABBDD; 2n = 6x = 42) is essential for geneticists and plant breeders. Particularly, the knowledge of how homologous chromosomes (equivalent chromosomes from the same genome) specifically recognize each other to pair at the beginning of meiosis, the cellular process to generate gametes in sexually reproducing organisms, is fundamental for plant breeding and has a big influence on the fertility of wheat plants. Initial homologous chromosome interactions contribute to specific recognition and pairing between homologues at the onset of meiosis. Understanding the molecular basis of these critical processes can help to develop genetic tools in a breeding context to promote interspecific chromosome associations in hybrids or interspecific genetic crosses to facilitate the transfer of desirable agronomic traits from related species into a crop like wheat. The terminal regions of chromosomes, which include telomeres and subtelomeres, participate in chromosome recognition and pairing. We present a detailed molecular analysis of subtelomeres of wheat chromosome arms 1AS, 4AS, 7AS, 7BS and 7DS. Results showed a high polymorphism in the subtelomeric region among homoeologues (equivalent chromosomes from related genomes) for all the features analyzed, including genes, transposable elements, repeats, GC content, predicted CpG islands, recombination hotspots and targeted sequence motifs for relevant DNA-binding proteins. These polymorphisms might be the molecular basis for the specificity of homologous recognition and pairing in initial chromosome interactions at the beginning of meiosis in wheat.

Tidying-up the plant nuclear space: domains, functions, and dynamics

Understanding how the packaging of chromatin in the nucleus is regulated and organized to guide complex cellular and developmental programmes, as well as responses to environmental cues is a major question in biology. Technological advances have allowed remarkable progress within this field over the last years. However, we still know very little about how the 3D genome organization within the cell nucleus contributes to the regulation of gene expression. The nuclear space is compartmentalized in several domains such as the nucleolus, chromocentres, telomeres, protein bodies, and the nuclear periphery without the presence of a membrane around these domains. The role of these domains and their possible impact on nuclear activities is currently under intense investigation. In this review, we discuss new data from research in plants that clarify functional links between the organization of different nuclear domains and plant genome function with an emphasis on the potential of this organization for gene regulation.

Chromatin dynamics during interphase and cell division: similarities and differences between model and crop plants

Genetic information in the cell nucleus controls organismal development and responses to the environment, and finally ensures its own transmission to the next generations. To achieve so many different tasks, the genetic information is associated with structural and regulatory proteins, which orchestrate nuclear functions in time and space. Furthermore, plant life strategies require chromatin plasticity to allow a rapid adaptation to abiotic and biotic stresses. Here, we summarize current knowledge on the organization of plant chromatin and dynamics of chromosomes during interphase and mitotic and meiotic cell divisions for model and crop plants differing as to genome size, ploidy, and amount of genomic resources available. The existing data indicate that chromatin changes accompany most (if not all) cellular processes and that there are both shared and unique themes in the chromatin structure and global chromosome dynamics among species. Ongoing efforts to understand the molecular mechanisms involved in chromatin organization and remodeling have, together with the latest genome editing tools, potential to unlock crop genomes for innovative breeding strategies and improvements of various traits.

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.

Chromatin Organization in Early Land Plants Reveals an Ancestral Association between H3K27me3, Transposons, and Constitutive Heterochromatin

Genome packaging by nucleosomes is a hallmark of eukaryotes. Histones and the pathways that deposit, remove, and read histone modifications are deeply conserved. Yet, we lack information regarding chromatin landscapes in extant representatives of ancestors of the main groups of eukaryotes, and our knowledge of the evolution of chromatin-related processes is limited. We used the bryophyte Marchantia polymorpha, which diverged from vascular plants circa 400 mya, to obtain a whole chromosome genome assembly and explore the chromatin landscape and three-dimensional genome organization in an early diverging land plant lineage. Based on genomic profiles of ten chromatin marks, we conclude that the relationship between active marks and gene expression is conserved across land plants. In contrast, we observed distinctive features of transposons and other repetitive sequences in Marchantia compared with flowering plants. Silenced transposons and repeats did not accumulate around centromeres. Although a large fraction of constitutive heterochromatin was marked by H3K9 methylation as in flowering plants, a significant proportion of transposons were marked by H3K27me3, which is otherwise dedicated to the transcriptional repression of protein-coding genes in flowering plants. Chromatin compartmentalization analyses of Hi-C data revealed that repressed B compartments were densely decorated with H3K27me3 but not H3K9 or DNA methylation as reported in flowering plants. We conclude that, in early plants, H3K27me3 played an essential role in heterochromatin function, suggesting an ancestral role of this mark in transposon silencing.

High Satellite Repeat Turnover in Great Apes Studied with Short- and Long-Read Technologies

Satellite repeats are a structural component of centromeres and telomeres, and in some instances, their divergence is known to drive speciation. Due to their highly repetitive nature, satellite sequences have been understudied and underrepresented in genome assemblies. To investigate their turnover in great apes, we studied satellite repeats of unit sizes up to 50 bp in human, chimpanzee, bonobo, gorilla, and Sumatran and Bornean orangutans, using unassembled short and long sequencing reads. The density of satellite repeats, as identified from accurate short reads (Illumina), varied greatly among great ape genomes. These were dominated by a handful of abundant repeated motifs, frequently shared among species, which formed two groups: 1) the (AATGG)n repeat (critical for heat shock response) and its derivatives; and 2) subtelomeric 32-mers involved in telomeric metabolism. Using the densities of abundant repeats, individuals could be classified into species. However, clustering did not reproduce the accepted species phylogeny, suggesting rapid repeat evolution. Several abundant repeats were enriched in males versus females; using Y chromosome assemblies or Fluorescent In Situ Hybridization, we validated their location on the Y. Finally, applying a novel computational tool, we identified many satellite repeats completely embedded within long Oxford Nanopore and Pacific Biosciences reads. Such repeats were up to 59 kb in length and consisted of perfect repeats interspersed with other similar sequences. Our results based on sequencing reads generated with three different technologies provide the first detailed characterization of great ape satellite repeats, and open new avenues for exploring their functions.

Whole-chromosome paints in maize reveal rearrangements, nuclear domains, and chromosomal relationships

Whole-chromosome painting probes were developed for each of the 10 chromosomes of maize by producing amplifiable libraries of unique sequences of oligonucleotides that can generate labeled probes through transcription reactions. These paints allow identification of individual homologous chromosomes for many applications as demonstrated in somatic root tip metaphase cells, in the pachytene stage of meiosis, and in interphase nuclei. Several chromosomal aberrations were examined as proof of concept for study of various rearrangements using probes that cover the entire chromosome and that label diverse varieties. The relationship of the supernumerary B chromosome and the normal chromosomes was examined with the finding that there is no detectable homology between any of the normal A chromosomes and the B chromosome. Combined with other chromosome-labeling techniques, a complete set of whole-chromosome oligonucleotide paints lays the foundation for future studies of the structure, organization, and evolution of genomes.

Out-of-position telomeres in meiotic leptotene appear responsible for chiasmate pairing in an inversion heterozygote in wheat (Triticum aestivum L.)

Chromosome pairing in meiosis usually starts in the vicinity of the telomere attachment to the nuclear membrane and congregation of telomeres in the leptotene bouquet is believed responsible for bringing homologue pairs together. In a heterozygote for an inversion of a rye (Secale cereale L.) chromosome arm in wheat, a distal segment of the normal homologue is capable of chiasmate pairing with its counterpart in the inverted arm, located near the centromere. Using 3D imaging confocal microscopy, we observed that some telomeres failed to be incorporated into the bouquet and occupied various positions throughout the entire volume of the nucleus, including the centromere pole. Rye telomeres appeared ca. 21 times more likely to fail to be included in the telomere bouquet than wheat telomeres. The frequency of the out-of-bouquet rye telomere position in leptotene was virtually identical to the frequency of telomeres deviating from Rabl's orientation in the nuclei of somatic cells, and was similar to the frequency of synapsis of the normal and inverted chromosome arms, but lower than the MI pairing frequency of segments of these two arms normally positioned across the volume of the nucleus. Out-of-position placement of the rye telomeres may be responsible for reduced MI pairing of rye chromosomes in hybrids with wheat and their disproportionate contribution to aneuploidy, but appears responsible for initiating chiasmate pairing of distantly positioned segments of homology in an inversion heterozygote.

3D Molecular Cytology of Hop (Humulus lupulus) Meiotic Chromosomes Reveals Non-disomic Pairing and Segregation, Aneuploidy, and Genomic Structural Variation

Hop (Humulus lupulus L.) is an important crop worldwide, known as the main flavoring ingredient in beer. The diversifying brewing industry demands variation in flavors, superior process properties, and sustainable agronomics, which are the focus of advanced molecular breeding efforts in hops. Hop breeders have been limited in their ability to create strains with desirable traits, however, because of the unusual and unpredictable inheritance patterns and associated non-Mendelian genetic marker segregation. Cytogenetic analysis of meiotic chromosome behavior has also revealed conspicuous and prevalent occurrences of multiple, atypical, non-disomic chromosome complexes, including those involving autosomes in late prophase. To explore the role of meiosis in segregation distortion, we undertook 3D cytogenetic analysis of hop pollen mother cells stained with DAPI and FISH. We used telomere FISH to demonstrate that hop exhibits a normal telomere clustering bouquet. We also identified and characterized a new sub-terminal 180 bp satellite DNA tandem repeat family called HSR0, located proximal to telomeres. Highly variable 5S rDNA FISH patterns within and between plants, together with the detection of anaphase chromosome bridges, reflect extensive departures from normal disomic signal composition and distribution. Subsequent FACS analysis revealed variable DNA content in a cultivated pedigree. Together, these findings implicate multiple phenomena, including aneuploidy, segmental aneuploidy, or chromosome rearrangements, as contributing factors to segregation distortion in hop.

Three-dimensional chromatin packing and positioning of plant genomes

Information and function of a genome are not only decorated with epigenetic marks in the linear DNA sequence but also in their non-random spatial organization in the nucleus. Recent research has revealed that three-dimensional (3D) chromatin organization is highly correlated with the functionality of the genome, contributing to many cellular processes. Driven by the improvements in chromatin conformation capture methods and visualization techniques, the past decade has been an exciting period for the study of plants' 3D genome structures, and our knowledge in this area has been substantially advanced. This Review describes our current understanding of plant chromatin organization and positioning beyond the nucleosomal level, and discusses future directions.

Variable Patterning of Chromatin Remodeling, Telomere Positioning, Synapsis, and Chiasma Formation of Individual Rye Chromosomes in Meiosis of Wheat-Rye Additions

Meiosis, the type of cell division that halves the chromosome number, shows a considerable degree of diversity among species. Unraveling molecular mechanisms of the meiotic machinery has been mainly based on meiotic mutants, where the effects of a change were assessed on chromosomes of the particular species. An alternative approach is to study the meiotic behavior of the chromosomes introgressed into different genetic backgrounds. As an allohexaploid, common wheat tolerates introgression of chromosomes from related species, such as rye. The behavior of individual pairs of rye homologues added to wheat has been monitored in meiotic prophase I and metaphase I. Chromosome 4R increased its length in early prophase I much more than other chromosomes studied, implying chromosome specific patterns of chromatin organization. Chromosome conformation affected clustering of telomeres but not their dispersion. Telomeres of the short arm of submetacentric chromosomes 4R, 5R, and 6R failed more often to be included in the telomere cluster either than the telomeres of the long arms or telomeres of metacentrics such as 2R, 3R, and 7R. The disturbed migration of the telomeres of 5RS and 6RS was associated with failure of synapsis and chiasma formation. However, despite the failed convergence of its telomere, the 4RS arm developed normal synapsis, perhaps because the strong increase of its length in early prophase I facilitated homologous encounters in intercalary regions. Surprisingly, chiasma frequencies in both arms of 4R were reduced. Similarly, the short arm of metacentric chromosome 2R often failed to form chiasmata despite normal synapsis. Chromosomes 1R, 3R, and 7R showed a regular meiotic behavior. These observations are discussed in the context of the behavior that these chromosomes show in rye itself.

Complex characterization of oat (Avena sativa L.) lines obtained by wide crossing with maize (Zea mays L.)

Background

The oat × maize addition (OMA) lines are used for mapping of the maize genome, the studies of centromere-specific histone (CENH3), gene expression, meiotic chromosome behavior and also for introducing maize C4 photosynthetic system to oat. The aim of our study was the identification and molecular-cytogenetic characterization of oat × maize hybrids.

Methods

Oat DH lines and oat × maize hybrids were obtained using the wide crossing of Avena sativa L. with Zea mays L. The plants identified as having a Grande-1 retrotransposon fragment, which produced seeds, were used for genomic in situ hybridization (GISH).

Results

A total of 138 oat lines obtained by crossing of 2,314 oat plants from 80 genotypes with maize cv. Waza were tested for the presence of maize chromosomes. The presence of maize chromatin was indicated in 66 lines by amplification of the PCR product (500 bp) generated using primers specific for the maize retrotransposon Grande-1. Genomic in situ hybridization (GISH) detected whole maize chromosomes in eight lines (40%). All of the analyzed plants possessed full complement of oat chromosomes. The number of maize chromosomes differed between the OMA lines. Four OMA lines possessed two maize chromosomes similar in size, three OMA—one maize chromosome, and one OMA—four maize chromosomes. In most of the lines, the detected chromosomes were labeled uniformly. The presence of six 45S rDNA loci was detected in oat chromosomes, but none of the added maize chromosomes in any of the lines carried 45S rDNA locus. Twenty of the analyzed lines did not possess whole maize chromosomes, but the introgression of maize chromatin in the oat chromosomes. Five of 66 hybrids were shorter in height, grassy type without panicles. Twenty-seven OMA lines were fertile and produced seeds ranging in number from 1–102 (in total 613). Sixty-three fertile DH lines, out of 72 which did not have an addition of maize chromosomes or chromatin, produced seeds in the range of 1–343 (in total 3,758). Obtained DH and OMA lines were fertile and produced seeds.

Discussion

In wide hybridization of oat with maize, the complete or incomplete chromosomes elimination of maize occur. Hybrids of oat and maize had a complete set of oat chromosomes without maize chromosomes, and a complete set of oat chromosomes with one to four retained maize chromosomes.

Meta-analysis of chromosome-scale crossover rate variation in eukaryotes and its significance to evolutionary genomics

Understanding the distribution of crossovers along chromosomes is crucial to evolutionary genomics because the crossover rate determines how strongly a genome region is influenced by natural selection on linked sites. Nevertheless, generalities in the chromosome-scale distribution of crossovers have not been investigated formally. We fill this gap by synthesizing joint information on genetic and physical maps across 62 animal, plant and fungal species. Our quantitative analysis reveals a strong and taxonomically widespread reduction of the crossover rate in the centre of chromosomes relative to their peripheries. We demonstrate that this pattern is poorly explained by the position of the centromere, but find that the magnitude of the relative reduction in the crossover rate in chromosome centres increases with chromosome length. That is, long chromosomes often display a dramatically low crossover rate in their centre, whereas short chromosomes exhibit a relatively homogeneous crossover rate. This observation is compatible with a model in which crossover is initiated from the chromosome tips, an idea with preliminary support from mechanistic investigations of meiotic recombination. Consequently, we show that organisms achieve a higher genome-wide crossover rate by evolving smaller chromosomes. Summarizing theory and providing empirical examples, we finally highlight that taxonomically widespread and systematic heterogeneity in crossover rate along chromosomes generates predictable broad-scale trends in genetic diversity and population differentiation by modifying the impact of natural selection among regions within a genome. We conclude by emphasizing that chromosome-scale heterogeneity in crossover rate should urgently be incorporated into analytical tools in evolutionary genomics, and in the interpretation of resulting patterns.

Nuclear Envelope-Associated Chromosome Dynamics during Meiotic Prophase I

Chromosome dynamics during meiotic prophase I are associated with a series of major events such as chromosomal reorganization and condensation, pairing/synapsis and recombination of the homologs, and chromosome movements at the nuclear envelope (NE). The NE is the barrier separating the nucleus from the cytoplasm and thus plays a central role in NE-associated chromosomal movements during meiosis. Previous studies have shown in various species that NE-linked chromosome dynamics are actually driven by the cytoskeleton. The linker of nucleoskeleton and cytoskeleton (LINC) complexes are important constituents of the NE that facilitate in the transfer of cytoskeletal forces across the NE to individual chromosomes. The LINCs consist of the inner and outer NE proteins Sad1/UNC-84 (SUN), and Klarsicht/Anc-1/Syne (KASH) domain proteins. Meiosis-specific adaptations of the LINC components and unique modifications of the NE are required during chromosomal movements. Nonetheless, the actual role of the NE in chromosomic dynamic movements in plants remains elusive. This review summarizes the findings of recent studies on meiosis-specific constituents and modifications of the NE and corresponding nucleoplasmic/cytoplasmic adaptors being involved in NE-associated movement of meiotic chromosomes, as well as describes the potential molecular network of transferring cytoplasm-derived forces into meiotic chromosomes in model organisms. It helps to gain a better understanding of the NE-associated meiotic chromosomal movements in plants.

Impact of Maternal Age on Oocyte and Embryo Competence

The overall success of human reproduction, either spontaneously or after IVF, is highly dependent upon maternal age. The main reasons for age-related infertility include reduced ovarian reserve and decreased oocyte/embryo competence due to aging insults, especially concerning an increased incidence of aneuploidies and possibly decreased mitochondrial activity. Age-related chromosomal abnormalities mainly arise because of meiotic impairments during oogenesis, following flawed chromosome segregation patterns such as non-disjunction, premature separation of sister chromatids, or the recent reverse segregation. In this review, we briefly discuss the main mechanisms putatively impaired by aging in the oocytes and the deriving embryos. We also report the main strategies proposed to improve the management of advanced maternal age women in IVF: fertility preservation through oocyte cryopreservation to prevent aging; optimization of the ovarian stimulation and enhancement of embryo selection to limit its effects; and oocyte donation to circumvent its consequences.

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.

Put your 3D glasses on: plant chromatin is on show

The three-dimensional organization of the eukaryotic nucleus and its chromosomal conformation have emerged as important features in the complex network of mechanisms behind gene activity and genome connectivity dynamics, which can be evidenced in the regionalized chromosomal spatial distribution and the clustering of diverse genomic regions with similar expression patterns. The development of chromatin conformation capture (3C) techniques has permitted the elucidation of commonalities between the eukaryotic phyla, as well as important differences among them. The growing number of studies in the field performed in plants has shed light on the structural and regulatory features of these organisms. For instance, it has been proposed that plant chromatin can be arranged into different conformations such as Rabl, Rosette-like, and Bouquet, and that both short- and long-range chromatin interactions occur in Arabidopsis. In this review, we compile the current knowledge about chromosome architecture characteristics in plants, as well as the molecular events and elements (including long non-coding RNAs, histone and DNA modifications, chromatin remodeling complexes, and transcription factors) shaping the genome three-dimensional conformation. Furthermore, we discuss the developmental outputs of genome topology-mediated gene expression regulation. It is becoming increasingly clear that new tools and techniques with higher resolution need to be developed and implemented in Arabidopsis and other model plants in order to better understand chromosome architecture dynamics, from an integrative perspective with other fields of plant biology such as development, stress biology, and finally agriculture.

A complete annotation of the chromosomes of the cellulase producer Trichoderma reesei provides insights in gene clusters, their expression and reveals genes required for fitness

BACKGROUND

Investigations on a few eukaryotic model organisms showed that many genes are non-randomly distributed on chromosomes. In addition, chromosome ends frequently possess genes that are important for the fitness of the organisms. Trichoderma reesei is an industrial producer of enzymes for food, feed and biorefinery production. Its seven chromosomes have recently been assembled, thus making an investigation of its chromosome architecture possible.

RESULTS

We manually annotated and mapped 9194 ORFs on their respective chromosomes and investigated the clustering of the major gene categories and of genes encoding carbohydrate-active enzymes (CAZymes), and the relationship between clustering and expression. Genes responsible for RNA processing and modification, amino acid metabolism, transcription, translation and ribosomal structure and biogenesis indeed showed loose clustering, but this had no impact on their expression. A third of the genes encoding CAZymes also occurred in loose clusters that also contained a high number of genes encoding small secreted cysteine-rich proteins. Five CAZyme clusters were located less than 50 kb apart from the chromosome ends. These genes exhibited the lowest basal (but not induced) expression level, which correlated with an enrichment of H3K9 methylation in the terminal 50 kb areas indicating gene silencing. No differences were found in the expression of CAZyme genes present in other parts of the chromosomes. The putative subtelomeric areas were also enriched in genes encoding secreted proteases, amino acid permeases, enzyme clusters for polyketide synthases (PKS)-non-ribosomal peptide synthase (NRPS) fusion proteins (PKS-NRPS) and proteins involved in iron scavenging. They were strongly upregulated during conidiation and interaction with other fungi.

CONCLUSIONS

Our findings suggest that gene clustering on the T. reesei chromosomes occurs but generally has no impact on their expression. CAZyme genes, located in subtelomers, however, exhibited a much lower basal expression level. The gene inventory of the subtelomers suggests a major role of competition for nitrogen and iron supported by antibiosis for the fitness of T. reesei. The availability of fully annotated chromosomes will facilitate the use of genetic crossings in identifying still unknown genes responsible for specific traits of T. reesei.

Centromere Associations in Meiotic Chromosome Pairing

Production of gametes of halved ploidy for sexual reproduction requires a specialized cell division called meiosis. The fusion of two gametes restores the original ploidy in the new generation, and meiosis thus stabilizes ploidy across generations. To ensure balanced distribution of chromosomes, pairs of homologous chromosomes (homologs) must recognize each other and pair in the first meiotic division. Recombination plays a key role in this in most studied species, but it is not the only actor and particular chromosomal regions are known to facilitate the meiotic pairing of homologs. In this review, we focus on the roles of centromeres and in particular on the clustering and pairwise associations of nonhomologous centromeres that precede stable pairing between homologs. Although details vary from species to species, it is becoming increasingly clear that these associations play active roles in the meiotic chromosome pairing process, analogous to those of the telomere bouquet.

A computational study of dsDNA pairs and vibrational resonance in separating water

This article investigates the relationship between molecular sequence and dependent interacting behavior of molecular segment pairs and secondly, sequence dependent, vibrational resonance in surrounding normal saline, protein-free water. The development of a molecular model to explore these systems phenomena, the results of several nanoscale molecular dynamics simulations, and analysis of behavior of interacting ΦX174 double-stranded DNA segment pair models in various configurations are presented. Fourier analysis revealed intriguing vibration frequencies within the solvent plane between the segments, while subsequent frequency domain transformation of the time domain waveforms revealed statistically significant resonating harmonic signals in the THz range.

The subtelomeric region is important for chromosome recognition and pairing during meiosis

The process of meiosis results in the formation of haploid daughter cells, each of which inherit a half of the diploid parental cells' genetic material. The ordered association of homologues (identical chromosomes) is a critical prerequisite for a successful outcome of meiosis. Homologue recognition and pairing are initiated at the chromosome ends, which comprise the telomere dominated by generic repetitive sequences, and the adjacent subtelomeric region, which harbours chromosome-specific sequences. In many organisms telomeres are responsible for bringing the ends of the chromosomes close together during early meiosis, but little is known regarding the role of the subtelomeric region sequence during meiosis. Here, the observation of homologue pairing between a pair of Hordeum chilense chromosomes lacking the subtelomeric region on one chromosome arm indicates that the subtelomeric region is important for the process of homologous chromosome recognition and pairing.

A role for nuclear envelope-bridging complexes in homology-directed repair

Persistent double-strand DNA breaks (DSBs) are recruited to the nuclear periphery, where they induce formation of associated nuclear envelope–spanning LINC complexes made up of the SUN protein Sad1 and the KASH protein Kms1. The LINC complex couples DSBs within the nucleus to cytoplasmic microtubules, which alters DSB repair pathway choice.

Unless efficiently and faithfully repaired, DNA double-strand breaks (DSBs) cause genome instability. We implicate a Schizosaccharomyces pombe nuclear envelope–spanning linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of the Sad1/Unc84 protein Sad1 and Klarsicht/Anc1/SYNE1 homology protein Kms1, in the repair of DSBs. An induced DSB associates with Sad1 and Kms1 in S/G2 phases of the cell cycle, connecting the DSB to cytoplasmic microtubules. DSB resection to generate single-stranded DNA and the ATR kinase drive the formation of Sad1 foci in response to DNA damage. Depolymerization of microtubules or loss of Kms1 leads to an increase in the number and size of DSB-induced Sad1 foci. Further, Kms1 and the cytoplasmic microtubule regulator Mto1 promote the repair of an induced DSB by gene conversion, a type of homology-directed repair. kms1 genetically interacts with a number of genes involved in homology-directed repair; these same gene products appear to attenuate the formation or promote resolution of DSB-induced Sad1 foci. We suggest that the connection of DSBs with the cytoskeleton through the LINC complex may serve as an input to repair mechanism choice and efficiency.

A maize root tip system to study DNA replication programmes in somatic and endocycling nuclei during plant development

The progress of nuclear DNA replication is complex in both time and space, and may reflect several levels of chromatin structure and 3-dimensional organization within the nucleus. To understand the relationship between DNA replication and developmental programmes, it is important to examine replication and nuclear substructure in different developmental contexts including natural cell-cycle progressions in situ. Plant meristems offer an ideal opportunity to analyse such processes in the context of normal growth of an organism. Our current understanding of large-scale chromosomal DNA replication has been limited by the lack of appropriate tools to visualize DNA replication with high resolution at defined points within S phase. In this perspective, we discuss a promising new system that can be used to visualize DNA replication in isolated maize (Zea mays L.) root tip nuclei after in planta pulse labelling with the thymidine analogue, 5-ethynyl-2'-deoxyuridine (EdU). Mixed populations of EdU-labelled nuclei are then separated by flow cytometry into sequential stages of S phase and examined directly using 3-dimensional deconvolution microscopy to characterize spatial patterns of plant DNA replication. Combining spatiotemporal analyses with studies of replication and epigenetic inheritance at the molecular level enables an integrated experimental approach to problems of mitotic inheritance and cellular differentiation.

Gathering up meiotic telomeres: a novel function of the microtubule-organizing center

During meiosis, telomeres cluster and promote homologous chromosome pairing. Telomere clustering depends on conserved SUN and KASH domain nuclear membrane proteins, which form a complex called the linker of nucleoskeleton and cytoskeleton (LINC) and connect telomeres with the cytoskeleton. It has been thought that LINC-mediated cytoskeletal forces induce telomere clustering. However, how cytoskeletal forces induce telomere clustering is not fully understood. Recent study of fission yeast has shown that the LINC complex forms the microtubule-organizing center (MTOC) at the telomere, which has been designated as the "telocentrosome", and that microtubule motors gather telomeres via telocentrosome-nucleated microtubules. This MTOC-dependent telomere clustering might be conserved in other eukaryotes. Furthermore, the MTOC-dependent clustering mechanism appears to function in various other biological events. This review presents an overview of the current understanding of the mechanism of meiotic telomere clustering and discusses the universality of the MTOC-dependent clustering mechanism.

Recombination-independent mechanisms and pairing of homologous chromosomes during meiosis in plants

Meiosis is the specialized eukaryotic cell division that permits the halving of ploidy necessary for gametogenesis in sexually reproducing organisms. This involves a single round of DNA replication followed by two successive divisions. To ensure balanced segregation, homologous chromosome pairs must migrate to opposite poles at the first meiotic division and this means that they must recognize and pair with each other beforehand. Although understanding of the mechanisms by which meiotic chromosomes find and pair with their homologs has greatly advanced, it remains far from being fully understood. With some notable exceptions such as male Drosophila, the recognition and physical linkage of homologs at the first meiotic division involves homologous recombination. However, in addition to this, it is clear that many organisms, including plants, have also evolved a series of recombination-independent mechanisms to facilitate homolog recognition and pairing. These implicate chromosome structure and dynamics, telomeres, centromeres, and, most recently, small RNAs. With a particular focus on plants, we present here an overview of understanding of these early, recombination-independent events that act in the pairing of homologous chromosomes during the first meiotic division.

A dynamic meiotic SUN belt includes the zygotene-stage telomere bouquet and is disrupted in chromosome segregation mutants of maize (Zea mays L.)

The nuclear envelope (NE) plays an essential role in meiotic telomere behavior and links the cytoplasm and nucleoplasm during homologous chromosome pairing and recombination in many eukaryotic species. Resident NE proteins including SUN (Sad-1/UNC-84) and KASH (Klarsicht/ANC-1/Syne-homology) domain proteins are known to interact forming the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex that connects chromatin to the cytoskeleton. To investigate the possible cross-kingdom conservation of SUN protein functions in plant meiosis, we immunolocalized maize SUN2 using 3D microscopy of pollen mother cells from maize (Zea mays L.), a large-genome plant model with a canonical NE zygotene-stage telomere bouquet. We detected SUN2 at the nuclear periphery and found that it exhibited a distinct belt-like structure that transitioned to a half-belt during the zygotene stage and back to a full belt during and beyond the pachytene stage. The zygotene-stage half-belt SUN structure was shown by 3D immuno-FISH to include the NE-associated telomere cluster that defines the bouquet stage and coincides with homologous chromosome synapsis. Microtubule and filamentous actin staining patterns did not show any obvious belt or a retracted-like structure other than a general enrichment of tubulin staining distributed widely around the nucleus and throughout the cytoplasm. Genetic disruption of the meiotic SUN belt staining patterns with three different meiosis-specific mutants, desynaptic (dy1), asynaptic1 (as1), and divergent spindle1 (dv1) provides additional evidence for the role of the nuclear envelope in meiotic chromosome behavior. Taking into account all of the observations from this study, we propose that the maize SUN belt is directly or indirectly involved in meiotic telomere dynamics, chromosome synapsis, and possibly integration of signals and forces across the meiotic prophase nuclear envelope.

Chromosome segregation in plant meiosis

Faithful chromosome segregation in meiosis is essential for ploidy stability over sexual life cycles. In plants, defective chromosome segregation caused by gene mutations or other factors leads to the formation of unbalanced or unreduced gametes creating aneuploid or polyploid progeny, respectively. Accurate segregation requires the coordinated execution of conserved processes occurring throughout the two meiotic cell divisions. Synapsis and recombination ensure the establishment of chiasmata that hold homologous chromosomes together allowing their correct segregation in the first meiotic division, which is also tightly regulated by cell-cycle dependent release of cohesin and monopolar attachment of sister kinetochores to microtubules. In meiosis II, bi-orientation of sister kinetochores and proper spindle orientation correctly segregate chromosomes in four haploid cells. Checkpoint mechanisms acting at kinetochores control the accuracy of kinetochore-microtubule attachment, thus ensuring the completion of segregation. Here we review the current knowledge on the processes taking place during chromosome segregation in plant meiosis, focusing on the characterization of the molecular factors involved.

Three-dimensional acrylamide fluorescence in situ hybridization for plant cells

Plant meiosis involves complex and dynamic processes that occur within the space inside the nucleus. Direct inspection of meiotic chromosomes by fluorescence microscopy has been used to investigate many of these processes. In particular, optical sectioning microscopy of fluorescence in situ hybridization (FISH)-stained nuclei provides three-dimensional spatial information about the organization and distribution of specific sequences and chromosomal loci within the nucleus. Here we provide a fully detailed three-dimensional (3D) acrylamide FISH method for the analysis of plant meiotic nuclei. Several examples illustrate the versatility of this technique for the investigation of meiotic telomere dynamics in maize, Arabidopsis, and oat. Additional examples of 3D FISH include chromosome painting in a maize chromosome-addition line of oat and telomere FISH with maize nuclei from plants expressing a fluorescently tagged fusion protein, histone H2B-mCherry.

Telomeres and human reproduction

Telomeres mediate biologic aging in organisms as diverse as plants, yeast, and mammals. We propose a telomere theory of reproductive aging that posits telomere shortening in the female germ line as the primary driver of reproductive aging in women. Experimental shortening of telomeres in mice, which normally do not exhibit appreciable oocyte aging, and which have exceptionally long telomeres, recapitulates the aging phenotype of human oocytes. Telomere shortening in mice reduces synapsis and chiasmata, increases embryo fragmentation, cell cycle arrest, apoptosis, spindle dysmorphologies, and chromosome abnormalities. Telomeres are shorter in the oocytes from women undergoing in vitro fertilization, who then produce fragmented, aneuploid embryos that fail to implant. In contrast, the testes are replete with spermatogonia that can rejuvenate telomere reserves throughout the life of the man by expressing telomerase. Differences in telomere dynamics across the life span of men and women may have evolved because of the difference in the inherent risks of aging on reproduction between men and women. Additionally, growing evidence links altered telomere biology to endometriosis and gynecologic cancers, thus future studies should examine the role of telomeres in pathologies of the reproductive tract.

Maize histone H2B-mCherry: a new fluorescent chromatin marker for somatic and meiotic chromosome research

Cytological studies of fluorescent proteins are rapidly yielding insights into chromatin structure and dynamics. Here we describe the production and cytological characterization of new transgenic maize lines expressing a fluorescent histone fusion protein, H2B-mCherry. The transgene is expressed under the control of the maize ubiquitin1 promoter, including its first exon and intron. Polymerase chain reaction-based genotyping and root-tip microscopy showed that most of the lines carrying the transgene also expressed it, producing bright uniform staining of nuclei. Further, plants showing expression in root tips at the seedling stage also showed expression during meiosis, late in the life cycle. Detailed high-resolution three-dimensional imaging of cells and nuclei from various somatic and meiotic cell types showed that H2B-mCherry produced remarkably clear images of chromatin and chromosome fiber morphology, as seen in somatic, male meiotic prophase, and early microgametophyte cells. H2B-mCherry also yielded distinct nucleolus staining and was shown to be compatible with fluorescence in situ hybridization. We found several instances where H2B-mCherry was superior to DAPI as a generalized chromatin stain. Our study establishes these histone H2B-mCherry lines as new biological reagents for visualizing chromatin structure, chromosome morphology, and nuclear dynamics in fixed and living cells in a model plant genetic system.

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.

Dynein-dependent processive chromosome motions promote homologous pairing in C. elegans meiosis

Meiotic chromosome segregation requires homologue pairing, synapsis, and crossover recombination, which occur during meiotic prophase. Telomere-led chromosome motion has been observed or inferred to occur during this stage in diverse species, but its mechanism and function remain enigmatic. In Caenorhabditis elegans, special chromosome regions known as pairing centers (PCs), rather than telomeres, associate with the nuclear envelope (NE) and the microtubule cytoskeleton. In this paper, we investigate chromosome dynamics in living animals through high-resolution four-dimensional fluorescence imaging and quantitative motion analysis. We find that chromosome movement is constrained before meiosis. Upon prophase onset, constraints are relaxed, and PCs initiate saltatory, processive, dynein-dependent motions along the NE. These dramatic motions are dispensable for homologous pairing and continue until synapsis is completed. These observations are consistent with the idea that motions facilitate pairing by enhancing the search rate but that their primary function is to trigger synapsis. This quantitative analysis of chromosome dynamics in a living animal extends our understanding of the mechanisms governing faithful genome inheritance.

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.

Inversions of chromosome arms 4AL and 2BS in wheat invert the patterns of chiasma distribution

In many species, including wheat, crossing over is distal, and the proximal regions of chromosome arms contribute little to genetic maps. This was thought to be a consequence of terminal initiation of synapsis favoring distal crossing over. However, in an inverted rye chromosome arm, the pattern of metaphase I chiasmata was also inverted, suggesting that crossover frequencies were specific to chromosome segments. Here, wheat chromosome arms 2BS and 4AL, with essentially entire arms inverted in reverse tandem duplications (rtd), were studied in the MI of meiosis. Inversion-duplication placed the recombining segments in the middle of the arms. While the overall pairing frequencies of the inverted-duplicated arms were considerably reduced relative to normal arms, chiasmata, if present, were always located in the same regions as in structurally normal arms, and relative chiasma frequencies remained the same. The frequencies of fragment or fragment + bridge configurations in AI and AII indicated that of the two tandemly arranged copies of segments in rtds, the more distal inverted segments were more likely to cross over than the segments in their original orientations. These observations show that also in wheat, relative crossover frequencies along chromosome arms are predetermined and independent of the segment location. The segments normally not licensed to cross over do not do so even when placed in seemingly most favorable positions for it.

Chromosome painting reveals asynaptic full alignment of homologs and HIM-8-dependent remodeling of X chromosome territories during Caenorhabditis elegans meiosis

During early meiotic prophase, a nucleus-wide reorganization leads to sorting of chromosomes into homologous pairs and to establishing associations between homologous chromosomes along their entire lengths. Here, we investigate global features of chromosome organization during this process, using a chromosome painting method in whole-mount Caenorhabditis elegans gonads that enables visualization of whole chromosomes along their entire lengths in the context of preserved 3D nuclear architecture. First, we show that neither spatial proximity of premeiotic chromosome territories nor chromosome-specific timing is a major factor driving homolog pairing. Second, we show that synaptonemal complex-independent associations can support full lengthwise juxtaposition of homologous chromosomes. Third, we reveal a prominent elongation of chromosome territories during meiotic prophase that initiates prior to homolog association and alignment. Mutant analysis indicates that chromosome movement mediated by association of chromosome pairing centers (PCs) with mobile patches of the nuclear envelope (NE)–spanning SUN-1/ZYG-12 protein complexes is not the primary driver of territory elongation. Moreover, we identify new roles for the X chromosome PC (X-PC) and X-PC binding protein HIM-8 in promoting elongation of X chromosome territories, separable from their role(s) in mediating local stabilization of pairing and association of X chromosomes with mobile SUN-1/ZYG-12 patches. Further, we present evidence that HIM-8 functions both at and outside of PCs to mediate chromosome territory elongation. These and other data support a model in which synapsis-independent elongation of chromosome territories, driven by PC binding proteins, enables lengthwise juxtaposition of chromosomes, thereby facilitating assessment of their suitability as potential pairing partners.

Successful sexual reproduction relies on the ability of germ cells to faithfully segregate homologous chromosomes in meiosis, which requires accurate sorting of chromosomes into homologous pairs and alignment of homologs along their entire lengths. The mechanisms underlying homolog sorting and alignment are not well understood, partly because of a scarcity of studies investigating homolog alignment at the level of whole chromosomes. This study provides a global view of the organization of chromosome territories during early meiotic prophase in the nematode Caenorhabditis elegans. We applied chromosome painting to visualize the entire lengths of chromosomes. Our study provides several conceptual advances. First, our study excluded several possible mechanisms as primary drivers of chromosome sorting. Second, our analysis has revealed both a robust capacity for full-lengthwise alignment between homologous chromosomes prior to the stabilization of pairing by the synaptonemal complex as well as a dramatic elongation of chromosome territories that could enable this alignment. We also identified a factor required for the elongation of chromosome territories. Elongation of chromosome territories could enable lengthwise juxtaposition of chromosomes, thereby facilitating assessment of their suitability as potential pairing partners by promoting utilization of information about chromosome identity that is distributed along the length of a chromosome.