Stretching the rules: monocentric chromosomes with multiple centromere domains

Eukaryotic Centromere Remodeling: Plasticity, Dynamics, and Holocentromere Formation

Eukaryotic centromeres highlight the remarkable plasticity of eukaryotic chromosomes through their conserved functionality and sequence divergence. Holocentric chromosomes, where centromere activity is distributed along the entire chromosome length, offer a unique model for investigating the molecular mechanisms underlying adaptive evolution between centromeres and chromosomes. In this review, we summarise and speculate on the multiple changes and prerequisites potentially involved in the evolution of holocentromeres. The interplay between environmental factors, chromosomal rearrangements, and centromere plasticity drives the transition from regional to holocentric characteristics. The centromeric histone H3 (CenH3) protein mediates neocentromere formation by recognising non-centromeric chromosomal regions with appropriate AT content, thereby facilitating chromosome restructuring in the transition from regional to holocentric chromosomes. Dynamic changes in repetitive sequences provide functional sites for centromere assembly, chromosomal recombination and repair and centromere spreading and maturation. Epigenetic modifications maintain functional coordination among multiple centromeric units by modulating chromatin states, CenH3 localisation, and kinetochore assembly. This review provides a comprehensive framework for understanding the evolutionary mechanisms of holocentromeres derived from monocentromere and offers insights into the design of artificial centromeres.

Advances and Challenges in Haploid Induction for Warm-Season Legumes

Haploid induction via doubled haploid (DH) technology is pivotal for achieving true homozygosity in plant breeding; however, Fabaceae species lag in establishing effective haploidization methods. This review explores recent advances in DH techniques for warm-season legumes, including soybean, cowpea, pigeon pea, common bean, peanut, mung bean, and winged bean, highlighting key challenges and perspectives. While anther culture, cold pretreatment, and MS-based medium with growth regulators demonstrate potential, fully reproducible protocols remain elusive. Advances in microspore-derived embryogenesis have improved regeneration consistency in soybeans, cowpeas, and peanuts. Key areas for improving DH technology include optimizing regeneration conditions, such as shortening the callus phase and accelerating embryo and shoot development. Recent advancements in CRISPR/Cas9-mediated genome modifications, targeting genes like CENH3, MTL, and DMP, offer promising strategies to enhance efficiency and overcome resistance to conventional methods. Integrating molecular tools with haploid induction and conventional breeding techniques can significantly improve legume breeding and productivity.

CENP-A/CENP-B uncoupling in the evolutionary reshuffling of centromeres in equids

BACKGROUND

While CENP-A is the epigenetic determinant of the centromeric function, the role of CENP-B, a centromeric protein binding a specific DNA sequence, the CENP-B-box, remains elusive. In the few mammalian species analyzed so far, the CENP-B box is contained in the major satellite repeat that is present at all centromeres, with the exception of the Y chromosome. We previously demonstrated that, in the genus Equus, numerous centromeres lack any satellite repeat.

RESULTS

In four Equus species, CENP-B is expressed but does not bind the majority of satellite-based centromeres, or the satellite-free ones, while it is localized at several ancestral, now-inactive, centromeres. Centromeres lacking CENP-B are functional and recruit normal amounts of CENP-A and CENP-C. The absence of CENP-B is related to the lack of CENP-B boxes rather than to peculiar features of the protein itself. CENP-B boxes are present in a previously undescribed repeat which is not the major satellite bound by CENP-A. Comparative sequence analysis suggests that this satellite was centromeric in the equid ancestor, lost centromeric function during evolution, and gave rise to a shorter CENP-A bound repeat not containing the CENP-B box but enriched in dyad symmetries.

CONCLUSIONS

We propose that the uncoupling between CENP-B and CENP-A may have played a role in the extensive evolutionary reshuffling of equid centromeres. This study provides new insights into the complexity of centromere organization in a largely biodiverse world where the majority of mammalian species still have to be studied.

Structural insights into mechanisms of zinc scavenging by the Candida albicans zincophore Pra1

Candida albicans causes more than 400,000 life-threatening, and half a billion mucosal infections annually. In response to infection, the host limits availability of essential micronutrients, including zinc, to restrict growth of the invading pathogen. As assimilation of zinc is essential for C. albicans pathogenicity, its limitation induces the secretion of the zincophore protein Pra1 to scavenge zinc from the host. Pra1 also plays a number of important roles in host-pathogen interactions and is conserved in most fungi. However, the structure of fungal zincophores is not known. Here, we present the first cryogenic electron microscopy structures of C. albicans Pra1 in its apo- and zinc-bound states, at 2.8 and 2.5 Å resolution respectively. Our work reveals a hexameric ring-like assembly with multiple zinc binding sites. Through genetic studies, we show that one of these zinc binding sites is essential for C. albicans growth under zinc restriction but does not affect the inflammatory properties of Pra1. These data provide a foundation for future work to explore the structural basis of Pra1-mediated host-pathogen interactions, C. albicans zinc uptake, as well therapeutics development.

Core promoterome of barley embryo

Precise localization and dissection of gene promoters are key to understanding transcriptional gene regulation and to successful bioengineering applications. The core RNA polymerase II initiation machinery is highly conserved among eukaryotes, leading to a general expectation of equivalent underlying mechanisms. Still, less is known about promoters in the plant kingdom. In this study, we employed cap analysis of gene expression (CAGE) at three embryonic developmental stages in barley to accurately map, annotate, and quantify transcription initiation events. Unsupervised discovery of de novo sequence clusters grouped promoters based on characteristic initiator and position-specific core-promoter motifs. This grouping was complemented by the annotation of transcription factor binding site (TFBS) motifs. Integration with genome-wide epigenomic data sets and gene ontology (GO) enrichment analysis further delineated the chromatin environments and functional roles of genes associated with distinct promoter categories. The TATA-box presence governs all features explored, supporting the general model of two separate genomic regulatory environments. We describe the extent and implications of alternative transcription initiation events, including those that are specific to developmental stages, which can affect the protein sequence or the presence of regions that regulate translation. The generated promoterome dataset provides a valuable genomic resource for enhancing the functional annotation of the barley genome. It also offers insights into the transcriptional regulation of individual genes and presents opportunities for the informed manipulation of promoter architecture, with the aim of enhancing traits of agronomic importance.

Chromosome-specific barcode system with centromeric repeat in cultivated soybean and wild progenitor

Wild soybean Glycine soja is the progenitor of cultivated soybean Glycine max Information on soybean functional centromeres is limited despite extensive genome analysis. These species are an ideal model for studying centromere dynamics for domestication and breeding. We performed a detailed chromatin immunoprecipitation analysis using centromere-specific histone H3 protein to delineate two distinct centromeric DNA sequences with unusual repeating units with monomer sizes of 90-92 bp (CentGm-1) and 413-bp (CentGm-4) shorter and longer than standard nucleosomes. These two unrelated DNA sequences with no sequence similarity are part of functional centromeres in both species. Our results provide a comparison of centromere properties between a cultivated and a wild species under the effect of the same kinetochore protein. Possible sequence homogenization specific to each chromosome could highlight the mechanism for evolutionary conservation of centromeric properties independent of domestication and breeding. Moreover, a unique barcode system to track each chromosome is developed using CentGm-4 units. Our results with a unifying centromere composition model using CentGm-1 and CentGm-4 superfamilies could have far-reaching implications for comparative and evolutionary genome research.

Plant kinetochore complex: composition, function, and regulation

The kinetochore complex, an important protein assembly situated on the centromere, plays a pivotal role in chromosome segregation during cell division. Like in animals and fungi, the plant kinetochore complex is important for maintaining chromosome stability, regulating microtubule attachment, executing error correction mechanisms, and participating in signaling pathways to ensure accurate chromosome segregation. This review summarizes the composition, function, and regulation of the plant kinetochore complex, emphasizing the interactions of kinetochore proteins with centromeric DNAs (cenDNAs) and RNAs (cenRNAs). Additionally, the applications of the centromeric histone H3 variant (the core kinetochore protein CENH3, first identified as CENP-A in mammals) in the generation of ploidy-variable plants and synthesis of plant artificial chromosomes (PACs) are discussed. The review serves as a comprehensive roadmap for researchers delving into plant kinetochore exploration, highlighting the potential of kinetochore proteins in driving technological innovations in synthetic genomics and plant biotechnology.

Structure and evolution of metapolycentromeres

Metapolycentromeres consist of multiple sequential domains of centromeric chromatin associated with a centromere-specific variant of histone H3 (CENP-A), functioning collectively as a single centromere. To date, they have been revealed in nine flowering plant, five insect and six vertebrate species. In this paper, we focus on their structure and possible mechanisms of emergence and evolution. The metapolycentromeres may vary in the number of centromeric domains and in their genetic content and epigenetic modifications. However, these variations do not seem to affect their function. The emergence of metapolycentromeres has been attributed to multiple Robertsonian translocations and segmental duplications. Conditions of genomic instability, such as interspecific hybridization and malignant neoplasms, are suggested as triggers for the de novo emergence of metapolycentromeres. Addressing the "centromere paradox" - the rapid evolution of centromeric DNA and proteins despite their conserved cellular function - we explore the centromere drive hypothesis as a plausible explanation for the dynamic evolution of centromeres in general, and in particular the emergence of metapolycentromeres and holocentromeres. Apparently, metapolycentromeres are more common across different species than it was believed until recently. Indeed, a systematic review of the available cytogenetic publications allowed us to identify 27 candidate species with metapolycentromeres. Тhe list of the already established and newly revealed candidate species thus spans 27 species of flowering plants and eight species of gymnosperm plants, five species of insects, and seven species of vertebrates. This indicates an erratic phylogenetic distribution of the species with metapolycentromeres and may suggest an independent emergence of the metapolycentromeres in the course of evolution. However, the current catalog of species with identified and likely metapolycentromeres remains too short to draw reliable conclusions about their evolution, particularly in the absence of knowledge about related species without metapolycentromeres for comparative analysis. More studies are necessary to shed light on the mechanisms of metapolycentromere formation and evolution.

Immunodetection of tubulin and centromeric histone H3 (CENH3) proteins in Glycine species

BACKGROUND

The centromeres appear as primary constrictions on monocentric metaphase chromosomes; where sister chromatids are held together and assemble the proteinaceous kitechore complex at which microtubule proteins attach during nuclear divisions for pulling sister chromatids to opposite cell poles. The movement of chromosomes is usually governed by structural proteins that are either species-specific or highly conserved, such as the centromere-specific histone H3 (CENH3) and tubulin proteins, respectively.

METHODS AND RESULTS

We aimed to detect these proteins across eight different Glycine species by an immunofluorescence assay using specific antibodies. Furthermore, with the α-tubulin antibody we traced the dynamics of microtubules during the mitotic cell cycle in Glycine max. With two-color immunofluorescence staining, we showed that both proteins interact during nuclear division.

CONCLUSIONS

Finally, we proved that in different diploid and tetraploid Glycine species CENH3 can be detected in functional centromeres with spatial proximity of microtubule proteins.

Centromere sequence-independent but biased loading of subgenome-specific CENH3 variants in allopolyploid Arabidopsis suecica

Centromeric nucleosomes are determined by the replacement of the canonical histone H3 with the centromere-specific histone H3 (CENH3) variant. Little is known about the centromere organization in allopolyploid species where different subgenome-specific CENH3s and subgenome-specific centromeric sequences coexist. Here, we analyzed the transcription and centromeric localization of subgenome-specific CENH3 variants in the allopolyploid species Arabidopsis suecica. Synthetic A. thaliana x A. arenosa hybrids were generated and analyzed to mimic the early evolution of A. suecica. Our expression analyses indicated that CENH3 has generally higher expression levels in A. arenosa compared to A. thaliana, and this pattern persists in the hybrids. We also demonstrated that despite a different centromere DNA composition, the centromeres of both subgenomes incorporate CENH3 encoded by both subgenomes, but with a positive bias towards the A. arenosa-type CENH3. The intermingled arrangement of both CENH3 variants demonstrates centromere plasticity and may be an evolutionary adaption to handle more than one CENH3 variant in the process of allopolyploidization.

Centromere diversity: How different repeat-based holocentromeres may have evolved

In addition to monocentric eukaryotes, which have a single localized centromere on each chromosome, there are holocentric species, with extended repeat-based or repeat-less centromeres distributed over the entire chromosome length. At least two types of repeat-based holocentromeres exist, one composed of many small repeat-based centromere units (small unit-type), and another one characterized by a few large centromere units (large unit-type). We hypothesize that the transposable element-mediated dispersal of hundreds of short satellite arrays formed the small centromere unit-type holocentromere in Rhynchospora pubera. The large centromere unit-type of the plant Chionographis japonica is likely a product of simultaneous DNA double-strand breaks (DSBs), which initiated the de novo formation of repeat-based holocentromeres via insertion of satellite DNA, derived from extra-chromosomal circular DNAs (eccDNAs). The number of initial DSBs along the chromosomes must be higher than the number of centromere units since only a portion of the breaks will have incorporated eccDNA at an appropriate position to serve as future centromere unit sites. Subsequently, preferential incorporation of the centromeric histone H3 variant at these positions is assumed. The identification of repeat-based holocentromeres across lineages will unveil the centromere plasticity and elucidate the mechanisms underlying the diverse formation of holocentromeres.

How diverse a monocentric chromosome can be? Repeatome and centromeric organization of Juncus effusus (Juncaceae)

Juncus is the largest genus of Juncaceae and was considered holocentric for a long time. Recent findings, however, indicated that 11 species from different clades of the genus have monocentric chromosomes. Thus, the Juncus centromere organization and evolution need to be reassessed. We aimed to investigate the major repetitive DNA sequences of two accessions of Juncus effusus and its centromeric structure by employing whole-genome analyses, fluorescent in situ hybridization, CENH3 immunodetection, and chromatin immunoprecipitation sequencing. We showed that the repetitive fraction of the small J. effusus genome (~270 Mbp/1C) is mainly composed of Class I and Class II transposable elements (TEs) and satellite DNAs. Three identified satellite DNA families were mainly (peri)centromeric, with two being associated with the centromeric protein CENH3, but not strictly centromeric. Two types of centromere organization were discerned in J. effusus: type 1 was characterized by a single CENH3 domain enriched with JefSAT1-155 or JefSAT2-180, whereas type 2 showed multiple CENH3 domains interrupted by other satellites, TEs or genes. Furthermore, while type 1 centromeres showed a higher degree of satellite identity along the array, type 2 centromeres had less homogenized arrays along the multiple CENH3 domains per chromosome. Although the analyses confirmed the monocentric organization of J. effusus chromosomes, our data indicate a more dynamic arrangement of J. effusus centromeres than observed for other plant species, suggesting it may constitute a transient state between mono- and holocentricity.

The role of centromeric repeats and transcripts in kinetochore assembly and function

Centromeres are the chromosomal domains, where the kinetochore protein complex is formed, mediating proper segregation of chromosomes during cell division. Although the function of centromeres has remained conserved during evolution, centromeric DNA is highly variable, even in closely related species. In addition, the composition of the kinetochore complexes varies among organisms. Therefore, it is assumed that the centromeric position is determined epigenetically, and the centromeric histone H3 (CENH3) serves as an epigenetic marker. The loading of CENH3 onto centromeres depends on centromere-licensing factors, chaperones, and transcription of centromeric repeats. Several proteins that regulate CENH3 loading and kinetochore assembly interact with the centromeric transcripts and DNA in a sequence-independent manner. However, the functional aspects of these interactions are not fully understood. This review discusses the variability of centromeric sequences in different organisms and the regulation of their transcription through the RNA Pol II and RNAi machinery. The data suggest that the interaction of proteins involved in CENH3 loading and kinetochore assembly with centromeric DNA and transcripts plays a role in centromere, and possibly neocentromere, formation in a sequence-independent manner.

KNL1 and NDC80 represent new universal markers for the detection of functional centromeres in plants

Centromere is the chromosomal site of kinetochore assembly and microtubule attachment for chromosome segregation. Given its importance, markers that allow specific labeling of centromeric chromatin throughout the cell cycle and across all chromosome types are sought for facilitating various centromere studies. Antibodies against the N-terminal region of CENH3 are commonly used for this purpose, since CENH3 is the near-universal marker of functional centromeres. However, because the N-terminal region of CENH3 is highly variable among plant species, antibodies directed against this region usually function only in a small group of closely related species. As a more versatile alternative, we present here antibodies targeted to the conserved domains of two outer kinetochore proteins, KNL1 and NDC80. Sequence comparison of these domains across more than 350 plant species revealed a high degree of conservation, particularly within a six amino acid motif, FFGPVS in KNL1, suggesting that both antibodies would function in a wide range of plant species. This assumption was confirmed by immunolabeling experiments in angiosperm (monocot and dicot) and gymnosperm species, including those with mono-, holo-, and meta-polycentric chromosomes. In addition to centromere labeling on condensed chromosomes during cell division, both antibodies detected the corresponding regions in the interphase nuclei of most species tested. These results demonstrated that KNL1 and NDC80 are better suited for immunolabeling centromeres than CENH3, because antibodies against these proteins offer incomparably greater versatility across different plant species which is particularly convenient for studying the organization and function of the centromere in non-model species.

Injection-based hairy root induction and plant regeneration techniques in Brassicaceae

BACKGROUND

Hairy roots constitute a valuable tissue culture system for species that are difficult to propagate through conventional seed-based methods. Moreover, the generation of transgenic plants derived from hairy roots can be facilitated by employing carefully designed hormone-containing media.

RESULTS

We initiated hairy root formation in the rare crucifer species Asperuginoides axillaris via an injection-based protocol using the Agrobacterium strain C58C1 harboring a hairy root-inducing (Ri) plasmid and successfully regenerated plants from established hairy root lines. Our study confirms the genetic stability of both hairy roots and their derived regenerants and highlights their utility as a permanent source of mitotic chromosomes for cytogenetic investigations. Additionally, we have developed an effective embryo rescue protocol to circumvent seed dormancy issues in A. axillaris seeds. By using inflorescence primary stems of Arabidopsis thaliana and Cardamine hirsuta as starting material, we also established hairy root lines that were subsequently used for regeneration studies.

CONCLUSION

We developed efficient hairy root transformation and regeneration protocols for various crucifers, namely A. axillaris, A. thaliana, and C. hirsuta. Hairy roots and derived regenerants can serve as a continuous source of plant material for molecular and cytogenetic analyses.

Pan-centromere reveals widespread centromere repositioning of soybean genomes

Centromere repositioning refers to a de novo centromere formation at another chromosomal position without sequence rearrangement. This phenomenon was frequently encountered in both mammalian and plant species and has been implicated in genome evolution and speciation. To understand the dynamic of centromeres on soybean genome, we performed the pan-centromere analysis using CENH3-ChIP-seq data from 27 soybean accessions, including 3 wild soybeans, 9 landraces, and 15 cultivars. Building upon the previous discovery of three centromere satellites in soybean, we have identified two additional centromere satellites that specifically associate with chromosome 1. These satellites reveal significant rearrangements in the centromere structures of chromosome 1 across different accessions, consequently impacting the localization of CENH3. By comparative analysis, we reported a high frequency of centromere repositioning on 14 out of 20 chromosomes. Most newly emerging centromeres formed in close proximity to the native centromeres and some newly emerging centromeres were apparently shared in distantly related accessions, suggesting their emergence is independent. Furthermore, we crossed two accessions with mismatched centromeres to investigate how centromere positions would be influenced in hybrid genetic backgrounds. We found that a significant proportion of centromeres in the S9 generation undergo changes in size and position compared to their parental counterparts. Centromeres preferred to locate at satellites to maintain a stable state, highlighting a significant role of centromere satellites in centromere organization. Taken together, these results revealed extensive centromere repositioning in soybean genome and highlighted how important centromere satellites are in constraining centromere positions and supporting centromere function.

Decoding allelic diversity, transcript variants and transcriptional complexity of CENH3 gene in Brassica oleracea var. botrytis

Histone proteins play a critical role in the primary organization of nucleosomes, which is the fundamental unit of chromatin. Among the five types of the histones, histone H3 has multiple variants, and the number differs among the species. Amongst histone H3 variants, centromeric histone H3 (CENH3) is crucial for centromere identification and proper chromosomal segregation during cell division. In the present study, we have identified 17 putative histone H3 genes of Brassica oleracea. Furthermore, we have done a detailed characterization of the CENH3 gene of B. oleracea. We showed that a single CENH3 gene exhibits allelic diversity with at least two alleles and alternative splicing pattern. Also, we have identified a CENH3 gene-specific co-dominant cleaved amplified polymorphic sequence marker SNP34(A/C) to distinguish CENH3 alleles and follow their expression in leaf and flower tissues. The gene structure analysis of the CENH3 gene revealed the conserved 5'-CAGCAG-3' sequence at the intron 3-exon 4 junction in B. oleracea, which serves as an alternative splicing site with one-codon (alanine) addition/deletion. However, this one-codon alternative splicing feature is not conserved in the CENH3 genes of wild allied Brassica species. Our finding suggests that transcriptional complexity and alternative splicing might play a key role in the transcriptional regulation and function of the CENH3 gene in B. oleracea. Altogether, data generated from the present study can serve as a primary information resource and can be used to engineer CENH3 gene towards developing haploid inducer lines in B. oleracea.

The gap-free genome of mulberry elucidates the architecture and evolution of polycentric chromosomes

Mulberry is a fundamental component of the global sericulture industry, and its positive impact on our health and the environment cannot be overstated. However, the mulberry reference genomes reported previously remained unassembled or unplaced sequences. Here, we report the assembly and analysis of the telomere-to-telomere gap-free reference genome of the mulberry species, Morus notabilis, which has emerged as an important reference in mulberry gene function research and genetic improvement. The mulberry gap-free reference genome produced here provides an unprecedented opportunity for us to study the structure and function of centromeres. Our results revealed that all mulberry centromeric regions share conserved centromeric satellite repeats with different copies. Strikingly, we found that M. notabilis is a species with polycentric chromosomes and the only reported polycentric chromosome species up to now. We propose a compelling model that explains the formation mechanism of new centromeres and addresses the unsolved scientific question of the chromosome fusion-fission cycle in mulberry species. Our study sheds light on the functional genomics, chromosome evolution, and genetic improvement of mulberry species.

Holocentromeres can consist of merely a few megabase-sized satellite arrays

The centromere is the chromosome region where microtubules attach during cell division. In contrast to monocentric chromosomes with one centromere, holocentric species usually distribute hundreds of centromere units along the entire chromatid. We assembled the chromosome-scale reference genome and analyzed the holocentromere and (epi)genome organization of the lilioid Chionographis japonica. Remarkably, each of its holocentric chromatids consists of only 7 to 11 evenly spaced megabase-sized centromere-specific histone H3-positive units. These units contain satellite arrays of 23 and 28 bp-long monomers capable of forming palindromic structures. Like monocentric species, C. japonica forms clustered centromeres in chromocenters at interphase. In addition, the large-scale eu- and heterochromatin arrangement differs between C. japonica and other known holocentric species. Finally, using polymer simulations, we model the formation of prometaphase line-like holocentromeres from interphase centromere clusters. Our findings broaden the knowledge about centromere diversity, showing that holocentricity is not restricted to species with numerous and small centromere units.

Assembly of the 81.6 Mb centromere of pea chromosome 6 elucidates the structure and evolution of metapolycentric chromosomes

Centromeres in the legume genera Pisum and Lathyrus exhibit unique morphological characteristics, including extended primary constrictions and multiple separate domains of centromeric chromatin. These so-called metapolycentromeres resemble an intermediate form between monocentric and holocentric types, and therefore provide a great opportunity for studying the transitions between different types of centromere organizations. However, because of the exceedingly large and highly repetitive nature of metapolycentromeres, highly contiguous assemblies needed for these studies are lacking. Here, we report on the assembly and analysis of a 177.6 Mb region of pea (Pisum sativum) chromosome 6, including the 81.6 Mb centromere region (CEN6) and adjacent chromosome arms. Genes, DNA methylation profiles, and most of the repeats were uniformly distributed within the centromere, and their densities in CEN6 and chromosome arms were similar. The exception was an accumulation of satellite DNA in CEN6, where it formed multiple arrays up to 2 Mb in length. Centromeric chromatin, characterized by the presence of the CENH3 protein, was predominantly associated with arrays of three different satellite repeats; however, five other satellites present in CEN6 lacked CENH3. The presence of CENH3 chromatin was found to determine the spatial distribution of the respective satellites during the cell cycle. Finally, oligo-FISH painting experiments, performed using probes specifically designed to label the genomic regions corresponding to CEN6 in Pisum, Lathyrus, and Vicia species, revealed that metapolycentromeres evolved via the expansion of centromeric chromatin into neighboring chromosomal regions and the accumulation of novel satellite repeats. However, in some of these species, centromere evolution also involved chromosomal translocations and centromere repositioning.

Investigating the Origin and Evolution of Polyploid Trifolium medium L. Karyotype by Comparative Cytogenomic Methods

Trifolium medium L. is a wild polyploid relative of the agriculturally important red clover that possesses traits promising for breeding purposes. To date, T. medium also remains the only clover species with which agriculturally important red clover has successfully been hybridized. Even though allopolyploid origin has previously been suggested, little has in fact been known about the T. medium karyotype and its origin. We researched T. medium and related karyotypes using comparative cytogenomic methods, such as fluorescent in situ hybridization (FISH) and RepeatExplorer cluster analysis. The results indicate an exceptional karyotype diversity regarding numbers and mutual positions of 5S and 26S rDNA loci and centromeric repeats in populations of T. medium ecotypes and varieties. The observed variability among T. medium ecotypes and varieties suggests current karyotype instability that can be attributed to ever-ongoing battle between satellite DNA together with genomic changes and rearrangements enhanced by post-hybridization events. Comparative cytogenomic analyses of a T. medium hexaploid variety and diploid relatives revealed stable karyotypes with a possible case of chromosomal rearrangement. Moreover, the results provided evidence of T. medium having autopolyploid origin.

Chromosome-scale genome assemblies and annotations for Poales species Carex cristatella, Carex scoparia, Juncus effusus, and Juncus inflexus

The majority of sequenced genomes in the monocots are from species belonging to Poaceae, which include many commercially important crops. Here, we expand the number of sequenced genomes from the monocots to include the genomes of 4 related cyperids: Carex cristatella and Carex scoparia from Cyperaceae and Juncus effusus and Juncus inflexus from Juncaceae. The high-quality, chromosome-scale genome sequences from these 4 cyperids were assembled by combining whole-genome shotgun sequencing of Nanopore long reads, Illumina short reads, and Hi-C sequencing data. Some members of the Cyperaceae and Juncaceae are known to possess holocentric chromosomes. We examined the repeat landscapes in our sequenced genomes to search for potential repeats associated with centromeres. Several large satellite repeat families, comprising 3.2–9.5% of our sequenced genomes, showed dispersed distribution of large satellite repeat clusters across all Carex chromosomes, with few instances of these repeats clustering in the same chromosomal regions. In contrast, most large Juncus satellite repeats were clustered in a single location on each chromosome, with sporadic instances of large satellite repeats throughout the Juncus genomes. Recognizable transposable elements account for about 20% of each of the 4 genome assemblies, with the Carex genomes containing more DNA transposons than retrotransposons while the converse is true for the Juncus genomes. These genome sequences and annotations will facilitate better comparative analysis within monocots.

The Newly Sequenced Genome of Pisum sativum Is Replete with Potential G-Quadruplex-Forming Sequences-Implications for Evolution and Biological Regulation

G-quadruplexes (G4s) have been long considered rare and physiologically unimportant in vitro curiosities, but recent methodological advances have proved their presence and functions in vivo. Moreover, in addition to their functional relevance in bacteria and animals, including humans, their importance has been recently demonstrated in evolutionarily distinct plant species. In this study, we analyzed the genome of Pisum sativum (garden pea, or the so-called green pea), a unique member of the Fabaceae family. Our results showed that this genome contained putative G4 sequences (PQSs). Interestingly, these PQSs were located nonrandomly in the nuclear genome. We also found PQSs in mitochondrial (mt) and chloroplast (cp) DNA, and we experimentally confirmed G4 formation for sequences found in these two organelles. The frequency of PQSs for nuclear DNA was 0.42 PQSs per thousand base pairs (kbp), in the same range as for cpDNA (0.53/kbp), but significantly lower than what was found for mitochondrial DNA (1.58/kbp). In the nuclear genome, PQSs were mainly associated with regulatory regions, including 5'UTRs, and upstream of the rRNA region. In contrast to genomic DNA, PQSs were located around RNA genes in cpDNA and mtDNA. Interestingly, PQSs were also associated with specific transposable elements such as TIR and LTR and around them, pointing to their role in their spreading in nuclear DNA. The nonrandom localization of PQSs uncovered their evolutionary and functional significance in the Pisum sativum genome.

Prospects of telomere-to-telomere assembly in barley: Analysis of sequence gaps in the MorexV3 reference genome

The first gapless, telomere-to-telomere (T2T) sequence assemblies of plant chromosomes were reported recently. However, sequence assemblies of most plant genomes remain fragmented. Only recent breakthroughs in accurate long-read sequencing have made it possible to achieve highly contiguous sequence assemblies with a few tens of contigs per chromosome, that is a number small enough to allow for a systematic inquiry into the causes of the remaining sequence gaps and the approaches and resources needed to close them. Here, we analyse sequence gaps in the current reference genome sequence of barley cv. Morex (MorexV3). Optical map and sequence raw data, complemented by ChIP-seq data for centromeric histone variant CENH3, were used to estimate the abundance of centromeric, ribosomal DNA, and subtelomeric repeats in the barley genome. These estimates were compared with copy numbers in the MorexV3 pseudomolecule sequence. We found that almost all centromeric sequences and 45S ribosomal DNA repeat arrays were absent from the MorexV3 pseudomolecules and that the majority of sequence gaps can be attributed to assembly breakdown in long stretches of satellite repeats. However, missing sequences cannot fully account for the difference between assembly size and flow cytometric genome size estimates. We discuss the prospects of gap closure with ultra-long sequence reads.

Evolution of holocentric chromosomes: Drivers, diversity, and deterrents

Centromeres are specialized chromosomal regions that recruit kinetochore proteins and mediate spindle microtubule attachment to ensure faithful chromosome segregation during mitosis and meiosis. Centromeres can be restricted to one region of the chromosome. Named "monocentromere", this type represents the most commonly found centromere organization across eukaryotes. Alternatively, centromeres can also be assembled at sites chromosome-wide. This second type is called "holocentromere". Despite their early description over 100 years ago, research on holocentromeres has lagged behind that of monocentromeres. Nevertheless, the application of next generation sequencing approaches and advanced microscopic technologies enabled recent advances understanding the molecular organization and regulation of holocentromeres in different organisms. Here we review the current state of research on holocentromeres focusing on evolutionary considerations. First, we provide a brief historical perspective on the discovery of holocentric chromosomes. We then discuss models/drivers that have been proposed over the years to explain the evolutionary transition from mono- to holocentric chromosomes. We continue to review the description of holocentric chromosomes in diverse eukaryotic groups and then focus our discussion on a specific and recently characterized type of holocentromere organization in insects that functions independently of the otherwise essential centromeric marker protein CenH3, thus providing novel insights into holocentromere evolution in insects. Finally, we propose reasons to explain why the holocentric trait is not more frequent across eukaryotes despite putative selective advantages.

Integration of Genomic and Cytogenetic Data on Tandem DNAs for Analyzing the Genome Diversity Within the Genus Hedysarum L. (Fabaceae)

The section Multicaulia is the largest clade in the genus Hedysarum L. (Fabaceae). Representatives of the sect. Multicaulia are valuable plants used for medicinal and fodder purposes. The taxonomy and phylogeny of the sect. Multicaulia are still ambiguous. To clarify the species relationships within sect. Multicaulia, we, for the first time, explored repeatomes of H. grandiflorum Pall., H. zundukii Peschkova, and H. dahuricum Turcz. using next-generation sequencing technologies and a subsequent bioinformatic analysis by RepeatExplorer/TAREAN pipelines. The comparative repeatome analysis showed that mobile elements made up 20-24% (Class I) and about 2-2.5% (Class II) of their repetitive DNAs. The amount of ribosomal DNA varied from 1 to 2.6%, and the content of satellite DNA ranged from 2.7 to 5.1%. For each species, five high confident putative tandem DNA repeats and 5-10 low confident putative DNA repeats were identified. According to BLAST, these repeats demonstrated high sequence similarity within the studied species. FISH-based mapping of 35S rDNA, 5S rDNA, and satDNAs made it possible to detect new effective molecular chromosome markers for Hedysarum species and construct the species karyograms. Comparison of the patterns of satDNA localization on chromosomes of the studied species allowed us to assess genome diversity within the sect. Multicaulia. In all studied species, we revealed intra- and interspecific variabilities in patterns of the chromosomal distribution of molecular chromosome markers. In H. gmelinii Ledeb. and H. setigerum Turcz. ex Fisch. et Meyer, similar subgenomes were detected, which confirmed the polyploid status of their genomes. Our findings demonstrated a close genomic relationship among six studied species indicating their common origin and confirmed the taxonomic status of H. setigerum as a subspecies of H. gmelinii as well as the validity of combining the sect. Multicaulia and Subacaulia into one sect. Multicaulia.

Satellitome comparison of two oedipodine grasshoppers highlights the contingent nature of satellite DNA evolution

BACKGROUND

The full catalog of satellite DNA (satDNA) within a same genome constitutes the satellitome. The Library Hypothesis predicts that satDNA in relative species reflects that in their common ancestor, but the evolutionary mechanisms and pathways of satDNA evolution have never been analyzed for full satellitomes. We compare here the satellitomes of two Oedipodine grasshoppers (Locusta migratoria and Oedaleus decorus) which shared their most recent common ancestor about 22.8 Ma ago.

RESULTS

We found that about one third of their satDNA families (near 60 in every species) showed sequence homology and were grouped into 12 orthologous superfamilies. The turnover rate of consensus sequences was extremely variable among the 20 orthologous family pairs analyzed in both species. The satDNAs shared by both species showed poor association with sequence signatures and motives frequently argued as functional, except for short inverted repeats allowing short dyad symmetries and non-B DNA conformations. Orthologous satDNAs frequently showed different FISH patterns at both intra- and interspecific levels. We defined indices of homogenization and degeneration and quantified the level of incomplete library sorting between species.

CONCLUSIONS

Our analyses revealed that satDNA degenerates through point mutation and homogenizes through partial turnovers caused by massive tandem duplications (the so-called satDNA amplification). Remarkably, satDNA amplification increases homogenization, at intragenomic level, and diversification between species, thus constituting the basis for concerted evolution. We suggest a model of satDNA evolution by means of recursive cycles of amplification and degeneration, leading to mostly contingent evolutionary pathways where concerted evolution emerges promptly after lineages split.

The Highly Repeat-Diverse (Peri) Centromeres of White Lupin (Lupinus albus L.)

Plant genomes are known to be mainly composed of repetitive DNA sequences. Regardless of the non-genic function of these sequences, they are important for chromosome structure and stability during cell-cycle. Based on the recent available whole-genome assembly of white lupin (Lupinus albus L.; WL), we have in silico annotated and in situ mapped the main classes of DNA repeats identified with RepeatExplorer. A highly diverse and an abundance of satellite DNAs were found representing more than 10 families, where three of them were highly associated with CENH3-immunoprecipitated chromatin. Applying a strategy of several re-hybridization steps with different combinations of satDNA, rDNA, and LTR-RTs probes, we were able to construct a repeat-based chromosome map for the identification of most chromosome pairs. Two families of LTR retrotransposons, Ty1/copia SIRE and Ty3/gypsy Tekay, were highly abundant at pericentromeric regions, while the centromeric retrotransposon of WL (CRWL) from the CRM clade showed strong centromere-specific localization in most chromosomes and was also highly enriched with CENH3-immunoprecipitated chromatin. FISH mapping of repeat DNA showed some incongruences with the reference genome, which can be further used for improving the current version of the genome. Our results demonstrate that despite the relatively small genome of WL, a high diversity of pericentromeric repeats was found, emphasizing the rapid evolution of repeat sequences in plant genomes.

Chromosome and Genome Diversity in the Genus Trifolium (Fabaceae)

Trifolium L. is an economically important genus that is characterized by variable karyotypes relating to its ploidy level and basic chromosome numbers. The advent of genomic resources combined with molecular cytogenetics provides an opportunity to develop our understanding of plant genomes in general. Here, we summarize the current state of knowledge on Trifolium genomes and chromosomes and review methodologies using molecular markers that have contributed to Trifolium research. We discuss possible future applications of cytogenetic methods in research on the Trifolium genome and chromosomes.

Centromere size scales with genome size across Eukaryotes

Previous studies on grass species suggested that the total centromere size (sum of all centromere sizes in a cell) may be determined by the genome size, possibly because stable scaling is important for proper cell division. However, it is unclear whether this relationship is universal. Here we analyze the total centromere size using the CenH3-immunofluorescence area as a proxy in 130 taxa including plants, animals, fungi, and protists. We verified the reliability of our methodological approach by comparing our measurements with available ChIP-seq-based measurements of the size of CenH3-binding domains. Data based on these two independent methods showed the same positive relationship between the total centromere size and genome size. Our results demonstrate that the genome size is a strong predictor (R-squared = 0.964) of the total centromere size universally across Eukaryotes. We also show that this relationship is independent of phylogenetic relatedness and centromere type (monocentric, metapolycentric, and holocentric), implying a common mechanism maintaining stable total centromere size in Eukaryotes.

Chromosomal variations of Lycoris species revealed by FISH with rDNAs and centromeric histone H3 variant associated DNAs

Lycoris species have various chromosome numbers and karyotypes, but all have a constant total number of chromosome major arms. In addition to three fundamental types, including metacentric (M-), telocentric (T-), and acrocentric (A-) chromosomes, chromosomes in various morphology and size were also observed in natural populations. Both fusion and fission translocation have been considered as main mechanisms leading to the diverse karyotypes among Lycoris species, which suggests the centromere organization playing a role in such arrangements. We detected several chromosomal structure changes in Lycoris including centric fusion, inversion, gene amplification, and segment deletion by using fluorescence in situ hybridization (FISH) probing with rDNAs. An antibody against centromere specific histone H3 (CENH3) of L. aurea (2n = 14, 8M+6T) was raised and used to obtain CENH3-associated DNA sequences of L. aurea by chromatin immunoprecipitation (ChIP) cloning method. Immunostaining with anti-CENH3 antibody could label the centromeres of M-, T-, and A-type chromosomes. Immunostaining also revealed two centromeres on one T-type chromosome and a centromere on individual mini-chromosome. Among 10,000 ChIP clones, 500 clones which showed abundant in L. aurea genome by dot-blotting analysis were FISH mapped on chromosomes to examine their cytological distribution. Five of these 500 clones could generate intense FISH signals at centromeric region on M-type but not T-type chromosomes. FISH signals of these five clones rarely appeared on A-type chromosomes. The five ChIP clones showed similarity in DNA sequences and could generate similar but not identical distribution patterns of FISH signals on individual chromosomes. Furthermore, the distinct distribution patterns of FISH signals on each chromosome generated by these five ChIP clones allow to identify individual chromosome, which is considered difficult by conventional staining approaches. Our results suggest a different organization of centromeres of the three chromosome types in Lycoris species.

The Genomics of Plant Satellite DNA

The twenty-first century began with a certain indifference to the research of satellite DNA (satDNA). Neither genome sequencing projects were able to accurately encompass the study of satDNA nor classic methodologies were able to go further in undertaking a better comprehensive study of the whole set of satDNA sequences of a genome. Nonetheless, knowledge of satDNA has progressively advanced during this century with the advent of new analytical techniques. The enormous advantages that genome-wide approaches have brought to its analysis have now stimulated a renewed interest in the study of satDNA. At this point, we can look back and try to assess more accurately many of the key questions that were left unsolved in the past about this enigmatic and important component of the genome. I review here the understanding gathered on plant satDNAs over the last few decades with an eye on the near future.

The Centromere Histone Is Conserved and Associated with Tandem Repeats Sharing a Conserved 19-bp Box in the Holocentromere of Meloidogyne Nematodes

Although centromeres have conserved function, centromere-specific histone H3 (CenH3) and centromeric DNA evolve rapidly. The centromere drive model explains this phenomenon as a consequence of the conflict between fast-evolving DNA and CenH3, suggesting asymmetry in female meiosis as a crucial factor. We characterized evolution of the CenH3 protein in three closely related, polyploid mitotic parthenogenetic species of the Meloidogyne incognita group, and in the distantly related meiotic parthenogen Meloidogyne hapla. We identified duplication of the CenH3 gene in a putative sexual ancestral Meloidogyne. We found that one CenH3 (αCenH3) remained conserved in all extant species, including in distant Meloidogyne hapla, whereas the other evolved rapidly and under positive selection into four different CenH3 variants. This pattern of CenH3 evolution in Meloidogyne species suggests the subspecialization of CenH3s in ancestral sexual species. Immunofluorescence performed on mitotic Meloidogyne incognita revealed a dominant role of αCenH3 on its centromere, whereas the other CenH3s have lost their function in mitosis. The observed αCenH3 chromosome distribution disclosed cluster-like centromeric organization. The ChIP-Seq analysis revealed that in M. incognita αCenH3-associated DNA dominantly comprises tandem repeats, composed of divergent monomers which share a completely conserved 19-bp long box. Conserved αCenH3-associated DNA is also confirmed in the related mitotic Meloidogyne incognita group species suggesting preservation of both centromere protein and DNA constituents. We hypothesize that the absence of centromere drive in mitosis might allow for CenH3 and its associated DNA to achieve an equilibrium in which they can persist for long periods of time.

The Evolutionary Dynamics of Repetitive DNA and Its Impact on the Genome Diversification in the Genus Sorghum

Polyploidization is an evolutionary event leading to structural changes of the genome(s), particularly allopolyploidization, which combines different genomes of distinct species. The tetraploid species, Sorghum halepense, is assumed an allopolyploid species formed by hybridization between diploid S. bicolor and S. propinquum. The repeat profiles of S. bicolor, S. halepense, and their relatives were compared to elucidate the repeats' role in shaping their genomes. The repeat frequencies and profiles of the three diploid accessions (S. bicolor, S. bicolor ssp. verticilliflorum, and S. bicolor var. technicum) and two tetraploid accessions (S. halepense) are similar. However, the polymorphic distribution of the subtelomeric satellites preferentially enriched in the tetraploid S. halepense indicates drastic genome rearrangements after the allopolyploidization event. Verified by CENH3 chromatin immunoprecipitation (ChIP)-sequencing and fluorescence in situ hybridization (FISH) analysis the centromeres of S. bicolor are mainly composed of the abundant satellite SorSat137 (CEN38) and diverse CRMs, Athila of Ty3_gypsy and Ty1_copia-SIRE long terminal repeat (LTR) retroelements. A similar centromere composition was found in S. halepense. The potential contribution of S. bicolor in the formation of tetraploid S. halepense is discussed.

Unequal contribution of two paralogous CENH3 variants in cowpea centromere function

In most diploids the centromere-specific histone H3 (CENH3), the assembly site of active centromeres, is encoded by a single copy gene. Persistance of two CENH3 paralogs in diploids species raises the possibility of subfunctionalization. Here we analysed both CENH3 genes of the  diploid dryland crop cowpea. Phylogenetic analysis suggests that gene duplication of CENH3 occurred independently during the speciation of Vigna unguiculata. Both functional CENH3 variants are transcribed, and the corresponding proteins are intermingled in subdomains of different types of centromere sequences in a tissue-specific manner together with the kinetochore protein CENPC. CENH3.2 is removed from the generative cell of mature pollen, while CENH3.1 persists. CRISPR/Cas9-based inactivation of CENH3.1 resulted in delayed vegetative growth and sterility, indicating that this variant is needed for plant development and reproduction. By contrast, CENH3.2 knockout individuals did not show obvious defects during vegetative and reproductive development. Hence, CENH3.2 of cowpea is likely at an early stage of pseudogenization and less likely undergoing subfunctionalization.

Analysis of the small chromosomal Prionium serratum (Cyperid) demonstrates the importance of reliable methods to differentiate between mono- and holocentricity

For a long time, the Cyperid clade (Thurniceae-Juncaceae-Cyperaceae) was considered a group of species possessing holocentromeres exclusively. The basal phylogenetic position of Prionium serratum (Thunb.) Drège (Thurniceae) within Cyperids makes this species an important specimen to understand the centromere evolution within this clade. In contrast to the expectation, the chromosomal distribution of the centromere-specific histone H3 (CENH3), alpha-tubulin and different centromere-associated post-translational histone modifications (H3S10ph, H3S28ph and H2AT120ph) demonstrate a monocentromeric organisation of P. serratum chromosomes. Analysis of the high-copy repeat composition resulted in the identification of two centromere-localised satellite repeats. Hence, monocentricity was the ancestral condition for the Juncaceae-Cyperaceae-Thurniaceae Cyperid clade, and holocentricity in this clade has independently arisen at least twice after differentiation of the three families, once in Juncaceae and the other one in Cyperaceae. In this context, methods suitable for the identification of holocentromeres are discussed.

CenH3 distribution reveals extended centromeres in the model beetle Tribolium castaneum

Centromeres are chromosomal domains essential for kinetochore assembly and correct chromosome segregation. Inconsistent in their underlying DNA sequences, centromeres are defined epigenetically by the presence of the centromere-specific histone H3 variant CenH3. Most of the analyzed eukaryotes have monocentric chromosomes in which CenH3 proteins deposit into a single, primary constriction visible at metaphase chromosomes. Contrary to monocentrics, evolutionary sporadic holocentric chromosomes lack a primary constriction and have kinetochore activity distributed along the entire chromosome length. In this work, we identified cCENH3 protein, the centromeric H3 histone of the coleopteran model beetle Tribolium castaneum. By ChIP-seq analysis we disclosed that cCENH3 chromatin assembles upon a repertoire of repetitive DNAs. cCENH3 in situ mapping revealed unusually elongated T. castaneum centromeres that comprise approximately 40% of the chromosome length. Being the longest insect regional centromeres evidenced so far, T. castaneum centromeres are characterized by metapolycentric structure composed of several individual cCENH3-containing domains. We suggest that the model beetle T. castaneum with its metapolycentromeres could represent an excellent model for further studies of non-canonical centromeres in insects.

High allelic diversity of the centromere-specific histone H3 (CENH3) in the legume sainfoin (Onobrychis viciifolia)

The centromere is a structurally and functionally specialized region on each eukaryotic chromosome and is essential for accurate and complete segregation of chromosomes during cell division. Centromeric nucleosomes differ from canonical nucleosomes by replacement of the histone H3 with its centromere-specific variant CENH3. CENH3 is essential for active centromeres in most eukaryotes. Homologs of CENH3 are identified in many organisms. Sainfoin (Onobrychis viciifolia) is an agriculturally important perennial forage and is a legume of the Fabaceae family. There is very limited information on the structure of the sainfoin genome and no data are available on its centromere structure. Here, we aim to characterize the sainfoin CENH3 homolog (OvCENH3). Using a sequence homology-based strategy with gene-specific primers, we were able to clone transcripts from sainfoin total RNA. The amplified clones were sequenced and compared by bioinformatics tools. Four distinct alleles of OvCENH3 were detected. Our study provides the first structural features on sainfoin centromeres with a possible allotetraploid origin for sainfoin. We discuss and compare our findings with that for other important legume species.

Global analysis of repetitive DNA from unassembled sequence reads using RepeatExplorer2

RepeatExplorer2 is a novel version of a computational pipeline that uses graph-based clustering of next-generation sequencing reads for characterization of repetitive DNA in eukaryotes. The clustering algorithm facilitates repeat identification in any genome by using relatively small quantities of short sequence reads, and additional tools within the pipeline perform automatic annotation and quantification of the identified repeats. The pipeline is integrated into the Galaxy platform, which provides a user-friendly web interface for script execution and documentation of the results. Compared to the original version of the pipeline, RepeatExplorer2 provides automated annotation of transposable elements, identification of tandem repeats and enhanced visualization of analysis results. Here, we present an overview of the RepeatExplorer2 workflow and provide procedures for its application to (i) de novo repeat identification in a single species, (ii) comparative repeat analysis in a set of species, (iii) development of satellite DNA probes for cytogenetic experiments and (iv) identification of centromeric repeats based on ChIP-seq data. Each procedure takes approximately 2 d to complete. RepeatExplorer2 is available at https://repeatexplorer-elixir.cerit-sc.cz .

Centromeres under Pressure: Evolutionary Innovation in Conflict with Conserved Function

Centromeres are essential genetic elements that enable spindle microtubule attachment for chromosome segregation during mitosis and meiosis. While this function is preserved across species, centromeres display an array of dynamic features, including: (1) rapidly evolving DNA; (2) wide evolutionary diversity in size, shape and organization; (3) evidence of mutational processes to generate homogenized repetitive arrays that characterize centromeres in several species; (4) tolerance to changes in position, as in the case of neocentromeres; and (5) intrinsic fragility derived by sequence composition and secondary DNA structures. Centromere drive underlies rapid centromere DNA evolution due to the "selfish" pursuit to bias meiotic transmission and promote the propagation of stronger centromeres. Yet, the origins of other dynamic features of centromeres remain unclear. Here, we review our current understanding of centromere evolution and plasticity. We also detail the mutagenic processes proposed to shape the divergent genetic nature of centromeres. Changes to centromeres are not simply evolutionary relics, but ongoing shifts that on one side promote centromere flexibility, but on the other can undermine centromere integrity and function with potential pathological implications such as genome instability.

Extraordinary Sequence Diversity and Promiscuity of Centromeric Satellites in the Legume Tribe Fabeae

Satellite repeats are major sequence constituents of centromeres in many plant and animal species. Within a species, a single family of satellite sequences typically occupies centromeres of all chromosomes and is absent from other parts of the genome. Due to their common origin, sequence similarities exist among the centromere-specific satellites in related species. Here, we report a remarkably different pattern of centromere evolution in the plant tribe Fabeae, which includes genera Pisum, Lathyrus, Vicia, and Lens. By immunoprecipitation of centromeric chromatin with CENH3 antibodies, we identified and characterized a large and diverse set of 64 families of centromeric satellites in 14 species. These families differed in their nucleotide sequence, monomer length (33-2,979 bp), and abundance in individual species. Most families were species-specific, and most species possessed multiple (2-12) satellites in their centromeres. Some of the repeats that were shared by several species exhibited promiscuous patterns of centromere association, being located within CENH3 chromatin in some species, but apart from the centromeres in others. Moreover, FISH experiments revealed that the same family could assume centromeric and noncentromeric positions even within a single species. Taken together, these findings suggest that Fabeae centromeres are not shaped by the coevolution of a single centromeric satellite with its interacting CENH3 proteins, as proposed by the centromere drive model. This conclusion is also supported by the absence of pervasive adaptive evolution of CENH3 sequences retrieved from Fabeae species.

"Lessons from the extremes: Epigenetic and genetic regulation in point monocentromere and holocentromere establishment on artificial chromosomes"

The formation of de novo centromeres on artificial chromosomes in humans (HACs) and fission yeast (SpYACs) has provided much insights to the epigenetic and genetic control on regional centromere establishment and maintenance. Similarly, the use of artificial chromosomes in point centromeric budding yeast Saccharomyces cerevisiae (ScYACs) and holocentric Caenorhabditis elegans (WACs) has revealed epigenetic regulation in the originally thought purely genetically-determined point centromeres and some centromeric DNA sequence features in holocentromeres, respectively. These relatively extreme and less characterized centromere organizations, on the endogenous chromosomes and artificial chromosomes, will be discussed and compared to the more well-studied regional centromere systems. This review will highlight some of the common epigenetic and genetic features in different centromere architectures, including the presence of the centromeric histone H3 variant, CENP-A or CenH3, centromeric and pericentric transcription, AT-richness and repetitiveness of centromeric DNA sequences.

Super-Resolution Microscopy Reveals Diversity of Plant Centromere Architecture

Centromeres are essential for proper chromosome segregation to the daughter cells during mitosis and meiosis. Chromosomes of most eukaryotes studied so far have regional centromeres that form primary constrictions on metaphase chromosomes. These monocentric chromosomes vary from point centromeres to so-called "meta-polycentromeres", with multiple centromere domains in an extended primary constriction, as identified in Pisum and Lathyrus species. However, in various animal and plant lineages centromeres are distributed along almost the entire chromosome length. Therefore, they are called holocentromeres. In holocentric plants, centromere-specific proteins, at which spindle fibers usually attach, are arranged contiguously (line-like), in clusters along the chromosomes or in bands. Here, we summarize findings of ultrastructural investigations using immunolabeling with centromere-specific antibodies and super-resolution microscopy to demonstrate the structural diversity of plant centromeres. A classification of the different centromere types has been suggested based on the distribution of spindle attachment sites. Based on these findings we discuss the possible evolution and advantages of holocentricity, and potential strategies to segregate holocentric chromosomes correctly.

What makes a centromere?

Centromeres are the eukaryotic chromosomal sites at which the kinetochore forms and attaches to spindle microtubules to orchestrate chromosomal segregation in mitosis and meiosis. Although centromeres are essential for cell division, their sequences are not conserved and evolve rapidly. Centromeres vary dramatically in size and organization. Here we categorize their diversity and explore the evolutionary forces shaping them. Nearly all centromeres favor AT-rich DNA that is gene-free and transcribed at a very low level. Repair of frequent centromere-proximal breaks probably contributes to their rapid sequence evolution. Point centromeres are only ~125 bp and are specified by common protein-binding motifs, whereas short regional centromeres are 1-5 kb, typically have unique sequences, and may have pericentromeric repeats adapted to facilitate centromere clustering. Transposon-rich centromeres are often ~100-300 kb and are favored by RNAi machinery that silences transposons, by suppression of meiotic crossovers at centromeres, and by the ability of some transposons to target centromeres. Megabase-length satellite centromeres arise in plants and animals with asymmetric female meiosis that creates centromere competition, and favors satellite monomers one or two nucleosomes in length that position and stabilize centromeric nucleosomes. Holocentromeres encompass the length of a chromosome and may differ dramatically between mitosis and meiosis. We propose a model in which low level transcription of centromeres facilitates the formation of non-B DNA that specifies centromeres and promotes loading of centromeric nucleosomes.

A reference genome for pea provides insight into legume genome evolution

We report the first annotated chromosome-level reference genome assembly for pea, Gregor Mendel's original genetic model. Phylogenetics and paleogenomics show genomic rearrangements across legumes and suggest a major role for repetitive elements in pea genome evolution. Compared to other sequenced Leguminosae genomes, the pea genome shows intense gene dynamics, most likely associated with genome size expansion when the Fabeae diverged from its sister tribes. During Pisum evolution, translocation and transposition differentially occurred across lineages. This reference sequence will accelerate our understanding of the molecular basis of agronomically important traits and support crop improvement.

Fluorescence in situ hybridization in plants: recent developments and future applications

Fluorescence in situ hybridization (FISH) was developed more than 30 years ago and has been the most paradigm-changing technique in cytogenetic research. FISH has been used to answer questions related to structure, mutation, and evolution of not only individual chromosomes but also entire genomes. FISH has served as an important tool for chromosome identification in many plant species. This review intends to summarize and discuss key technical development and applications of FISH in plants since 2006. The most significant recent advance of FISH is the development and application of probes based on synthetic oligonucleotides (oligos). Oligos specific to a repetitive DNA sequence, to a specific chromosomal region, or to an entire chromosome can be computationally identified, synthesized in parallel, and fluorescently labeled. Oligo probes designed from conserved DNA sequences from one species can be used among genetically related species, allowing comparative cytogenetic mapping of these species. The advances with synthetic oligo probes will significantly expand the applications of FISH especially in non-model plant species. Recent achievements and future applications of FISH and oligo-FISH are discussed.

Fluorescence in situ hybridization in plants: recent developments and future applications

Fluorescence in situ hybridization (FISH) was developed more than 30 years ago and has been the most paradigm-changing technique in cytogenetic research. FISH has been used to answer questions related to structure, mutation, and evolution of not only individual chromosomes but also entire genomes. FISH has served as an important tool for chromosome identification in many plant species. This review intends to summarize and discuss key technical development and applications of FISH in plants since 2006. The most significant recent advance of FISH is the development and application of probes based on synthetic oligonucleotides (oligos). Oligos specific to a repetitive DNA sequence, to a specific chromosomal region, or to an entire chromosome can be computationally identified, synthesized in parallel, and fluorescently labeled. Oligo probes designed from conserved DNA sequences from one species can be used among genetically related species, allowing comparative cytogenetic mapping of these species. The advances with synthetic oligo probes will significantly expand the applications of FISH especially in non-model plant species. Recent achievements and future applications of FISH and oligo-FISH are discussed.

State-of-the-art and novel developments of in vivo haploid technologies

The ability to generate (doubled) haploid plants significantly accelerates the crop breeding process. Haploids have been induced mainly through the generation of plants from cultivated gametophic (haploid) cells and tissues, i.e., in vitro haploid technologies, or through the selective loss of a parental chromosome set upon inter- or intraspecific hybridization. Here, we focus our review on the mechanisms responsible for the in vivo formation of haploids in the context of inter- and intraspecific hybridization. The application of a modified CENH3 for uniparental genome elimination, the IG1 system used for paternal as well as the BBM-like and the patatin-like phospholipase essential for maternal haploidy induction are discussed in detail.

Comparative Dissection of Three Giant Genomes: Allium cepa, Allium sativum, and Allium ursinum

Knowledge of the fascinating world of DNA repeats is continuously being enriched by newly identified elements and their hypothetical or well-established biological relevance. Genomic approaches can be used for comparative studies of major repeats in any group of genomes, regardless of their size and complexity. Such studies are particularly fruitful in large genomes, and useful mainly in crop plants where they provide a rich source of molecular markers or information on indispensable genomic components (e.g., telomeres, centromeres, or ribosomal RNA genes). Surprisingly, in Allium species, a comprehensive comparative study of repeats is lacking. Here we provide such a study of two economically important species, Allium cepa (onion), and A. sativum (garlic), and their distantly related A. ursinum (wild garlic). We present an overview and classification of major repeats in these species and have paid specific attention to sequence conservation and copy numbers of major representatives in each type of repeat, including retrotransposons, rDNA, or newly identified satellite sequences. Prevailing repeats in all three studied species belonged to Ty3/gypsy elements, however they significantly diverged and we did not detect them in common clusters in comparative analysis. Actually, only a low number of clusters was shared by all three species. Such conserved repeats were for example 5S and 45S rDNA genes and surprisingly a specific and quite rare Ty1/copia lineage. Species-specific long satellites were found mainly in A. cepa and A. sativum. We also show in situ localization of selected repeats that could potentially be applicable as chromosomal markers, e.g., in interspecific breeding.