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.

The holocentricity in the dioecious nutmeg (Myristica fragrans) is not based on major satellite repeats.

Holocentric species are characterized by the presence of centromeres throughout the length of the chromosomes. We confirmed the holocentricity of the dioecious, small chromosome-size species Myristica fragrans based on the chromosome-wide distribution of the centromere-specific protein KNL1, α-tubulin fibers, and the cell cycle-dependent histone H3 serine 28 phosphorylation (H3S28ph) mark. Each holocentromere is likely composed of, on average, ten centromere units, but none of the identified and in situ hybridized high-copy satellite repeats is centromere-specific. No sex-specific major repeats are present in the high-copy repeat composition of male or female plants, or a significant difference in genome size was detected. Therefore, it is unlikely that M. fragrans possesses heteromorphic sex chromosomes.

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.

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.

Helical chromonema coiling is conserved in eukaryotes.

Efficient chromatin condensation is required to transport chromosomes during mitosis and meiosis, forming daughter cells. While it is well accepted that these processes follow fundamental rules, there has been a controversial debate for more than 140 years on whether the higher-order chromatin organization in chromosomes is evolutionarily conserved. Here, we summarize historical and recent investigations based on classical and modern methods. In particular, classical light microscopy observations based on living, fixed, and treated chromosomes covering a wide range of plant and animal species, and even in single-cell eukaryotes suggest that the chromatids of large chromosomes are formed by a coiled chromatin thread, named the chromonema. More recently, these findings were confirmed by electron and super-resolution microscopy, oligo-FISH, molecular interaction data, and polymer simulation. Altogether, we describe common and divergent features of coiled chromonemata in different species. We hypothesize that chromonema coiling in large chromosomes is a fundamental feature established early during the evolution of eukaryotes to handle increasing genome sizes.

Meiotic segregation and post-meiotic drive of the Festuca pratensis B chromosome.

In many species, the transmission of B chromosomes (Bs) does not follow the Mendelian laws of equal segregation and independent assortment. This deviation results in transmission rates of Bs higher than 0.5, a process known as "chromosome drive". Here, we studied the behavior of the 103 Mbp-large B chromosome of Festuca pratensis during all meiotic and mitotic stages of microsporogenesis. Mostly, the B chromosome of F. pratensis segregates during meiosis like standard A chromosomes (As). In some cases, the B passes through meiosis in a non-Mendelian segregation leading to their accumulation already in meiosis. However, a true drive of the B happens during the first pollen mitosis, by which the B preferentially migrates to the generative nucleus. During second pollen mitosis, B divides equally between the two sperms. Despite some differences in the frequency of drive between individuals with different numbers of Bs, at least 82% of drive was observed. Flow cytometry-based quantification of B-containing sperm nuclei agrees with the FISH data.

Simultaneous EYFP-CENH3/H2B-DsRed Expression Is Impaired Differentially in Meristematic and Differentiated Nuclei of Arabidopsis Double Transformants.

Fluorescence live-cell microscopy is important in cell biology to perform artifact-free investigations. To analyze the dynamics of chromatin and centromeres at different stages of the cell cycle in nuclei and chromosomes, we performed simultaneous EYFP-CENH3/H2B-DsRed and single H2B-YFP transformations in Arabidopsis wild-type and cohesin T-DNA mutants. All constructs were under the control of the strong CaMV 35S promoter. While a strong silencing of fluorescence expression occurred differently in leaf and root tissues in the double transformants, nearly all single-transformed wild-type and most mutant cells showed H2B-YFP fluorescence. It seems that for an efficient co-expression of two fluorescence proteins, endogenous promoters and terminators should be used.

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.

Disruption of the standard kinetochore in holocentric Cuscuta species.

The segregation of chromosomes depends on the centromere. Most species are monocentric, with the centromere restricted to a single region per chromosome. In some organisms, the monocentric organization changed to holocentric, in which the centromere activity is distributed over the entire chromosome length. However, the causes and consequences of this transition are poorly understood. Here, we show that the transition in the genus Cuscuta was associated with dramatic changes in the kinetochore, a protein complex that mediates the attachment of chromosomes to microtubules. We found that in holocentric Cuscuta species, the KNL2 genes were lost; the CENP-C, KNL1, and ZWINT1 genes were truncated; the centromeric localization of CENH3, CENP-C, KNL1, MIS12, and NDC80 proteins was disrupted; and the spindle assembly checkpoint (SAC) degenerated. Our results demonstrate that holocentric Cuscuta species lost the ability to form a standard kinetochore and do not employ SAC to control the attachment of microtubules to chromosomes.

Helical coiling of metaphase chromatids.

Chromatids of mitotic chromosomes were suggested to coil into a helix in early cytological studies and this assumption was recently supported by chromosome conformation capture (3C) sequencing. Still, direct differential visualization of a condensed chromatin fibre confirming the helical model was lacking. Here, we combined Hi-C analysis of purified metaphase chromosomes, biopolymer modelling and spatial structured illumination microscopy of large fluorescently labeled chromosome segments to reveal the chromonema - a helically-wound, 400 nm thick chromatin thread forming barley mitotic chromatids. Chromatin from adjacent turns of the helix intermingles due to the stochastic positioning of chromatin loops inside the chromonema. Helical turn size varies along chromosome length, correlating with chromatin density. Constraints on the observable dimensions of sister chromatid exchanges further supports the helical chromonema model.

TurboID-based proteomic profiling of meiotic chromosome axes in Arabidopsis thaliana.

During meiotic prophase I, sister chromatids are arranged in a loop-base array along a proteinaceous structure, called the meiotic chromosome axis. This structure is essential for synapsis and meiotic recombination progression and hence formation of genetically diverse gametes. Proteomic studies in plants aiming to unravel the composition and regulation of meiotic axes are constrained by limited meiotic cells embedded in floral organs. Here we report TurboID (TbID)-based proximity labelling (PL) in meiotic cells of Arabidopsis thaliana. TbID fusion to the two meiotic chromosome axis proteins ASY1 and ASY3 enabled the identification of their proximate 'interactomes' based on affinity purification coupled with mass spectrometry. We identified 39 ASY1 and/or ASY3 proximate candidates covering most known chromosome axis-related proteins. Functional studies of selected candidates demonstrate that not only known meiotic candidates but also new meiotic proteins were uncovered. Hence, TbID-based PL in meiotic cells enables the identification of chromosome axis proximate proteins in A. thaliana.

Super-resolution microscopy reveals the number and distribution of topoisomerase IIα and CENH3 molecules within barley metaphase chromosomes.

Topoisomerase IIα (Topo IIα) and the centromere-specific histone H3 variant CENH3 are key proteins involved in chromatin condensation and centromere determination, respectively. Consequently, they are required for proper chromosome segregation during cell divisions. We combined two super-resolution techniques, structured illumination microscopy (SIM) to co-localize Topo IIα and CENH3, and photoactivated localization microscopy (PALM) to determine their molecule numbers in barley metaphase chromosomes. We detected a dispersed Topo IIα distribution along chromosome arms but an accumulation at centromeres, telomeres, and nucleolus-organizing regions. With a precision of 10-50 nm, we counted ~ 20,000-40,000 Topo IIα molecules per chromosome, 28% of them within the (peri)centromere. With similar precision, we identified ~13,500 CENH3 molecules per centromere where Topo IIα proteins and CENH3-containing chromatin intermingle. In short, we demonstrate PALM as a useful method to count and localize single molecules with high precision within chromosomes. The ultrastructural distribution and the detected amount of Topo IIα and CENH3 are instrumental for a better understanding of their functions during chromatin condensation and centromere determination.

The meiotic topoisomerase VI B subunit (MTOPVIB) is essential for meiotic DNA double-strand break formation in barley (Hordeum vulgare L.).

KEY MESSAGE: In barley (Hordeum vulgare), MTOPVIB is critical for meiotic DSB and accompanied SC and CO formation while dispensable for meiotic bipolar spindle formation. Homologous recombination during meiosis assures genetic variation in offspring. Programmed meiotic DNA double-strand breaks (DSBs) are repaired as crossover (CO) or non-crossover (NCO) during meiotic recombination. The meiotic topoisomerase VI (TopoVI) B subunit (MTOPVIB) plays an essential role in meiotic DSB formation critical for CO-recombination. More recently MTOPVIB has been also shown to play a role in meiotic bipolar spindle formation in rice and maize. Here, we describe a meiotic DSB-defective mutant in barley (Hordeum vulgare L.). CRISPR-associated 9 (Cas9) endonuclease-generated mtopVIB plants show complete sterility due to the absence of meiotic DSB, synaptonemal complex (SC), and CO formation leading to the occurrence of univalents and their unbalanced segregation into aneuploid gametes. In HvmtopVIB plants, we also frequently found the bi-orientation of sister kinetochores in univalents during metaphase I and the precocious separation of sister chromatids during anaphase I. Moreover, the near absence of polyads after meiosis II, suggests that despite being critical for meiotic DSB formation in barley, MTOPVIB seems not to be strictly required for meiotic bipolar spindle formation.

A Spontaneous Inversion of the X Chromosome Heterochromatin Provides a Tool for Studying the Structure and Activity of the Nucleolus in Drosophila melanogaster.

The pericentromeric heterochromatin is largely composed of repetitive sequences, making it difficult to analyze with standard molecular biological methods. At the same time, it carries many functional elements with poorly understood mechanisms of action. The search for new experimental models for the analysis of heterochromatin is an urgent task. In this work, we used the Rif1 mutation, which suppresses the underreplication of all types of repeated sequences, to analyze heterochromatin regions in polytene chromosomes of Drosophila melanogaster. In the Rif1 background, we discovered and described in detail a new inversion, In(1)19EHet, which arose on a chromosome already carrying the In(1)sc8 inversion and transferred a large part of X chromosome heterochromatin, including the nucleolar organizer to a new euchromatic environment. Using nanopore sequencing and FISH, we have identified the eu- and heterochromatin breakpoints of In(1)19EHet. The combination of the new inversion and the Rif1 mutation provides a promising tool for studies of X chromosome heterochromatin structure, nucleolar organization, and the nucleolar dominance phenomenon. In particular, we found that, with the complete polytenization of rDNA repeats, the nucleolus consists of a cloud-like structure corresponding to the classical nucleolus of polytene chromosomes, as well as an unusual intrachromosomal structure containing alternating transcriptionally active and inactive regions.

Image analysis workflows to reveal the spatial organization of cell nuclei and chromosomes.

Nucleus, chromatin, and chromosome organization studies heavily rely on fluorescence microscopy imaging to elucidate the distribution and abundance of structural and regulatory components. Three-dimensional (3D) image stacks are a source of quantitative data on signal intensity level and distribution and on the type and shape of distribution patterns in space. Their analysis can lead to novel insights that are otherwise missed in qualitative-only analyses. Quantitative image analysis requires specific software and workflows for image rendering, processing, segmentation, setting measurement points and reference frames and exporting target data before further numerical processing and plotting. These tasks often call for the development of customized computational scripts and require an expertise that is not broadly available to the community of experimental biologists. Yet, the increasing accessibility of high- and super-resolution imaging methods fuels the demand for user-friendly image analysis workflows. Here, we provide a compendium of strategies developed by participants of a training school from the COST action INDEPTH to analyze the spatial distribution of nuclear and chromosomal signals from 3D image stacks, acquired by diffraction-limited confocal microscopy and super-resolution microscopy methods (SIM and STED). While the examples make use of one specific commercial software package, the workflows can easily be adapted to concurrent commercial and open-source software. The aim is to encourage biologists lacking custom-script-based expertise to venture into quantitative image analysis and to better exploit the discovery potential of their images.Abbreviations: 3D FISH: three-dimensional fluorescence in situ hybridization; 3D: three-dimensional; ASY1: ASYNAPTIC 1; CC: chromocenters; CO: Crossover; DAPI: 4',6-diamidino-2-phenylindole; DMC1: DNA MEIOTIC RECOMBINASE 1; DSB: Double-Strand Break; FISH: fluorescence in situ hybridization; GFP: GREEN FLUORESCENT PROTEIN; HEI10: HUMAN ENHANCER OF INVASION 10; NCO: Non-Crossover; NE: Nuclear Envelope; Oligo-FISH: oligonucleotide fluorescence in situ hybridization; RNPII: RNA Polymerase II; SC: Synaptonemal Complex; SIM: structured illumination microscopy; ZMM (ZIP: MSH4: MSH5 and MER3 proteins); ZYP1: ZIPPER-LIKE PROTEIN 1.

Chromosome Numbers and Genome Sizes of All 36 Duckweed Species (Lemnaceae).

Usually, chromosome sets (karyotypes) and genome sizes are rather stable for distinct species and therefore of diagnostic value for taxonomy. In combination with (cyto)genomics, both features provide essential cues for genome evolution and phylogenetic relationship studies within and between taxa above the species level. We present for the first time a survey on chromosome counts and genome size measurement for one or more accessions from all 36 duckweed species and discuss the evolutionary impact and peculiarities of both parameters in duckweeds.

Recurrent Plant-Specific Duplications of KNL2 and Its Conserved Function as a Kinetochore Assembly Factor.

KINETOCHORE NULL2 (KNL2) plays key role in the recognition of centromeres and new CENH3 deposition. To gain insight into the origin and diversification of the KNL2 gene, we reconstructed its evolutionary history in the plant kingdom. Our results indicate that the KNL2 gene in plants underwent three independent ancient duplications in ferns, grasses and eudicots. Additionally, we demonstrated that previously unclassified KNL2 genes could be divided into two clades αKNL2 and ßKNL2 in eudicots and γKNL2 and δKNL2 in grasses, respectively. KNL2s of all clades encode the conserved SANTA domain, but only the αKNL2 and γKNL2 groups additionally encode the CENPC-k motif. In the more numerous eudicot sequences, signatures of positive selection were found in both αKNL2 and ßKNL2 clades, suggesting recent or ongoing adaptation. The confirmed centromeric localization of ßKNL2 and mutant analysis suggests that it participates in loading of new CENH3, similarly to αKNL2. A high rate of seed abortion was found in heterozygous ßKNL2 plants and the germinated homozygous mutants did not develop beyond the seedling stage. Taken together, our study provides a new understanding of the evolutionary diversification of the plant kinetochore assembly gene KNL2, and suggests that the plant-specific duplicated KNL2 genes are involved in centromere and/or kinetochore assembly for preserving genome stability.

H3K9 demethylases IBM1 and JMJ27 are required for male meiosis in Arabidopsis thaliana.

Dimethylation of histone H3 lysine 9 (H3K9me2), a crucial modification for heterochromatin formation and transcriptional silencing, is essential for proper meiotic prophase progression in mammals. We analyzed meiotic defects and generated genome-wide profiles of H3K9me2 and transcriptomes for the mutants of H3K9 demethylases. Moreover, we also identified proteins interacting with H3K9 demethylases. H3K9me2 is usually found at transposable elements and repetitive sequences but is absent from the bodies of protein-coding genes. In this study, we show that the Arabidopsis thaliana H3K9 demethylases IBM1 and JMJ27 cooperatively regulate crossover formation and chromosome segregation. They protect thousands of protein-coding genes from ectopic H3K9me2, including genes essential for meiotic prophase progression. In addition to removing H3K9me2, IBM1 and JMJ27 interact with the Precocious Dissociation of Sisters 5 (PDS5) cohesin complex cofactors. The pds5 mutant shared similar transcriptional alterations with ibm1 jmj27, including meiosis-essential genes, yet without affecting H3K9me2 levels. Hence, PDS5s, together with IBM1 and JMJ27, regulate male meiosis and gene expression independently of H3K9 demethylation. These findings uncover a novel role of H3K9me2 removal in meiosis and a new function of H3K9 demethylases and cohesin cofactors in meiotic transcriptional regulation.

Kinetochore size scales with chromosome size in bimodal karyotypes of Agavoideae.

BACKGROUND AND AIMS: In eukaryotes, the total kinetochore size (defined as a chromosomal region containing CENH3-positive nucleosomes) per nucleus strongly correlates with genome size, a relationship that has been hypothesized to stem from general intracellular scaling principles. However, if larger chromosomes within a karyotype required larger kinetochores to move properly, it could also be derived from the mechanics of cell division. METHODS: We selected seven species of the plant subfamily Agavoideae whose karyotypes are characterized by the presence of small and very large chromosomes. We visualized the kinetochore regions and chromosomes by immunolabelling with an anti-CENH3 antibody and DAPI (6'-diamidino-2-phenylindole) staining. We then employed 2D widefield and 3D super-resolution microscopy to measure chromosome and kinetochore areas and volumes, respectively. To assess the scaling relationship of kinetochore size to chromosome size inside a karyotype, we log-transformed the data and analysed them with linear mixed models which allowed us to control for the inherent hierarchical structure of the dataset (metaphases within slides and species). KEY RESULTS: We found a positive intra-karyotype relationship between kinetochore and chromosome size. The slope of the regression line of the observed relationship (0.277 for areas, 0.247 for volumes) was very close to the theoretical slope of 0.25 for chromosome width based on the expected physics of chromosome passage through the cytoplasm during cell division. We obtained similar results by reanalysing available data from human and maize. CONCLUSIONS: Our findings suggest that the total kinetochore size to genome size scaling observed across eukaryotes may also originate from the mechanics of cell division. Moreover, the potential causal link between kinetochore and chromosome size indicates that evolutionary mechanisms capable of leading kinetochore size changes to fixation, such as centromere drive, could promote the size evolution of entire chromosomes and genomes.

Corrigendum: Only the Rye Derived Part of the 1BL/1RS Hybrid Centromere Incorporates CENH3 of Wheat.

[This corrects the article DOI: 10.3389/fpls.2021.802222.].