The Enigma of Progressively Partial Endoreplication: New Insights Provided by Flow Cytometry and Next-Generation Sequencing

A chromosome-level, haplotype-phased Vanilla planifolia genome highlights the challenge of partial endoreplication for accurate whole-genome assembly

Vanilla planifolia, the species cultivated to produce one of the world's most popular flavors, is highly prone to partial genome endoreplication, which leads to highly unbalanced DNA content in cells. We report here the first molecular evidence of partial endoreplication at the chromosome scale by the assembly and annotation of an accurate haplotype-phased genome of V. planifolia. Cytogenetic data demonstrated that the diploid genome size is 4.09 Gb, with 16 chromosome pairs, although aneuploid cells are frequently observed. Using PacBio HiFi and optical mapping, we assembled and phased a diploid genome of 3.4 Gb with a scaffold N50 of 1.2 Mb and 59 128 predicted protein-coding genes. The atypical k-mer frequencies and the uneven sequencing depth observed agreed with our expectation of unbalanced genome representation. Sixty-seven percent of the genes were scattered over only 30% of the genome, putatively linking gene-rich regions and the endoreplication phenomenon. By contrast, low-coverage regions (non-endoreplicated) were rich in repeated elements but also contained 33% of the annotated genes. Furthermore, this assembly showed distinct haplotype-specific sequencing depth variation patterns, suggesting complex molecular regulation of endoreplication along the chromosomes. This high-quality, anchored assembly represents 83% of the estimated V. planifolia genome. It provides a significant step toward the elucidation of this complex genome. To support post-genomics efforts, we developed the Vanilla Genome Hub, a user-friendly integrated web portal that enables centralized access to high-throughput genomic and other omics data and interoperable use of bioinformatics tools.

Reference standards for flow cytometric estimation of absolute nuclear DNA content in plants

The estimation of nuclear DNA content has been by far the most popular application of flow cytometry in plants. Because flow cytometry measures relative fluorescence intensities of nuclei stained by a DNA fluorochrome, ploidy determination, and estimation of the nuclear DNA content in absolute units both require comparison to a reference standard of known DNA content. This implies that the quality of the results obtained depends on the standard selection and use. Internal standardization, when the nuclei of an unknown sample and the reference standard are isolated, stained, and measured simultaneously, is mandatory for precise measurements. As DNA peaks representing G1 /G0 nuclei of the sample and standard appear on the same histogram of fluorescence intensity, the quotient of their position on the fluorescence intensity axis provides the quotient of DNA amounts. For the estimation of DNA amounts in absolute units, a number of well-established standards are now available to cover the range of known plant genome sizes. Since there are different standards in use, the standard and the genome size assigned to it has always to be reported. When none of the established standards fits, the introduction of a new standard species is needed. For this purpose, the regression line approach or simultaneous analysis of the candidate standard with several established standards should be prioritized. Moreover, the newly selected standard organism has to fulfill a number of requirements: it should be easy to identify and maintain, taxonomically unambiguous, globally available, with known genome size stability, lacking problematic metabolites, suitable for isolation of sufficient amounts of nuclei, and enabling measurements with low coefficients of variation of DNA peaks, hence suitable for the preparation of high quality samples.

Application-based guidelines for best practices in plant flow cytometry

Flow cytometry (FCM) is currently the most widely-used method to establish nuclear DNA content in plants. Since simple, 1-3-parameter, flow cytometers, which are sufficient for most plant applications, are commercially available at a reasonable price, the number of laboratories equipped with these instruments, and consequently new FCM users, has greatly increased over the last decade. This paper meets an urgent need for comprehensive recommendations for best practices in FCM for different plant science applications. We discuss advantages and limitations of establishing plant ploidy, genome size, DNA base composition, cell cycle activity, and level of endoreduplication. Applications of such measurements in plant systematics, ecology, molecular biology research, reproduction biology, tissue cultures, plant breeding, and seed sciences are described. Advice is included on how to obtain accurate and reliable results, as well as how to manage troubleshooting that may occur during sample preparation, cytometric measurements, and data handling. Each section is followed by best practice recommendations; tips as to what specific information should be provided in FCM papers are also provided.

Repeat proliferation and partial endoreplication jointly shape the patterns of genome size evolution in orchids

Although the evolutionary drivers of genome size change are known, the general patterns and mechanisms of plant genome size evolution are yet to be established. Here we aim to assess the relative importance of proliferation of repetitive DNA, chromosomal variation (including polyploidy), and the type of endoreplication for genome size evolution of the Pleurothallidinae, the most species-rich orchid lineage. Phylogenetic relationships between 341 Pleurothallidinae representatives were refined using a target enrichment hybrid capture combined with high-throughput sequencing approach. Genome size and the type of endoreplication were assessed using flow cytometry supplemented with karyological analysis and low-coverage Illumina sequencing for repeatome analysis on a subset of samples. Data were analyzed using phylogeny-based models. Genome size diversity (0.2-5.1 Gbp) was mostly independent of profound chromosome count variation (2n = 12-90) but tightly linked with the overall content of repetitive DNA elements. Species with partial endoreplication (PE) had significantly greater genome sizes, and genomic repeat content was tightly correlated with the size of the non-endoreplicated part of the genome. In PE species, repetitive DNA is preferentially accumulated in the non-endoreplicated parts of their genomes. Our results demonstrate that proliferation of repetitive DNA elements and PE together shape the patterns of genome size diversity in orchids.

Integrative Study of Genotypic and Phenotypic Diversity in the Eurasian Orchid Genus Neotinea

Knowledge of population variation across species' ranges is a prerequisite for correctly assessing the overall variability of any group of organisms and provides an invaluable basis for unraveling evolutionary history, optimizing taxonomy and devising effective conservation strategies. Here, we examine the genus Neotinea, which represents a relatively recently delimited monophyletic genus of orchids, for which a detailed study of its overall variability was lacking. We applied a suite of biosystematic methods, consisting of flow cytometry, multivariate and geometric morphometrics, and analysis of genomic SNP data, to identify phylogenetic lineages within the genus, to delineate phenotypic variation relevant to these lineages, and to identify potential cryptic taxa within lineages. We found clear differentiation into four major lineages corresponding to the groups usually recognized within the genus: Neotinea maculata as a distinct and separate taxon, the Neotinea lactea group comprising two Mediterranean taxa N. lactea and Neotinea conica, the Neotinea ustulata group comprising two phenologically distinct varieties, and the rather complex Neotinea tridentata group comprising two major lineages and various minor lineages of unclear taxonomic value. N. conica constitutes both a monophyletic group within N. lactea and a distinct phenotype within the genus and merits its proposed subspecies-level recognition. By contrast, the spring and summer flowering forms of N. ustulata (var. ustulata and var. aestivalis) were confirmed to be distinct only morphologically, not phylogenetically. The most complex pattern emerged in the N. tridentata group, which splits into two main clades, one containing lineages from the Balkans and eastern Mediterranean and the other consisting of plants from Central Europe and the central Mediterranean. These individual lineages differ in genome size and show moderate degrees of morphological divergence. The tetraploid Neotinea commutata is closely related to the N. tridentata group, but our evidence points to an auto- rather than an allopolyploid origin. Our broad methodological approach proved effective in recognizing cryptic lineages among the orchids, and we propose the joint analysis of flow cytometric data on genome size and endopolyploidy as a useful and beneficial marker for delineating orchid species with partial endoreplication.

DNA replication and chromosome positioning throughout the interphase in three-dimensional space of plant nuclei

Despite much recent progress, our understanding of the principles of plant genome organization and its dynamics in three-dimensional space of interphase nuclei remains surprisingly limited. Notably, it is not clear how these processes could be affected by the size of a plant's nuclear genome. In this study, DNA replication timing and interphase chromosome positioning were analyzed in seven Poaceae species that differ in their genome size. To provide a comprehensive picture, a suite of advanced, complementary methods was used: labeling of newly replicated DNA by ethynyl-2'-deoxyuridine, isolation of nuclei at particular cell cycle phases by flow cytometric sorting, three-dimensional immunofluorescence in situ hybridization, and confocal microscopy. Our results revealed conserved dynamics of DNA replication in all species, and a similar replication timing order for telomeres and centromeres, as well as for euchromatin and heterochromatin regions, irrespective of genome size. Moreover, stable chromosome positioning was observed while transitioning through different stages of interphase. These findings expand upon earlier studies in suggesting that a more complex interplay exists between genome size, organization of repetitive DNA sequences along chromosomes, and higher order chromatin structure and its maintenance in interphase, albeit controlled by currently unknown factors.

Comparing DNA replication programs reveals large timing shifts at centromeres of endocycling cells in maize roots

Plant cells undergo two types of cell cycles–the mitotic cycle in which DNA replication is coupled to mitosis, and the endocycle in which DNA replication occurs in the absence of cell division. To investigate DNA replication programs in these two types of cell cycles, we pulse labeled intact root tips of maize (Zea mays) with 5-ethynyl-2’-deoxyuridine (EdU) and used flow sorting of nuclei to examine DNA replication timing (RT) during the transition from a mitotic cycle to an endocycle. Comparison of the sequence-based RT profiles showed that most regions of the maize genome replicate at the same time during S phase in mitotic and endocycling cells, despite the need to replicate twice as much DNA in the endocycle and the fact that endocycling is typically associated with cell differentiation. However, regions collectively corresponding to 2% of the genome displayed significant changes in timing between the two types of cell cycles. The majority of these regions are small with a median size of 135 kb, shift to a later RT in the endocycle, and are enriched for genes expressed in the root tip. We found larger regions that shifted RT in centromeres of seven of the ten maize chromosomes. These regions covered the majority of the previously defined functional centromere, which ranged between 1 and 2 Mb in size in the reference genome. They replicate mainly during mid S phase in mitotic cells but primarily in late S phase of the endocycle. In contrast, the immediately adjacent pericentromere sequences are primarily late replicating in both cell cycles. Analysis of CENH3 enrichment levels in 8C vs 2C nuclei suggested that there is only a partial replacement of CENH3 nucleosomes after endocycle replication is complete. The shift to later replication of centromeres and possible reduction in CENH3 enrichment after endocycle replication is consistent with a hypothesis that centromeres are inactivated when their function is no longer needed.

In traditional cell division, or mitosis, a cell’s genetic material is duplicated and then split between two daughter cells. In contrast, in some specialized cell types, the DNA is duplicated a second time without an intervening division step, resulting in cells that carry twice as much DNA. This phenomenon, which is called the endocycle, is common during plant development. At each step, DNA replication follows an ordered program in which highly compacted DNA is unraveled and replicated in sections at different times during the synthesis (S) phase. In plants, it is unclear whether traditional and endocycle programs are the same, especially since endocycling cells are typically in the process of differentiation. Using root tips of maize, we found that in comparison to replication in the mitotic cell cycle, there is a small portion of the genome whose replication in the endocycle is shifted in time, usually to later in S phase. Some of these regions are scattered around the genome and mostly coincide with active genes. However, the most prominent shifts occur in centromeres. The shift to later replication in centromeres is noteworthy because they orchestrate the process of separating duplicated chromosomes into daughter cells, a function that is not needed in the endocycle.

Endoreplication - a means to an end in cell growth and stress response

Endoreplication, also called endoreduplication or endopolyploidization, is a cell cycle variant in which the genome is re-replicated in the absence of mitosis causing cellular polyploidization. Despite the common occurrence of endoreplication in plants and the tremendous extent in specific tissues and cell types such as the endosperm, the underlying molecular regulation and the physiological consequences have only now started to be understood. Endoreplication is often associated with cell differentiation and withdrawal from mitotic cycles. Recent studies have underlined the importance of endoreplication as a stress response and we summarize here this progress with particular focus on future perspectives offered by the recent advances in genomics and biotechnology.

Diversity in genome size and GC content shows adaptive potential in orchids and is closely linked to partial endoreplication, plant life-history traits and climatic conditions

In angiosperms, genome size and nucleobase composition (GC content) exhibit pronounced variation with possible adaptive consequences. The hyperdiverse orchid family possessing the unique phenomenon of partial endoreplication (PE) provides a great opportunity to search for interactions of both genomic traits with the evolutionary history of the family. Using flow cytometry, we report values of both genomic traits and the type of endoreplication for 149 orchid species and compare these with a suite of life-history traits and climatic niche data using phylogeny-based statistics. The evolution of genomic traits was further studied using the Brownian motion (BM) and Ornstein-Uhlenbeck (OU) models to access their adaptive potential. Pronounced variation in genome size (341-54 878 Mb), and especially in GC content (23.9-50.5%), was detected among orchids. Diversity in both genomic traits was closely related to the type of endoreplication, plant growth form and climatic conditions. GC content was also associated with the type of dormancy. In all tested scenarios, OU models always outperformed BM models. Unparalleled GC content variation was discovered in orchids, setting new limits for plants. Our study indicates that diversity in both genome size and GC content has adaptive consequences and is tightly linked with evolutionary transitions to PE.

Flow cytometric seed screen data are consistent with models of chromosome inheritance in asymmetrically compensating allopolyploids

Angiosperms have evolved a mechanism of double fertilization, which results in the production of a separate embryo (new individual) and endosperm (nutritive tissue). The flow cytometric seed screen (FCSS) was developed to infer plant reproduction modes based on endosperm-to-embryo DNA content ratio (P_ind ). A ratio of 1.5 indicates sexual reproduction, whereas higher values of ≥2.0 are consistent with apomixis. Although FCSS has been successfully applied to the study of sexual and asexual plants, the limits of FCSS and particularly its potential for determination of reproduction modes in hemisexual plants have not been explored. Here, we evaluated the application of FCSS to the study of reproduction modes in two asymmetrically compensating allopolyploids (ACAs), Onosma arenaria and Rosa canina. These two species are characterized by the presence of asexually inherited univalent-forming and sexually inherited bivalent-forming chromosome sets. They both use asymmetric meiosis, which eliminates univalent-forming chromosome sets from the male gamete and retains them in the female gamete. Different chromosomal behavior in male and female meiosis in these plants is reflected in different theoretically derived P_ind values, which deviate from a sexual 1.5 value. Here, we determined P_ind FCSS-based values in seeds of ACAs, and compared the results to sexual species. As expected, we determined that the mean P_ind is 1.51, 1.52, and 1.52 in the sexual plants, that is, Capsella bursa-pastoris, Crataegus monogyna, and O. pseudoarenaria, respectively. In the ACAs, different mean P_ind values were determined for O. arenaria (1.61) and R. canina (1.82). These values are consistent with the theoretical P_ind values determined based on models of chromosome inheritance. This study highlights the precision of flow cytometry in determining DNA content and it's utility in screening reproduction modes. Additionally, it advocates for more in-depth investigations into rapid screening of accessions where the P_ind ratio has deviated from the 1.5 value typical of sexual species, which may indicate meiotic irregularities.

Evolutionary and functional potential of ploidy increase within individual plants: somatic ploidy mapping of the complex labellum of sexually deceptive bee orchids

Background and Aims

Recent tissue-level observations made indirectly via flow cytometry suggest that endoreplication (duplication of the nuclear genome within the nuclear envelope in the absence of subsequent cell division) is widespread within the plant kingdom. Here, we also directly observe ploidy variation among cells within individual petals, relating size of nucleus to cell micromorphology and (more speculatively) to function.

Methods

We compared the labella (specialized pollinator-attracting petals) of two European orchid genera: Dactylorhiza has a known predisposition to organismal polyploidy, whereas Ophrys exhibits exceptionally complex epidermal patterning that aids pseudocopulatory pollination. Confocal microscopy using multiple staining techniques allowed us to observe directly both the sizes and the internal structures of individual nuclei across each labellum, while flow cytometry was used to test for progressively partial endoreplication.

Key Results

In Dactylorhiza, endoreplication was comparatively infrequent, reached only low levels, and appeared randomly located across the labellum, whereas in Ophrys endoreplication was commonplace, being most frequent in large peripheral trichomes. Endoreplicated nuclei reflected both endomitosis and endocycling, the latter reaching the third round of genome doubling (16C) to generate polytene nuclei. All Ophrys individuals studied exhibited progressively partial endoreplication.

Conclusions

Comparison of the two genera failed to demonstrate the hypothesized pattern of frequent polyploid speciation in genera showing extensive endoreplication. Endoreplication in Ophrys appears more strongly positively correlated with cell size/complexity than with cell location or secretory role. Epigenetic control of gene overexpression by localized induction of endoreplication within individual plant organs may represent a significant component of a plant's developmental programme, contributing substantially to organ plasticity.

Induction and recovery of copy number variation in banana through gamma irradiation and low-coverage whole-genome sequencing

Traditional breeding methods are hindered in bananas due to the fact that major cultivars are sterile, parthenocarpic, triploid and thus clonally propagated. This has resulted in a narrow genetic base and limited resilience to biotic and abiotic stresses. Mutagenesis of in vitro propagated bananas is one method to introduce novel alleles and broaden genetic diversity. We previously established a method for the induction and recovery of single nucleotide mutations generated with the chemical mutagen EMS. However, officially released mutant banana varieties have been created using gamma rays, a mutagen that can produce large genomic insertions and deletions (indels). Such dosage mutations may be important for generating observable phenotypes in polyploids. In this study, we establish a low-coverage whole-genome sequencing approach in triploid bananas to recover large genomic indels caused by treatment with gamma irradiation. We first evaluated the commercially released mutant cultivar 'Novaria' and found that it harbours multiple predicted deletions, ranging from 0.3 to 3.8 million base pairs (Mbp). In total, predicted deletions span 189 coding regions. To evaluate the feasibility of generating and maintaining new mutations, we developed a pipeline for mutagenesis and screening for copy number variation in Cavendish bananas using the cultivar 'Williams'. Putative mutations were recovered in 70% of lines treated with 20 Gy and 60% of the lines treated with 40 Gy. While deletion events predominate, insertions were identified in 20 Gy-treated material. Based on these results, we believe this approach can be scaled up to support large breeding projects.

Integrating restriction site-associated DNA sequencing (RAD-seq) with morphological cladistic analysis clarifies evolutionary relationships among major species groups of bee orchids

BACKGROUND AND AIMS

Bee orchids (Ophrys) have become the most popular model system for studying reproduction via insect-mediated pseudo-copulation and for exploring the consequent, putatively adaptive, evolutionary radiations. However, despite intensive past research, both the phylogenetic structure and species diversity within the genus remain highly contentious. Here, we integrate next-generation sequencing and morphological cladistic techniques to clarify the phylogeny of the genus.

METHODS

At least two accessions of each of the ten species groups previously circumscribed from large-scale cloned nuclear ribosomal internal transcibed spacer (nrITS) sequencing were subjected to restriction site-associated sequencing (RAD-seq). The resulting matrix of 4159 single nucleotide polymorphisms (SNPs) for 34 accessions was used to construct an unrooted network and a rooted maximum likelihood phylogeny. A parallel morphological cladistic matrix of 43 characters generated both polymorphic and non-polymorphic sets of parsimony trees before being mapped across the RAD-seq topology.

KEY RESULTS

RAD-seq data strongly support the monophyly of nine out of ten groups previously circumscribed using nrITS and resolve three major clades; in contrast, supposed microspecies are barely distinguishable. Strong incongruence separated the RAD-seq trees from both the morphological trees and traditional classifications; mapping of the morphological characters across the RAD-seq topology rendered them far more homoplastic.

CONCLUSIONS

The comparatively high level of morphological homoplasy reflects extensive convergence, whereas the derived placement of the fusca group is attributed to paedomorphic simplification. The phenotype of the most recent common ancestor of the extant lineages is inferred, but it post-dates the majority of the character-state changes that typify the genus. RAD-seq may represent the high-water mark of the contribution of molecular phylogenetics to understanding evolution within Ophrys; further progress will require large-scale population-level studies that integrate phenotypic and genotypic data in a cogent conceptual framework.

DNA Remodeling by Strict Partial Endoreplication in Orchids, an Original Process in the Plant Kingdom

DNA remodeling during endoreplication appears to be a strong developmental characteristic in orchids. In this study, we analyzed DNA content and nuclei in 41 species of orchids to further map the genome evolution in this plant family. We demonstrate that the DNA remodeling observed in 36 out of 41 orchids studied corresponds to strict partial endoreplication. Such process is developmentally regulated in each wild species studied. Cytometry data analyses allowed us to propose a model where nuclear states 2C, 4E, 8E, etc. form a series comprising a fixed proportion, the euploid genome 2C, plus 2-32 additional copies of a complementary part of the genome. The fixed proportion ranged from 89% of the genome in Vanilla mexicana down to 19% in V. pompona, the lowest value for all 148 orchids reported. Insterspecific hybridization did not suppress this phenomenon. Interestingly, this process was not observed in mass-produced epiphytes. Nucleolar volumes grow with the number of endocopies present, coherent with high transcription activity in endoreplicated nuclei. Our analyses suggest species-specific chromatin rearrangement. Towards understanding endoreplication, V. planifolia constitutes a tractable system for isolating the genomic sequences that confer an advantage via endoreplication from those that apparently suffice at diploid level.