Sources of inversion variation in the small single copy (SSC) region of chloroplast genomes

Comparative plastome assembly of the yellow ironweed (Verbesina alternifolia) using Nanopore and Illumina reads

Chloroplast genomes (plastomes) represent a very important source of valuable information for phylogenetic and biogeographic reconstructions. The use of short reads (as those produced from Illumina sequencing), along with de novo read assembly, has been considered the "gold standard" for plastome reconstruction. However, short reads often cannot reconstruct long repetitive regions in chloroplast genomes. Long Nanopore (ONT) reads can help bridging long repetitive regions but are by far more error-prone than those produced by Illumina sequencing. Verbesina is the largest genus of tribe Heliantheae (Asteraceae) and includes species of economic importance as ornamental or as invasive weeds. However, no complete chloroplast genomes have been published yet for the genus. We utilized Illumina and Nanopore sequencing data and different assembly strategies to reconstruct the plastome of Verbesina alternifolia and evaluated the usefulness of the Nanopore assemblies. The two plastome sequence assemblages, one obtained with the Nanopore sequencing and the other inferred with Illumina reads, were identical, except for missing bases in homonucleotide regions. The best-assembled plastome of V. alternifolia was 152,050 bp in length and contained 80, 29, and four unique protein-coding genes, tRNAs, and rRNAs, respectively. When used as reference for mapping Illumina reads, all plastomes performed similarly. In a phylogenetic analysis including 28 other plastomes from closely related taxa (from the Heliantheae alliance), the two Verbesina chloroplast genomes grouped together and were nested among the other members of the tribe Heliantheae s.str. Our study highlights the usefulness of the Nanopore technology for assembling rapidly and cost-effectively chloroplast genomes, especially in taxonomic groups with paucity of publicly available plastomes.

Genetic characterization and phylogenetic analysis of the Nigella sativa (black seed) plastome

In this study, the complete plastome sequence of Nigella sativa (black seed), was analyzed for the first time. The plastome spans approximately 154,120 bp, comprising four sections: the Large Single-Copy (LSC) (85,538 bp), the Small Single-Copy (SSC) (17,984 bp), and two Inverted Repeat (IR) regions (25,299 bp). A comparative study of N. sativa's plastome with ten other species from various genera in the Ranunculaceae family reveals substantial structural variations. The contraction of the inverted repeat region in N. sativa influences the boundaries of single-copy regions, resulting in a shorter plastome size than other species. When comparing the plastome of N. sativa with those of its related species, significant divergence is observed, particularly except for N. damascena. Among these, the plastome of A. glaucifolium displays the highest average pairwise sequence divergence (0.2851) with N. sativa, followed by A. raddeana (0.2290) and A. coerulea (0.1222). Furthermore, the study identified 12 distinct hotspot regions characterized by elevated Pi values (> 0.1). These regions include trnH-GUG-psbA, matK-trnQ-UUG, psbK-trnR-UCU, atpF-atpI, rpoB-psbD, ycf3-ndhJ, ndhC-cemA, petA-psaJ, trnN-GUU-ndhF, trnV-GAC-rps12, ycf2-trnI-CAU, and ndhA-ycf1. Approximately, 24 tandem and 48 palindromic and forward repeats were detected in N. sativa plastome. The analysis revealed 32 microsatellites with the majority being mononucleotide repeats. In the N. sativa plastome, phenylalanine had the highest number of codons (1982 codons), while alanine was the least common amino acid with 260 codons. A phylogenetic tree, constructed using protein-coding genes, revealed a distinct monophyletic clade comprising N. sativa and N. damascene, closely aligned with the Cimicifugeae tribe and exhibiting robust support. This plastome provides valuable genetic information for precise species identification, phylogenetic resolution, and evolutionary studies of N. sativa.

Comparative analysis of codon usage bias in chloroplast genomes of ten medicinal species of Rutaceae

Rutaceae family comprises economically important plants due to their extensive applications in spices, food, oil, medicine, etc. The Rutaceae plants is able to better utilization through biotechnology. Modern biotechnological approaches primarily rely on the heterologous expression of functional proteins in different vectors. However, several proteins are difficult to express outside their native environment. The expression potential of functional genes in heterologous systems can be maximized by replacing the rare synonymous codons in the vector with preferred optimal codons of functional genes. Codon usage bias plays a critical role in biogenetic engineering-based research and development. In the current study, 727 coding sequences (CDSs) obtained from the chloroplast genomes of ten Rutaceae plant family members were analyzed for codon usage bias. The nucleotide composition analysis of codons showed that these codons were rich in A/T(U) bases and preferred A/T(U) endings. Analyses of neutrality plots, effective number of codons (ENC) plots, and correlations between ENC and codon adaptation index (CAI) were conducted, which revealed that natural selection is a major driving force for the Rutaceae plant family's codon usage bias, followed by base mutation. In the ENC vs. CAI plot, codon usage bias in the Rutaceae family had a negligible relationship with gene expression level. For each sample, we screened 12 codons as preferred and high-frequency codons simultaneously, of which GCU encoding Ala, UUA encoding Leu, and AGA encoding Arg were the most preferred codons. Taken together, our study unraveled the synonymous codon usage pattern in the Rutaceae family, providing valuable information for the genetic engineering of Rutaceae plant species in the future.

Nuclear Magnetic Resonance Fingerprints and Mini DNA Markers for the Authentication of Cinnamon Species Ingredients Used in Food and Natural Health Products

Cinnamomum verum (syn C. zeylanicum) is considered 'true' cinnamon. However, it is reported that less expensive sources of cinnamon from C. cassia (syn C. aromaticum), C. loureiroi, and C. burmannii (toxic coumarin) may be used in the place of C. verum. We lack the quality assurance tools that are required to differentiate C. verum from other cinnamon species when verifying that the correct species is sourced from ingredient suppliers. The current research on cinnamon species authentication using DNA tools is limited to a few species and the use of high-quality DNA extracted from raw leaf materials. The cinnamon bark traded in the supply chain contains much less DNA and poorer-quality DNA than leaves. Our research advances DNA methods to authenticate cinnamon, as we utilized full-length chloroplast genomes via a genome skimming approach for C. burmannii and C. cassia to facilitate the design of optimal mini DNA markers. Furthermore, we developed and validated the use of NMR fingerprints for several commercial cinnamon species, including the quantification of 16 molecules. NMR fingerprints provided additional data that were useful for quality assessment in cinnamon extract powders and product consistency. Both the new mini DNA markers and NMR fingerprints were tested on commercial cinnamon products.

More than a spiny morphology: plastome variation in the prickly pear cacti (Opuntieae)

BACKGROUND

Plastid genomes (plastomes) have long been recognized as highly conserved in their overall structure, size, gene arrangement and content among land plants. However, recent studies have shown that some lineages present unusual variations in some of these features. Members of the cactus family are one of these lineages, with distinct plastome structures reported across disparate lineages, including gene losses, inversions, boundary movements or loss of the canonical inverted repeat (IR) region. However, only a small fraction of cactus diversity has been analysed so far.

METHODS

Here, we investigated plastome features of the tribe Opuntieae, the remarkable prickly pear cacti, which represent one of the most diverse and important lineages of Cactaceae. We assembled de novo the plastome of 43 species, representing a comprehensive sampling of the tribe, including all seven genera, and analysed their evolution in a phylogenetic comparative framework. Phylogenomic analyses with different datasets (full plastome sequences and genes only) were performed, followed by congruence analyses to assess signals underlying contentious nodes.

KEY RESULTS

Plastomes varied considerably in length, from 121 to 162 kbp, with striking differences in the content and size of the IR region (contraction and expansion events), including a lack of the canonical IR in some lineages and the pseudogenization or loss of some genes. Overall, nine different types of plastomes were reported, deviating in the presence of the IR region or the genes contained in the IR. Overall, plastome sequences resolved phylogenetic relationships within major clades of Opuntieae with high bootstrap values but presented some contentious nodes depending on the dataset analysed (e.g. whole plastome vs. genes only). Congruence analyses revealed that most plastidial regions lack phylogenetic resolution, while few markers are supporting the most likely topology. Likewise, alternative topologies are driven by a handful of plastome markers, suggesting recalcitrant nodes in the phylogeny.

CONCLUSIONS

Our study reveals a dynamic nature of plastome evolution across closely related lineages, shedding light on peculiar features of plastomes. Variation of plastome types across Opuntieae is remarkable in size, structure and content and can be important for the recognition of species in some major clades. Unravelling connections between the causes of plastome variation and the consequences for species biology, physiology, ecology, diversification and adaptation is a promising and ambitious endeavour in cactus research. Although plastome data resolved major phylogenetic relationships, the generation of nuclear genomic data is necessary to confront these hypotheses and assess the recalcitrant nodes further.

Complete chloroplast genomes of six neotropical palm species, structural comparison, and evolutionary dynamic patterns

The Arecaceae family has a worldwide distribution, especially in tropical and subtropical regions. We sequenced the chloroplast genomes of Acrocomia intumescens and A. totai, widely used in the food and energy industries; Bactris gasipaes, important for palm heart; Copernicia alba and C. prunifera, worldwide known for wax utilization; and Syagrus romanzoffiana, of great ornamental potential. Copernicia spp. showed the largest chloroplast genomes (C. prunifera: 157,323 bp and C. alba: 157,192 bp), while S. romanzoffiana and B. gasipaes var. gasipaes presented the smallest (155,078 bp and 155,604 bp). Structurally, great synteny was detected among palms. Conservation was also observed in the distribution of single sequence repeats (SSR). Copernicia spp. presented less dispersed repeats, without occurrence in the small single copy (SSC). All RNA editing sites were C (cytidine) to U (uridine) conversions. Overall, closely phylogenetically related species shared more sites. Almost all nodes of the phylogenetic analysis showed a posterior probability (PP) of 1.0, reaffirming the close relationship between Acrocomia species. These results elucidate the conservation among palm chloroplast genomes, but point to subtle structural changes, providing support for the evolutionary dynamics of the Arecaceae family.

Chloroplast genome sequence of Nothapodytes nimmoniana (Icacinaceae), a vulnerable reservoir of camptothecin from Western Ghats

Nothapodytes nimmoniana belongs to family Icacinaceae and is a major source of compound Camptothecin. The global demand for Camptothecin has caused large-scale exploitation of N. nimmoniana from its wild habitat in Western Ghats of India, thereby making it vulnerable. The species is known to exhibit genetic diversity among the populations in Western Ghats. In this study, we report plastome sequence of N. nimmoniana, first for the genus. For the study, the species was collected from Western Ghats of Maharashtra. The plastome of N. nimmoniana was 150,726 bp in length and exhibited typical quadripartite structure with 83,771 bp LSC, 18,513 bp SSC and 24,221 IR region. The plastome was characterized by presence of 124 unique genes, 87 protein coding genes, 29 tRNA genes and four rRNA genes. Further, the plastome was compared with the available basal lamiid plastomes for gene order and composition. N. nimmoniana plastome exhibited SSC region in an inverted configuration. Phylogenomic study placed N. nimmoniana sister to Mappia mexicana. The SSR markers identified in this study, might help to distinguish genetically diverse populations, prioritizing the populations which need immediate conservation effects as well as for checking adulteration.

Characterisation of the complete mitochondrial genome of Taraxacum mongolicum revealed five repeat-mediated recombinations

We reported the mitochondrial genome of Taraxacum mongolicum for the first time. Five pairs of repeats that can mediate recombination were validated, leading to multiple conformations of genome. Taraxacum mongolicum belongs to the Asteraceae family and has important pharmaceutical value. To explore the possible interaction between the organelle genomes, we assembled the complete mitochondrial genome (mitogenome) of T. mongolicum using Illumina and Oxford Nanopore sequencing data. This genome corresponded to a circular molecule 304,467 bp long. It encodes 52 unique genes including 31 protein-coding, 3 ribosomal RNA (rRNA) and 18 transfer RNA (tRNA) genes. In addition to the single circular conformation, the existence of alternative conformations mediated by five repetitive sequences in the mitogenome was identified and validated. Recombination mediated by the inverted repeats resulted in two conformations. Conversely, recombination mediated by the two direct repeats broke one large circular molecule into two subgenomic circular molecules. Furthermore, we identified 12 homologous fragments by comparing the sequences of mitogenome and plastome, including eight complete tRNA genes. Lastly, we identified a total of 278 RNA-editing sites in protein-coding sequences based on RNA-seq data. Among them, cox1 and nad5 gene has the most sites (21), followed by the nad2 gene with 19 sites. We successfully validated 213 predicted RNA-editing sites using PCR amplification and Sanger sequencing. This project reported the first mitogenome of T. mongolicum and demonstrated its multiple conformations generated by repeat-mediated recombination. This genome could provide critical information for the molecular breeding of T. mongolicum, and also be used as a reference genome for other species of the genus Taraxacum.

Distinctive origin and evolution of endemic thistle of Korean volcanic island: Structural organization and phylogenetic relationships with complete chloroplast genome

Unlike other Cirsium in Korea, Cirsium nipponicum (Island thistle) is distributed only on Ulleung Island, a volcanic island off the east coast of the Korean Peninsula, and a unique thistle with none or very small thorns. Although many researchers have questioned the origin and evolution of C. nipponicum, there is not much genomic information to estimate it. We thus assembled the complete chloroplast of C. nipponicum and reconstructed the phylogenetic relationships within the genus Cirsium. The chloroplast genome was 152,586 bp, encoding 133 genes consisting of 8 rRNA genes, 37 tRNA genes, and 88 protein-coding genes. We found 833 polymorphic sites and eight highly variable regions in chloroplast genomes of six Cirsium species by calculating nucleotide diversity, as well as 18 specific variable regions distinguished C. nipponicum from other Cirsium. As a result of phylogenetic analysis, C. nipponicum was closer to C. arvense and C. vulgare than native Cirsium in Korea: C. rhinoceros and C. japonicum. These results indicate that C. nipponicum is likely introduced through the north Eurasian root, not the mainland, and evolved independently in Ulleung Island. This study contributes to further understanding the evolutionary process and the biodiversity conservation of C. nipponicum on Ulleung Island.

Chloroplast genome assemblies and comparative analyses of commercially important Vaccinium berry crops

Vaccinium is a large genus of shrubs that includes a handful of economically important berry crops. Given the numerous hybridizations and polyploidization events, the taxonomy of this genus has remained the subject of long debate. In addition, berries and berry-based products are liable to adulteration, either fraudulent or unintentional due to misidentification of species. The availability of more genomic information could help achieve higher phylogenetic resolution for the genus, provide molecular markers for berry crops identification, and a framework for efficient genetic engineering of chloroplasts. Therefore, in this study we assembled five Vaccinium chloroplast sequences representing the economically relevant berry types: northern highbush blueberry (V. corymbosum), southern highbush blueberry (V. corymbosum hybrids), rabbiteye blueberry (V. virgatum), lowbush blueberry (V. angustifolium), and bilberry (V. myrtillus). Comparative analyses showed that the Vaccinium chloroplast genomes exhibited an overall highly conserved synteny and sequence identity among them. Polymorphic regions included the expansion/contraction of inverted repeats, gene copy number variation, simple sequence repeats, indels, and single nucleotide polymorphisms. Based on their in silico discrimination power, we suggested variants that could be developed into molecular markers for berry crops identification. Phylogenetic analysis revealed multiple origins of highbush blueberry plastomes, likely due to the hybridization events that occurred during northern and southern highbush blueberry domestication.

Plastome-based phylogeny and biogeography of Lactuca L. (Asteraceae) support revised lettuce gene pool categories

This study generated and analyzed complete plastome and internal transcribed spacer (ITS) data of 46 Lactuca species, 13 African endemic (AE) Lactuca species, and 15 species from eight related genera in Lactucinae. The new plastome and nuclear ITS sequences were then used to reconstruct the phylogenetic relationships of Lactuca species. The whole-plastome data were used to estimate divergence time and ancestral area reconstruction of the identified major Lactuca lineages. The results showed that Lactuca species are generally similar in plastome size, Guanine and Cytosine (GC) content, gene structure, and categories, although crop lettuce (Lactuca sativa L.) and its gene pool relatives were found to have one unique pseudogene (ψ ndhF), and accD, atpF, cemA, clpP, and rpl22 showed signs of positive selection. Our phylogenomic analysis demonstrated that Lactuca is monophyletic after excluding Lactuca alatipes Collett and Hemsl and AE Lactuca species. AE Lactuca species are morphologically distinct from core Lactuca lineage and need to be excluded from Lactua. The core Lactuca species most likely originated from Asia-Temperate W ~6.82 Mya and then dispersed globally and formed nine clades. Finally, the lettuce gene pool concept was amended according to the phylogenetic and historical biogeographic analyses. This study revised the circumscription of Lactuca, revealed robust phylogenetic relationships within the genus, and provided insights into Lactucinae phylogeny. The lettuce gene pool species could be used as potential genetic resources for lettuce breeding.

Does IR-loss promote plastome structural variation and sequence evolution?

Plastids are one of the main distinguishing characteristics of the plant cell. The plastid genome (plastome) of most autotrophic seed plants possesses a highly conserved quadripartite structure containing a large single-copy (LSC) and a small single-copy (SSC) region separated by two copies of the inverted repeat (termed as IR_A and IR_B). The IRs have been inferred to stabilize the plastid genome via homologous recombination-induced repair mechanisms. IR loss has been documented in seven autotrophic flowering plant lineages and two autotrophic gymnosperm lineages, and the plastomes of these species (with a few exceptions) are rearranged to a great extent. However, some plastomes containing normal IRs also show high structural variation. Therefore, the role of IRs in maintaining plastome stability is still controversial. In this study, we first integrated and compared genome structure and sequence evolution of representative plastomes of all nine reported IR-lacking lineages and those of their closest relative(s) with canonical inverted repeats (CRCIRs for short) to explore the role of the IR in maintaining plastome structural stability and sequence evolution. We found the plastomes of most IR-lacking lineages have experienced significant structural rearrangement, gene loss and duplication, accumulation of novel small repeats, and acceleration of synonymous substitution compared with those of their CRCIRs. However, the IR-lacking plastomes show similar structural variation and sequence evolution rate, and even less rearrangement distance, dispersed repeat number, tandem repeat number, indels frequency and GC3 content than those of IR-present plastomes with variation in Geraniaceae. We argue that IR loss is not a driver of these changes but is instead itself a consequence of other processes that more broadly shape both structural and sequence-level plastome evolution.

New plastome structural rearrangements discovered in core Tillandsioideae (Bromeliaceae) support recently adopted taxonomy

Full plastome sequences for land plants have become readily accessible thanks to the development of Next Generation Sequencing (NGS) techniques and powerful bioinformatic tools. Despite this vast amount of genomic data, some lineages remain understudied. Full plastome sequences from the highly diverse (>1,500 spp.) subfamily Tillandsioideae (Bromeliaceae, Poales) have been published for only three (i.e., Guzmania, Tillandsia, and Vriesea) out of 22 currently recognized genera. Here, we focus on core Tillandsioideae, a clade within subfamily Tillandsioideae, and explore the contribution of individual plastid markers and data categories to inform deep divergences of a plastome phylogeny. We generated 37 high quality plastome assemblies and performed a comparative analysis in terms of plastome structure, size, gene content and order, GC content, as well as number and type of repeat motifs. Using the obtained phylogenetic context, we reconstructed the evolution of these plastome attributes and assessed if significant shifts on the evolutionary traits' rates have occurred in the evolution of the core Tillandsioideae. Our results agree with previously published phylogenetic hypotheses based on plastid data, providing stronger statistical support for some recalcitrant nodes. However, phylogenetic discordance with previously published nuclear marker-based hypotheses was found. Several plastid markers that have been consistently used to address phylogenetic relationships within Tillandsioideae were highly informative for the retrieved plastome phylogeny and further loci are here identified as promising additional markers for future studies. New lineage-specific plastome rearrangements were found to support recently adopted taxonomic groups, including large inversions, as well as expansions and contractions of the inverted repeats. Evolutionary trait rate shifts associated with changes in size and GC content of the plastome regions were found across the phylogeny of core Tillandsioideae.

Software Choice and Sequencing Coverage Can Impact Plastid Genome Assembly-A Case Study in the Narrow Endemic Calligonum bakuense

Most plastid genome sequences are assembled from short-read whole-genome sequencing data, yet the impact that sequencing coverage and the choice of assembly software can have on the accuracy of the resulting assemblies is poorly understood. In this study, we test the impact of both factors on plastid genome assembly in the threatened and rare endemic shrub Calligonum bakuense. We aim to characterize the differences across plastid genome assemblies generated by different assembly software tools and levels of sequencing coverage and to determine if these differences are large enough to affect the phylogenetic position inferred for C. bakuense compared to congeners. Four assembly software tools (FastPlast, GetOrganelle, IOGA, and NOVOPlasty) and seven levels of sequencing coverage across the plastid genome (original sequencing depth, 2,000x, 1,000x, 500x, 250x, 100x, and 50x) are compared in our analyses. The resulting assemblies are evaluated with regard to reproducibility, contig number, gene complement, inverted repeat length, and computation time; the impact of sequence differences on phylogenetic reconstruction is assessed. Our results show that software choice can have a considerable impact on the accuracy and reproducibility of plastid genome assembly and that GetOrganelle produces the most consistent assemblies for C. bakuense. Moreover, we demonstrate that a sequencing coverage between 500x and 100x can reduce both the sequence variability across assembly contigs and computation time. When comparing the most reliable plastid genome assemblies of C. bakuense, a sequence difference in only three nucleotide positions is detected, which is less than the difference potentially introduced through software choice.

The complete mitochondrial genome of okra (Abelmoschus esculentus): using nanopore long reads to investigate gene transfer from chloroplast genomes and rearrangements of mitochondrial DNA molecules

BACKGROUND

Okra (Abelmoschus esculentus L. Moench) is an economically important crop and is known for its slimy juice, which has significant scientific research value. The A. esculentus chloroplast genome has been reported; however, the sequence of its mitochondrial genome is still lacking.

RESULTS

We sequenced the plastid and mitochondrial genomes of okra based on Illumina short reads and Nanopore long reads and conducted a comparative study between the two organelle genomes. The plastid genome of okra is highly structurally conserved, but the mitochondrial genome of okra has been confirmed to have abundant subgenomic configurations. The assembly results showed that okra's mitochondrial genome existed mainly in the form of two independent molecules, which could be divided into four independent molecules through two pairs of long repeats. In addition, we found that four pairs of short repeats could mediate the integration of the two independent molecules into one complete molecule at a low frequency. Subsequently, we also found extensive sequence transfer between the two organelles of okra, where three plastid-derived genes (psaA, rps7 and psbJ) remained intact in the mitochondrial genome. Furthermore, psbJ, psbF, psbE and psbL were integrated into the mitochondrial genome as a conserved gene cluster and underwent pseudogenization as nonfunctional genes. Only psbJ retained a relatively complete sequence, but its expression was not detected in the transcriptome data, and we speculate that it is still nonfunctional. Finally, we characterized the RNA editing events of protein-coding genes located in the organelle genomes of okra.

CONCLUSIONS

In the current study, our results not only provide high-quality organelle genomes for okra but also advance our understanding of the gene dialogue between organelle genomes and provide information to breed okra cultivars efficiently.

Structural and evolutive features of the Plinia phitrantha and P. cauliflora plastid genomes and evolutionary relationships within tribe Myrteae (Myrtaceae)

Plinia phitrantha and P. cauliflora are Myrtaceae species with recognized horticultural and pharmacological potential. Nevertheless, studies on molecular genetics and the evolution of these species are absent in the literature. In this study, we report the complete plastid genome sequence of these species and an analysis of structural and evolutive features of the plastid genome within the tribe Myrteae. The two plastid genomes present the conserved quadripartite structure and are similar to already reported plastid genomes of Myrteae species concerning the size, number, and order of the genes. A total of 69-70 SSR loci, 353 single nucleotide polymorphisms, and 574 indels were identified in P. phitrantha and P. caulifora. Observed evolutive features of the plastid genomes support the development of programs for the conservation and breeding of Plinia. The phylogenomic analysis based on the complete plastid genome sequence of 15 Myrteae species presented a robust phylogenetic signal and evolutive traits of the tribe. Ten hotspots of nucleotide diversity were identified, evidence of purifying selection was observed in 27 genes, and relative conservation of the plastid genomes was confirmed for Myrteae. Altogether, the outcomes of the present study provide support for planning conservation, breeding, and biotechnological programs for Plinia species.

Plastid phylogenomics of the Sansevieria Clade of Dracaena (Asparagaceae) resolves a recent radiation

Best known as low maintenance houseplants, sansevierias are a diverse group of flowering plants native to Africa, Madagascar, the Arabian Peninsula, and the Indian subcontinent. Traditionally recognised as a distinct genus, Sansevieria was recently merged with the larger genus Dracaena based on molecular phylogenetic data. Within the Sansevieria Clade of Dracaena, taxonomic uncertainties remain despite attempts to unravel the relationships between the species. To investigate the evolutionary relationships, morphological evolution and biogeographical history in the group, we aim to reconstruct a robust dated phylogenetic hypothesis. Using genome skimming, a chloroplast genome (cpDNA) dataset and a nuclear ribosomal (nrDNA) dataset were generated. The sampling included representatives of all sections and informal groups previously described in Sansevieria based on morphology. Analysis of the cpDNA dataset using a maximum likelihood approach resulted in a well-supported phylogeny. The time-calibrated phylogeny indicated a recent radiation with five main clades emerging in the Pliocene. Two strongly supported clades align with previously defined groups, i.e., Sansevieria section Dracomima, characterised by the Dracomima-type inflorescence, and the Zeylanica informal group, native to the Indian subcontinent. Other previously defined groups were shown to be polyphyletic; a result of convergent evolution of the identifying characters. Switches between flat and cylindrical leaves occurred multiple times in the evolution of the Sansevieria Clade. Similarly, the Cephalantha-type inflorescence has originated multiple times from an ancestor with a Sansevieria-type inflorescence. Analysis of the nrDNA dataset resulted in a phylogenetic hypothesis with low resolution, yet it supported the same two groups confirmed by the cpDNA dataset. This study furthers our understanding of the evolution of the Sansevieria Clade, which will benefit taxonomic and applied research, and aid conservation efforts.

Assembly of the Complete Mitochondrial Genome of Chinese Plum (Prunus salicina): Characterization of Genome Recombination and RNA Editing Sites

Despite the significant progress that has been made in the genome sequencing of Prunus, this area of research has been lacking a systematic description of the mitochondrial genome of this genus for a long time. In this study, we assembled the mitochondrial genome of the Chinese plum (Prunus salicina) using Illumina and Oxford Nanopore sequencing data. The mitochondrial genome size of P. salicina was found to be 508,035 base pair (bp), which is the largest reported in the Rosaceae family to date, and P. salicina was shown to be 63,453 bp longer than sweet cherry (P. avium). The P. salicina mitochondrial genome contained 37 protein-coding genes (PCGs), 3 ribosomal RNA (rRNA) genes, and 16 transfer RNA (tRNA) genes. Two plastid-derived tRNA were identified. We also found two short repeats that captured the nad3 and nad6 genes and resulted in two copies. In addition, nine pairs of repeat sequences were identified as being involved in the mediation of genome recombination. This is crucial for the formation of subgenomic configurations. To characterize RNA editing sites, transcriptome data were used, and we identified 480 RNA editing sites in protein-coding sequences. Among them, the initiation codon of the nad1 gene confirmed that an RNA editing event occurred, and the genomic encoded ACG was edited as AUG in the transcript. Combined with previous reports on the chloroplast genome, our data complemented our understanding of the last part of the organelle genome of plum, which will facilitate our understanding of the evolution of organelle genomes.

Calophyllaceae plastomes, their structure and insights in relationships within the clusioids

A complete chloroplast genome is not yet available for numerous species of plants. Among the groups that lack plastome information is the clusioid clade (Malpighiales), which includes five families: Bonnetiaceae, Calophyllaceae, Clusiaceae, Hypericaceae, and Podostemaceae. With around 2200 species, it has few published plastomes and most of them are from Podostemaceae. Here we assembled and compared six plastomes from members of the clusioids: five from Calophyllaceae (newly sequenced) and one from Clusiaceae. Putative regions for evolutionary studies were identified and the newly assembled chloroplasts were analyzed with other available chloroplasts for the group, focusing on Calophyllaceae. Our results mostly agree with recent studies which found a general conserved structure, except for the two Podostemaceae species that have a large inversion (trnK-UUU–rbcL) and lack one intron from ycf3. Within Calophyllaceae we observed a longer LSC and reduced IRs in Mahurea exstipulata, resulting in some genic rearrangement, and a short inversion (psbJ–psbE) in Kielmeyera coriacea. Phylogenetic analyses recovered the clusioids and the five families as monophyletic and revealed that conflicts in relationships reported in the literature for the group agree with nodes concentrating uninformative or conflicting gene trees. Our study brings new insights about clusioid plastome architecture and its evolution.

Plastome evolution in the Caesalpinia group (Leguminosae) and its application in phylogenomics and populations genetics

The chloroplast genomes of Caesalpinia group species are structurally conserved, but sequence level variation is useful for both phylogenomic and population genetic analyses. Variation in chloroplast genomes (plastomes) has been an important source of information in plant biology. The Caesalpinia group has been used as a model in studies correlating ecological and genomic variables, yet its intergeneric and infrageneric relationships are not fully solved, despite densely sampled phylogenies including nuclear and plastid loci by Sanger sequencing. Here, we present the de novo assembly and characterization of plastomes from 13 species from the Caesalpinia group belonging to eight genera. A comparative analysis was carried out with 13 other plastomes previously available, totalizing 26 plastomes and representing 15 of the 26 known Caesalpinia group genera. All plastomes showed a conserved quadripartite structure and gene repertoire, except for the loss of four ndh genes in Erythrostemon gilliesii. Thirty polymorphic regions were identified for inter- or intrageneric analyses. The 26 aligned plastomes were used for phylogenetic reconstruction, revealing a well-resolved topology, and dividing the Caesalpinia group into two fully supported clades. Sixteen microsatellite (cpSSR) loci were selected from Cenostigma microphyllum for primer development and at least two were cross-amplified in different Leguminosae subfamilies by in vitro or in silico approaches. Four loci were used to assess the genetic diversity of C. microphyllum in the Brazilian Caatinga. Our results demonstrate the structural conservation of plastomes in the Caesalpinia group, offering insights into its systematics and evolution, and provides new genomic tools for future phylogenetic, population genetics, and phylogeographic studies.

Towards the Well-Tempered Chloroplast DNA Sequences

With the development of next-generation sequencing technology and bioinformatics tools, the process of assembling DNA sequences has become cheaper and easier, especially in the case of much shorter organelle genomes. The number of available DNA sequences of complete chloroplast genomes in public genetic databases is constantly increasing and the data are widely used in plant phylogenetic and biotechnological research. In this work, we investigated possible inconsistencies in the stored form of publicly available chloroplast genome sequence data. The impact of these inconsistencies on the results of the phylogenetic analysis was investigated and the bioinformatic solution to identify and correct inconsistencies was implemented. The whole procedure was demonstrated using five plant families (Apiaceae, Asteraceae, Campanulaceae, Lamiaceae and Rosaceae) as examples.

Complete chloroplast genome of Eucampia zodiacus (Mediophyceae, Bacillariophyta)

The cosmopolitan phytoplankton species Eucampia zodiacus Ehrenberg 1839 is a common harmful algal bloom (HAB) species with significant negative ecological impact. However, molecular information for this HAB species is limited. In this study, the complete chloroplast genome (cpDNA) of E. zodiacus was constructed for the first time. The circular genome was 118,107 bp in length, containing a pair of inverted repeats (IR) (6991 bp each). The overall GC content of E. zodiacus cpDNA was 31.64%. It encoded 169 genes, including 131 protein-coding genes (PCGs), 30 tRNA genes, one tmRNA gene, one ncRNA gene, and six rRNA genes in IR regions. Phylogenetic analysis using concatenated PCGs of 56 diatom cpDNAs strongly supported that E. zodiacus was closely related to Cerataulina daemon, which belongs to the same family Hemiaulaceae according to AlgaeBase. Syntenic analysis revealed nearly identical gene order between the two cpDNAs, except for an inversion in the small single-copy (SSC) region.

NOVOWrap: An automated solution for plastid genome assembly and structure standardization

Plastid genomes play an important role in genomics and evolutionary biology. Next-generation sequencing has revolutionized plastid genomic data acquisition to the point that genome assembly has become a bottleneck for widespread utilization of plastid genome data. To solve this problem, we developed an open-source, cross-platform tool known as, NOVOWrap, which includes both command-line and graphical interfaces for automatically assembling plastid genomes on personal computers. With minimal inputs, settings, and user intervention, NOVOWrap can automatically assemble plastid genomes, validate results and standardize the structure using affordable computer resources. The performance of this software has been successfully benchmarked against the plastid genomes of 11 species belonging to lycopods, gymnosperms, and angiosperms. By liberating researchers from laborious and cumbersome computer manipulations and create reliable and standardized genomic data, NOVOWrap is expected to accelerate plastid genome assembly, ease the process of data exchange, and contribute to downstream analysis.

Plastid Genomes of Flowering Plants: Essential Principles

The plastid genome (plastome ) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy, and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations is allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter, we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and the tools of plastid genetic engineering.

Chloroplot: An Online Program for the Versatile Plotting of Organelle Genomes

Understanding the complexity of genomic structures and their unique architecture is linked with the power of visualization tools used to represent these features. Such tools should be able to provide a realistic and scalable version of genomic content. Here, we present an online organelle plotting tool focused on chloroplasts, which were developed to visualize the exclusive structure of these genomes. The distinguished unique features of this program include its ability to represent the Single Short Copy (SSC) regions in reverse complement, which allows the depiction of the codon usage bias index for each gene, along with the possibility of the minor mismatches between inverted repeat (IR) regions and user-specified plotting layers. The versatile color schemes and diverse functionalities of the program are specifically designed to reflect the accurate scalable representation of the plastid genomes. We introduce a Shiny app website for easy use of the program; a more advanced application of the tool is possible by further development and modification of the downloadable source codes provided online. The software and its libraries are completely coded in R, available at https://irscope.shinyapps.io/chloroplot/.

GetOrganelle: a fast and versatile toolkit for accurate de novo assembly of organelle genomes

GetOrganelle is a state-of-the-art toolkit to accurately assemble organelle genomes from whole genome sequencing data. It recruits organelle-associated reads using a modified "baiting and iterative mapping" approach, conducts de novo assembly, filters and disentangles the assembly graph, and produces all possible configurations of circular organelle genomes. For 50 published plant datasets, we are able to reassemble the circular plastomes from 47 datasets using GetOrganelle. GetOrganelle assemblies are more accurate than published and/or NOVOPlasty-reassembled plastomes as assessed by mapping. We also assemble complete mitochondrial genomes using GetOrganelle. GetOrganelle is freely released under a GPL-3 license ( https://github.com/Kinggerm/GetOrganelle ).

GetOrganelle: a fast and versatile toolkit for accurate de novo assembly of organelle genomes

GetOrganelle is a state-of-the-art toolkit to accurately assemble organelle genomes from whole genome sequencing data. It recruits organelle-associated reads using a modified "baiting and iterative mapping" approach, conducts de novo assembly, filters and disentangles the assembly graph, and produces all possible configurations of circular organelle genomes. For 50 published plant datasets, we are able to reassemble the circular plastomes from 47 datasets using GetOrganelle. GetOrganelle assemblies are more accurate than published and/or NOVOPlasty-reassembled plastomes as assessed by mapping. We also assemble complete mitochondrial genomes using GetOrganelle. GetOrganelle is freely released under a GPL-3 license ( https://github.com/Kinggerm/GetOrganelle ).

Complete Chloroplast Genomes of Anthurium huixtlense and Pothos scandens (Pothoideae, Araceae): Unique Inverted Repeat Expansion and Contraction Affect Rate of Evolution

The subfamily Pothoideae belongs to the ecologically important plant family Araceae. Here, we report the chloroplast genomes of two species of the subfamily Pothoideae: Anthurium huixtlense (size: 163,116 bp) and Pothos scandens (size: 164,719 bp). The chloroplast genome of P. scandens showed unique contraction and expansion of inverted repeats (IRs), thereby increasing the size of the large single-copy region (LSC: 102,956 bp) and decreasing the size of the small single-copy region (SSC: 6779 bp). This led to duplication of many single-copy genes due to transfer to IR regions from the small single-copy (SSC) region, whereas some duplicate genes became single copy due to transfer to large single-copy regions. The rate of evolution of protein-coding genes was affected by the contraction and expansion of IRs; we found higher mutation rates for genes that exist in single-copy regions as compared to those in IRs. We found a 2.3-fold increase of oligonucleotide repeats in P. scandens when compared with A. huixtlense, whereas amino acid frequency and codon usage revealed similarities. The ratio of transition to transversion mutations was 2.26 in P. scandens and 2.12 in A. huixtlense. Transversion mutations mostly translated in non-synonymous substitutions. The phylogenetic inference of the limited species showed the monophyly of the Araceae subfamilies. Our study provides insight into the molecular evolution of chloroplast genomes in the subfamily Pothoideae and family Araceae.

The Loss of the Inverted Repeat in the Putranjivoid Clade of Malpighiales

The typical plastid genome (plastome) of photosynthetic angiosperms comprises a pair of Inverted Repeat regions (IRs), which separate a Large Single Copy region (LSC) from a Small Single Copy region (SSC). The independent losses of IRs have been documented in only a few distinct plant lineages. The majority of these taxa show uncommonly high levels of plastome structural variations, while a few have otherwise conserved plastomes. For a better understanding of the function of IRs in stabilizing plastome structure, more taxa that have lost IRs need to be investigated. We analyzed the plastomes of eight species from two genera of the putranjivoid clade of Malpighiales using Illumina paired-end sequencing, the de novo assembly strategy GetOrganelle, as well as a combination of two annotation methods. We found that all eight plastomes of the putranjivoid clade have lost their IRB, representing the fifth case of IR loss within autotrophic angiosperms. Coinciding with the loss of the IR, plastomes of the putranjivoid clade have experienced significant structural variations including gene and intron losses, multiple large inversions, as well as the translocation and duplication of plastome segments. However, Balanopaceae, one of the close relatives of the putranjivoid clade, exhibit a relatively conserved plastome organization with canonical IRs. Our results corroborate earlier reports that the IR loss and additional structural reorganizations are closely linked, hinting at a shared mechanism that underpins structural disturbances.

Plastome Structural Conservation and Evolution in the Clusioid Clade of Malpighiales

The clusioid clade of Malpighiales is comprised of five families: Bonnetiaceae, Calophyllaceae, Clusiaceae, Hypericaceae and Podostemaceae. Recent studies have found the plastome structure of Garcinia mangostana L. from Clusiaceae was conserved, while plastomes of five riverweed species from Podostemaceae showed significant structural variations. The diversification pattern of plastome structure of the clusioid clade worth a thorough investigation. Here we determined five complete plastomes representing four families of the clusioid clade. Our results found that the plastomes of the early diverged three families (Clusiaceae, Bonnetiaceae and Calophyllaceae) in the clusioid clade are relatively conserved, while the plastomes of the other two families show significant variations. The Inverted Repeat (IR) regions of Tristicha trifaria and Marathrum foeniculaceum (Podostemaceae) are greatly reduced following the loss of the ycf1 and ycf2 genes. An inversion over 50 kb spanning from trnK-UUU to rbcL in the LSC region is shared by Cratoxylum cochinchinense (Hypericaceae), T. trifaria and Ma. foeniculaceum (Podostemaceae). The large inversed colinear block in Hypericaceae and Podostemaceae contains all the genes in the 50-kb inversed colinear block in a clade of Papilionoideae, with two extra genes (trnK-UUU and matK) at one end. Another endpoint of both inversions in the two clusioids families and Papilionoideae is located between rbcL and accD. This study greatly helped to clarify the plastome evolution in the clusioid clade.

PACVr: plastome assembly coverage visualization in R

BACKGROUND

Plastid genomes typically display a circular, quadripartite structure with two inverted repeat regions, which challenges automatic assembly procedures. The correct assembly of plastid genomes is a prerequisite for the validity of subsequent analyses on genome structure and evolution. The average coverage depth of a genome assembly is often used as an indicator of assembly quality. Visualizing coverage depth across a draft genome is a critical step, which allows users to inspect the quality of the assembly and, where applicable, identify regions of reduced assembly confidence. Despite the interplay between genome structure and assembly quality, no contemporary, user-friendly software tool can visualize the coverage depth of a plastid genome assembly while taking its quadripartite genome structure into account. A software tool is needed that fills this void.

RESULTS

We introduce 'PACVr', an R package that visualizes the coverage depth of a plastid genome assembly in relation to the circular, quadripartite structure of the genome as well as the individual plastome genes. By using a variable window approach, the tool allows visualizations on different calculation scales. It also confirms sequence equality of, as well as visualizes gene synteny between, the inverted repeat regions of the input genome. As a tool for plastid genomics, PACVr provides the functionality to identify regions of coverage depth above or below user-defined threshold values and helps to identify non-identical IR regions. To allow easy integration into bioinformatic workflows, PACVr can be invoked from a Unix shell, facilitating its use in automated quality control. We illustrate the application of PACVr on four empirical datasets and compare visualizations generated by PACVr with those of alternative software tools.

CONCLUSIONS

PACVr provides a user-friendly tool to visualize (a) the coverage depth of a plastid genome assembly on a circular, quadripartite plastome map and in relation to individual plastome genes, and (b) gene synteny across the inverted repeat regions. It contributes to optimizing plastid genome assemblies and increasing the reliability of publicly available plastome sequences. The software, example datasets, technical documentation, and a tutorial are available with the package at https://cran.r-project.org/package=PACVr.

Can we use it? On the utility of de novo and reference-based assembly of Nanopore data for plant plastome sequencing

The chloroplast genome harbors plenty of valuable information for phylogenetic research. Illumina short-read data is generally used for de novo assembly of whole plastomes. PacBio or Oxford Nanopore long reads are additionally employed in hybrid approaches to enable assembly across the highly similar inverted repeats of a chloroplast genome. Unlike for PacBio, plastome assemblies based solely on Nanopore reads are rarely found, due to their high error rate and non-random error profile. However, the actual quality decline connected to their use has rarely been quantified. Furthermore, no study has employed reference-based assembly using Nanopore reads, which is common with Illumina data. Using Leucanthemum Mill. as an example, we compared the sequence quality of seven chloroplast genome assemblies of the same species, using combinations of two sequencing platforms and three analysis pipelines. In addition, we assessed the factors which might influence Nanopore assembly quality during sequence generation and bioinformatic processing. The consensus sequence derived from de novo assembly of Nanopore data had a sequence identity of 99.59% compared to Illumina short-read de novo assembly. Most of the errors detected were indels (81.5%), and a large majority of them is part of homopolymer regions. The quality of reference-based assembly is heavily dependent upon the choice of a close-enough reference. When using a reference with 0.83% sequence divergence from the studied species, mapping of Nanopore reads results in a consensus comparable to that from Nanopore de novo assembly, and of only slightly inferior quality compared to a reference-based assembly with Illumina data. For optimal de novo assembly of Nanopore data, appropriate filtering of contaminants and chimeric sequences, as well as employing moderate read coverage, is essential. Based on these results, we conclude that Nanopore long reads are a suitable alternative to Illumina short reads in plastome phylogenomics. Few errors remain in the finalized assembly, which can be easily masked in phylogenetic analyses without loss in analytical accuracy. The easily applicable and cost-effective technology might warrant more attention by researchers dealing with plant chloroplast genomes.

Chloroplot: An Online Program for the Versatile Plotting of Organelle Genomes

Understanding the complexity of genomic structures and their unique architecture is linked with the power of visualization tools used to represent these features. Such tools should be able to provide a realistic and scalable version of genomic content. Here, we present an online organelle plotting tool focused on chloroplasts, which were developed to visualize the exclusive structure of these genomes. The distinguished unique features of this program include its ability to represent the Single Short Copy (SSC) regions in reverse complement, which allows the depiction of the codon usage bias index for each gene, along with the possibility of the minor mismatches between inverted repeat (IR) regions and user-specified plotting layers. The versatile color schemes and diverse functionalities of the program are specifically designed to reflect the accurate scalable representation of the plastid genomes. We introduce a Shiny app website for easy use of the program; a more advanced application of the tool is possible by further development and modification of the downloadable source codes provided online. The software and its libraries are completely coded in R, available at https://irscope.shinyapps.io/chloroplot/.

Long-Reads Reveal That the Chloroplast Genome Exists in Two Distinct Versions in Most Plants

The chloroplast genome usually has a quadripartite structure consisting of a large single copy region and a small single copy region separated by two long inverted repeats. It has been known for some time that a single cell may contain at least two structural haplotypes of this structure, which differ in the relative orientation of the single copy regions. However, the methods required to detect and measure the abundance of the structural haplotypes are labor-intensive, and this phenomenon remains understudied. Here, we develop a new method, Cp-hap, to detect all possible structural haplotypes of chloroplast genomes of quadripartite structure using long-read sequencing data. We use this method to conduct a systematic analysis and quantification of chloroplast structural haplotypes in 61 land plant species across 19 orders of Angiosperms, Gymnosperms, and Pteridophytes. Our results show that there are two chloroplast structural haplotypes which occur with equal frequency in most land plant individuals. Nevertheless, species whose chloroplast genomes lack inverted repeats or have short inverted repeats have just a single structural haplotype. We also show that the relative abundance of the two structural haplotypes remains constant across multiple samples from a single individual plant, suggesting that the process which maintains equal frequency of the two haplotypes operates rapidly, consistent with the hypothesis that flip-flop recombination mediates chloroplast structural heteroplasmy. Our results suggest that previous claims of differences in chloroplast genome structure between species may need to be revisited.

Characterizing gene tree conflict in plastome-inferred phylogenies

Evolutionary relationships among plants have been inferred primarily using chloroplast data. To date, no study has comprehensively examined the plastome for gene tree conflict. Using a broad sampling of angiosperm plastomes, we characterize gene tree conflict among plastid genes at various time scales and explore correlates to conflict (e.g., evolutionary rate, gene length, molecule type). We uncover notable gene tree conflict against a backdrop of largely uninformative genes. We find alignment length and tree length are strong predictors of concordance, and that nucleotides outperform amino acids. Of the most commonly used markers, matK, greatly outperforms rbcL; however, the rarely used gene rpoC2 is the top-performing gene in every analysis. We find that rpoC2 reconstructs angiosperm phylogeny as well as the entire concatenated set of protein-coding chloroplast genes. Our results suggest that longer genes are superior for phylogeny reconstruction. The alleviation of some conflict through the use of nucleotides suggests that stochastic and systematic error is likely the root of most of the observed conflict, but further research on biological conflict within plastome is warranted given documented cases of heteroplasmic recombination. We suggest that researchers should filter genes for topological concordance when performing downstream comparative analyses on phylogenetic data, even when using chloroplast genomes.

Comparative genomics of 11 complete chloroplast genomes of Senecioneae (Asteraceae) species: DNA barcodes and phylogenetics

BACKGROUND

Majority of the species within Senecioneae are classified in Senecio, making it the tribe's largest genus. Certain intergeneric relationships within the tribe are vaguely defined, with the genus Senecio being partly linked to this ambiguity. Infrageneric relationships within Senecio remain largely unknown and consequently, the genus has undergone continuous expansion and contraction over the recent past due to addition and removal of taxa. Dendrosenecio, an endemic genus in Africa, is one of its segregate genera. To heighten the understanding of species divergence and phylogeny within the tribe, the complete chloroplast genomes of the first five Senecio and six Dendrosenecio species were sequenced and analyzed in this study.

RESULTS

The entire length of the complete chloroplast genomes was ~ 150 kb and ~ 151 kb in Dendrosenecio and Senecio respectively. Characterization of the 11 chloroplast genomes revealed a significant degree of similarity particularly in their organization, gene content, repetitive sequence composition and patterns of codon usage. The chloroplast genomes encoded an equal number of unique genes out of which 80 were protein-coding genes, 30 transfer ribonucleic acid, and four ribosomal ribonucleic acid genes. Based on comparative sequence analyses, the level of divergence was lower in Dendrosenecio. A total of 331 and 340 microsatellites were detected in Senecio and Dendrosenecio, respectively. Out of which, 25 and five chloroplast microsatellites (cpSSR) were identified as potentially valuable molecular markers. Also, through whole chloroplast genome comparisons and DNA polymorphism tests, ten divergent hotspots were identified. Potential primers were designed creating genomic tools to further molecular studies within the tribe. Intergeneric relationships within the tribe were firmly resolved using genome-scale dataset in partitioned and unpartitioned schemes. Two main clades, corresponding to two subtribes within the Senecioneae, were formed with the genus Ligularia forming a single clade while the other had Dendrosenecio, Pericallis, Senecio and Jacobaea. A sister relationship was revealed between Dendrosenecio and Pericallis whereas Senecio, and Jacobaea were closely placed in a different clade.

CONCLUSION

Besides emphasizing on the potential of chloroplast genome data in resolving intergeneric relationships within Senecioneae, this study provides genomic resources to facilitate species identification and phylogenetic reconstructions within the respective genera.

Complete Chloroplast Genome Sequence of Justicia flava: Genome Comparative Analysis and Phylogenetic Relationships among Acanthaceae

The complete chloroplast genome of J. flava, an endangered medicinal plant in Saudi Arabia, was sequenced and compared with cp genome of three Acanthaceae species to characterize the cp genome, identify SSRs, and also detect variation among the cp genomes of the sampled Acanthaceae. NOVOPlasty was used to assemble the complete chloroplast genome from the whole genome data. The cp genome of J. flava was 150, 888bp in length with GC content of 38.2%, and has a quadripartite structure; the genome harbors one pair of inverted repeat (IRa and IRb 25, 500bp each) separated by large single copy (LSC, 82, 995 bp) and small single copy (SSC, 16, 893 bp). There are 132 genes in the genome, which includes 80 protein coding genes, 30 tRNA, and 4 rRNA; 113 are unique while the remaining 19 are duplicated in IR regions. The repeat analysis indicates that the genome contained all types of repeats with palindromic occurring more frequently; the analysis also identified total number of 98 simple sequence repeats (SSR) of which majority are mononucleotides A/T and are found in the intergenic spacer. The comparative analysis with other cp genomes sampled indicated that the inverted repeat regions are conserved than the single copy regions and the noncoding regions show high rate of variation than the coding region. All the genomes have ndhF and ycf1 genes in the border junction of IRb and SSC. Sequence divergence analysis of the protein coding genes showed that seven genes (petB, atpF, psaI, rpl32, rpl16, ycf1, and clpP) are under positive selection. The phylogenetic analysis revealed that Justiceae is sister to Ruellieae. This study reported the first cp genome of the largest genus in Acanthaceae and provided resources for studying genetic diversity of J. flava as well as resolving phylogenetic relationships within the core Acanthaceae.

Sequencing the Plastid Genome of Giant Ragweed (Ambrosia trifida, Asteraceae) From a Herbarium Specimen

We report the first plastome sequence of giant ragweed (Ambrosia trifida); with this new genome information, we assessed the phylogeny of Asteraceae and the transcriptional profiling against glyphosate resistance in giant ragweed. Assembly and genic features show a normal angiosperm quadripartite plastome structure with no signatures of deviation in gene directionality. Comparative analysis revealed large inversions across the plastome of giant ragweed and the previously sequenced members of the plant family. Asteraceae plastid genomes contain two inversions of 22.8 and 3.3 kb; the former is located between trnS-GCU and trnG-UCC genes, and the latter between trnE-UUC and trnT-GGU genes. The plastid genome sequences of A. trifida and the related species, Ambrosia artemisiifolia, are identical in gene content and arrangement, but they differ in length. The phylogeny is well-resolved and congruent with previous hypotheses about the phylogenetic relationship of Asteraceae. Transcriptomic analysis revealed divergence in the relative expressions at the exonic and intronic levels, providing hints toward the ecological adaptation of the genus. Giant ragweed shows various levels of glyphosate resistance, with introns displaying higher expression patterns at resistant time points after the assumed herbicide treatment.

Prevalence of isomeric plastomes and effectiveness of plastome super-barcodes in yews (Taxus) worldwide

Taxus (yew) is both the most species-rich and taxonomically difficult genus in Taxaceae. To date, no study has elucidated the complexities of the plastid genome (plastome) or examined the possibility of whole plastomes as super-barcodes across yew species worldwide. In this study, we sequenced plastomes from two to three individuals for each of the 16 recognized yew species (including three potential cryptics) and Pseudotaxus chienii. Our comparative analyses uncovered several gene loss events that independently occurred in yews, resulting in a lower plastid gene number than other Taxaceous genera. In Pseudotaxus and Taxus, we found two isomeric arrangements that differ by the orientation of a 35 kb fragment flanked by "trnQ-IRs". These two arrangements exist in different ratios within each sampled individual, and intraspecific shifts in major isomeric arrangements are first reported here in Taxus. Moreover, we demonstrate that entire plastomes can be used to successfully discriminate all Taxus species with 100% support, suggesting that they are useful as super-barcodes for species identification. We also propose that accD and rrn16-rrn23 are promising special barcodes to discriminate yew species. Our newly developed Taxus plastomic sequences provide a resource for super-barcodes and conservation genetics of several endangered yews and serve as comprehensive data to improve models of plastome complexity in Taxaceae as a whole and authenticate Taxus species.

Detecting useful genetic markers and reconstructing the phylogeny of an important medicinal resource plant, Artemisia selengensis, based on chloroplast genomics

Artemisia selengenesis is not only a health food, but also a well-known traditional Chinese medicine. Only a fraction of the chloroplast (cp) genome data of Artemisia has been reported and chloroplast genomic materials have been widely used in genomic evolution studies, molecular marker development, and phylogenetic analysis of the genus Artemisia, which makes evolutionary studies, genetic improvement, and phylogenetic identification very difficult. In this study, the complete chloroplast genome of A. selengensis was compared with that of other species within Artemisia and phylogenetic analyses was conducted with other genera in the Asteraceae family. The results showed that A. selengensis is an AT-rich species and has a typical quadripartite structure that is 151,215 bp in length. Comparative genome analyses demonstrated that the available chloroplast genomes of species of Artemisia were well conserved in terms of genomic length, GC contents, and gene organization and order. However, some differences, which may indicate evolutionary events, were found, such as a re-inversion event within the Artemisia genus, an unequal duplicate phenomenon of the ycf1 gene because of the expansion and contraction of the IR region, and the fast-evolving regions. Repeated sequences analysis showed that Artemisia chloroplast genomes presented a highly similar pattern of SSR or LDR distribution. A total of 257 SSRs and 42 LDRs were identified in the A. selengensis chloroplast genome. The phylogenetic analysis showed that A. selengensis was sister to A. gmelinii. The findings of this study will be valuable in further studies to understand the genetic diversity and evolutionary history of Asteraceae.

Flip-flop organization in the chloroplast genome of Capsosiphon fulvescens (Ulvophyceae, Chlorophyta)

To better understand organelle genome evolution of the ulvophycean green alga Capsosiphon fulvescens, we sequenced and characterized its complete chloroplast genome. The circular chloroplast genome was 111,561 bp in length with 31.3% GC content that contained 108 genes including 77 protein-coding genes, two copies of rRNA operons, and 27 tRNAs. In this analysis, we found the two types of isoform, called heteroplasmy, were likely caused by a flip-flop organization. The flip-flop mechanism may have caused structural variation and gene conversion in the chloroplast genome of C. fulvescens. In a phylogenetic analysis based on all available ulvophycean chloroplast genome data, including a new C. fulvescens genome, we found three major conflicting signals for C. fulvescens and its sister taxon Pseudoneochloris marina within 70 individual genes: (i) monophyly with Ulotrichales, (ii) monophyly with Ulvales, and (iii) monophyly with the clade of Ulotrichales and Ulvales. Although the 70-gene concatenated phylogeny supported monophyly with Ulvales for both species, these complex phylogenetic signals of individual genes need further investigations using a data-rich approach (i.e., organelle genome data) from broader taxon sampling.

Genetic, evolutionary and phylogenetic aspects of the plastome of annatto (Bixa orellana L.), the Amazonian commercial species of natural dyes

The plastome of B. orellana reveals specific evolutionary features, unique RNA editing sites, molecular markers and the position of Bixaceae within Malvales. Annatto (Bixa orellana L.) is a native species of tropical Americas with center of origin in Brazilian Amazonia. Its seeds accumulate the apocarotenoids, bixin and norbixin, which are only found in high content in this species. The seeds of B. orellana are commercially valued by the food industry because its dyes replace synthetic ones from the market due to potential carcinogenic risks. The increasing consumption of B. orellana seeds for dye extraction makes necessary the increase of productivity, which is possible accessing the genetic basis and searching for elite genotypes. The identification and characterization of molecular markers are essential to analyse the genetic diversity of natural populations and to establish suitable strategies for conservation, domestication, germplasm characterization and genetic breeding. Therefore, we sequenced and characterized in detail the plastome of B. orellana. The plastome of B. orellana is a circular DNA molecule of 159,708 bp with a typical quadripartite structure and 112 unique genes. Additionally, a total of 312 SSR loci were identified in the plastome of B. orellana. Moreover, we predicted in 23 genes a total of 57 RNA-editing sites of which 11 are unique for B. orellana. Furthermore, our plastid phylogenomic analyses, using the plastome sequences available in the plastid database belonging to species of order Malvales, indicate a closed relationship between Bixaceae and Malvaceae, which formed a sister group to Thymelaeaceae. Finally, our study provided useful data to be employed in several genetic and biotechnological approaches in B. orellana and related species of the family Bixaceae.

Characterizing gene tree conflict in plastome-inferred phylogenies

Evolutionary relationships among plants have been inferred primarily using chloroplast data. To date, no study has comprehensively examined the plastome for gene tree conflict. Using a broad sampling of angiosperm plastomes, we characterize gene tree conflict among plastid genes at various time scales and explore correlates to conflict (e.g., evolutionary rate, gene length, molecule type). We uncover notable gene tree conflict against a backdrop of largely uninformative genes. We find alignment length and tree length are strong predictors of concordance, and that nucleotides outperform amino acids. Of the most commonly used markers, matK, greatly outperforms rbcL; however, the rarely used gene rpoC2 is the top-performing gene in every analysis. We find that rpoC2 reconstructs angiosperm phylogeny as well as the entire concatenated set of protein-coding chloroplast genes. Our results suggest that longer genes are superior for phylogeny reconstruction. The alleviation of some conflict through the use of nucleotides suggests that stochastic and systematic error is likely the root of most of the observed conflict, but further research on biological conflict within plastome is warranted given documented cases of heteroplasmic recombination. We suggest that researchers should filter genes for topological concordance when performing downstream comparative analyses on phylogenetic data, even when using chloroplast genomes.

Phylogenomic and Comparative Analyses of Complete Plastomes of Croomia and Stemona (Stemonaceae)

The monocot genus Croomia (Stemonaceae) comprises three herbaceous perennial species that exhibit EA (Eastern Asian)–ENA (Eastern North American) disjunct distribution. However, due to the lack of effective genomic resources, its evolutionary history is still weakly resolved. In the present study, we conducted comparative analysis of the complete chloroplast (cp) genomes of three Croomia species and two Stemona species. These five cp genomes proved highly similar in overall size (154,407–155,261 bp), structure, gene order and content. All five cp genomes contained the same 114 unique genes consisting of 80 protein-coding genes, 30 tRNA genes and 4 rRNA genes. Gene content, gene order, AT content and IR/SC boundary structures were almost the same among the five Stemonaceae cp genomes, except that the Stemona cp genome was found to contain an inversion in cemA and petA. The lengths of five genomes varied due to contraction/expansion of the IR/SC borders. A/T mononucleotides were the richest Simple Sequence Repeats (SSRs). A total of 46, 48, 47, 61 and 60 repeats were identified in C. japonica, C. heterosepala, C. pauciflora, S. japonica and S. mairei, respectively. A comparison of pairwise sequence divergence values across all introns and intergenic spacers revealed that the ndhF–rpl32, psbM–trnD and trnS–trnG regions are the fastest-evolving regions. These regions are therefore likely to be the best choices for molecular evolutionary and systematic studies at low taxonomic levels in Stemonaceae. Phylogenetic analyses of the complete cp genomes and 78 protein-coding genes strongly supported the monophyly of Croomia. Two Asian species were identified as sisters that likely diverged in the Early Pleistocene (1.62 Mya, 95% HPD: 1.125–2.251 Mya), whereas the divergence of C. pauciflora dated back to the Late Miocene (4.77 Mya, 95% HPD: 3.626–6.162 Mya). The availability of these cp genomes will provide valuable genetic resources for further population genetics and phylogeographic studies on Croomia.

Comparative Analysis of Complete Chloroplast Genomes of Anemoclema, Anemone, Pulsatilla, and Hepatica Revealing Structural Variations Among Genera in Tribe Anemoneae (Ranunculaceae)

Structural rearrangements of Anemone species' chloroplast genome has been reported based on genetic mapping of restriction sites but has never been confirmed by genomic studies. We used a next-generation sequencing method to characterize the complete chloroplast genomes of five species in the tribe Anemoneae. Plastid genomes were assembled using de novo assembling methods combined with conventional Sanger sequencing to fill the gaps. The gene order of the chloroplast genomes of tribe Anemoneae was compared with that of other Ranunculaceae species. Multiple inversions and transpositions were detected in tribe Anemoneae. Anemoclema, Anemone, Hepatica, and Pulsatilla shared the same gene order, which contained three inversions in the large single copy region (LSC) compared to other Ranunculaceae genera. Archiclematis, Clematis, and Naravelia shared the same gene order containing two inversions and one transposition in LSC. A roughly 4.4 kb expansion region in inverted repeat (IR) regions was detected in tribe Anemoneae, suggesting that this expansion event may be a synapomorphy for this group. Plastome phylogenomic analyses using parsimony and a Bayesian method with implementation of partitioned models generated a well resolved phylogeny of Ranunculaceae. These results suggest that evaluation of chloroplast genomes may result in improved resolution of family phylogenies. Samples of Anemone, Hepatica, and Pulsatilla were tested to form paraphyletic grades within tribe Anemoneae. Anemoclema was a sister clade to Clematis. Structual variation of the plastid genome within tribe Anemoneae provided strong phylogenetic information for Ranunculaceae.

Complete chloroplast genome of glyphosate resistant Sonchus oleraceus L. from Australia, with notes on the small single copy (SSC) region orientation

Sonchus oleraceus, common sowthistle, is an asteraceous weed in Australian agricultural systems and has recently developed resistance to glyphosate. We present the complete chloroplast sequence of S. oleracueus reconstructed from Illumina whole genome shotgun sequencing. This is the first complete chloroplast genome available for the genus Sonchus. The complete chloroplast sequence is 151,808 bp long. A Bayesian phylogeny of the chloroplast coding regions of the tribe Cichorieae (Asteraceae) is presented. The S. oleraceus chloroplast genome is deposited at GenBank under accession number MG878405.

Gene losses and partial deletion of small single-copy regions of the chloroplast genomes of two hemiparasitic Taxillus species

Numerous variations are known to occur in the chloroplast genomes of parasitic plants. We determined the complete chloroplast genome sequences of two hemiparasitic species, Taxillus chinensis and T. sutchuenensis, using Illumina and PacBio sequencing technologies. These species are the first members of the family Loranthaceae to be sequenced. The complete chloroplast genomes of T. chinensis and T. sutchuenensis comprise circular 121,363 and 122,562 bp-long molecules with quadripartite structures, respectively. Compared with the chloroplast genomes of Nicotiana tabacum and Osyris alba, all ndh genes as well as three ribosomal protein genes, seven tRNA genes, four ycf genes, and the infA gene of these two species have been lost. The results of the maximum likelihood and neighbor-joining phylogenetic trees strongly support the theory that Loranthaceae and Viscaceae are monophyletic clades. This research reveals the effect of a parasitic lifestyle on the chloroplast structure and genome content of T. chinensis and T. sutchuenensis, and enhances our understanding of the discrepancies in terms of assembly results between Illumina and PacBio.

Strategies for complete plastid genome sequencing

Plastid sequencing is an essential tool in the study of plant evolution. This high-copy organelle is one of the most technically accessible regions of the genome, and its sequence conservation makes it a valuable region for comparative genome evolution, phylogenetic analysis and population studies. Here, we discuss recent innovations and approaches for de novo plastid assembly that harness genomic tools. We focus on technical developments including low-cost sequence library preparation approaches for genome skimming, enrichment via hybrid baits and methylation-sensitive capture, sequence platforms with higher read outputs and longer read lengths, and automated tools for assembly. These developments allow for a much more streamlined assembly than via conventional short-range PCR. Although newer methods make complete plastid sequencing possible for any land plant or green alga, there are still challenges for producing finished plastomes particularly from herbarium material or from structurally divergent plastids such as those of parasitic plants.

The complete chloroplast genome sequence of Gentiana lawrencei var. farreri (Gentianaceae) and comparative analysis with its congeneric species

Background

The chloroplast (cp) genome is useful in plant systematics, genetic diversity analysis, molecular identification and divergence dating. The genus Gentiana contains 362 species, but there are only two valuable complete cp genomes. The purpose of this study is to report the characterization of complete cp genome of G. lawrencei var. farreri, which is endemic to the Qinghai-Tibetan Plateau (QTP).

Methods

Using high throughput sequencing technology, we got the complete nucleotide sequence of the G. lawrencei var. farreri cp genome. The comparison analysis including genome difference and gene divergence was performed with its congeneric species G. straminea. The simple sequence repeats (SSRs) and phylogenetics were studied as well.

Results

The cp genome of G. lawrencei var. farreri is a circular molecule of 138,750 bp, containing a pair of 24,653 bp inverted repeats which are separated by small and large single-copy regions of 11,365 and 78,082 bp, respectively. The cp genome contains 130 known genes, including 85 protein coding genes (PCGs), eight ribosomal RNA genes and 37 tRNA genes. Comparative analyses indicated that G. lawrencei var. farreri is 10,241 bp shorter than its congeneric species G. straminea. Four large gaps were detected that are responsible for 85% of the total sequence loss. Further detailed analyses revealed that 10 PCGs were included in the four gaps that encode nine NADH dehydrogenase subunits. The cp gene content, order and orientation are similar to those of its congeneric species, but with some variation among the PCGs. Three genes, ndhB, ndhF and clpP, have high nonsynonymous to synonymous values. There are 34 SSRs in the G. lawrencei var. farreri cp genome, of which 25 are mononucleotide repeats: no dinucleotide repeats were detected. Comparison with the G. straminea cp genome indicated that five SSRs have length polymorphisms and 23 SSRs are species-specific. The phylogenetic analysis of 48 PCGs from 12 Gentianales taxa cp genomes clearly identified three clades, which indicated the potential of cp genomes in phylogenetics.

Discussion

The “missing” sequence of G. lawrencei var. farreri mainly consistent of ndh genes which could be dispensable under chilling-stressed conditions in the QTP. The complete cp genome sequence of G. lawrencei var. farreri provides intragenic information that will contribute to genetic and phylogenetic research in the Gentianaceae.

Parallel evolution of highly conserved plastid genome architecture in red seaweeds and seed plants

Background

The red algae (Rhodophyta) diverged from the green algae and plants (Viridiplantae) over one billion years ago within the kingdom Archaeplastida. These photosynthetic lineages provide an ideal model to study plastid genome reduction in deep time. To this end, we assembled a large dataset of the plastid genomes that were available, including 48 from the red algae (17 complete and three partial genomes produced for this analysis) to elucidate the evolutionary history of these organelles.

Results

We found extreme conservation of plastid genome architecture in the major lineages of the multicellular Florideophyceae red algae. Only three minor structural types were detected in this group, which are explained by recombination events of the duplicated rDNA operons. A similar high level of structural conservation (although with different gene content) was found in seed plants. Three major plastid genome architectures were identified in representatives of 46 orders of angiosperms and three orders of gymnosperms.

Conclusions

Our results provide a comprehensive account of plastid gene loss and rearrangement events involving genome architecture within Archaeplastida and lead to one over-arching conclusion: from an ancestral pool of highly rearranged plastid genomes in red and green algae, the aquatic (Florideophyceae) and terrestrial (seed plants) multicellular lineages display high conservation in plastid genome architecture. This phenomenon correlates with, and could be explained by, the independent and widely divergent (separated by >400 million years) origins of complex sexual cycles and reproductive structures that led to the rapid diversification of these lineages.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0299-5) contains supplementary material, which is available to authorized users.