Leveraging plastomes for comparative analysis and phylogenomic inference within Scutellarioideae (Lamiaceae)

Comparative analysis of medicinal plants Scutellaria baicalensis and common adulterants based on chloroplast genome sequencing

BACKGROUND

Scutellaria baicalensis Georgi has been extensively used as a medicinal herb in China for over 2000 years. They may be intentionally or inadvertently substituted or blended with comparable species in the local market, threatening clinical medication safety. Molecular markers are effective tools to prevent misidentification and eliminate doping and falsification among Scutellaria plants. This study screened four highly variable regions to identify Scutellaria and its adulterants. In addition, a phylogenetic analysis was performed using the complete cp genome combined with published Scutellaria species samples. Moreover, a comparative analysis of the cp genomes was conducted to investigate the cp genome evolution of S. baicalensis.

RESULTS

The complete cp genome of five species of Scutellaria was sequenced for the first time, and four previously published Scutellaria species were re-sequenced. They all exhibited a conserved quadripartite structure in their cp genomes, including two distinct regions, namely a small and large single copy region, respectively, and two inverted repeats encompassing the majority of ribosomal RNA genes. Furthermore, the nine species exhibited high conservation from aspects of the genome structure, codon usage, repeat sequences, and gene content. Four highly variable regions (matK-rps16, ndhC-trnV-UAC, psbE-petL, and rps16-trnQ-UUG) may function as potential molecular markers for differentiating S. baicalensis from its adulterants. Additionally, the monophyly of Scutellaria was ascertained and could be reclassified into two subgenera, subgenus Anaspis and subgenus Scutellaria, as evidenced by the phylogenetic analyses on sequences of cp genome and shared protein-coding sequences. According to the molecular clock analysis, it has been inferred that the divergence of Scutellaria occurred at approximately 4.0 Mya during the Pliocene Epoch.

CONCLUSION

Our study provides an invaluable theoretical basis for further Scutellaria species identification, phylogenetics, and evolution analysis.

Comparative plastome genomics and phylogenetic relationships of the genus Trollius

Trollius, a genus in the Ranunculaceae family, has significant medicinal and ornamental value. It is widely distributed in China with 16 different species accepted. However, due to the lack of enough samples and information sites, the molecular phylogenetic relationships of Trollius have been unresolved till now. Here we sequenced, assembled and annotated the plastomes of 16 Trollius species to investigate their genomic characteristics, inverted repeat (IR) boundaries, sequence repeats, and hypervariable loci. In addition, the phylogenetic relationships of this genus was reconstructed based on the whole plastomes and the protein-coding sequences data-sets. The plastomes of Trollius ranged between 159,597 bp and 160,202 bp in length, and contained 113 unique genes, including 79 protein coding, 30 tRNA, and 4 rRNA. The IR boundaries were relatively conserved within the genus Trollius. 959 simple sequence repeats and 657 long sequence repeats were detected in the Trollius plastomes. We identified 12 highly polymorphic loci (Pi > 0.0115) that can be used as plastid markers in molecular identification and phylogenetic investigation of the genus. Besides, Trollius was a monophyletic group with the earliest divergence clade being Trollius lilacinus Bunge, and the remaining species were divided into two strongly-supported clades. The phylogeny in our study supported the traditional classification systems based on the color of sepal, but not the previous classification system based on the types and relative lengths of the nectaries, and distribution. The genomic resources provided in our study can be used in the taxonomy of the genus Trollius, promoting the development and utilization of this genus.

The complete chloroplast genome of Callicarpa dichotoma (Lour.) K.Koch (Lamiaceae)

Callicarpa dichotoma (Lour.) K.Koch is a shrub species with distribution from East Asia to Southeast Asia. We assembled and annotated for the first time the complete chloroplast (cp) genome of C. dichotoma. The cp genome of C. dichotoma is 154,110 bp long with the GC content of 38.09% and consists of four subregions: a large single-copy (LSC) region of 84,915 bp, a small single-copy (SSC) region of 17,783 bp and a pair of inverted repeats (IRs) of 25,706 bp each. The cp genome of C. dichotoma encodes a total of 114 unique genes, comprising 80 protein-coding genes, 30 tRNA genes, and four rRNA genes. Phylogenetic trees based on the coding sequences strongly support the position of C. dichotoma within the genus Callicarpa, confirming the previously reported monophyly of the genus.

The complete chloroplast genome of Scutellaria barbata D. Don 1825 revealed the phylogenetic relationships of the Scutellaria genus

Scutellaria barbata D. Don 1825 is an important medicinal plant distributed in wetlands about 2000 m above sea level and used to treat various diseases. The complete chloroplast genome of S. barbata is 152,050 bp with four subregions consisting of a large single-copy region (84,053 bp), a small single-copy region (17,517 bp), and a pair of inverted repeats (25,240 bp). In the chloroplast genome of S. barbata, 131 genes were detected, comprising 87 protein-encoding genes, eight ribosomal RNA (rRNA) genes, and 36 transfer RNA (tRNA) genes. Phylogenetic analysis based on the complete chloroplast genome and protein-coding DNA sequences of 27 related taxa of the genus (out group included Holmskioldia sanguinea and Tinnea aethiopica) indicates that S. barbata was made a clade with S. orthocalyx, and S. meehanioides was a sister to them. The first chloroplast genome of S. barbata was reported in this work, serving as a potential reference for important medicinal plants within the Scutellaria genus.

Development of genomic resources for Wenchengia alternifolia (Lamiaceae) based on genome skimming data

Wenchengia alternifolia (Lamiaceae), the sole species of the genus Wenchengia is extremely rare and is currently listed as Critically Endangered (CR) on the IUCN Red List. The species had long been considered endemic to Hainan Island, China and was once believed to be extinct until a small remnant population was rediscovered at the type locality in 2010. Four more populations were later found on Hainan and in Vietnam. In order to develop genomic resources for further studies on population genetics and conservation biology of this rare species, we identified infraspecific molecular markers in the present study, using genome skimming data of five individuals collected from two populations on Hainan Island and three populations in Vietnam respectively. The length of plastome of the five individuals varied from 152,961 bp to 150,204 bp, and exhibited a typical angiosperm quadripartite structure. Six plastid hotspot regions with the Pi > 0.01 (trnH-psbA, psbA-trnK, rpl22, ndhE, ndhG-ndhI and rps15-ycf1), 1621 polymorphic gSSRs, as well as 1657 candidate SNPs in 237 variant nuclear genes were identified, thereby providing important information for further genetic studies.

Characterization of the plastome of Physaliscordata and comparative analysis of eight species of Physalis sensu stricto

In this study, we sequenced, assembled, and annotated the plastome of Physaliscordata Mill. and compared it with seven species of the genus Physalis sensu stricto. Sequencing, annotating, and comparing plastomes allow us to understand the evolutionary mechanisms associated with physiological functions, select possible molecular markers, and identify the types of selection that have acted in different regions of the genome. The plastome of P.cordata is 157,000 bp long and presents the typical quadripartite structure with a large single-copy (LSC) region of 87,267 bp and a small single-copy (SSC) region of 18,501 bp, which are separated by two inverted repeat (IRs) regions of 25,616 bp each. These values are similar to those found in the other species, except for P.angulata L. and P.pruinosa L., which presented an expansion of the LSC region and a contraction of the IR regions. The plastome in all Physalis species studied shows variation in the boundary of the regions with three distinct types, the percentage of the sequence identity between coding and non-coding regions, and the number of repetitive regions and microsatellites. Four genes and 10 intergenic regions show promise as molecular markers and eight genes were under positive selection. The maximum likelihood analysis showed that the plastome is a good source of information for phylogenetic inference in the genus, given the high support values and absence of polytomies. In the Physalis plastomes analyzed here, the differences found, the positive selection of genes, and the phylogenetic relationships do not show trends that correspond to the biological or ecological characteristics of the species studied.

Pharmacophylogenetic study of Scutellaria baicalensis and its substitute medicinal species based on the chloroplast genomics, metabolomics, and active ingredient

The genetic relationships among the species in Scutellaria genus remain unclear because of the variation in the number of species and complex trait. The usage of S. baicalensis and its four substitute medicinal species (S. amoena, S. hypericifolia, S. likiangensis, and S. viscidula) in traditional medicines make their specialized metabolism important in China, but interspecific genetic and chemical differences have rarely been reported for these species. In this study, the chloroplast genomes of four substitute species for S. baicalensis were assembled, and comparative and phylogenetic analyses were performed with these species and other Scutellaria relatives. In addition, metabolomics analyses were performed and the contents of the main active compounds were determined to reveal the interspecific chemical diversity of S. baicalensis and its four substitute species. The full lengths of their chloroplast genomes ranged from 151,574 to 151,816 bp with an average GC content of 38.34%, and a total of 113 genes were annotated. In the chloroplast genomes of S. baicalensis and its four substitutes, one hypervariable region (petA-psbL) is proposed as a potential DNA barcode. Phylogenetic analysis showed that the subdivision of the genus Scutellaria should be reconsidered. The metabolomics and content determination analyses showed that the four species exhibit a metabolism similar to that of S. baicalensis in different parts. Except for the roots of S. likiangensis, all parts of the substitute species showed high contents of baicalin. Genetic and chemical analyses of four substitute medicinal species for S. baicalensis were performed here for the first time, and their pharmacophylogenetic relationships were further explored, providing a scientific basis for the subsequent development of the medicinal value and resource utilization of Scutellaria.

Chloroplast Genomes of Genus Tilia: Comparative Genomics and Molecular Evolution

Tilia is a complex genus in the family Malvaceae that has high ecological and economical values. Owing to the lack of sufficient distinguishable morphological and molecular characteristics, interspecific relationships in this genus are not clear. Chloroplast (cp) genomes are small, meanwhile most angiosperms usually undergo matrilineal inheritance. Consequently, they can be used in molecular systematics and phylogenetic analyses. Here, we sequenced and assembled cp genomes from T. endochrysea, T. tomentosa, T. miqueliana, T. americana and T. cordata, and compared them with those of seven previously reported Tilia species. Similar gene contents, gene orders and GC contents existed among the 12 cp genomes, which ranged from 162,564 to 162,855 bp and encoded 113 unique genes. Abundant simple sequence repeats (119–127) and dispersed repeats (97–135) were detected in Tilia cp genomes. In total, 11 hypervariable regions were identified that could be suitable for species identification and phylogenetic studies. A phylogenetic analysis of Malvaceae based on 5 hypervariable genes (matK + ndhF + rpoB + rpoC2+ycf1) revealed that all eight subfamilies were monophyletic groups. Additionally, the genus Tilia was divided into three groups on the basis of all 521 molecular variation loci. The current study provides valuable insights into the genomic evolution of the genus Tilia.

The complete chloroplast genome of Callicarpa rubella Lindl. (Lamiaceae) from Guangxi, China: genome structure and phylogenetic analysis

The relationship between Callicarpa rubella Lindl. and its infraspecific taxa has troubled researchers for a long time. Here, we reported for the first time the complete chloroplast (cp) genome of C. rubella to investigate its phylogenetic position and provide more sequencing information for further studies about the relationship between C. rubella and its related species. The cp genome of C. rubella was 154,202 bp in length and displayed a typical quadripartite angiosperm structure, containing two inverted repeat (IR) regions of 25,701 bp, a large single-copy (LSC) region of 84,968 bp and a small single-copy (SSC) region of 17,832 bp. It contained 87 protein-coding genes, 37 tRNA genes and 8 rRNA genes. The analysis fully resolved C. rubella was in a clade with C. bodinieri and C. nudiflora. The results indicated Callicarpa formed a sister relationship with Dicrastylis parvifolia in Lamiaceae.

Analysis of the complete chloroplast genomes of Scutellaria tsinyunensis and Scutellaria tuberifera (Lamiaceae)

Scutellaria Linn. is a perennial herb with about 300 species. This genus has high medicinal value and many are used in Traditional Chinese Medicine (TCM). In this study, we sequenced and assembled the complete chloroplast genomes of Scutellaria tsinyunensis and S. tuberifera. Subsequently, we conducted a comprehensive comparative genomics analysis with 12 other published Scutellaria species. These genomes all had a conserved quartile structure, and the gene contents, gene sequences and GC contents are highly similar. The study on the genetic characteristics and nucleotide substitution rate of different genes found that the protein-coding genes of chloroplasts have differed greatly. Most genes are under purifying selection, but the rps12 gene may have undergone positive selection. Besides, we identified three hypervariable regions as potential markers for Scutellaria taxa, which could play an important role in species identification of Scutellaria. Phylogenetic analysis showed that the 14 Scutellaria taxa were divided into two major clades. Moreover, the variation of IR regions is closely related to the evolutionary history as was reconstructed based on SNPs. In conclusion, we provided two high-quality chloroplast reference genomes of Scutellaria, this reliable information and genomic resources are valuable for developing of efficient DNA barcodes as reconstruction of chloroplast evolutionary history of the genus.

Comparative analysis of plastomes in Oxalidaceae: Phylogenetic relationships and potential molecular markers

The wood sorrel family, Oxalidaceae, is mainly composed of annual or perennial herbs, a few shrubs, and trees distributed from temperate to tropical zones. Members of Oxalidaceae are of high medicinal, ornamental, and economic value. Despite the rich diversity and value of Oxalidaceae, few molecular markers or plastomes are available for phylogenetic analysis of the family. Here, we reported four new whole plastomes of Oxalidaceae and compared them with plastomes of three species in the family, as well as the plastome of Rourea microphylla in the closely related family Connaraceae. The eight plastomes ranged in length from 150,673 bp (Biophytum sensitivum) to 156,609 bp (R. microphylla). Genome annotations revealed a total of 129-131 genes, including 83-84 protein-coding genes, eight rRNA genes, 37 tRNA genes, and two to three pseudogenes. Comparative analyses showed that the plastomes of these species have minor variations at the gene level. The smaller plastomes of herbs B. sensitivum and three Oxalis species are associated with variations in IR region sizes, intergenic region variation, and gene or intron loss. We identified sequences with high variation that may serve as molecular markers in taxonomic studies of Oxalidaceae. The phylogenetic trees of selected superrosid representatives based on 76 protein-coding genes corroborated the Oxalidaceae position in Oxalidales and supported it as a sister to Connaraceae. Our research also supported the monophyly of the COM (Celastrales, Oxalidales, and Malpighiales) clade.

Systematic Placement of the Enigmatic Southeast Asian Genus Paralamium and an Updated Phylogeny of Tribe Pogostemoneae (Lamiaceae Subfamily Lamioideae)

Paralamium (Lamiaceae) is a monotypic genus within the subfamily Lamioideae and has a sporadic distribution in subtropical mountains of southeast Asia. Although recent studies have greatly improved our understanding of generic relationships within Lamioideae, the second most species-rich subfamily of Lamiaceae, the systematic position of Paralamium within the subfamily remains unclear. In this study, we investigate the phylogenetic placement of the genus using three datasets: (1) a 69,276 bp plastome alignment of Lamiaceae; (2) a five chloroplast DNA region dataset of tribe Pogostemoneae, and (3) a nuclear ribosomal internal transcribed spacer region dataset of Pogostemoneae. These analyses demonstrate that Paralamium is a member of Pogostemoneae and sister to the monotypic genus Craniotome. In addition, generic-level phylogenetic relationships within Pogostemoneae are also discussed, and a dichotomous key for genera within Pogostemoneae is provided.

Traditional uses, ten-years research progress on phytochemistry and pharmacology, and clinical studies of the genus Scutellaria

ETHNOPHARMACOLOGICAL RELEVANCE

Scutellaria (Lamiaceae), which includes approximately 360-469 accepted species, is widespread in Europe, North America, East Asia, and South America. Several species have a long history being used as traditional medicines to treat respiratory, peptic, neurological, and hepatic and gall diseases. The phytochemistry and pharmacology of the genus Scutellaria have been developed dramatically in the past ten years, and the traditional uses and clinical studies of the genus have not been systematically summarized. Therefore, it is especially valuable to review the current state of knowledge to provide a basis for further exploration of its medicinal potential.

AIM OF THE REVIEW

The review aims to provide updated information on the ethnopharmacology, the ten-year research progress of phytochemistry and pharmacology, and clinical studies of Scutellaria and to explore the potential medicinal values and further studies of Scutellaria.

MATERIALS AND METHODS

This review is based on published studies and books from the library and electronic sources, including SciFinder, Scopus, PubMed, Web of Science, Baidu Scholar, CNKI, the online ethnobotanical database, and ethnobotanical monographs. This literature is related to ethnopharmacology, the ten-year research progress on the phytochemistry and pharmacology, and clinical studies of Scutellaria.

RESULTS

A total of 50 species, 5 subspecies and 17 varieties of the genus Scutellaria are used as traditional medicine with various biological activities. In the past ten years, 208 chemical constituents have been identified from 16 species and 1 variety of the genus Scutellaria, such as neo-clerodane diterpenoids, sesterterpenoids, terpenoids, flavonoids. Pharmacological research has demonstrated that the extracts and compounds identified from this genus exhibit extensive biological activities, including anticancer, antioxidant, anti-inflammatory, antiviral and antibacterial activities, effects on cardiovascular, cerebrovascular diseases as well as hepatoprotective and neuroprotective effects. The species S. baicalensis, S. barbata, and S. lateriflora and the main compounds baicalein, baicalin and wogonin are involved in clinical trials, which point the way for us to conduct further studies, such as study on the anticancer, antihypertensive, anti-infective, anti-inflammatory, neuroprotective and other effects of Scutellaria.

CONCLUSIONS

The species included in the genus Scutellaria can be used to treat cancer, infection, hepatic disorders, cardiovascular and cerebrovascular diseases, neurodegenerative diseases, and other diseases. Some indications in traditional medicines have been confirmed by modern pharmacological studies, such as anticancer, anti-inflammatory, anti-infective activity, and hepatoprotective and neuroprotective effects. The available literature indicated that most of the bioactivities could be attributed to flavonoids and neo-clerodane diterpenoids. Although there are some uses of Scutellaria in clinical practice, the existing research on this genus is still limited. In order to expand the development of medicinal resources of Scutellaria, the already studied species in this genus are recommended for more comprehensive investigation on their active substances, pharmacological mechanisms, quality control, clinical use and new drug research. Additionally, it is necessary to study species that their chemical composition or pharmacological activity have not yet been investigated, especially those used in folk medicine.

Three complete plastome sequences from the families of Lamiaceae, Mazaceae, and Phrymaceae (Lamiales)

In this study, we report the first complete plastome sequence of Vitex rotundifolia (Lamiaceae) (MT937186). In addition, the plastome sequences of Phryma leptostachya subsp. asiatica (Phrymaceae) (153,324 bp; MT948145) and Mazus pumilus (Mazaceae) (152,847 bp; MT937187) are also included. The gene orders and structures of the three plastomes are collinear with those of the typical plastome of angiosperm. The plastome size of V. rotundifolia is 154,370 bp in length and consists of a large single-copy region of 85,079 bp and a small single-copy region of 17,917 bp, which are separated by a pair of 25,687 bp-long inverted repeat regions. In addition, the plastome sizes of P. leptostachya subsp. asiatica and M. pumilus are 153,324 bp and 152,847 bp, respectively. The three plastomes contain 113 genes, including 79 protein-coding, 30 tRNA, and four rRNA genes. Sixteen genes contain one intron and two genes have two introns. A total of 41 simple sequence repeat loci was identified in the V. rotundifolia plastome. Phylogenetic analysis shows that Viticoideae is a sister group of the last of Lamiaceae except Nepetoideae. The Mazaceae are a sister group of Lamiaceae, while Phrymaceae form a sister group to the Paulowniaceae-Orobanchaceae clade.

An updated tribal classification of Lamiaceae based on plastome phylogenomics

BACKGROUND

A robust molecular phylogeny is fundamental for developing a stable classification and providing a solid framework to understand patterns of diversification, historical biogeography, and character evolution. As the sixth largest angiosperm family, Lamiaceae, or the mint family, consitutes a major source of aromatic oil, wood, ornamentals, and culinary and medicinal herbs, making it an exceptionally important group ecologically, ethnobotanically, and floristically. The lack of a reliable phylogenetic framework for this family has thus far hindered broad-scale biogeographic studies and our comprehension of diversification. Although significant progress has been made towards clarifying Lamiaceae relationships during the past three decades, the resolution of a phylogenetic backbone at the tribal level has remained one of the greatest challenges due to limited availability of genetic data.

RESULTS

We performed phylogenetic analyses of Lamiaceae to infer relationships at the tribal level using 79 protein-coding plastid genes from 175 accessions representing 170 taxa, 79 genera, and all 12 subfamilies. Both maximum likelihood and Bayesian analyses yielded a more robust phylogenetic hypothesis relative to previous studies and supported the monophyly of all 12 subfamilies, and a classification for 22 tribes, three of which are newly recognized in this study. As a consequence, we propose an updated phylogenetically informed tribal classification for Lamiaceae that is supplemented with a detailed summary of taxonomic history, generic and species diversity, morphology, synapomorphies, and distribution for each subfamily and tribe.

CONCLUSIONS

Increased taxon sampling conjoined with phylogenetic analyses based on plastome sequences has provided robust support at both deep and shallow nodes and offers new insights into the phylogenetic relationships among tribes and subfamilies of Lamiaceae. This robust phylogenetic backbone of Lamiaceae will serve as a framework for future studies on mint classification, biogeography, character evolution, and diversification.