Species Delimitation and Interspecific Relationships of the Genus Orychophragmus (Brassicaceae) Inferred from Whole Chloroplast Genomes

Comparative Analysis and Phylogeny of the Complete Chloroplast Genomes of Nine Cynanchum (Apocynaceae) Species

Cynanchum belongs to the Apocynaceae family and is a morphologically diverse genus that includes around 200 shrub or perennial herb species. Despite the utilization of CPGs, few molecular phylogenetic studies have endeavored to elucidate infrafamilial relationships within Cynanchum through extensive taxon sampling. In this research, we constructed a phylogeny and estimated divergence time based on the chloroplast genomes (CPGs) of nine Cynanchum species. We sequenced and annotated nine chloroplast (CP) genomes in this study. The comparative analysis of these genomes from these Cynanchum species revealed a typical quadripartite structure, with a total sequence length ranging from 158,283 to 161,241 base pairs (bp). The CP genome (CPG) was highly conserved and moderately differentiated. Through annotation, we identified a total of 129-132 genes. Analysis of the boundaries of inverted repeat (IR) regions showed consistent positioning: the rps19 gene was located in the IRb region, varying from 46 to 50 bp. IRb/SSC junctions were located between the trnN and ndhF genes. We did not detect major expansions or contractions in the IR region or rearrangements or insertions in the CPGs of the nine Cynanchum species. The results of SSR analysis revealed a variation in the number of SSRs, ranging from 112 to 150. In five types of SSRs, the largest number was mononucleotide repeats, and the smallest number was hexanucleotide repeats. The number of long repeats in the cp genomes of nine Cynanchum species was from 35 to 80. In nine species of Cynanchum, the GC3s values ranged from 26.80% to 27.00%, indicating a strong bias towards A/U-ending codons. Comparative analyses revealed four hotspot regions in the CPG, ndhA-ndhH, trnI-GAU-rrn16, psbI-trnS-GCU, and rps7-ndhB, which could potentially serve as molecular markers. In addition, phylogenetic tree construction based on the CPG indicated that the nine Cynanchum species formed a monophyletic group. Molecular dating suggested that Cynanchum diverged from its sister genus approximately 18.87 million years ago (Mya) and species diversification within the Cynanchum species primarily occurred during the recent Miocene epoch. The divergence time estimation presented in this study will facilitate future research on Cynanchum, aid in species differentiation, and facilitate diverse investigations into this economically and ecologically important genus.

Characterization of the complete chloroplast genome of the rare medicinal plant: Mandragora caulescens (Solanaceae)

In this study, we assembled high-quality chloroplast genomes of Mandragora caulescens through a reference-guided approach using high-throughput Illumina sequencing reads. The resulting chloroplast genome assembly displayed a typical quadripartite structural organization, comprising a large single-copy (LSC) region of 85,233 bp, two inverted repeat (IR) regions of 25,685 bp each, and a small single-copy (SSC) region of 18,207 bp. The chloroplast genome harbored 141 complete genes, and its overall GC content was 38.0%. In maximum-likelihood (ML) and Bayesian inference (BI) trees, the 19 Solanaceae species formed a monophyletic group, dividing into two main clades. M. caulescens and Nicandra physalodes formed a monophyletic group, suggesting a close relationship between the two species. The M. caulescens cp genome presented in this study lays a good foundation for further genetic and genomic studies of the Solanaceae.

Phylogenetic analysis and divergence time estimation of Lycium species in China based on the chloroplast genomes

BACKGROUND

Lycium is an economically and ecologically important genus of shrubs, consisting of approximately 70 species distributed worldwide, 15 of which are located in China. Despite the economic and ecological importance of Lycium, its phylogeny, interspecific relationships, and evolutionary history remain relatively unknown. In this study, we constructed a phylogeny and estimated divergence time based on the chloroplast genomes (CPGs) of 15 species, including subspecies, of the genus Lycium from China.

RESULTS

We sequenced and annotated 15 CPGs in this study. Comparative analysis of these genomes from these Lycium species revealed a typical quadripartite structure, with a total sequence length ranging from 154,890 to 155,677 base pairs (bp). The CPGs was highly conserved and moderately differentiated. Through annotation, we identified a total of 128-132 genes. Analysis of the boundaries of inverted repeat (IR) regions showed consistent positioning: the junctions of the IRb/LSC region were located in rps19 in all Lycium species, IRb/SSC between the ycf1 and ndhF genes, and SSC/IRa within the ycf1 gene. Sequence variation in the SSC region exceeded that in the IR region. We did not detect major expansions or contractions in the IR region or rearrangements or insertions in the CPGs of the 15 Lycium species. Comparative analyses revealed five hotspot regions in the CPG: trnR(UCU), atpF-atpH, ycf3-trnS(GGA), trnS(GGA), and trnL-UAG, which could potentially serve as molecular markers. In addition, phylogenetic tree construction based on the CPG indicated that the 15 Lycium species formed a monophyletic group and were divided into two typical subbranches and three minor branches. Molecular dating suggested that Lycium diverged from its sister genus approximately 17.7 million years ago (Mya) and species diversification within the Lycium species of China primarily occurred during the recent Pliocene epoch.

CONCLUSION

The divergence time estimation presented in this study will facilitate future research on Lycium, aid in species differentiation, and facilitate diverse investigations into this economically and ecologically important genus.

Wasabi Gone Wild? Origin and Characterization of the Complete Plastomes of Ulleung Island Wasabi (Eutrema japonicum; Brassicaceae) and Other Cultivars in Korea

Korean wasabi occurs naturally on the young oceanic, volcanic Ulleung Island off the east coast of the Korean Peninsula. Although the Ulleung Island wasabi is reported as Eutrema japonicum and has been suggested to be morphologically identical to cultivars in Korea, very little is known about its taxonomic identity and relationship with other cultivars. In this study, we sequenced the complete chloroplast DNA sequences of three naturally occurring Ulleung Island wasabi plants and six cultivars ('Daewang', 'Daruma', 'Micado', 'Orochi', 'Green Thumb', and 'Shogun') from continental Korea and determined the taxonomic identity of Korean wasabi on Ulleung Island. The size and organization of the complete chloroplast genomes of the nine accessions were nearly identical to those of previously reported wasabi cultivars. In addition, phylogenetic analysis based on the complete plastomes suggested that Ulleung Island wasabi most likely comprises various wasabi cultivars with three chlorotypes ('Shogun', 'Green Thumb', and a unique Chusan type). Based on the complete plastomes, we identified eight chlorotypes for the major wasabi cultivars and the Ulleung Island wasabi. Two major groups (1-'Mazuma' and 'Daruma', and 2-'Fujidaruma'/'Shimane No. 3'/Ulleung Island wasabi/five cultivars in Korea) were also identified based on mother line genealogical history. Furthermore, different types of variations (mutations, insertions/deletions (indels), mononucleotide repeats, and inversions) in plastomes were identified to distinguish different cultivar lines and five highly divergent hotspots. The nine newly obtained complete plastomes are valuable organelle genomic resources for species identification and infraspecific phylogeographic studies on wild and cultivated wasabi.

Species delimitation of tea plants (Camellia sect. Thea) based on super-barcodes

BACKGROUND

The era of high throughput sequencing offers new paths to identifying species boundaries that are complementary to traditional morphology-based delimitations. De novo species delimitation using traditional or DNA super-barcodes serve as efficient approaches to recognizing putative species (molecular operational taxonomic units, MOTUs). Tea plants (Camellia sect. Thea) form a group of morphologically similar species with significant economic value, providing the raw material for tea, which is the most popular nonalcoholic caffeine-containing beverage in the world. Taxonomic challenges have arisen from vague species boundaries in this group.

RESULTS

Based on the most comprehensive sampling of C. sect. Thea by far (165 individuals of 39 morphospecies), we applied three de novo species delimitation methods (ASAP, PTP, and mPTP) using plastome data to provide an independent evaluation of morphology-based species boundaries in tea plants. Comparing MOTU partitions with morphospecies, we particularly tested the congruence of MOTUs resulting from different methods. We recognized 28 consensus MOTUs within C. sect. Thea, while tentatively suggesting that 11 morphospecies be discarded. Ten of the 28 consensus MOTUs were uncovered as morphospecies complexes in need of further study integrating other evidence. Our results also showed a strong imbalance among the analyzed MOTUs in terms of the number of molecular diagnostic characters.

CONCLUSION

This study serves as a solid step forward for recognizing the underlying species boundaries of tea plants, providing a needed evidence-based framework for the utilization and conservation of this economically important plant group.

Characteristics of chloroplast and mitochondrial genomes and intracellular gene transfer in the Korean endemic shrub, Sophora koreensis Nakai (Fabaceae)

Sophora koreensis Nakai, an endemic species distributed only in the Korean Peninsula, is of great geographical, economic, and taxonomic importance. Although its complete chloroplast (cp) genome sequence has been reported, its mitochondrial (mt) genome sequence has not yet been studied. Therefore, in this study, we aimed to investigate its mt genome sequence and compare it with those reported for other Fabaceae species. Total genomic DNA was extracted from fresh S. koreensis leaves collected from natural habitats in Gangwon-do Province, South Korea. This was followed by polymerase chain reaction (PCR) amplification of cpDNA insertions in the mt genome and the detection of microsatellites and dispersed repeats in the cp and mt genomes. Finally, the cp and mt genomes of S. koreensis were compared with those reported for other Fabaceae species. The cp sequence of S. koreensis showed identical gene orders and contents as those previously reported. Only six substitutions and one deletion were detected with 99 % homology. Conversely, the complete mt genome sequence, which was 517,845 bp in length and encoded 61 genes, including 43 protein-coding, 15 transfer RNAs, and 3 ribosomal RNA genes, was considerably different from that of S. japonica in terms of gene order and composition. Further, the mt genome of S. koreensis included ca. 7 and 3 kb insertions, representing an intracellular gene transfer (IGT) event, and the regions with these insertions were determined to be originally present in the cp genome. This IGT event was also confirmed via PCR amplification. IGT events can be induced via biological gene expression control or the use of repetitive sequences, and they provide important insights into the evolutionary lineage of S. koreensis. However, further studies are needed to clarify the gene transfer mechanisms between the two organelles.

Insights into the phylogenetic relationships and species boundaries of the Myricaria squamosa complex (Tamaricaceae) based on the complete chloroplast genome

Myricaria plants are widely distributed in Eurasia and are helpful for windbreak and embankment protection. Current molecular evidence has led to controversy regarding species boundaries within the Myricaria genus and interspecific phylogenetic relationships between three specific species-M. bracteata, M. paniculata and M. squamosa-which have remained unresolved. This study treated these three unresolved taxa as a species complex, named the M. squamosa complex. The genome skimming approach was used to determine 35 complete plastome sequences and nuclear ribosomal DNA sequences for the said complex and other closely related species, followed by de novo assembly. Comparative analyses were conducted across Myricaria to identify the genome size, gene content, repeat type and number, SSR (simple sequence repeat) abundance, and codon usage bias of chloroplast genomes. Tree-based species delimitation results indicated that M. bracteata, M. paniculata and M. squamosa could not be distinguished and formed two monophyletic lineages (P1 and P2) that were clustered together. Compared to plastome-based species delimitation, the standard nuclear DNA barcode had the lowest species resolution, and the standard chloroplast DNA barcode and group-specific barcodes delimitated a maximum of four out of the five species. Plastid phylogenomics analyses indicated that the monophyletic M. squamosa complex is comprised of two evolutionarily significant units: one in the western Tarim Basin and the other in the eastern Qinghai-Tibet Plateau. This finding contradicts previous species discrimination and promotes the urgent need for taxonomic revision of the threatened genus Myricaria. Dense sampling and plastid genomes will be essential in this effort. The super-barcodes and specific barcode candidates outlined in this study will aid in further studies of evolutionary history.

Comparative analysis of the chloroplast genomes of Rosa species and RNA editing analysis

BACKGROUND

The genus Rosa (Rosaceae) contains approximately 200 species, most of which have high ecological and economic values. Chloroplast genome sequences are important for studying species differentiation, phylogeny, and RNA editing.

RESULTS

In this study, the chloroplast genomes of three Rosa species, Rosa hybrida, Rosa acicularis, and Rosa rubiginosa, were assembled and compared with other reported Rosa chloroplast genomes. To investigate the RNA editing sites in R. hybrida (commercial rose cultivar), we mapped RNA-sequencing data to the chloroplast genome and analyzed their post-transcriptional features. Rosa chloroplast genomes presented a quadripartite structure and had highly conserved gene order and gene content. We identified four mutation hotspots (ycf3-trnS, trnT-trnL, psbE-petL, and ycf1) as candidate molecular markers for differentiation in the Rosa species. Additionally, 22 chloroplast genomic fragments with a total length of 6,192 bp and > 90% sequence similarity with their counterparts were identified in the mitochondrial genome, representing 3.96% of the chloroplast genome. Phylogenetic analysis including all sections and all subgenera revealed that the earliest divergence in the chloroplast phylogeny roughly distinguished species of sections Pimpinellifoliae and Rosa and subgenera Hulthemia. Moreover, DNA- and RNA-sequencing data revealed 19 RNA editing sites, including three synonymous and 16 nonsynonymous, in the chloroplast genome of R. hybrida that were distributed among 13 genes.

CONCLUSIONS

The genome structure and gene content of Rosa chloroplast genomes are similar across various species. Phylogenetic analysis based on the Rosa chloroplast genomes has high resolution. Additionally, a total of 19 RNA editing sites were validated by RNA-Seq mapping in R. hybrida. The results provide valuable information for RNA editing and evolutionary studies of Rosa and a basis for further studies on genomic breeding of Rosa species.

Comparative chloroplast genomics of 34 species in subtribe Swertiinae (Gentianaceae) with implications for its phylogeny

BACKGROUND

Subtribe Swertiinae, a medicinally significant and highly speciose Subtribe of family Gentianaceae. Despite previous extensive studies based on both morphology and molecular data, intergeneric and infrageneric relationships within subtribe Swertiinae remain controversial.

METHODS

Here, we employed four newly generated Swertia chloroplast genomes with thirty other published genomes to elucidate their genomic characteristics.

RESULTS

The 34 chloroplast genomes were small and ranged in size from 149,036 to 154,365 bp, each comprising two inverted repeat regions (size range 25,069-26,126 bp) that separated large single-copy (80,432-84,153 bp) and small single-copy (17,887-18,47 bp) regions, and all the chloroplast genomes showed similar gene orders, contents, and structures. These chloroplast genomes contained 129-134 genes each, including 84-89 protein-coding genes, 37 tRNAs, and 8 rRNAs. The chloroplast genomes of subtribe Swertiinae appeared to have lost some genes, such as rpl33, rpl2 and ycf15 genes. Comparative analyses revealed that two mutation hotspot regions (accD-psaI and ycf1) could serve as effective molecular markers for further phylogenetic analyses and species identification in subtribe Swertiinae. Positive selection analyses showed that two genes (ccsA and psbB) had high Ka/Ks ratios, indicating that chloroplast genes may have undergone positive selection in their evolutionary history. Phylogenetic analysis showed that the 34 subtribe Swertiinae species formed a monophyletic clade, with Veratrilla, Gentianopsis and Pterygocalyx located at the base of the phylogenetic tree. Some genera of this subtribe, however, were not monophyletic, including Swertia, Gentianopsis, Lomatogonium, Halenia, Veratrilla and Gentianopsis. In addition, our molecular phylogeny was consistent with taxonomic classification of subtribe Swertiinae in the Roate group and Tubular group. The results of molecular dating showed that the divergence between subtrib Gentianinae and subtrib Swertiinae was estimated to occur in 33.68 Ma. Roate group and Tubular group in subtribe Swertiinae approximately diverged in 25.17 Ma.

CONCLUSION

Overall, our study highlighted the taxonomic utility of chloroplast genomes in subtribe Swertiinae, and the genetic markers identified here will facilitate future studies on the evolution, conservation, population genetics, and phylogeography of subtribe Swertiinae species.

Genome assembly of the Brassicaceae diploid Orychophragmus violaceus reveals complex whole-genome duplication and evolution of dihydroxy fatty acid metabolism

Orychophragmus violaceus is a Brassicaceae species widely cultivated in China, particularly as a winter cover crop in northern China because of its low-temperature tolerance and low water demand. Recently, O. violaceus has also been cultivated as a potential industrial oilseed crop because of its abundant 24-carbon dihydroxy fatty acids (diOH-FAs), which contribute to superior high-temperature lubricant properties. In this study, we performed de novo assembly of the O. violaceus genome. Whole-genome synteny analysis of the genomes of its relatives demonstrated that O. violaceus is a diploid that has undergone an extra whole-genome duplication (WGD) after the Brassicaceae-specific α-WGD event, with a basic chromosome number of x = 12. Formation of diOH-FAs is hypothesized to have occurred after the WGD event. Based on the genome and the transcriptome data from multiple stages of seed development, we predicted that OvDGAT1-1 and OvDGAT1-2 are candidate genes for the regulation of diOH-FA storage in O. violaceus seeds. These results may greatly facilitate the development of heat-tolerant and eco-friendly plant-based lubricants using O. violaceus seed oil and improve our understanding of the genomic evolution of Brassicaceae.

Comparative plastomes of Carya species provide new insights into the plastomes evolution and maternal phylogeny of the genus

Carya, in the Juglandiodeae subfamily, is to a typical temperate-subtropical forest-tree genus for studying the phylogenetic evolution and intercontinental disjunction between eastern Asia (EA) and North America (NA). Species of the genus have high economic values worldwide for their high-quality wood and the rich healthy factors of their nuts. Although previous efforts based on multiple molecular markers or genome-wide SNPs supported the monophyly of Carya and its two EA and NA major subclades, the maternal phylogeny of Carya still need to be comprehensively evaluated. The variation of Carya plastome has never been thoroughly characterized. Here, we novelly present 19 newly generated plastomes of congeneric Carya species, including the recently rediscovered critically endangered C. poilanei. The overall assessment of plastomes revealed highly conservative in the general structures. Our results indicated that remarkable differences in several plastome features are highly consistent with the EA-NA disjunction and showed the relatively diverse matrilineal sources among EA Carya compared to NA Carya. The maternal phylogenies were conducted with different plastome regions and full-length plastome datasets from 30 plastomes, representing 26 species in six genera of Juglandoideae and Myrica rubra (as root). Six out of seven phylogenetic topologies strongly supported the previously reported relationships among genera of Juglandoideae and the two subclades of EA and NA Carya, but displayed significant incongruencies between species within the EA and NA subclades. The phylogenetic tree generated from full-length plastomes demonstrated the optimal topology and revealed significant geographical maternal relationships among Carya species, especially for EA Carya within overlapping distribution areas. The full-length plastome-based phylogenetic topology also strongly supported the taxonomic status of five controversial species as separate species of Carya. Historical and recent introgressive hybridization and plastid captures might contribute to plastome geographic patterns and inconsistencies between topologies built from different datasets, while incomplete lineage sorting could account for the discordance between maternal topology and the previous nuclear genome data-based phylogeny. Our findings highlight full-length plastomes as an ideal tool for exploring maternal relationships among the subclades of Carya, and potentially in other outcrossing perennial woody plants, for resolving plastome phylogenetic relationships.

Variation and Evolution of the Whole Chloroplast Genomes of Fragaria spp. (Rosaceae)

Species identification is vital for protecting species diversity and selecting high-quality germplasm resources. Wild Fragaria spp. comprise rich and excellent germplasm resources; however, the variation and evolution of the whole chloroplast (cp) genomes in the genus Fragaria have been ignored. In the present study, 27 complete chloroplast genomes of 11 wild Fragaria species were sequenced using the Illumina platform. Then, the variation among complete cp genomes of Fragaria was analyzed, and phylogenetic relationships were reconstructed from those genome sequences. There was an overall high similarity of sequences, with some divergence. According to analysis with mVISTA, non-coding regions were more variable than coding regions. Inverted repeats (IRs) were observed to contract or expand to different degrees, which resulted in different sizes of cp genomes. Additionally, five variable loci, trnS-trnG, trnR-atpA, trnC-petN, rbcL-accD, and psbE-petL, were identified that could be used to develop DNA barcoding for identification of Fragaria species. Phylogenetic analyses based on the whole cp genomes supported clustering all species into two groups (A and B). Group A species were mainly distributed in western China, while group B contained several species from Europe and Americas. These results support allopolyploid origins of the octoploid species F. chiloensis and F. virginiana and the tetraploid species F. moupinensis and F. tibetica. The complete cp genomes of these Fragaria spp. provide valuable information for selecting high-quality Fragaria germplasm resources in the future.

Comparative Chloroplast Genomics and Phylogenetic Analysis of Zygophyllum (Zygophyllaceae) of China

The genus Zygophyllum comprises over 150 species within the plant family Zygophyllaceae. These species predominantly grow in arid and semiarid areas, and about 20 occur in northwestern China. In this study, we sampled 24 individuals of Zygophyllum representing 15 species and sequenced their complete chloroplast (cp) genomes. For comparison, we also sequenced cp genomes of two species of Peganum from China representing the closely allied family, Nitrariaceae. The 24 cp genomes of Zygophyllum were smaller and ranged in size from 104,221 to 106,286 bp, each containing a large single-copy (LSC) region (79,245-80,439 bp), a small single-copy (SSC) region (16,285-17,146 bp), and a pair of inverted repeat (IR) regions (3,792-4,466 bp). These cp genomes contained 111-112 genes each, including 74-75 protein-coding genes (PCGs), four ribosomal RNA genes, and 33 transfer RNA genes, and all cp genomes showed similar gene order, content, and structure. The cp genomes of Zygophyllum appeared to lose some genes such as ndh genes and rRNA genes, of which four rRNA genes were in the SSC region, not in the IR regions. However, the SC and IR regions had greater similarity within Zygophyllum than between the genus and Peganum. We detected nine highly variable intergenic spacers: matK-trnQ, psaC-rps15, psbZ-trnG, rps7-trnL, rps15-trnN, trnE-trnT, trnL-rpl32, trnQ-psbK, and trnS-trnG. Additionally, we identified 156 simple sequence repeat (cpSSR) markers shared among the genomes of the 24 Zygophyllum samples and seven cpSSRs that were unique to the species of Zygophyllum. These markers may be useful in future studies on genetic diversity and relationships of Zygophyllum and closely related taxa. Using the sequenced cp genomes, we reconstructed a phylogeny that strongly supported the division of Chinese Zygophyllum into herbaceous and shrubby clades. We utilized our phylogenetic results along with prior morphological studies to address several remaining taxonomic questions within Zygophyllum. Specifically, we found that Zygophyllum kaschgaricum is included within Zygophyllum xanthoxylon supporting the present treatment of the former genus Sarcozygium as a subgenus within Zygophyllum. Our results provide a foundation for future research on the genetic resources of Zygophyllum.

Decoding first complete chloroplast genome of toothbrush tree (Salvadora persica L.): insight into genome evolution, sequence divergence and phylogenetic relationship within Brassicales

BACKGROUND

Salvadora persica L. (Toothbrush tree - Miswak; family-Salvadoraceae) grows in the arid-land ecosystem and possesses economic and medicinal importance. The species, genus and the family have no genomic datasets available specifically on chloroplast (cp) genomics and taxonomic evolution. Herein, we have sequenced the complete chloroplast genome of S. persica for the first time and compared it with 11 related specie's cp genomes from the order Brassicales.

RESULTS

The S. persica cp genome was 153,379 bp in length containing a sizeable single-copy region (LSC) of 83,818 bp which separated from the small single-copy region (SSC) of 17,683 bp by two inverted repeats (IRs) each 25,939 bp. Among these genomes, the largest cp genome size (160,600 bp) was found in M. oleifera, while in S. persica it was the smallest (153,379 bp). The cp genome of S. persica encoded 131 genes, including 37 tRNA genes, eight rRNA genes and 86 protein-coding genes. Besides, S. persica contains 27 forward, 36 tandem and 19 palindromic repeats. The S. persica cp genome had 154 SSRs with the highest number in the LSC region. Complete cp genome comparisons showed an overall high degree of sequence resemblance between S. persica and related cp genomes. Some divergence was observed in the intergenic spaces of other species. Phylogenomic analyses of 60 shared genes indicated that S. persica formed a single clade with A. tetracantha with high bootstrap values. The family Salvadoraceae is closely related to Capparaceae and Petadiplandraceae rather than to Bataceae and Koberliniacaea.

CONCLUSION

The current genomic datasets provide pivotal genetic resources to determine the phylogenetic relationships, genome evolution and future genetic diversity-related studies of S. persica in complex angiosperm families.

Comparative analysis of the complete plastid genomes of Mangifera species and gene transfer between plastid and mitochondrial genomes

Mango is an important commercial fruit crop belonging to the genus Mangifera. In this study, we reported and compared four newly sequenced plastid genomes of the genus Mangifera, which showed high similarities in overall size (157,780–157,853 bp), genome structure, gene order, and gene content. Three mutation hotspots (trnG-psbZ, psbD-trnT, and ycf4-cemA) were identified as candidate DNA barcodes for Mangifera. These three DNA barcode candidate sequences have high species identification ability. We also identified 12 large fragments that were transferred from the plastid genome to the mitochondrial genome, and found that the similarity was more than 99%. The total size of the transferred fragment was 35,652 bp, accounting for 22.6% of the plastid genome. Fifteen intact chloroplast genes, four tRNAs and numerous partial genes and intergenic spacer regions were identified. There are many of these genes transferred from mitochondria to the chloroplast in other species genomes. Phylogenetic analysis based on whole plastid genome data provided a high support value, and the interspecies relationships within Mangifera were resolved well.

The History and Diversity of Rice Domestication as Resolved From 1464 Complete Plastid Genomes

The plastid is an essential organelle in autotrophic plant cells, descending from free-living cyanobacteria and acquired by early eukaryotic cells through endosymbiosis roughly one billion years ago. It contained a streamlined genome (plastome) that is uniparentally inherited and non-recombinant, which makes it an ideal tool for resolving the origin and diversity of plant species and populations. In the present study, a large dataset was amassed by de novo assembling plastomes from 295 common wild rice (Oryza rufipogon Griff.) and 1135 Asian cultivated rice (Oryza sativa L.) accessions, supplemented with 34 plastomes from other Oryza species. From this dataset, the phylogenetic relationships and biogeographic history of O. rufipogon and O. sativa were reconstructed. Our results revealed two major maternal lineages across the two species, which further diverged into nine well supported genetic clusters. Among them, the Or-wj-I/II/III and Or-wi-I/II genetic clusters were shared with cultivated (percentage for each cluster ranging 54.9%∼99.3%) and wild rice accessions. Molecular dating, phylogeographic analyses and reconstruction of population historical dynamics indicated an earlier origin of the Or-wj-I/II genetic clusters from East Asian with at least two population expansions, and later origins of other genetic clusters from multiple regions with one or more population expansions. These results supported a single origin of japonica rice (mainly in Or-wj-I/II) and multiple origins of indica rice (in all five clusters) for the history of rice domestication. The massive plastomic data set presented here provides an important resource for understanding the history and evolution of rice domestication as well as a genomic resources for use in future breeding and conservation efforts.

Plastomes of eight Ligusticum species: characterization, genome evolution, and phylogenetic relationships

BACKGROUND

The genus Ligusticum consists of approximately 60 species distributed in the Northern Hemisphere. It is one of the most taxonomically difficult taxa within Apiaceae, largely due to the varied morphological characteristics. To investigate the plastome evolution and phylogenetic relationships of Ligusticum, we determined the complete plastome sequences of eight Ligusticum species using a de novo assembly approach.

RESULTS

Through a comprehensive comparative analysis, we found that the eight plastomes were similar in terms of repeat sequence, SSR, codon usage, and RNA editing site. However, compared with the other seven species, L. delavayi exhibited striking differences in genome size, gene number, IR/SC borders, and sequence identity. Most of the genes remained under the purifying selection, whereas four genes showed relaxed selection, namely ccsA, rpoA, ycf1, and ycf2. Non-monophyly of Ligusticum species was inferred from the plastomes and internal transcribed spacer (ITS) sequences phylogenetic analyses.

CONCLUSION

The plastome tree and ITS tree produced incongruent tree topologies, which may be attributed to the hybridization and incomplete lineage sorting. Our study highlighted the advantage of plastome with mass informative sites in resolving phylogenetic relationships. Moreover, combined with the previous studies, we considered that the current taxonomy system of Ligusticum needs to be improved and revised. In summary, our study provides new insights into the plastome evolution, phylogeny, and taxonomy of Ligusticum species.

Characterization of Chloroplast Genomes From Two Salvia Medicinal Plants and Gene Transfer Among Their Mitochondrial and Chloroplast Genomes

Salvia species have been widely used as medicinal plants and have played an important role in the treatment and recovery of individuals with COVID-19. In this study, we reported two newly identified whole chloroplast genome sequences of Salvia medicinal plants (Salvia yangii and Salvia miltiorrhiza f. alba) and compared them with those of seven other reported Salvia chloroplast genomes. These were proven to be highly similar in terms of overall size, genome structure, gene content, and gene order. We identified 10 mutation hot spots (trnK-rps16, atpH-atpI, psaA-ycf3, ndhC-trnV, ndhF, rpl32-trnL, ndhG-ndhI, rps15-ycf1, ycf1a, and ycf1b) as candidate DNA barcodes for Salvia. Additionally, we observed the transfer of nine large-sized chloroplast genome fragments, with a total size of 49,895 bp (accounting for 32.97% of the chloroplast genome), into the mitochondrial genome as they shared >97% sequence similarity. Phylogenetic analyses of the whole chloroplast genome provided a high resolution of Salvia. This study will pave the way for the identification and breeding of Salvia medicinal plants and further phylogenetic evolutionary research on them as well.

The large single-copy (LSC) region functions as a highly effective and efficient molecular marker for accurate authentication of medicinal Dendrobium species

Having great medicinal values, Dendrobium species of "Fengdou" (DSFs) are a taxonomically complex group in Dendrobium genus including many closely related and recently diverged species. Traditionally used DNA markers have been proved to be insufficient in authenticating many species of this group. Here, we investigated 101 complete plastomes from 23 DSFs, comprising 72 newly sequenced and 29 documented, which all exhibited well-conserved genomic organization and gene order. Plastome-wide comparison showed the co-occurrence of single nucleotide polymorphisms (SNPs) and insertions/deletions (indels), which can be explained by both the repeat-associated and indel-associated mutation hypotheses. Moreover, guanine-cytosine (GC) content was found to be negatively correlated with the three divergence variables (SNPs, indels and repeats), indicating that GC content may reflect the level of the local sequence divergence. Our species authentication analyses revealed that the relaxed filtering strategies of sequence alignment had no negative impact on species identification. By assessing the maximum likelihood (ML) trees inferred from different datasets, we found that the complete plastome and large single-copy (LSC) datasets both successfully identified all 23 DSFs with the maximum bootstrap values. However, owing to the high efficiency of LSC in species identification, we recommend using LSC for accurate authentication of DSFs.

Comparative Chloroplast Genomics of Fritillaria (Liliaceae), Inferences for Phylogenetic Relationships between Fritillaria and Lilium and Plastome Evolution

Fritillaria is a genus that has important medicinal and horticultural values. The study involved the most comprehensive chloroplast genome samples referring to Old and New World clades of Fritillaria for marker selection and phylogenetic studies. We reported and compared eleven newly sequenced whole-plastome sequences of Fritillaria which proved highly similar in overall size (151,652–152,434 bp), genome structure, gene content, and order. Comparing them with other species of Liliales (6 out of 10 families) indicated the same similarity but showed some structural variations due to the contraction or expansion of the inverted repeat (IR) regions. A/T mononucleotides, palindromic, and forward repeats were the most common types. Six hypervariable regions (rps16-trnQ, rbcL-accD, accD-psaI, psaJ-rpl33, petD-rpoA, and rpl32-trnL) were discovered based on 26 Fritillaria whole-plastomes to be potential molecular markers. Based on the plastome data that were collected from 26 Fritillaria and 21 Lilium species, a phylogenomic study was carried out with three Cardiocrinum species as outgroups. Fritillaria was sister to Lilium with a high support value, and the interspecies relationships within subgenus Fritillaria were resolved very well. The six hypervariable regions can be used as candidate DNA barcodes of Fritillaria and the phylogenomic framework can guide extensive genomic sampling for further phylogenetic analyses.

Complete chloroplast genome sequences of four Allium species: comparative and phylogenetic analyses

The genus Allium is one of the largest monocotyledonous genera, containing over 850 species, and most of these species are found in temperate climates of the Northern Hemisphere. Furthermore, as a large number of new Allium species continue to be identified, phylogenetic classification based on morphological characteristics and a few genetic markers will gradually exhibit extremely low discriminatory power. In this study, we present the use of complete chloroplast genome sequences in genome-scale phylogenetic studies of Allium. We sequenced and assembled four Allium chloroplast genomes and retrieved five published chloroplast genomes from GenBank. All nine chloroplast genomes were used for genomic comparison and phylogenetic inference. The chloroplast genomes, ranging from 152,387 bp to 154,482 bp in length, exhibited conservation of genomic structure, and gene organization and order. Subsequently, we observed the expansion of IRs from the basal monocot Acorus americanus to Allium, identified 814 simple sequence repeats, 131 tandem repeats, 154 dispersed repeats and 109 palindromic repeats, and found six highly variable regions. The phylogenetic relationships of the Allium species inferred from the chloroplast genomes obtained high support, indicating that chloroplast genome data will be useful for further resolution of the phylogeny of the genus Allium.

Interspecific delimitation and relationships among four Ostrya species based on plastomes

BACKGROUND

The genus Ostrya (Betulaceae) contains eight species and four of them are distributed in China. However, studies based on limited informative sites of several chloroplast markers failed to resolve interspecific delimitation and relationships among the four Chinese species. In this study, we aimed to use the whole chloroplast genomes to address these two issues.

RESULTS

We assembled and annotated 33 complete chloroplast genomes (plastomes) of the four Chinese species, representing 17 populations across most of their geographical distributions. Each species contained samples of several individuals that cover most of geographic distributions of the species. All plastomes are highly conserved in genome structure and gene order, with a total length of 158-159 kb and 122 genes. Phylogenetic analyses of whole plastomes, non-coding regions and protein-coding genes produced almost the same topological relationships. In contrast to the well-delimitated species boundary inferred from the nuclear ITS sequence variations, three of the four species are non-monophyletic in the plastome trees, which is consistent with previous studies based on a few chloroplast markers.

CONCLUSIONS

The high incongruence between the ITS and plastome trees may suggest the widespread occurrences of hybrid introgression and incomplete lineage sorting during the divergence of these species. In addition, the plastomes with more informative sites compared with a few chloroplast markers still failed to resolve the phylogenetic relationships of the four species, and further studies involving population genomic data may be needed to better understand their evolutionary histories.

Species delimitation and interspecific relationships of the endangered herb genus Notopterygium inferred from multilocus variations

Species identification and discrimination is the basis of biodiversity research. In general, it is considered that numerous nucleotide variations (e.g., whole chloroplast genomes) can identify species with higher resolution than a few loci, e.g., partial chloroplast or nuclear gene fragments. In this study, we tested this hypothesis by sampling population genetics samples of the endangered herb genus Notopterygium. We sequenced the complete plastomes, five nuclear gene regions, three chloroplast DNA fragments, and a nuclear internal transcribed spacer (nrITS) region for 18 populations sampled throughout most of the geographic ranges of all six Notopterygium species. Species identification analysis showed that four DNA barcodes (matK, rbcL, trnS-trnG, and nrITS) and/or combinations of these markers achieved Notopterygium species discrimination at higher resolution than the general plastomes and nuclear gene sequences. In particular, nrITS had the highest discriminatory power among all of the individual markers. Molecular data sets and morphological evidence indicated that all six Notopterygium species could be reclassified unambiguously to four putative species clades. N. oviforme and N. franchetii had the closest relationship. Molecular dating showed that the origin and divergence of Notopterygium species was significantly associated with geological and climatic fluctuations during the middle of the Pliocene. In conclusion, our results suggest that a few nucleotide variations can achieve species discrimination with higher resolution than numerous plastomes and general nuclear gene fragments when discerning related Notopterygium species.

Accurate authentication of Dendrobium officinale and its closely related species by comparative analysis of complete plastomes

Owing to its great medicinal and ornamental values, Dendrobium officinale is frequently adulterated with other Dendrobium species on the market. Unfortunately, the utilization of the common DNA markers ITS, ITS2, and matK+rbcL is unable to distinguish D. officinale from 5 closely related species of it (D. tosaense, D. shixingense, D. flexicaule, D. scoriarum and D. aduncum). Here, we compared 63 Dendrobium plastomes comprising 40 newly sequenced plastomes of the 6 species and 23 previously published plastomes. The plastomes of D. officinale and its closely related species were shown to have conserved genome structure and gene content. Comparative analyses revealed that small single copy region contained higher variation than large single copy and inverted repeat regions, which was mainly attributed to the loss/retention of ndh genes. Furthermore, the intraspecific sequence variability among different Dendrobium species was shown to be diversified, which necessitates a cautious evaluation of genetic markers specific for different Dendrobium species. By evaluating the maximum likelihood trees inferred from different datasets, we found that the complete plastome sequence dataset had the highest discriminatory power for D. officinale and its closely related species, indicating that complete plastome sequences can be used to accurately authenticate Dendrobium species.

Plastomes of nine hornbeams and phylogenetic implications

Poor phylogenetic resolution and inconsistency of gene trees are major complications when attempting to construct trees of life for various groups of organisms. In this study, we addressed these issues in analyses of the genus Carpinus (hornbeams) of the Betulaceae. We assembled and annotated the chloroplast (cp) genomes (plastomes) of nine hornbeams representing main clades previously distinguished in this genus. All nine plastomes are highly conserved, with four regions, and about 158-160 kb long, including 121-123 genes. Phylogenetic analyses of whole plastome sequences, noncoding sequences, and the well-aligned coding genes resulted in high resolution of the sampled species in contrast to the failure based on a few cpDNA markers. Phylogenetic relationships in a few clades based only on the coding genes are slightly inconsistent with those based on the noncoding and total plastome datasets. Moreover, these plastome trees are highly incongruent with those based on bi-parentally inherited internal transcribed spacer (ITS) sequence variations. Such high inconsistencies suggest widespread occurrence of incomplete lineage sorting and hybrid introgression during diversification of these hornbeams.

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.

Taxonomy in the Kunming Institute of Botany (KIB): Progress during the past decade (2008-2018) and perspectives on future development

The development of new taxonomical theories and approaches, particularly molecular phylogenetics, has led to the expansion of traditional morphology-based taxonomy into the concept of "integrative taxonomy." Taxonomic knowledge has assumed greater significance in recent years, particularly because of growing concerns over the looming biodiversity crisis. Since its establishment in 1938, the Kunming Institute of Botany (KIB), which is located in Yunnan province in Southwest China, has focused attention on the taxonomy and conservation of the flora of China. For the forthcoming 80th anniversary of KIB, we review the achievements of researchers at KIB and their associates with respect to the taxonomy of land plants, fungi, and lichen. Major taxonomic advances are summarized for families of Calymperaceae, Cryphaeaceae, Lembophyllaceae, Neckeraceae, Polytrichaceae and Pottiaceae of mosses, Pteridaceae and Polypodiaceae of ferns, Taxaceae and Cycadaceae of gymnosperms, Asteraceae, Begoniaceae, Ericaceae, Euphorbiaceae, Gesneriaceae, Lamiaceae, Orchidaceae, Orobanchaceae, Poaceae, Theaceae and Urticaceae of angiosperms, Agaricaceae, Amanitaceae, Boletaceae, Cantharellaceae, Physalacriaceae Russulaceae, Suillaceae and Tuberaceae of fungi, and Ophioparmaceae and Parmeliaceae of lichens. Regarding the future development of taxonomy at KIB, we recommend that taxonomists continue to explore the biodiversity of China, integrate new theories and technologies with traditional taxonomic approaches, and engage in creative monographic work, with support from institutions, funding agencies, and the public.

Bioinformatic Workflows for Generating Complete Plastid Genome Sequences-An Example from Cabomba (Cabombaceae) in the Context of the Phylogenomic Analysis of the Water-Lily Clade

The sequencing and comparison of plastid genomes are becoming a standard method in plant genomics, and many researchers are using this approach to infer plant phylogenetic relationships. Due to the widespread availability of next-generation sequencing, plastid genome sequences are being generated at breakneck pace. This trend towards massive sequencing of plastid genomes highlights the need for standardized bioinformatic workflows. In particular, documentation and dissemination of the details of genome assembly, annotation, alignment and phylogenetic tree inference are needed, as these processes are highly sensitive to the choice of software and the precise settings used. Here, we present the procedure and results of sequencing, assembling, annotating and quality-checking of three complete plastid genomes of the aquatic plant genus Cabomba as well as subsequent gene alignment and phylogenetic tree inference. We accompany our findings by a detailed description of the bioinformatic workflow employed. Importantly, we share a total of eleven software scripts for each of these bioinformatic processes, enabling other researchers to evaluate and replicate our analyses step by step. The results of our analyses illustrate that the plastid genomes of Cabomba are highly conserved in both structure and gene content.

Interspecific Divergence of Two Sinalliaria (Brassicaceae) Species in Eastern China

How endemic species originated in eastern Asia has interested botanists for a long time. In this study, we combined experimental and computational modeling approaches to examine the morphological and genetic divergence and reproductive isolation of two tentative species of Sinalliaria (Brassicaceae) endemic to eastern China, S. limprichtiana and S. grandifolia. Most of the examined morphological characters (including hairs of leaf blades and stems, corolla length and width, and flower stalk length) were well-delineated between two species at the same ploidy level, and there was clear evidence of reproductive isolation between them (mainly due to post-pollination barriers) in the common garden environment. There were also strong and consistent divergences in the population genetic data. Coalescent simulations based on sequence variation of the nuclear genes suggest that interspecific divergence began during the Pleistocene when the climate oscillated in eastern Asia. Gene flow between two species appears to have been very limited and asymmetrical. Our results suggested that both species are well-differentiated and that the fast divergence between them might have been together shaped by both stochastic processes and habitat selection pressures.