Chloroplast genomes in Populus (Salicaceae): comparisons from an intensively sampled genus reveal dynamic patterns of evolution

The population chloroplast genomes of Populus reveal the phylogenetic relationship between three new taxa of sect. Leucoides and their parents

BACKGROUND

Poplars are important woody plants, which are widely distributed in the forests from the subtropics to the north of the Northern Hemisphere. Poplars have high ecological and economic value. However, there are frequent interspecific and intraspecific hybrids in Populus, resulting in a large number of intermediate taxa, which makes the morphological identification of Populus very challenging. Plastid genome is an important tool to study the evolutionary relationship of plants. Therefore, comparison and phylogenetic analysis were carried out based on the population chloroplast genomes of 34 individuals from 7 taxa.

RESULTS

In this study, seven newly assembled and annotated chloroplast genomes of Populus were reported. They all had typical quadripartite structures with the same GC content (37.6%), but there were differences within the population, and the genome size ranged from 155,736 bp to 156,812 bp. In all Populus species, 134 genes were identified, including 88 protein coding genes (PCGs), 37 tRNA and 8 rRNA genes. The gene sequences alignment of different taxa showed that the gene sequences and content were relatively conservative, there was no gene rearrangement, and only 3 highly variable regions (psbZ-trnG, ndhC-trnV and trnN-trnR) were identified, which can be used as molecular markers. Most PCGs had high codon usage bias and 3 positive selection genes (rps7, rps12 and rpl16) have been identified. The analysis of population genetic structure and phylogeny showed that the chloroplast genomes supported that Populus was a monophyletic taxon, which could be divided into four sections (Abaso, Turanga, Populus and ATL (Aigeiros, Tacamahaca and Leucoides)). Among them, P. dafengensis, P. butuoensis and P. szechuanica had the closest genetic relationship, P. gonggaensis and P. cathayana had the closest genetic relationship, it was speculated that the taxa of Sect. Tacamahaca may be the main female parent of the three new taxa from Sect. Leucoides.

CONCLUSION

In general, this study provides valuable insights for new species identification, phylogenetic relationships, breeding and resource development, and genetic diversity of Populus.

Species delimitation in the Populus laurifolia complex (Salicaceae) based on phylogenetic and morphometric evidence

Due to significant morphological differences and extensive interspecific hybridization, there are numerous species complexes with taxonomic challenges in the genus Populus. Integrative taxonomy, which combines evidence of morphology, molecular phylogeny, niche differentiation, and reproductive isolation, provides the most effective approaches for species delimitation. The Populus laurifolia complex, which belongs to Populus subg. Tacamahaca (Salicaceae), is distributed in the Altai Mountains and Tianshan Mountains. This complex exhibits morphological variability, making species delimitation challenging. Due to limited sampling and systematic studies, its taxonomy has remained unresolved. In this study, 337 specimens, along with online digital samples representing nearly all wild populations, were collected. Morphological analyses were performed to evaluate key traits and clarify species boundaries. Phylogenetic relationships were reconstructed using concatenation and coalescent methods based on 566,375 nuclear single-nucleotide polymorphisms (SNPs). Ecological niche differentiation was assessed, and ABBA-BABA analysis was used to examine interspecific hybridization. The results revealed that this complex, based on a series of significant character states, could be morphologically distinguished into three species-P. laurifolia (Populus pilosa considered a synonym of P. laurifolia), Populus talassica, and Populus pamirica-which also correspond to three well-supported clades in the phylogenetic trees. P. pamirica exhibits some degree of ecological niche differentiation from P. talassica and P. laurifolia, whereas the latter two show minimal differentiation. Gene flow within the complex remains limited. This research underscores the importance of integrating multiple lines of evidence in the classification of Populus, providing a framework for future taxonomic studies.

Insights into Aquilaria phylogenetics through comparative plastomic resources

The plastid is an essential organelle for its role in photosynthesis and energy production and its genomic information is always employed as important evolutionary markers to explore the relationship among species. Agarwood (Aquilaria), prized for its aromatic blend, finds extensive use in various cultures as incense and perfume. Despite its high economic importance, the phylogenetic status among Aquilaria based on plastomes remains ambiguous due to the lack of available plastomic resources. To bridge this knowledge gap, 22 Aquilaria plastomes were newly sequenced, similar variation patterns in this genus were determined, including a shared 16 bp extension of the rps19 gene and seven highly variable regions. The analysis highlighted the highest prevalence of the A/T motif among simple sequence repeats in these plastomes. Further phylogenetic analysis revealed Aquilaria's phylogenetic implications with an expanded dataset. This comprehensive plastomic resource not only enhances our understanding of Aquilaria evolution but also presents potential molecular markers for DNA barcoding.

Chloroplast genome analysis and evolutionary insights in the versatile medicinal plant Calendula officinalis L

Calendula officinalis L.is a versatile medicinal plant with numerous applications in various fields. However, its chloroplast genome structure, features, phylogeny, and patterns of evolution and mutation remain largely unexplored. This study examines the chloroplast genome, phylogeny, codon usage bias, and divergence time of C. officinalis, enhancing our understanding of its evolution and adaptation. The chloroplast genome of C. officinalis is a 150,465 bp circular molecule with a G + C content of 37.75% and comprises 131 genes. Phylogenetic analysis revealed a close relationship between C. officinalis, C. arvensis, and Osteospermum ecklonis. A key finding is the similarity in codon usage bias among these species, which, coupled with the divergence time analysis, supports their close phylogenetic proximity. This similarity in codon preference and divergence times underscores a parallel evolutionary adaptation journey for these species, highlighting the intricate interplay between genetic evolution and environmental adaptation in the Asteraceae family. Moreover unique evolutionary features in C. officinalis, possibly associated with certain genes were identified, laying a foundation for future research into the genetic diversity and medicinal value of C. officinalis.

Plastid genome of Passiflora tripartita var. mollissima (poro-poro) from Huánuco, Peru

Passiflora tripartita var. mollissima, known locally as poro-poro, is an important native fruit used in traditional Peruvian medicine with relevant agro-industrial and pharmaceutical potential for its antioxidant capacity for human health. However, to date, only a few genetic data are available, which limits exploring its genetic diversity and developing new genetic studies for its improvement. We report the poro-poro plastid genome to expand the knowledge of its molecular markers, evolutionary studies, molecular pathways, and conservation genetics. The complete chloroplast (cp) genome is 163,451 bp in length with a typical quadripartite structure, containing a large single-copy region of 85,525 bp and a small single-copy region of 13,518 bp, separated by a pair of inverted repeat regions (IR) of 32,204 bp, and the overall GC content was 36.87%. This cp genome contains 128 genes (110 genes were unique and 18 genes were found duplicated in each IR region), including 84 protein-coding genes, 36 transfer RNA-coding genes, eight ribosomal RNA-coding genes, and 13 genes with introns (11 genes with one intron and two genes with two introns). The inverted repeat region boundaries among species were similar in organization, gene order, and content, with a few revisions. The phylogenetic tree reconstructed based on single-copy orthologous genes and maximum likelihood analysis demonstrates poro-poro is most closely related to Passiflora menispermifolia and Passiflora oerstedii. In summary, our study constitutes a valuable resource for studying molecular evolution, phylogenetics, and domestication. It also provides a powerful foundation for conservation genetics research and plant breeding programs. To our knowledge, this is the first report on the plastid genome of Passiflora tripartita var. mollissima from Peru.

Comparative Analysis of the Chloroplast Genome of Sicyos angulatus with Other Seven Species of Cucurbitaceae Family

Sicyos angulatus (SA) is an annual plant from the Cucurbitaceae family that is native to the eastern part of North America. This study aims to assemble and annotate the chloroplast genome of S. angulatus, and then compare it with plastomes of the other species representing the Cucurbitaceae family. The chloroplast genome size of S. angulatus is 154,986 bp, including a pair of inverted repeats (IR) of 26,276 bp, and small single-copy region (SSC) of 18,079 bp and large single-copy region (LSC) of 84,355 bp. Compared to other Cucurbitaceae species, the chloroplast genome of S. angulatus is almost 4222 bp smaller than the plastome Gynostemma pentaphyllum. All other seven species have an identical set of tRNA (37), except Citrullus laevigata, which contains 36 tRNA. The IRa/LSC junction in all eight species is located upstream of rpl2 and downstream of trnH gene. Moreover, variation in the size of the gene and the presence of pseudogene ycf1 has been seen because of the IR contraction and expansion. The highest number of tandem repeats was seen in G. pentaphyllum, and then Corynocarpus leavigata. The sequence divergence analysis and topology of the phylogenetic tree indicate that S. angulatus is more similar to genus Citrullus as compared to genus Gynostemma. These findings contribute to developing the genomic marker for the purpose of future genetic studies.

Comparative Plastomes of Curcuma alismatifolia (Zingiberaceae) Reveal Diversified Patterns among 56 Different Cut-Flower Cultivars

Curcuma alismatifolia (Zingiberaceae) is an ornamental species with high economic value due to its recent rise in popularity among floriculturists. Cultivars within this species have mixed genetic backgrounds from multiple hybridization events and can be difficult to distinguish via morphological and histological methods alone. Given the need to improve identification resources, we carried out the first systematic study using plastomic data wherein genomic evolution and phylogenetic relationships from 56 accessions of C. alismatifolia were analyzed. The newly assembled plastomes were highly conserved and ranged from 162,139 bp to 164,111 bp, including 79 genes that code for proteins, 30 tRNA genes, and 4 rRNA genes. The A/T motif was the most common of SSRs in the assembled genomes. The Ka/Ks values of most genes were less than 1, and only two genes had Ka/Ks values above 1, which were rps15 (1.15), and ndhl (1.13) with petA equal to 1. The sequence divergence between different varieties of C. alismatifolia was large, and the percentage of variation in coding regions was lower than that in the non-coding regions. Such data will improve cultivar identification, marker assisted breeding, and preservation of germplasm resources.

The first report describes features of the chloroplast genome of Withania frutescens

Genomic studies not only help researcher not only to identify genomic features in organisms, but also facilitate understanding of evolutionary relationships. Species in the Withania genus have medicinal benefits, and one of them is Withania frutescens, which is used to treat various diseases. This report investigates the nucleotides and genic features of chloroplast genome of Withania frutescens and trying to clarify the evolutionary relationship with Withania sp and family Solanaceae. We found that the total size of Withania frutescens chloroplast genome was 153.771 kb (the smallest chloroplast genome in genus Withania). A large single-copy region (91.285 kb), a small single-copy region (18.373 kb) form the genomic region, and are distinct from each other by a large inverted repeat (22.056 kb). 137 chloroplast genes are found including 4 rRNAs, 38 tRNAs and 83 protein-coding genes. The Withania frutescens chloroplast genome as well as four closest relatives was compared for features such as structure, nucleotide composition, simple sequence repeats (SSRs) and codon bias. Compared to other Withania species, Withania frutescens has unique characteristics. It has the smallest chloroplast genome of any Withania species, isoleucine is the major amino acid, and tryptophan is the minor, In addition, there are no ycf3 and ycf4 genes, fourth, there are only fifteen replicative genes, while in most other species there are more. Using fast minimum evolution and neighbor joining, we have reconstructed the trees to confirm the relationship with other Solanacaea species. The Withania frutescens chloroplast genome is submitted under accession no. ON153173.

A comparative plastomic analysis of Ziziphus jujuba var. spinosa (Bunge) Hu ex H. F. Chow and implication of the origin of Chinese jujube

Comparative plastomics can be used to explicitly dissect various types of plastome variation. In the present study, the plastome variation pattern of Ziziphus jujuba var. spinosa (also called sour jujube) and its phylogenomic relationship with Chinese jujube were investigated. Plastomes of 21 sour jujube individuals were sequenced and assembled. The length of the sour jujube plastomes ranged between 159399 and 161279 bp. The plastomes exhibited collinearity of structure, gene order and content. The most divergent regions were located in the intergenic spacers, such as trnR-UCU-atpA and psbZ-trnG-UCC. Sliding window analysis demonstrated that the sequence variation among the sour jujube plastomes was relatively low. Sixty-two to 76 SSRs with 4 motif types were identified in the sour jujube plastomes with a predominant motif type of A/T. Three protein-coding genes exhibited higher nonsynonymous/synonymous substitution ratios, indicating that these genes may undergo positive selection. A total of 80 SNPs were detected and 1266 potential RNA editing sites of 23 protein-coding genes were predicted. In the phylogenomic tree constructed, sour jujube has a sister relationship to Chinese jujube, which indicates that Chinese jujube may have originated or been domesticated from sour jujube. The present study explicitly investigated the individual-level plastome variation of sour jujube and provides potential valuable molecular markers for future genetic-related study of this lineage.

The complete chloroplast genome sequence of Cyathula officinalis and comparative analysis with four related species

Cyathula officinalis is a medicinal and edible herb, which can remove blood stasis, stimulate menstrual flow, and ease joint movement. In this study, the complete chloroplast genome of Cyathula officinalis was sequenced, assembled, and analyzed. Compared with the chloroplast genomes of Cyathula capitata, Achyranthes bidentata, Achyranthes longifolianine and Achyranthes aspera, the basic characteristics, codon usage bias, repeat sequences, simple sequence repeats, and phylogenetic tree were analyzed. In addition, according to nucleotide diversity analysis and sequence alignment, DNA barcoding and allele-specific PCR primers were designed to identify and distinguish Cyathula officinalis from its fake drugs, which has effectively practical significance for the authentication of "Chuan Niuxi" crude drug in the market.

Comparative Analyses of 35 Complete Chloroplast Genomes from the Genus Dalbergia (Fabaceae) and the Identification of DNA Barcodes for Tracking Illegal Logging and Counterfeit Rosewood

The genus Dalbergia contains more than 200 species, several of which are trees that produce traditional medicines and extremely high-value timber commonly referred to as rosewood. Due to the rarity of these species in the wild, the high value of the timber, and a growing international illicit trade, CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) has listed the entire genus in appendix II and the species Dalbergia nigra in appendix I because species in this genus are considered at risk of extinction. Given this, and the fact that species or even genus level determination is nearly impossible from cut timber morphology, alternative molecular methods are needed to identify and track intercepted rosewood. To better identify rosewood using molecular methods, we sequenced and assembled eight chloroplast genomes including D. nigra as well as conducted comparative analyses with all other available chloroplast genomes in Dalbergia and closely related lineages. From these analyses, numerous repeats including simple sequence repeats (SSR) and conserved nucleotide polymorphisms unique to subclades within the genus were detected. From phylogenetic analysis based on the CDS from 77 chloroplast genes, the groups Siam rosewood and scented rosewood resolved as monophyletic, supporting the morphological traits used to delimit these species. In addition, several instances of paraphyly and polyphyly resulting from mismatches between taxonomic determinations and phylogenetic tree topology were identified. Ultimately, the highly variable regions in the chloroplast genomes will provide useful plastid markers for further studies regarding the identification, phylogeny, and population genetics of Dalbergia species, including those frequently intercepted in illegal trade.

An integrated study of Violae Herba (Viola philippica) and five adulterants by morphology, chemical compositions and chloroplast genomes: insights into its certified plant origin

BACKGROUND

Viola philippica Cav. is the only original plant for Violae Herba, as described in the Chinese Pharmacopoeia. The quality of this crude drug is affected by several adulterants from congeneric Viola species, and the authentic plant origin of Violae Herba is still controversial. Genome-based identification offers abundant genetic information and potential molecular markers that can be used for the authentication of closely related species. This study aims to investigate the certified origin of Violae Herba and to develop more effective markers for these easily confused species at the genetic level.

METHODS

We compared the morphology and chemical composition of 18 batches of commercial samples and six widespread medicinal Viola plants used as Violae Herba or its substitutes by TLC and HPLC-Triple-TOF-MS/MS analyses. The complete chloroplast genomes of these species were sequenced and analyzed, including the general features, repeat sequences, mutational hotspots and phylogeny. The complete chloroplast genomes used as superbarcodes and some specific barcodes screened from mutational hotspots were tested for their ability to distinguish Viola species.

RESULTS

A comparative study showed that Violae Herba is a multi-origin traditional Chinese medicine. Commercial decoction pieces and the standard reference drug were mainly derived from V. prionantha, clashing with the record in the Chinese Pharmacopoeia. Chloroplast genome analyses of V. philippica and five adulterants indicated that sequence divergence was relatively low within Viola species. By tree-based approaches, the complete chloroplast genomes showed a better discrimination ability and phylogenetic resolution for each Viola species. These results indicate that the whole chloroplast genomes can be used as superbarcodes to differentiate Viola medicinal plants. More specific DNA barcodes could be further developed from the Viola chloroplast genomes for more efficient and rapid identification of commercial Violae Herba and its adulterants.

CONCLUSIONS

This study has implications for chloroplast genome-based phylogenetic analysis and the authentication of multiple Viola species used as Violae Herba. The legal origin recorded in the Chinese Pharmacopoeia should be further revised to V. prionantha, in line with the commercial Violae Herba in the TCM markets.

Comparison of Magnoliaceae Plastomes: Adding Neotropical Magnolia to the Discussion

Chloroplast genomes are considered to be highly conserved. Nevertheless, differences in their sequences are an important source of phylogenetically informative data. Chloroplast genomes are increasingly applied in evolutionary studies of angiosperms, including Magnoliaceae. Recent studies have focused on resolving the previously debated classification of the family using a phylogenomic approach and chloroplast genome data. However, most Neotropical clades and recently described species have not yet been included in molecular studies. We performed sequencing, assembly, and annotation of 15 chloroplast genomes from Neotropical Magnoliaceae species. We compared the newly assembled chloroplast genomes with 22 chloroplast genomes from across the family, including representatives from each genus and section. Family-wide, the chloroplast genomes presented a length of about 160 kb. The gene content in all species was constant, with 145 genes. The intergenic regions showed a higher level of nucleotide diversity than the coding regions. Differences were higher among genera than within genera. The phylogenetic analysis in Magnolia showed two main clades and corroborated that the current infrageneric classification does not represent natural groups. Although chloroplast genomes are highly conserved in Magnoliaceae, the high level of diversity of the intergenic regions still resulted in an important source of phylogenetically informative data, even for closely related taxa.

Molecular Karyotyping on Populus simonii × P. nigra and the Derived Doubled Haploid

The molecular karyotype could represent the basic genetic make-up in a cell nucleus of an organism or species. A doubled haploid (DH) is a genotype formed from the chromosome doubling of haploid cells. In the present study, molecular karyotype analysis of the poplar hybrid Populus simonii × P. nigra (P. xiaohei) and the derived doubled haploids was carried out with labeled telomeres, rDNA, and two newly repetitive sequences as probes by fluorescence in situ hybridization (FISH). The tandem repeats, pPC349_XHY and pPD284_XHY, with high-sequence homology were used, and the results showed that they presented the colocalized distribution signal in chromosomes. For P. xiaohei, pPD284_XHY produced hybridizations in chromosomes 1, 5, 8, and 9 in the hybrid. The combination of pPD284_XHY, 45S rDNA, and 5S rDNA distinctly distinguished six pairs of chromosomes, and the three pairs of chromosomes showed a significant difference in the hybridization between homologous chromosomes. The repeat probes used produced similar FISH hybridizations in the DH; nevertheless, pPD284_XHY generated an additional hybridization site in the telomere region of chromosome 14. Moreover, two pairs of chromosomes showed differential hybridization distributions between homologous chromosomes. Comparisons of the distinguished chromosomes between hybrid and DH poplar showed that three pairs of chromosomes in the DH presented hybridization patterns that varied from those of the hybrid. The No. 8 chromosome in DH and one of the homologous chromosomes in P. xiaohei shared highly similar FISH patterns, which suggested the possibility of intact or mostly partial transfer of the chromosome between the hybrid and DH. Our study will contribute to understanding the genetic mechanism of chromosomal variation in P. xiaohei and derived DH plants.