Comparative studies on codon usage pattern of chloroplasts and their host nuclear genes in four plant species

Comparative genomics on chloroplasts of Rubus (Rosaceae)

Rubus, the largest genus in Rosaceae, contains over 1400 species that distributed in multiple habitats across the world, with high species diversity in the temperate regions of Northern Hemisphere. Multiple Rubus species are cultivated for their valuable fruits. However, the intrageneric classification and phylogenetic relationships are still poorly understood. In this study, we sequenced, assembled, and characterized 17 plastomes of Rubus, and conducted comparative genomics integrating with 47 previously issued plastomes of this genus. The 64 plastomes of Rubus exhibited typical quadripartite structure with sizes ranging from 155,144 to 156,700 bp, and contained 132 genes including 87 protein-coding genes, 37 tRNA genes and eight rRNA genes. All plastomes are conservative in the gene order, the frequency of different types of long repeats and simple sequence repeats (SSRs), the codon usage, and the selection pressure of protein-coding genes. However, there are also some differences in the Rubus plastomes, including slight contraction and expansion of the IRs, a variation in the numbers of SSRs and long repeats, and some genes in certain clades undergoing intensified or relaxed purifying selection. Phylogenetic analysis based on whole plastomes showed that the monophyly of Rubus was strongly supported and resolved it into six clades corresponding to six subgenera. Moreover, we identified 12 highly variable regions that could be potential molecular markers for phylogenetic, population genetic, and barcoding studies. Overall, our study provided insight into plastomic structure and sequence diversification of Rubus, which could be beneficial for future studies on identification, evolution, and phylogeny in this genus.

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.

Decoding the complete organelle genomic architecture of Stewartia gemmata: an early-diverging species in Theaceae

BACKGROUND

Theaceae, comprising 300 + species, holds significance in biodiversity, economics, and culture, notably including the globally consumed tea plant. Stewartia gemmata, a species of the earliest diverging tribe Stewartieae, is critical to offer insights into Theaceae's origin and evolutionary history.

RESULT

We sequenced the complete organelle genomes of Stewartia gemmata using short/long reads sequencing technologies. The chloroplast genome (158,406 bp) exhibited a quadripartite structure including the large single-copy region (LSC), a small single-copy region (SSC), and a pair of inverted repeat regions (IRs); 114 genes encoded 80 proteins, 30 tRNAs, and four rRNAs. The mitochondrial genome (681,203 bp) exhibited alternative conformations alongside a monocyclic structure: 61 genes encoding 38 proteins, 20 tRNAs, three rRNAs, and RNA editing-impacting genes, including ATP6, RPL16, COX2, NAD4L, NAD5, NAD7, and RPS1. Comparative analyses revealed frequent recombination events and apparent rRNA gene gains and losses in the mitochondrial genome of Theaceae. In organelle genomes, the protein-coding genes exhibited a strong A/U bias at codon endings; ENC-GC3 analysis implies selection-driven codon bias. Transposable elements might facilitate interorganelle sequence transfer. Phylogenetic analysis confirmed Stewartieae's early divergence within Theaceae, shedding light on organelle genome characteristics and evolution in Theaceae.

CONCLUSIONS

We studied the detailed characterization of organelle genomes, including genome structure, composition, and repeated sequences, along with the identification of lateral gene transfer (LGT) events and complexities. The discovery of a large number of repetitive sequences and simple sequence repeats (SSRs) has led to new insights into molecular phylogenetic markers. Decoding the Stewartia gemmata organellar genome provides valuable genomic resources for further studies in tea plant phylogenomics and evolutionary biology.

Comprehensive analysis of complete chloroplast genome sequence of Plantago asiatica L. (Plantaginaceae)

Plantago asiatica L. is a representative individual species of Plantaginaceae, whose high reputation is owed to its edible and medicinal values. However, the phylogeny and genes of the P. asiatica chloroplast have not yet been well described. Here we report the findings of a comprehensive analysis of the P. asiatica chloroplast genome. The P. asiatica chloroplast genome is 164,992 bp, circular, and has a GC content of 37.98%. The circular genome contains 141 genes, including 8 rRNAs, 38 tRNAs, and 95 protein-coding genes. Seventy-two simple sequence repeats are detected. Comparative chloroplast genome analysis of six related species suggests that a higher similarity exists in the coding region than the non-coding region, and differences in the degree of preservation is smaller between P. asiatica and Plantago depressa than among others. Our phylogenetic analysis illustrates P. asiatica has a relatively close relationship with P. depressa, which was also divided into different clades with Plantago ovata and Plantago lagopus in the genus Plantago. This analysis of the P. asiatica chloroplast genome contributes to an improved deeply understanding of the evolutionary relationships among Plantaginaceae.

Novel molecular markers for Taxodium breeding from the chloroplast genomes of four artificial Taxodium hybrids

Taxodium "Zhongshanshan" are a group of intraspecific Taxodium hybrids with superparental dominance and high ecological and economic value in southern China. Identifying the parentage of hybrids, especially the male parent, is critically important for genetic studies. However, the large nuclear genomes of members of the genus Taxodium pose a major challenge for the development of molecular markers. Here, we developed novel molecular markers by conducting a comparative analysis of the chloroplast genomes of four artificial Taxodium hybrids and their parents. The lengths of the whole chloroplast genome ranged from 131,942 to 132,128 bp, and the total guanine (GC) content of the chloroplast genomes ranged from 34.6% to 35.81%. A total of 120 unique genes were identified, including 83 protein-coding genes, 33 transfer RNAs, and four ribosomal RNAs. There were 69-71 simple sequence repeats were detected in the four hybrids. Phylogenetic analysis revealed that these hybrids clustered with their paternal parents. Similar findings were obtained by analysis of the GC content of protein-coding genes. Molecular markers were developed using the highly variable regions of the chloroplast genomes, and polymerase chain reaction (PCR) assays revealed that these markers were effective for identifying the male parents of these hybrids. Our findings indicate for the first time that the chloroplast genomes of Taxodium are paternally inherited. Generally, these molecular markers could facilitate breeding and genetic studies of Taxodium.

Comparative chloroplast genomics and insights into the molecular evolution of Tanaecium (Bignonieae, Bignoniaceae)

Species of Tanaecium (Bignonieae, Bignoniaceae) are lianas distributed in the Neotropics and centered in the Amazon. Members of the genus exhibit exceptionally diverse flower morphology and pollination systems. Here, we sequenced, assembled, and annotated 12 complete and four partial chloroplast genomes representing 15 Tanaecium species and more than 70% of the known diversity in the genus. Gene content and order were similar in all species of Tanaecium studied, with genome sizes ranging between 158,470 and 160,935 bp. Tanaecium chloroplast genomes have 137 genes, including 80-81 protein-coding genes, 37 tRNA genes, and four rRNA genes. No rearrangements were found in Tanaecium plastomes, but two different patterns of boundaries between regions were recovered. Tanaecium plastomes show nucleotide variability, although only rpoA was hypervariable. Multiple SSRs and repeat regions were detected, and eight genes were found to have signatures of positive selection. Phylogeny reconstruction using 15 Tanaecium plastomes resulted in a strongly supported topology, elucidating several relationships not recovered previously and bringing new insights into the evolution of the genus.

Comparative and phylogenetic analysis of complete chloroplast genomes from seven Neocinnamomum taxa (Lauraceae)

The genus Neocinnamomum is considered to be one of the most enigmatic groups in Lauraceae, mainly distributed in tropical and subtropical regions of Southeast Asia. The genus contains valuable oilseed and medicinal tree species. However, there are few studies on the genus Neocinnamomum at present, and its interspecific relationship is still unclear. In order to explore the genetic structure and evolutionary characteristics of the Neocinnamomum chloroplast genome and to resolve the species relationships within the genus, comparative genomic and phylogenetic analyses were performed on the whole chloroplast genome sequences of 51 samples representing seven Neocinnamomum taxa. The whole Neocinnamomum chloroplast genome size ranged from 150,753-150,956 bp, with a GC content of 38.8%-38.9%. A total of 128 genes were annotated within the Neocinnamomum chloroplast genome, including 84 protein coding genes, 8 rRNA genes, and 36 tRNA genes. Between 71-82 SSRs were detected, among which A/T base repeats were the most common. The chloroplast genome contained a total of 31 preferred codons. Three highly variable regions, trnN-GUU-ndhF, petA-psbJ, and ccsA-ndhD, were identified with Pi values > 0.004. Based on the whole chloroplast genome phylogenetic tree, the phylogenetic relationships among the seven Neocinnamomum taxa were determined. N. delavayi and N. fargesii were the most closely related species, and N. lecomtei was identified as the most basal taxon. In this study, the characteristics and sequence variation of the chloroplast genomes of seven Neocinnamomum taxa were revealed, and the genetic relationship among the species was clarified. The results of this study will provide a reference for subsequent molecular marker development and phylogenetic research of Neocinnamomum.

Comparative chloroplast genome analysis of Sambucus L. (Viburnaceae): inference for phylogenetic relationships among the closely related Sambucus adnata Wall. ex DC Sambucus javanica Blume

Sambucus L. is found in the family Viburnaceae (syn. Adoxaceae) and encompasses approximately 29 accepted species. The complex morphology of these species has caused continued confusion concerning their nomenclature, classification, and identification. Despite previous attempts to resolve taxonomic complexities in the Sambucus genus, there are still unclear phylogenetic relationships among several species. In this study, the newly obtained plastome of Sambucus williamsii Hance. as well as the populations of Sambucus canadensis L., Sambucus javanica Blume, and Sambucus adnata Wall. ex DC were sequenced, and their sizes, structural similarity, gene order, gene number, and guanine-cytosine (GC) contents were analyzed. The phylogenetic analyses were conducted using the whole chloroplast genomes and protein-coding genes (PCGs). The findings revealed that the chloroplast genomes of Sambucus species exhibited typical quadripartite double-stranded DNA molecules. Their lengths ranged from 158,012 base pairs (bp) (S. javanica) to 158,716 bp (S. canadensis L). Each genome comprised a pair of inverted repeats (IRs), which separated the large single-copy (LSC) and small single-copy (SSC) regions. In addition, the plastomes contained 132 genes, encompassing 87 protein-coding, 37 tRNA, and four rRNA genes. In the simple sequence repeat (SSR) analysis, A/T mononucleotides had the highest proportion, with the most repetitive sequences observed in S. williamsii. The comparative genome analyses showed high similarities in structure, order, and gene contents. The hypervariable regions in the studied chloroplast genomes were trnT-GGU, trnF-GAA, psaJ, trnL-UAG, ndhF, and ndhE, which may be used as candidate barcodes for species discrimination in Sambucus genus. Phylogenetic analyses supported the monophyly of Sambucus and revealed the separation of S. javanica and S. adnata populations. Sambucus chinensis Lindl. was nested within S. javanica in the same clade, collaborating their conspecific treatment. These outcomes indicate that the chloroplast genome of Sambucus plants is a valuable genetic resource for resolving taxonomic discrepancies at the lower taxonomic levels and can be applied in molecular evolutionary studies.

Comparative chloroplast genome analysis of Sambucus L. (Viburnaceae): inference for phylogenetic relationships among the closely related Sambucus adnata Wall. ex DC Sambucus javanica Blume

Sambucus L. is found in the family Viburnaceae (syn. Adoxaceae) and encompasses approximately 29 accepted species. The complex morphology of these species has caused continued confusion concerning their nomenclature, classification, and identification. Despite previous attempts to resolve taxonomic complexities in the Sambucus genus, there are still unclear phylogenetic relationships among several species. In this study, the newly obtained plastome of Sambucus williamsii Hance. as well as the populations of Sambucus canadensis L., Sambucus javanica Blume, and Sambucus adnata Wall. ex DC were sequenced, and their sizes, structural similarity, gene order, gene number, and guanine–cytosine (GC) contents were analyzed. The phylogenetic analyses were conducted using the whole chloroplast genomes and protein-coding genes (PCGs). The findings revealed that the chloroplast genomes of Sambucus species exhibited typical quadripartite double-stranded DNA molecules. Their lengths ranged from 158,012 base pairs (bp) (S. javanica) to 158,716 bp (S. canadensis L). Each genome comprised a pair of inverted repeats (IRs), which separated the large single-copy (LSC) and small single-copy (SSC) regions. In addition, the plastomes contained 132 genes, encompassing 87 protein-coding, 37 tRNA, and four rRNA genes. In the simple sequence repeat (SSR) analysis, A/T mononucleotides had the highest proportion, with the most repetitive sequences observed in S. williamsii. The comparative genome analyses showed high similarities in structure, order, and gene contents. The hypervariable regions in the studied chloroplast genomes were trnT-GGU, trnF-GAA, psaJ, trnL-UAG, ndhF, and ndhE, which may be used as candidate barcodes for species discrimination in Sambucus genus. Phylogenetic analyses supported the monophyly of Sambucus and revealed the separation of S. javanica and S. adnata populations. Sambucus chinensis Lindl. was nested within S. javanica in the same clade, collaborating their conspecific treatment. These outcomes indicate that the chloroplast genome of Sambucus plants is a valuable genetic resource for resolving taxonomic discrepancies at the lower taxonomic levels and can be applied in molecular evolutionary studies.

Codon Usage Analyses Reveal the Evolutionary Patterns among Plastid Genes of Saxifragales at a Larger-Sampling Scale

Saxifragales is a 15-family order of early-divergent Eudicots with a rich morphological diversity and an ancient rapid radiation. Codon usage bias (CUB) analyses have emerged as an essential tool for understanding the evolutionary dynamics in genes. Thus far, the codon utilization patterns had only been reported in four separate genera within Saxifragales. This study provides a comprehensive assessment of the codon manipulation based on 50 plastid genes, covering 11 constituent families at a larger sampling scale. Our results first showed a high preference for AT bases and AT-ending codons. We then used effective number of codons (ENC) to assess a range of codon bias levels in the plastid genes. We also detected high-informative intrafamilial differences of ENC in three families. Subsequently, parity rule 2 (PR2) plot analyses revealed both family-unique and order-shared bias patterns. Most importantly, the ENC plots and neutrality analyses collectively supported the dominant roles of selection in the CUB of Saxifragales plastid genes. Notably, the phylogenetic affinities inferred by both ML and BI methods were consistent with each other, and they all comprised two primary clades and four subclades. These findings significantly enhance our understanding of the evolutionary processes of the Saxifrage order, and could potentially inspire more CUB analyses at higher taxonomic levels.

Codon usage bias and genetic diversity in chloroplast genomes of Elaeagnus species (Myrtiflorae: Elaeagnaceae)

Codon usage bias (CUB) reveals the characteristics of species and can be utilized to understand their evolutionary relationship, increase the target genes' expression in the heterologous receptor plants, and further provide theoretic assistance for correlative study on molecular biology and genetic breeding. The chief aim of this work was to analyze the CUB in chloroplast (cp.) genes in nine Elaeagnus species to provide references for subsequent studies. The codons of Elaeagnus cp. genes preferred to end with A/T bases rather than with G/C bases. Most of the cp. genes were prone to mutation, while the rps7 genes were identical in sequences. Natural selection was inferred to have a powerful impact on the CUB in Elaeagnus cp. genomes, and their CUB was extremely strong. In addition, the optimal codons were identified in the nine cp. genomes based on the relative synonymous codon usage (RSCU) values, and the optimal codon numbers were between 15 and 19. The clustering analyses based on RSCU were contrasted with the maximum likelihood (ML)-based phylogenetic tree derived from coding sequences, suggesting that the t-distributed Stochastic Neighbor Embedding clustering method was more appropriate for evolutionary relationship analysis than the complete linkage method. Moreover, the ML-based phylogenetic tree based on the conservative matK genes and the whole cp. genomes had visible differences, indicating that the sequences of specific cp. genes were profoundly affected by their surroundings. Following the clustering analysis, Arabidopsis thaliana was considered the optimal heterologous expression receptor plant for the Elaeagnus cp. genes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01289-6.

Characterization of the Evolutionary Pressure on Anisodus tanguticus Maxim. with Complete Chloroplast Genome Sequence

Anisodus tanguticus Maxim. (Solanaceae), a traditional endangered Tibetan herb, is endemic to the Qinghai-Tibet Plateau. Here, we report the de novo assembled chloroplast (cp) genome sequences of A. tanguticus (155,765 bp). The cp contains a pair of inverted repeated (IRa and IRb) regions of 25,881 bp that are separated by a large single copy (LSC) region (86,516 bp) and a small single copy SSC (17,487 bp) region. A total of 132 functional genes were annotated in the cp genome, including 87 protein-coding genes, 37 tRNA genes, and 8 rRNA genes. Moreover, 199 simple sequence repeats (SSR) and 65 repeat structures were detected. Comparative plastome analyses revealed a conserved gene order and high similarity of protein-coding sequences. The A. tanguticus cp genome exhibits contraction and expansion, which differs from Przewalskia tangutica and other related Solanaceae species. We identified 30 highly polymorphic regions, mostly belonging to intergenic spacer regions (IGS), which may be suitable for the development of robust and cost-effective markers for inferring the phylogeny of the genus Anisodus and family Solanaceae. Analysis of the Ka/Ks ratios of the Hyoscyameae tribe revealed significant positive selection exerted on the cemA, rpoC2, and clpP genes, which suggests that protein metabolism may be an important strategy for A. tanguticus and other species in Hyoscyameae in adapting to the adverse environment on the Qinghai-Tibetan Plateau. Phylogenetic analysis revealed that A. tanguticus clustered closer with Hyoscyamus niger than P. tangutica. Our results provide reliable genetic information for future exploration of the taxonomy and phylogenetic evolution of the Hyoscyameae tribe and related species.

Analysis of codon usage patterns in Haloxylon ammodendron based on genomic and transcriptomic data

Haloxylon ammodendron, a xero-halophytic shrub of Chenopodiaceae, is a dominant species in deserts, which has a strong drought and salt tolerance and plays an important role in sand fixation. However, the codon usage bias (CUB) in H. ammodendron is still unclear at present. In this study, the codon usage patterns of 38,657 coding sequences (CDSs) in the newly released whole-genome sequence data of H. ammodendron and 3,948 CDSs in the previously obtained transcriptome sequencing data were compared and analyzed. The results showed that the CDSs with the total guanineandcytosine(GC)content in the range of 40% ∼ 45% was the most in the genome and transcriptome. Among which, the GC1, GC2, and GC3 contents of genomic CDSs were 50.83%, 40.56%, and 40.23%, respectively, and those of CDSs in the transcriptome were 47.16%, 39.02%, and 39.59%, respectively. Therefore, the bases in H. ammodendron were rich in adenine and thymine, and the overallcodonusage was biasedtoward A- and U-ending codons. The analysis of neutrality plot, effective number of codon (ENC) plot, and parity rule 2 (PR2) bias plot showed that both natural selection and mutation pressure had great influences on the CUB of H. ammodendron, but natural selection was the most important determinant. Besides, gene expression level and the function and protein length of some specific genes also had influences on the codon usage pattern. Finally, a total of 25 common optimal codons were found in the genomic and transcriptomic data, and AU/GC-ending codons ratio was 24:1. It should be noted that the salt-tolerant unigenes had similar codon usage, and the highly expressed genes had higher usage frequency of optimal codons and lower GC content than the lowly expressed genes. In addition, there was no difference in the ENC values of salt-tolerant unigenes in H. ammodendron, and the expression level of the genes had no correlation with CAI. This study will help to elucidate the formation mechanism of H. ammodendron codon usage bias, and make contributions to the identification of new genes and the genetic engineering study on H. ammodendron.

The chloroplast genome of Salix floderusii and characterization of chloroplast regulatory elements

Salix floderusii is a rare alpine tree species in the Salix genus. Unfortunately, no extensive germplasm identification, molecular phylogeny, and chloroplast genomics of this plant have been conducted. We sequenced the chloroplast (cp) genome of S. floderusii for the first time using second-generation sequencing technology. The cp genome was 155,540 bp long, including a large single-copy region (LSC, 84,401 bp), a small single-copy region (SSC, 16,221 bp), and inverted repeat regions (IR, 54,918 bp). A total of 131 genes were identified, including 86 protein genes, 37 tRNA genes, and 8 rRNA genes. The S. floderusii cp genome contains 1 complement repeat, 24 forward repeats, 17 palindromic repeats, and 7 reverse repeats. Analysis of the IR borders showed that the IRa and IRb regions of S. floderusii and Salix caprea were shorter than those of Salix cinerea, which may affect plastome evolution. Furthermore, four highly variable regions were found, including the rpl22 coding region, psbM/trnD-GUC non-coding region, petA/psbJ non-coding region, and ycf1 coding region. These high variable regions can be used as candidate molecular markers and as a reference for identifying future Salix species. In addition, phylogenetic analysis indicated that the cp genome of S. floderusii is sister to Salix cupularis and belongs to the Subgenus Vetrix. Genes (Sf-trnI, Sf-PpsbA, aadA, Sf-TpsbA, Sf-trnA) obtained via cloning were inserted into the pBluescript II SK (+) to yield the cp expression vectors, which harbored the selectable marker gene aadA. The results of a spectinomycin resistance test indicated that the cp expression vector had been successfully constructed. Moreover, the aadA gene was efficiently expressed under the regulation of predicted regulatory elements. The present study provides a solid foundation for establishing subsequent S. floderusii cp transformation systems and developing strategies for the genetic improvement of S. floderusii.

Molecular Structure and Phylogenetic Analyses of the Complete Chloroplast Genomes of Three Medicinal Plants Conioselinum vaginatum, Ligusticum sinense, and Ligusticum jeholense

Most plants of Ligusticum have an important medicinal and economic value with a long history, Ligusticum sinense and L. jeholense ("Gaoben") has long been used in traditional Chinese medicine for the treatment of carminative, dispelling cold, dehumidification, and analgesia. While in the market Conioselinum vaginatum (Xinjiang Gaoben) is substitution for Gaoben, and occupies a higher market share. These three Gaoben-related medicinal materials are similar in morphology, and are difficult to distinguish from each other by the commonly used DNA barcodes. The chloroplast genome has been widely used for molecular markers, evolutionary biology, and barcoding identification. In this study, the complete chloroplast genome sequences of C. vaginatum, L. sinense, and L. jeholense were reported. The results showed that the complete chloroplast genomes of these three species have typical quadripartite structures, which were comprised of 148,664, 148,539, and 148,497 bp. A total of 114 genes were identified, including 81 protein-coding genes (PCGs), 29 tRNA genes, and four rRNA genes. Our study indicated that highly variable region ycf2-trnL and accD-ycf4 that can be used as specific DNA barcodes to distinguish and identify C. vaginatum, L. sinense, and L. jeholense. In addition, phylogenetic study showed that C. vaginatum nested in Ligusticum and as a sister group of L. sinense and L. jeholense, which suggested these two genera are both in need of revision. This study offer valuable information for future research in the identification of Gaoben-related medicinal materials and will benefit for further phylogenetic study of Apiaceae.

Complete chloroplast genome of Ilex dabieshanensis: Genome structure, comparative analyses with three traditional Ilex tea species, and its phylogenetic relationships within the family Aquifoliaceae

Ilex dabieshanensis K. Yao & M. B. Deng is not only a highly valued tree species for landscaping, it is also a good material for making kuding tea due to its anti-inflammatory and lipid-lowering medicinal properties. Utilizing next-generation and long-read sequencing technologies, we assembled the whole chloroplast genome of I. dabieshanensis. The genome was 157,218 bp in length, exhibiting a typical quadripartite structure with a large single copy (LSC: 86,607 bp), a small single copy (SSC: 18,427 bp) and a pair of inverted repeat regions (IRA and IRB: each of 26,092 bp). A total of 121 predicted genes were encoded, including 113 distinctive (79 protein-coding genes, 30 tRNAs, and 4 rRNAs) and 8 duplicated (8 protein-coding genes) located in the IR regions. Overall, 132 SSRs and 43 long repeats were detected and could be used as potential molecular markers. Comparative analyses of four traditional Ilex tea species (I. dabieshanensis, I. paraguariensis, I. latifolia and I. cornuta) revealed seven divergent regions: matK-rps16, trnS-psbZ, trnT-trnL, atpB-rbcL, petB-petD, rpl14-rpl16, and rpl32-trnL. These variations might be applicable for distinguishing different species within the genus Ilex. Phylogenetic reconstruction strongly suggested that I. dabieshanensis formed a sister clade to I. cornuta and also showed a close relationship to I. latifolia. The generated chloroplast genome information in our study is significant for Ilex tea germplasm identification, phylogeny and genetic improvement.

The complete chloroplast genome sequence of the medicinal plant Abrus pulchellus subsp. cantoniensis: genome structure, comparative and phylogenetic relationship analysis

Abrus pulchellus subsp. cantoniensis, an endemic medicinal plant in southern China, is clinically used to treat jaundice hepatitis, cholecystitis, stomachache and breast carbuncle. Here, we assembled and analyzed the first complete chloroplast (cp) genome of A. pulchellus subsp. cantoniensis. The A. pulchellus subsp. cantoniensis cp genome size is 156,497 bp with 36.5% GC content. The cp genome encodes 130 genes, including 77 protein-coding genes, 30 tRNA genes and four rRNA genes, of which 19 genes are duplicated in the inverted repeats (IR) regions. A total of 30 codons exhibited codon usage bias with A/U-ending. Moreover, 53 putative RNA editing sites were predicted in 20 genes, all of which were cytidine to thymine transitions. Repeat sequence analysis identified 45 repeat structures and 125 simple-sequence repeats (SSRs) in A. pulchellus subsp. cantoniensis cp genome. In addition, 19 mononucleotides (located in atpB, trnV-UAC, ycf3, atpF, rps16, rps18, clpP, rpl16, trnG-UCC and ndhA) and three compound SSRs (located in ndhA, atpB and rpl16) showed species specificity between A. pulchellus subsp. cantoniensis and Abrus precatorius, which might be informative sources for developing molecular markers for species identification. Furthermore, phylogenetic analysis inferred that A. pulchellus subsp. cantoniensis was closely related to A. precatorius, and the genus Abrus formed a subclade with Canavalia in the Millettioid/Phaseoloid clade. These data provide a valuable resource to facilitate the evolutionary relationship and species identification of this species.

Comparative analysis of chloroplast genomes reveals phylogenetic relationships and intraspecific variation in the medicinal plant Isodon rubescens

Isodon rubescens (Hemsley) H. Hara (Lamiaceae) is a traditional Chinese medicine plant that has been used to treat various human diseases and conditions such as inflammation, respiratory and gastrointestinal bacterial infections, and malignant tumors. However, the contents of the main active components of I. rubescens from different origins differ significantly, which greatly affected its quality. Therefore, a molecular method to identify and classify I. rubescens is needed. Here, we report the DNA sequence of the chloroplast genome of I. rubescens collected from Lushan, Henan province. The genome is 152,642 bp in length and has a conserved structure that includes a pair of IR regions (25,726 bp), a LSC region (83,527 bp) and a SSC region (17,663 bp). The chloroplast genome contains 113 unique genes, four rRNA genes, 30 tRNA genes, and 79 protein-coding genes, 23 of which contain introns. The protein-coding genes account for a total of 24,412 codons, and most of them are A/T biased usage. We identified 32 simple sequence repeats (SSRs) and 48 long repeats. Furthermore, we developed valuable chloroplast molecular resources by comparing chloroplast genomes from three Isodon species, and both mVISTA and DnaSP analyses showed that rps16-trnQ, trnS-trnG, and ndhC-trnM are candidate regions that will allow the identification of intraspecific differences within I. rubescens. Also 14 candidate fragments can be used to identify interspecific differences between species in Isodon. A phylogenetic analysis of the complete chloroplast genomes of 24 species in subfamily Nepetoideae was performed using the maximum likelihood method, and shows that I. rubescens clustered closer to I. serra than I. lophanthoides. Interestingly, our analysis showed that I. rubescens (MW018469.1) from Xianyang, Shaanxi Province (IR-X), is closer to I. serra than to the other two I. rubescens accessions. These results strongly indicate that intraspecific diversity is present in I. rubescens. Therefore, our results provide further insight into the phylogenetic relationships and interspecific diversity of species in the genus Isodon.

Plastome structure, phylogenomics and evolution of plastid genes in Swertia (Gentianaceae) in the Qing-Tibetan Plateau

BACKGROUND

The genus Swertia is of great medicinal importance and one of the most taxonomically challenging taxa within Gentianaceae, largely due to the morphological similarities of species within this genus and with its closely related genera. Previous molecular studies confirmed its polyphyly but suffered from low phylogenetic resolutions because only limited sequence loci were used. Thus, we conducted the structural, gene evolutionary, and phylogenetic analyses of 11 newly obtained plastomes of Swertia. Our result greatly improved the phylogenetic resolutions in Swertia, shed new light on the plastome evolution and phylogenetic relationships of this genus.

RESULTS

The 11 Swertia plastomes together with the published seven species proved highly similar in overall size, structure, gene order, and content, but revealed some structural variations caused by the expansion and contraction of the IRb region into the LSC region, due to the heterogeneous length of the ψycf1. The gene rps16 was found to be in a state flux with pseudogenes or completely lost. Similar situation was also documented in other genera of Gentianaceae. This might imply loss of the gene in the common ancestor of Gentianaceae. The distribution plot of ENC vs. GC3 showed all these plastomes arranging very close in the Wright line with an expected ENC value (49-52%), suggesting the codon usage of Swertia was mainly constrained by a GC mutation bias. Most of the genes remained under the purifying selection, however, the cemA was identified under positive selection, possibly reflecting an adaptive response to low CO₂ atmospheric conditions during the Late Miocene. Our phylogenomic analyses, based on 74 protein-coding genes (CDS), supported the polyphyly of Swertia with its close allies in the subtribe Swertiinae, presumably due to recent rapid radiation. The topology inferred from our phylogenetic analyses partly supported the current taxonomic treatment. Finally, several highly variable loci were identified, which can be used in future phylogenetic studies and accurate identification of medicinal genuineness of Swertia.

CONCLUSIONS

Our study confirmed the polyphyly of Swertia and demonstrated the power of plastome phylogenomics in improvement of phylogenetic resolution, thus contributing to a better understanding of the evolutionary history of Swertia.

Comparative Analysis the Complete Chloroplast Genomes of Nine Musa Species: Genomic Features, Comparative Analysis, and Phylogenetic Implications

Musa (family Musaceae) is monocotyledonous plants in order Zingiberales, which grows in tropical and subtropical regions. It is one of the most important tropical fruit trees in the world. Herein, we used next-generation sequencing technology to assemble and perform in-depth analysis of the chloroplast genome of nine new Musa plants for the first time, including genome structure, GC content, repeat structure, codon usage, nucleotide diversity and etc. The entire length of the Musa chloroplast genome ranged from 167,975 to 172,653 bp, including 113 distinct genes comprising 79 protein-coding genes, 30 transfer RNA (tRNA) genes and four ribosomal RNA (rRNA) genes. In comparative analysis, we found that the contraction and expansion of the inverted repeat (IR) regions resulted in the doubling of the rps19 gene. The several non-coding sites (psbI-atpA, atpH-atpI, rpoB-petN, psbM-psbD, ndhf-rpl32, and ndhG-ndhI) and three genes (ycf1, ycf2, and accD) showed significant variation, indicating that they have the potential of molecular markers. Phylogenetic analysis based on the complete chloroplast genome and coding sequences of 77 protein-coding genes confirmed that Musa can be mainly divided into two groups. These genomic sequences provide molecular foundation for the development and utilization of Musa plants resources. This result may contribute to the understanding of the evolution pattern, phylogenetic relationships as well as classification of Musa plants.

Structural characterization of Platanthera ussuriensis chloroplast genome and comparative analyses with other species of Orchidaceae

BACKGROUND

The genus Tulotis has been classified into the genus Platanthera in the present taxonomic studies since the morphological characteristics of this genus is very similar to that of Platanthera. Platanthera ussuriensis, formerly named as Tulotis ussuriensis, is a small terrestrial orchid species and has been listed as wild plant under State protection (category II) in China. An improved understanding of the genomic information will enable future applications of conservation strategy as well as phylogenetic studies for this rare orchid species. The objective of this research was to characterize and compare the chloroplast genome of P. ussuriensis with other closely related species of Orchidaceae.

RESULTS

The chloroplast genome sequence of P. ussuriensis is 155,016 bp in length, which included a pair of inverted repeats (IRs) of 26,548 bp that separated a large single copy (LSC) region of 83,984 bp and a small single copy (SSC) region of 17,936 bp. The annotation contained a total of 132 genes, including 86 protein-coding genes, 38 tRNA genes and 8 rRNA genes. The simple sequence repeat (SSR) analysis showed that there were 104 SSRs in the chloroplast genome of P. ussuriensis. RNA editing sites recognition indicated 72 RNA editing events occurred, and all codon changes were C to T conversions. Comparative genomics showed that the chloroplast sequence of Platanthera related species were relatively conserved, while there were still some high variation regions that could be used as molecular markers. Moreover, Platanthera related species showed similar IR/SSC and IR/LSC borders. The phylogenetic analysis suggested that P. ussuriensis had a closer evolutionary relationship with P. japonica followed by the remaining Platanthera species.

CONCLUSION

Orchidaceae is a key group of biodiversity protection and also a hot spot group in the plant taxonomy and evolution studies due to their characteristics of high specialization and rapid evolution. This research determined the complete chloroplast genome of P. ussuriensis for the first time, and compared the sequence with other closely related orchid species. These results provide a foundation for future genomic and molecular evolution of the Orchidaceae species, and provide insights into the development of conservation strategy for Platanthera species.

Comparative and Phylogenetic Analysis of Complete Plastomes among Aristidoideae Species (Poaceae)

Aristidoideae is a subfamily in the PACMAD clade of family Poaceae, including three genera, Aristida, Stipagrostis, and Sartidia. In this study, the plastomes of Aristida adscensionis and Stipagrostis pennata were newly sequenced, and a total of 16 Aristidoideae plastomes were compared. All plastomes were conservative in genome size, gene number, structure, and IR boundary. Repeat sequence analysis showed that forward and palindrome repeats were the most common repeat types. The number of SSRs ranged from 30 (Sartidia isaloensis) to 54 (Aristida purpurea). Codon usage analysis showed that plastome genes preferred to use codons ending with A/T. A total of 12 highly variable regions were screened, including four protein coding sequences (matK, ndhF, infA, and rpl32) and eight non-coding sequences (rpl16-1-rpl16-2, ccsA-ndhD, trnY-GUA-trnD-GUC, ndhF-rpl32, petN-trnC-GCA, trnT-GGU-trnE-UUC, trnG-GCC-trnfM-CAU, and rpl32-trnL-UAG). Furthermore, the phylogenetic position of this subfamily and their intergeneric relationships need to be illuminated. All Maximum Likelihood and Bayesian Inference trees strongly support the monophyly of Aristidoideae and each of three genera, and the clade of Aristidoideae and Panicoideae was a sister to other subfamilies in the PACMAD clade. Within Aristidoideae, Aristida is a sister to the clade composed of Stipagrostis and Sartidia. The divergence between C₄ Stipagrostis and C₃ Sartidia was estimated at 11.04 Ma, which may be associated with the drought event in the Miocene period. Finally, the differences in carbon fixation patterns, geographical distributions, and ploidy may be related to the difference of species numbers among these three genera. This study provides insights into the phylogeny and evolution of the subfamily Aristidoideae.

Genome-wide analysis of codon usage in sesame (Sesamum indicum L.)

Sesamum indicum is an ancient oil crop grown in tropical and subtropical areas of the world. We have analyzed 23,538 coding sequences (CDS) of S. indicum to understand the factors shaping codon usage in this important oil crop plant. We identified eleven highly preferred codons in S. indicum that have AT-endings. The slope of a neutrality plot was less than one while effective number of codons (ENC) plot showed distribution above and below the standard curve. There is a significant relationship between protein length and relative synonymous codon usage (RSCU) at the primary axis while there is a weak correlation between protein length and Nc values. Correspondence analysis conducted on RSCU values differentiated CDS based on their GC content and their characteristic feature and showed a discrete distribution. Moreover, by determining codon usage, we found out that majority of the lignan biosynthesis related genes showed a weaker codon usage bias. These results provide insights into understanding codon evolution in sesame.

Strategies and Patterns of Codon Bias in Molluscum Contagiosum Virus

Trends associated with codon usage in molluscum contagiosum virus (MCV) and factors governing the evolution of codon usage have not been investigated so far. In this study, attempts were made to decipher the codon usage trends and discover the major evolutionary forces that influence the patterns of codon usage in MCV with special reference to sub-types 1 and 2, MCV-1 and MCV-2, respectively. Three hypotheses were tested: (1) codon usage patterns of MCV-1 and MCV-2 are identical; (2) SCUB (synonymous codon usage bias) patterns of MCV-1 and MCV-2 slightly deviate from that of human host to avoid affecting the fitness of host; and (3) translational selection predominantly shapes the SCUB of MCV-1 and MCV-2. Various codon usage indices viz. relative codon usage value, effective number of codons and codon adaptation index were calculated to infer the nature of codon usage. Correspondence analysis and correlation analysis were performed to assess the relative contribution of silent base contents and significance of codon usage indices in defining bias in codon usage. Among the tested hypotheses, only the second and third hypotheses were accepted.

Comparative analysis of codon usage patterns in chloroplast genomes of five Miscanthus species and related species

Miscanthus is not only a perennial fiber biomass crop, but also valuable breeding resource for its low-nutrient requirements, photosynthetic efficiency and strong adaptability to environment. In the present study, the codon usage patterns of five different Miscanthus plants and other two related species were systematically analyzed. The results indicated that the cp genomes of the seven representative species were preference to A/T bases and A/T-ending codons. In addition, 21 common high-frequency codons and 4–11 optimal codons were detected in the seven chloroplast genomes. The results of ENc-plot, PR2-plot and neutrality analysis revealed the codon usage patterns of the seven chloroplast genomes are influenced by multiple factors, in which nature selection is the main influencing factor. Comparative analysis of the codon usage frequencies between the seven representative species and four model organisms suggested that Arabidopsis thaliana, Populus trichocarpa and Saccharomyces cerevisiae could be considered as preferential appropriate exogenous expression receptors. These results might not only provide important reference information for evolutionary analysis, but also shed light on the way to improve the expression efficiency of exogenous gene in transgenic research based on codon optimization.

The complete chloroplast genome sequence of Rubus hirsutus Thunb. and a comparative analysis within Rubus species

Rubus hirsutus is a type of tonifying kidney-essence herb that belongs to the Rosaceae family, and has been commonly used to treat multiple diseases, such as polyuria, impotence, and infertility. In this study, we determined the complete chloroplast sequence of R. hirsutus and conduced a comparative analysis within the genus Rubus. The assembled chloroplast (cp.) genome is 156,380 bp in length with a GC content of 37.0% and shares a conserved quadripartite structure within the other cp. genomes in this genus. A total of 132 unique genes were annotated in the cp. genome of R. hirsutus, which contained 87 protein-coding genes, 37 tRNAs, and eight rRNAs. Seventeen duplicated genes were identified in the inverted repeats region. Furthermore, 70 simple sequence repeats and 35 long repeats were detected in total in the R. hirsutus chloroplast genome. Eight mutational hotspots were identified in the cp. genome of this species with higher nucleotide variations in non-coding regions than those of coding regions. Furthermore, the gene order, codon usage, and repeat sequence distribution were highly consistent in Rubus according to the results of a comparative analysis. A phylogenetic analysis indicated that there was a sister relationship between R. hirsutus and R. chingii. Overall, the complete chloroplast genome of R. hirsutus and the comparative analysis will help to further the evolutionary study, conservation, phylogenetic reconstruction, and development of molecular barcodes for the genus Rubus.

Free energy of mRNA positively correlates with GC content in chloroplast transcriptomes of edible legumes

In the present study, the results of nucleotide composition analysis showed that the legume chloroplast (cp) transcriptomes were AT rich. From the neutrality plot, we observed that natural selection might have played a major role, while mutation pressure played a minor role in the CUB of cp transcriptomes. Highly significant (p < 0.05) negative correlation was found between mRNA free energy (mFE) and scaled chi-square for entire mRNA in Cicer arietinum and Lens culinaris suggesting that the release of higher energy by entire mRNA molecule might be associated with higher degree of codon usage bias in these two crop plants. Further, highly significant (p < 0.01, p < 0.05) positive correlation of mFE for entire mRNA was found with GC3 and that of mFE for 39 bases with GC, GC1, GC2 and GC3 contents among all the legumes. This indicated that higher GC content might induce the release of more free energy by cp transcriptomes.

The complete chloroplast genome of Stauntonia chinensis and compared analysis revealed adaptive evolution of subfamily Lardizabaloideae species in China

BACKGROUND

Stauntonia chinensis DC. belongs to subfamily Lardizabaloideae, which is widely grown throughout southern China. It has been used as a traditional herbal medicinal plant, which could synthesize a number of triterpenoid saponins with anticancer and anti-inflammatory activities. However, the wild resources of this species and its relatives were threatened by over-exploitation before the genetic diversity and evolutionary analysis were uncovered. Thus, the complete chloroplast genome sequences of Stauntonia chinensis and comparative analysis of chloroplast genomes of Lardizabaloideae species are necessary and crucial to understand the plastome evolution of this subfamily.

RESULTS

A series of analyses including genome structure, GC content, repeat structure, SSR component, nucleotide diversity and codon usage were performed by comparing chloroplast genomes of Stauntonia chinensis and its relatives. Although the chloroplast genomes of eight Lardizabaloideae plants were evolutionary conserved, the comparative analysis also showed several variation hotspots, which were considered as highly variable regions. Additionally, pairwise Ka/Ks analysis showed that most of the chloroplast genes of Lardizabaloideae species underwent purifying selection, whereas 25 chloroplast protein coding genes were identified with positive selection in this subfamily species by using branch-site model. Bayesian and ML phylogeny on CCG (complete chloroplast genome) and CDs (coding DNA sequences) produced a well-resolved phylogeny of Lardizabaloideae plastid lineages.

CONCLUSIONS

This study enhanced the understanding of the evolution of Lardizabaloideae and its relatives. All the obtained genetic resources will facilitate future studies in DNA barcode, species discrimination, the intraspecific and interspecific variability and the phylogenetic relationships of subfamily Lardizabaloideae.

Chloroplast genome assembly of Handroanthus impetiginosus: comparative analysis and molecular evolution in Bignoniaceae

Bignoniaceae species have conserved chloroplast structure, with hotspots of nucleotide diversity. Several genes are under positive selection, and can be targets for evolutionary studies. Bignoniaceae is one of the most species-rich family of woody plants in Neotropical seasonally dry forests. Here we report the assembly of Handroanthus impetiginosus chloroplast genome and evolutionary comparative analyses of ten Bignoniaceae species representing the genera for which whole-genome chloroplast sequences were available. The chloroplast genome of H. impetiginosus is 159,462 bp in size and has a similar structure compared to the other nine species. The total number of genes was slightly variable amongst the Bignoniaceae, ranging from 124 in H. impetiginosus to 144 in Anemopaegma acutifolium. The inverted repeat (IR) size was variable, ranging from 24,657 bp (Tecomaria capensis) to 40,481 bp (A. acutifolium), due to the contraction and retraction at its boundaries. However, gene boundaries were very similar among the ten species. We found 98 forward and palindromic dispersed repeats, and 85 simple sequence repeats (SSRs). In general, chloroplast sequences were highly conserved, with few nucleotide diversity hotspots in the genes accD, clpP, rpoA, ycf1, ycf2. The phylogenetic analysis based on 77 coding genes was highly consistent with Angiosperm Phylogeny Group (APG) IV. Our results also indicate that most genes are under negative selection or neutral evolution. We found no evidence of branch-site selection, implying that H. impetiginosus is not evolving faster than the other species analyzed, notwithstanding we found site positive selection signal in several genes. These genes can provide targets for evolutionary studies in Bignoniaceae and Lamiales species.

Transit From Autotrophism to Heterotrophism: Sequence Variation and Evolution of Chloroplast Genomes in Orobanchaceae Species

The family Orobanchaceae including autotrophic, hemiparasitic, and holoparasitic species, is becoming a key taxa to study the evolution of chloroplast genomes in different lifestyles. But the early evolutionary trajectory in the transit from autotrophism to hemiparasitism still maintains unclear for the inadequate sampling. In this study, we compared 50 complete chloroplast genomes in Orobanchaceae, containing four newly sequenced plastomes from hemiparasitic Pedicularis, to elucidate the sequence variation patterns in the evolution of plastomes. Contrasted to the sequence and structural hypervariabilities in holoparasites, hemiparasitic plastomes exhibited high similarity to those of autotrophs in gene and GC contents. They are generally characterized with functional or physical loss of ndh/tRNA genes and the inverted small-single-copy region. Gene losses in Orobanchaceae were lineage-specific and convergent, possibly related to structural reconfiguration and expansion/contraction of the inverted region. Pseudogenization of ndh genes was unique in hemiparasites. At least in Pedicularis, the ndhF gene might be most sensitive to the environmental factors and easily pseudogenized when autotrophs transit to hemiparasites. And the changes in gene contents and structural variation potentially deeply rely on the feeding type. Selective pressure, together with mutational bias, was the dominant factor of shaping the codon usage patterns. The relaxed selective constraint, potentially with genome-based GC conversion (gBGC) and preferential codon usage, drive the fluctuation of GC contents among taxa with different lifestyles. Phylogenetic analysis in Orobanchaceae supported that parasitic species were single-originated while holoparasites were multiple-originated. Overall, the comparison of plastomes provided a good opportunity to understand the evolution process in Orobanchaceae with different lifestyles.

Analysis of codon usage bias of chloroplast genes in Oryza species : Codon usage of chloroplast genes in Oryza species

The codon usage bias in chloroplast genes of Oryza species was low and AT rich. The pattern of codon usage was different among Oryza species and mainly influenced by mutation pressure and natural selection. Codon usage bias (CUB) is the unequal usage of synonymous codons in which some codons are more preferred to others in the coding sequences of genes. It shows a species-specific property. We studied the patterns of codon usage and the factors that influenced the CUB of protein-coding chloroplast (cp) genes in 18 Oryza species as no work was yet reported. The nucleotide composition analysis revealed that the overall GC content of cp genes in different species of Oryza was lower than 50%, i.e., Oryza cp genes were AT rich. Synonymous codon usage order (SCUO) suggested that CUB was weak in the cp genes of different Oryza species. A highly significant correlation was observed between overall nucleotides and its constituents at the third codon position suggesting that both, mutation pressure and natural selection, might influence the CUB. Correspondence analysis (COA) revealed that codon usage pattern differed across Oryza species. In the neutrality plot, a narrow range of GC3 distribution was recorded and some points were diagonally distributed in all the plots, suggesting that natural selection and mutation pressure might have influenced the CUB. The slope of the regression line was < 0.5, augmenting our inference that natural selection might have played a major role, while mutation pressure had a minor role in shaping the CUB of cp genes. The magnitudes of mutation pressure and natural selection on cp genes varied across Oryza species.

Molecular structure and phylogenetic analyses of the complete chloroplast genomes of three original species of Pyrrosiae Folium

Pyrrosia petiolosa, Pyrrosia lingua and Pyrrosia sheareri are recorded as original plants of Pyrrosiae Folium (PF) and commonly used as Chinese herbal medicines. Due to the similar morphological features of PF and its adulterants, common DNA barcodes cannot accurately distinguish PF species. Knowledge of the chloroplast (cp) genome is widely used in species identification, molecular marker and phylogenetic analyses. Herein, we determined the complete cp genomes of three original species of PF via high-throughput sequencing technologies. The three cp genomes exhibited a typical quadripartite structure with sizes ranging from 158 165 to 163 026 bp. The cp genomes of P. petiolosa and P. lingua encoded 130 genes, whilst that of P. sheareri encoded 131 genes. The complete cp genomes were compared, and five highly divergent regions of petA-psbJ, matK-rps16, ndhC-trnM, psbM-petN and psaC-ndhE were screened as potential DNA barcodes for identification of Pyrrosia genus species. The phylogenetic tree we obtained indicated that P. petiolosa and P. lingua are clustered in a single clade and, thus, share a close relationship. This study provides invaluable information for further studies on the species identification, taxonomy and phylogeny of Pyrrosia genus species.

Water lily (Nymphaea thermarum) genome reveals variable genomic signatures of ancient vascular cambium losses

For more than 225 million y, all seed plants were woody trees, shrubs, or vines. Shortly after the origin of angiosperms ∼140 million y ago (MYA), the Nymphaeales (water lilies) became one of the first lineages to deviate from their ancestral, woody habit by losing the vascular cambium, the meristematic population of cells that produces secondary xylem (wood) and phloem. Many of the genes and gene families that regulate differentiation of secondary tissues also regulate the differentiation of primary xylem and phloem, which are produced by apical meristems and retained in nearly all seed plants. Here, we sequenced and assembled a draft genome of the water lily Nymphaea thermarum, an emerging system for the study of early flowering plant evolution, and compared it to genomes from other cambium-bearing and cambium-less lineages (e.g., monocots and Nelumbo). This revealed lineage-specific patterns of gene loss and divergence. Nymphaea is characterized by a significant contraction of the HD-ZIP III transcription factors, specifically loss of REVOLUTA, which influences cambial activity in other angiosperms. We also found the Nymphaea and monocot copies of cambium-associated CLE signaling peptides display unique substitutions at otherwise highly conserved amino acids. Nelumbo displays no obvious divergence in cambium-associated genes. The divergent genomic signatures of convergent loss of vascular cambium reveals that even pleiotropic genes can exhibit unique divergence patterns in association with independent events of trait loss. Our results shed light on the evolution of herbaceousness-one of the key biological innovations associated with the earliest phases of angiosperm evolution.

Complete Chloroplast Genomes of Chlorophytum comosum and Chlorophytum gallabatense: Genome Structures, Comparative and Phylogenetic Analysis

The genus Chlorophytum includes many economically important species well-known for medicinal, ornamental, and horticultural values. However, to date, few molecular genomic resources have been reported for this genus. Therefore, there is limited knowledge of phylogenetic studies, and the available chloroplast (cp) genome of Chlorophytum (C. rhizopendulum) does not provide enough information on this genus. In this study, we present genomic resources for C. comosum and C. gallabatense, which had lengths of 154,248 and 154,154 base pairs (bp), respectively. They had a pair of inverted repeats (IRa and IRb) of 26,114 and 26,254 bp each in size, separating the large single-copy (LSC) region of 84,004 and 83,686 bp from the small single-copy (SSC) region of 18,016 and 17,960 bp in C. comosum and C. gallabatense, respectively. There were 112 distinct genes in each cp genome, which were comprised of 78 protein-coding genes, 30 tRNA genes, and four rRNA genes. The comparative analysis with five other selected species displayed a generally high level of sequence resemblance in structural organization, gene content, and arrangement. Additionally, the phylogenetic analysis confirmed the previous phylogeny and produced a phylogenetic tree with similar topology. It showed that the Chlorophytum species (C. comosum, C. gallabatense and C. rhizopendulum) were clustered together in the same clade with a closer relationship than other plants to the Anthericum ramosum. This research, therefore, presents valuable records for further molecular evolutionary and phylogenetic studies which help to fill the gap in genomic resources and resolve the taxonomic complexes of the genus.

The Chloroplast Genome of Carya illinoinensis: Genome Structure, Adaptive Evolution, and Phylogenetic Analysis

Research Highlights: For the first time, the complete chloroplast (cp) genome of Carya illinoinensis cv. ‘Pawnee’ was de novo assembled. Comprehensive analysis the cp genome of C. illinoinensis revealed potential cpDNA markers for intraspecies identification, genes involved in adaptation, and its phylogenetic position. Background and Objectives: C. illinoinensis is an economically important nut tree in the family Juglandaceae. Cp-derived markers are helpful for genetic research, but they still need to be developed in C. illinoinensis. Additionally, the adaptation and phylogenetic relationships of C. illinoinensis have not been revealed based on the complete cp genome. Materials and Methods: Chloroplast genomic DNA of C. illinoinensis cv. ‘Pawnee’ was extracted and subjected to Illumina sequencing. Results: The cp genome is 160,819 bp in size, exhibiting a typical quadripartite structure with a large single copy (LSC) of 90,022 bp, a small single copy (SSC) of 18,791 bp, and a pair of inverted repeats (IRA and IRB) regions of 26,003 bp each. The genome was predicted to encode 112 unique genes, including 79 protein-coding genes, 29 tRNAs, and four rRNAs, with 19 duplicates in the IR regions. In total, 213 SSRs and 44 long repeats were identified in the cp genome. A comparison of two different C. illinoinensis genotypes, ‘Pawnee’ and 87MX3-2.11, obtained 143 SNPs and 74 indels. The highly variable regions such as atpF, clpP, and ndhA genes, and matK-rps16, trnS-trnG, and trnT-psbD intergenic spacers might be helpful for future intraspecific identification. Positive selection was acting on the ccsA and rps12 cp genes based on the Ka/Ks ratios. Phylogenetic analysis indicated that C. illinoinensis forms a sister clade to Asian Carya species, represented by C. kweichowensis and Annamocarya sinensis. Conclusions: The genome information in our study will have significance for further research on the intraspecies identification and genetic improvement of C. illinoinensis.

Selection constraints determine preference for A/U-ending codons in Taxus contorta

Unequal utilization of synonymous codons is a well-known phenomenon among living organisms. This phenomenon plays a major role in the enhancement of the accuracy and efficiency of translation. Gymnosperms are rarely paid attention in this aspect. Understanding the degree of and determining the forces influencing codon usage bias (CUB) in Taxus contorta, an endangered Himalayan gymnosperm, will prove useful in interpreting the evolutionary characteristics of this species. Using RNAseq data, 93 790 assembled transcripts were clustered into 32 701 unigenes. Around 13 061 full-length sequences were utilized for the analysis of CUB. Compositional properties showed that GC-content ranged from 28.76% to 65.22%, with an average value of 44.28%, suggesting an AT-rich genome. The mean effective number of codons (ENC) value revealed that CUB is not strong in T. contorta. The preferred codons tended to be A/U ending, whereas the avoided codons tended to be G/C ending. A P2 index of 0.54 and a Mutation Responsive Index (MRI) value of -0.02 in addition to the results revealed by the neutrality, ENC, and parity plots showed that natural selection is a predominating factor governing CUB. Mutational pressure, gene length, hydropathiciy, aromaticity, and nucleotide composition influence CUB weakly.

Complete Chloroplast Genomes of Ampelopsis humulifolia and Ampelopsis japonica: Molecular Structure, Comparative Analysis, and Phylogenetic Analysis

Ampelopsis humulifolia (A. humulifolia) and Ampelopsis japonica (A. japonica), which belong to the family Vitaceae, are valuably used as medicinal plants. The chloroplast (cp) genomes have been recognized as a convincing data for marker selection and phylogenetic studies. Therefore, in this study we reported the complete cp genome sequences of two Ampelopsis species. Results showed that the cp genomes of A. humulifolia and A. japonica were 161,724 and 161,430 bp in length, respectively, with 37.3% guanine-cytosine (GC) content. A total of 114 unique genes were identified in each cp genome, comprising 80 protein-coding genes, 30 tRNA genes, and 4 rRNA genes. We determined 95 and 99 small sequence repeats (SSRs) in A. humulifolia and A. japonica, respectively. The location and distribution of long repeats in the two cp genomes were identified. A highly divergent region of psbZ (Photosystem II reaction center protein Z) -trnG (tRNA-Glycine) was found and could be treated as a potential marker for Vitaceae, and then the corresponding primers were designed. Additionally, phylogenetic analysis showed that Vitis was closer to Tetrastigma than Ampelopsis. In general, this study provides valuable genetic resources for DNA barcoding marker identification and phylogenetic analyses of Ampelopsis.

Phylogenetic analysis of Fritillaria cirrhosa D. Don and its closely related species based on complete chloroplast genomes

Fritillaria cirrhosa D. Don, whose bulb is used in a well-known traditional Chinese medicine to relieve cough and eliminate phlegm, is one of the most important medicinal plants of Fritillaria L. The species is widely distributed among the alpine regions in southwestern China and possesses complex morphological variations in different distributions. A series of newly related species were reported, based on obscure morphological differences. As a result, F. cirrhosa and its closely related species constitute a taxonomically complex group. However, it is difficult to accurately identify these species and reveal their phylogenetic relationships using traditional taxonomy. Molecular markers and gene fragments have been adopted but they are not able to afford sufficient phylogenetic resolution in the genus. Here, we report the complete chloroplast genome sequences of F. cirrhosa and its closely related species using next generation sequencing (NGS) technology. Eight plastid genomes ranged from 151,058 bp to 152,064 bp in length and consisted of 115 genes. Gene content, gene order, GC content, and IR/SC boundary structures were highly similar among these genomes. SSRs and five large repeat sequences were identified and the total number of them ranged from 73 to 79 and 63 to 75, respectively. Six highly divergent regions were successfully identified that could be used as potential genetic markers of Fritillaria. Phylogenetic analyses revealed that eight Fritillaria species were clustered into three clades with strong supports and F. cirrhosa was closely related to F. przewalskii and F. sinica. Overall, this study indicated that the complete chloroplast genome sequence was an efficient tool for identifying species in taxonomically complex groups and exploring their phylogenetic relationships.

Analyzing and Characterizing the Chloroplast Genome of Salix wilsonii

Salix wilsonii is an important ornamental willow tree widely distributed in China. In this study, an integrated circular chloroplast genome was reconstructed for S. wilsonii based on the chloroplast reads screened from the whole-genome sequencing data generated with the PacBio RSII platform. The obtained pseudomolecule was 155,750 bp long and had a typical quadripartite structure, comprising a large single copy region (LSC, 84,638 bp) and a small single copy region (SSC, 16,282 bp) separated by two inverted repeat regions (IR, 27,415 bp). The S. wilsonii chloroplast genome encoded 115 unique genes, including four rRNA genes, 30 tRNA genes, 78 protein-coding genes, and three pseudogenes. Repetitive sequence analysis identified 32 tandem repeats, 22 forward repeats, two reverse repeats, and five palindromic repeats. Additionally, a total of 118 perfect microsatellites were detected, with mononucleotide repeats being the most common (89.83%). By comparing the S. wilsonii chloroplast genome with those of other rosid plant species, significant contractions or expansions were identified at the IR-LSC/SSC borders. Phylogenetic analysis of 17 willow species confirmed that S. wilsonii was most closely related to S. chaenomeloides and revealed the monophyly of the genus Salix. The complete S. wilsonii chloroplast genome provides an additional sequence-based resource for studying the evolution of organelle genomes in woody plants.

Determination of the evolutionary pressure on Camellia oleifera on Hainan Island using the complete chloroplast genome sequence

Camellia oleifera is one of the four largest woody edible oil plants in the world with high ecological and medicinal values. Due to frequent interspecific hybridization, it was difficult to study its genetics and evolutionary history. This study used C. oleifera that was collected on Hainan Island to conduct our research. The unique island environment makes the quality of tea oil higher than that of other species grown in the mainland. Moreover, a long-term geographic isolation might affect gene structure. In order to better understand the molecular biology of this species, protect excellent germplasm resources, and promote the population genetics and phylogenetic studies of Camellia plants, high-throughput sequencing technology was used to obtain the chloroplast genome sequence of Hainan C. oleifera. The results showed that the whole chloroplast genome of C. oleifera in Hainan was 156,995 bp in length, with a typical quadripartite structure of a large single copy (LSC) region of 86,648 bp, a small single copy (SSC) region of 18,297 bp, and a pair of inverted repeats (IRs) of 26,025 bp. The whole genome encoded a total of 141 genes (115 different genes), including 88 protein-coding genes, 45 tRNA genes, and eight rRNA genes. Among these genes, nine genes contained one intron, two genes contained two introns, and four overlapping genes were also detected. The total GC content of Hainan C. oleifera's chloroplast genome was 37.29%. The chloroplast genome structure characteristics of Hainan C. oleifera were compared with mainland C. oleifera and those of the other eight closely related Theaceae species; it was found that the contractions and expansions of the IR/LSC and IR/SSC regions affected the length of chloroplast genome. The chloroplast genome sequences of these Theaceae species were highly similar. A comparative analysis indicated that the Theaceae species were conserved in structure and evolution. A total of 51 simple sequence repeat (SSR) loci were detected in the chloroplast genome of Hainan C. oleifera, and all Camellia plants did not have pentanucleotide repeats, which could be used as a good marker in phylogenetic studies. We also detected seven long repeats, the base composition of all repeats was biased toward A/T, which was consistent with the codon bias. It was found that Hainan C. oleifera had a similar evolutionary relationship with C. crapnelliana, through the use of codons and phylogenetic analysis. This study can provide an effective genomic resource for the evolutionary history of Theaceae family.

Analysis of codon usage patterns in "Lonicerae Flos" (Lonicera macranthoides Hand. -Mazz.) based on transcriptome data

Lonicera macranthoides Hand. -Mazz. is an important medicinal and economical plant in China, however, the codon usage bias (CUB) in L. macranthoides genes is still unknown. In this study, L. macranthoides transcriptome sequencing has been completed, and codon usage patterns in 36,090 reconstructed genes from the L. macranthoides transcriptome were examined. The mean GC content and GC3 value is 44.9% and 43.1%, respectively, which indicates that nucleotide contents of L. macranthoides genome is somewhat AT rich, and its codon bias pattern tends to use A/T-ending codons. According to neutrality plot, ENC plot, PR2-Bias plot and correspondence analysis, we know that both compositional constraint under selection and mutation could affect the CUB in L. macranthoides, and the mutation is the most determinant factor. Meanwhile, gene expression levels can influence its codon usage pattern. Furthermore, we identified 29 optimal codons and most of them ended with A/U. The study will lay a foundation for future research on gene prediction, genetic engineering and molecular evolution in L. macranthoides.

The complete chloroplast genome sequence of watercress (Nasturtium officinale R. Br.): Genome organization, adaptive evolution and phylogenetic relationships in Cardamineae

Watercress (Nasturtium officinale R. Br.), an aquatic leafy vegetable of the Brassicaceae family, is known as a nutritional powerhouse. Here, we de novo sequenced and assembled the complete chloroplast (cp) genome of watercress based on combined PacBio and Illumina data. The cp genome is 155,106 bp in length, exhibiting a typical quadripartite structure including a pair of inverted repeats (IRA and IRB) of 26,505 bp separated by a large single copy (LSC) region of 84,265 bp and a small single copy (SSC) region of 17,831 bp. The genome contained 113 unique genes, including 79 protein-coding genes, 30 tRNAs and 4 rRNAs, with 20 duplicate in the IRs. Compared with the prior cp genome of watercress deposited in GenBank, 21 single nucleotide polymorphisms (SNPs) and 27 indels were identified, mainly located in noncoding sequences. A total of 49 repeat structures and 71 simple sequence repeats (SSRs) were detected. Codon usage showed a bias for A/T-ending codons in the cp genome of watercress. Moreover, 45 RNA editing sites were predicted in 16 genes, all for C-to-U transitions. A comparative plastome study with Cardamineae species revealed a conserved gene order and high similarity of protein-coding sequences. Analysis of the Ka/Ks ratios of Cardamineae suggested positive selection exerted on the ycf2 gene in watercress, which might reflect specific adaptations of watercress to its particular living environment. Phylogenetic analyses based on complete cp genomes and common protein-coding genes from 56 species showed that the genus Nasturtium was a sister to Cardamine in the Cardamineae tribe. Our study provides valuable resources for future evolution, population genetics and molecular biology studies of watercress.

Complete chloroplast genome sequence of Betula platyphylla: gene organization, RNA editing, and comparative and phylogenetic analyses

BACKGROUND

Betula platyphylla is a common tree species in northern China that has high economic and medicinal value. Our laboratory has been devoted to genome research on B. platyphylla for approximately 10 years. As primary organelle genomes, the complete genome sequences of chloroplasts are important to study the divergence of species, RNA editing and phylogeny. In this study, we sequenced and analyzed the complete chloroplast (cp) genome sequence of B. platyphylla.

RESULTS

The complete cp genome of B. platyphylla was 160,518 bp in length, which included a pair of inverted repeats (IRs) of 26,056 bp that separated a large single copy (LSC) region of 89,397 bp and a small single copy (SSC) region of 19,009 bp. The annotation contained a total of 129 genes, including 84 protein-coding genes, 37 tRNA genes and 8 rRNA genes. There were 3 genes using alternative initiation codons. Comparative genomics showed that the sequence of the Fagales species cp genome was relatively conserved, but there were still some high variation regions that could be used as molecular markers. The IR expansion event of B. platyphylla resulted in larger cp genomes and rps19 pseudogene formation. The simple sequence repeat (SSR) analysis showed that there were 105 SSRs in the cp genome of B. platyphylla. RNA editing sites recognition indicated that at least 80 RNA editing events occurred in the cp genome. Most of the substitutions were C to U, while a small proportion of them were not. In particular, three editing loci on the rRNA were converted to more than two other bases that had never been reported. For synonymous conversion, most of them increased the relative synonymous codon usage (RSCU) value of the codons. The phylogenetic analysis suggested that B. platyphylla had a closer evolutionary relationship with B. pendula than B. nana.

CONCLUSIONS

In this study, we not only obtained and annotated the complete cp genome sequence of B. platyphylla, but we also identified new RNA editing sites and predicted the phylogenetic relationships among Fagales species. These findings will facilitate genomic, genetic engineering and phylogenetic studies of this important species.

Intergeneric Relationships within the Early-Diverging Angiosperm Family Nymphaeaceae Based on Chloroplast Phylogenomics

The order Nymphaeales, consisting of three families with a record of eight genera, has gained significant interest from botanists, probably due to its position as a basal angiosperm. The phylogenetic relationships within the order have been well studied; however, a few controversial nodes still remain in the Nymphaeaceae. The position of the Nuphar genus and the monophyly of the Nymphaeaceae family remain uncertain. This study adds to the increasing number of the completely sequenced plastid genomes of the Nymphaeales and applies a large chloroplast gene data set in reconstructing the intergeneric relationships within the Nymphaeaceae. Five complete chloroplast genomes were newly generated, including a first for the monotypic Euryale genus. Using a set of 66 protein-coding genes from the chloroplast genomes of 17 taxa, the phylogenetic position of Nuphar was determined and a monophyletic Nymphaeaceae family was obtained with convincing statistical support from both partitioned and unpartitioned data schemes. Although genomic comparative analyses revealed a high degree of synteny among the chloroplast genomes of the ancient angiosperms, key minor variations were evident, particularly in the contraction/expansion of the inverted-repeat regions and in RNA-editing events. Genome structure, and gene content and arrangement were highly conserved among the chloroplast genomes. The intergeneric relationships defined in this study are congruent with those inferred using morphological data.

Differences in Codon Usage Bias between Photosynthesis-Related Genes and Genetic System-Related Genes of Chloroplast Genomes in Cultivated and Wild Solanum Species

Solanum is one of the largest genera, including two important crops-potato (Solanum tuberosum) and tomato (Solanum lycopersicum). In this study we compared the chloroplast codon usage bias (CUB) among 12 Solanum species, between photosynthesis-related genes (Photo-genes) and genetic system-related genes (Genet-genes), and between cultivated species and wild relatives. The Photo-genes encode proteins for photosystems, the photosynthetic electron transport chain, and RuBisCO, while the Genet-genes encode proteins for ribosomal subunits, RNA polymerases, and maturases. The following findings about the Solanum chloroplast genome CUB were obtained: (1) the nucleotide composition, gene expression, and selective pressure are identified as the main factors affecting chloroplast CUB; (2) all these 12 chloroplast genomes prefer A/U over G/C and pyrimidines over purines at the third-base of codons; (3) Photo-genes have higher codon adaptation indexes than Genet-genes, indicative of a higher gene expression level and a stronger adaptation of Photo-genes; (4) gene function is the primary factor affecting CUB of Photo-genes but not Genet-genes; (5) Photo-genes prefer pyrimidine over purine, whereas Genet-genes favor purine over pyrimidine, at the third position of codons; (6) Photo-genes are mainly affected by the selective pressure, whereas Genet-genes are under the underlying mutational bias; (7) S. tuberosum is more similar with Solanum commersonii than with Solanum bulbocastanum; (8) S. lycopersicum is greatly different from the analyzed seven wild relatives; (9) the CUB in codons for valine, aspartic acid, and threonine are the same between the two crop species, S. tuberosum and S. lycopersicum. These findings suggest that the chloroplast CUB contributed to the differential requirement of gene expression activity and function between Photo-genes and Genet-genes and to the performance of cultivated potato and tomato.

Genome-wide analysis of codon usage bias in four sequenced cotton species

Codon usage bias (CUB) is an important evolutionary feature in a genome which provides important information for studying organism evolution, gene function and exogenous gene expression. The CUB and its shaping factors in the nuclear genomes of four sequenced cotton species, G. arboreum (A₂), G. raimondii (D₅), G. hirsutum (AD₁) and G. barbadense (AD₂) were analyzed in the present study. The effective number of codons (ENC) analysis showed the CUB was weak in these four species and the four subgenomes of the two tetraploids. Codon composition analysis revealed these four species preferred to use pyrimidine-rich codons more frequently than purine-rich codons. Correlation analysis indicated that the base content at the third position of codons affect the degree of codon preference. PR2-bias plot and ENC-plot analyses revealed that the CUB patterns in these genomes and subgenomes were influenced by combined effects of translational selection, directional mutation and other factors. The translational selection (P2) analysis results, together with the non-significant correlation between GC12 and GC3, further revealed that translational selection played the dominant role over mutation pressure in the codon usage bias. Through relative synonymous codon usage (RSCU) analysis, we detected 25 high frequency codons preferred to end with T or A, and 31 low frequency codons inclined to end with C or G in these four species and four subgenomes. Finally, 19 to 26 optimal codons with 19 common ones were determined for each species and subgenomes, which preferred to end with A or T. We concluded that the codon usage bias was weak and the translation selection was the main shaping factor in nuclear genes of these four cotton genomes and four subgenomes.

Codon usage and codon pair patterns in non-grass monocot genomes

BACKGROUND AND AIMS

Studies on codon usage in monocots have focused on grasses, and observed patterns of this taxon were generalized to all monocot species. Here, non-grass monocot species were analysed to investigate the differences between grass and non-grass monocots.

METHODS

First, studies of codon usage in monocots were reviewed. The current information was then extended regarding codon usage, as well as codon-pair context bias, using four completely sequenced non-grass monocot genomes (Musa acuminata, Musa balbisiana, Phoenix dactylifera and Spirodela polyrhiza) for which comparable transcriptome datasets are available. Measurements were taken regarding relative synonymous codon usage, effective number of codons, derived optimal codon and GC content and then the relationships investigated to infer the underlying evolutionary forces.

KEY RESULTS

The research identified optimal codons, rare codons and preferred codon-pair context in the non-grass monocot species studied. In contrast to the bimodal distribution of GC3 (GC content in third codon position) in grasses, non-grass monocots showed a unimodal distribution. Disproportionate use of G and C (and of A and T) in two- and four-codon amino acids detected in the analysis rules out the mutational bias hypothesis as an explanation of genomic variation in GC content. There was found to be a positive relationship between CAI (codon adaptation index; predicts the level of expression of a gene) and GC3. In addition, a strong correlation was observed between coding and genomic GC content and negative correlation of GC3 with gene length, indicating a strong impact of GC-biased gene conversion (gBGC) in shaping codon usage and nucleotide composition in non-grass monocots.

CONCLUSION

Optimal codons in these non-grass monocots show a preference for G/C in the third codon position. These results support the concept that codon usage and nucleotide composition in non-grass monocots are mainly driven by gBGC.