Plastome sequences help to improve the systematic position of trinerved Lindera species in the family Lauraceae

Intraspecific differentiation of Lindera obtusiloba as revealed by comparative plastomic and evolutionary analyses

Lindera obtusiloba Blume is the northernmost tree species in the family Lauraceae, and it is a key species in understanding the evolutionary history of this family. The species of L. obtusiloba in East Asia has diverged into the Northern and Southern populations, which are geographically separated by an arid belt. Though the morphological differences between populations have been observed and well documented, intraspecific variations at the plastomic level have not been systematically investigated to date. Here, ten chloroplast genomes of L. obtusiloba individuals were sequenced and analyzed along with three publicly available plastomes. Comparative plastomic analysis suggests that both the Northern and the Southern populations share similar overall structure, gene order, and GC content in their plastomes although the size of the plasome and the level of intraspecific variability do vary between the two populations. The Northern have relatively larger plastomes while the Southern population possesses higher intraspecific variability, which could be attributed to the complexity of the geological environments in the South. Phylogenomic analyses also support the split of the Northern and Southern clades among L. obtusiloba individuals. However, there is no obvious species boundary between var. obtusiloba and var. heterophylla in the Southern population, indicating that gene flow could still occur between these two varieties, and this could be used as a good example of reticulate evolution. It is also found that a few photosynthesis-related genes are under positive selection, which is mainly related to the geological and environmental differences between the Northern and the Southern regions. Our results provide a reference for phylogenetic analysis within species and suggest that phylogenomic analyses with a sufficient number of nuclear and chloroplast genomic target loci from widely distributed individuals could provide a deeper understanding of the population evolution of the widespread species.

Comparative Analysis of Chloroplast Genomes of "Tiantai Wu-Yao" (Lindera aggregata) and Taxa of the Same Genus and Different Genera

Lindera aggregata is a species of the Lauraceae family, which has important medicinal, economic and ornamental values. In this study, we sequenced, assembled and annotated the chloroplast genome of L. aggregata and reannotated and corrected eight unverified annotations in the same genus. The chloroplast genomes taxa from Lindera and from different genera of Lauraceae were compared and analyzed, and their phylogenetic relationship and divergence time were speculated. All the 36 chloroplast genomes had typical quadripartite structures that ranged from 150,749 to 154,736 bp in total length. These genomes encoded 111-112 unique genes, including 78-79 protein-coding genes, 29-30 tRNA and 4 rRNA. Furthermore, there were 78-97 SSRs loci in these genomes, in which mononucleotide repeats were the most abundant; there were 24-49 interspersed repeats, and forward repeat types were the most frequent. The codon bias patterns of all species tended to use codons ending with A or U. Five and six highly variable regions were identified within genus and between genera, respectively, and three common regions (ycf1, ndhF-rpl32 and rpl32-trnL) were identified, which can be used as important DNA markers for phylogeny and species identification. According to the evaluation of the Ka/Ks ratio, most of the genes were under purifying selection, and only 10 genes were under positive selection. Finally, through the construction of the evolutionary tree of 39 chloroplast genomes, the phylogenetic relationship of Lauraceae was clarified and the evolutionary relationship of Lindera was revealed. The species of genus Lindera experienced rapid adaptive radiation from Miocene to Pleistocene. The results provided valuable insights for the study of chloroplast genomes in the Lauraceae family, especially in the genus Lindera.

Comparative analyses of eight complete plastid genomes of two hemiparasitic Cassytha vines in the family Lauraceae

Cassytha is the sole genus of hemiparasitic vines (ca. 20 spp.) belonging to the Cassytheae tribe of the Lauraceae family. It is extensively distributed in tropical and subtropical regions. In this study, we determined the complete plastid genome sequences of C. filiformis and C. larsenii, which do not possess the typical quadripartite structure. The length of C. filiformis plastomes ranged from 114,215 to 114,618 bp, whereas that of C. larsenii plastomes ranged from 114,900 to 114,988 bp. Comparative genomic analysis revealed 1,013 mutation sites, four large intragenomic deletions, and five highly variable regions in the eight plastome sequences. Phylogenetic analyses based on 61 complete plastomes of Laurales species, 19 ITS sequences, and trnK barcodes from 91 individuals of Cassytha spp. confirmed a non-basal group comprising individuals of C. filiformis, C. larsenii, and C. pubescens in the family Lauraceae and proposed a sister relationship between C. filiformis and C. larsenii. Further morphological comparisons indicated that the presence or absence of hairs on the haustoria and the shape or size of fruits were useful traits for differentiating C. filiformis and C. larsenii.

DNA barcoding of Cymbidium by genome skimming: Call for next-generation nuclear barcodes

Cymbidium is an orchid genus that has undergone rapid radiation and has high ornamental, economic, ecological and cultural importance, but its classification based on morphology is controversial. The plastid genome (plastome), as an extension of plant standard DNA barcodes, has been widely used as a potential molecular marker for identifying recently diverged species or complicated plant groups. In this study, we newly generated 237 plastomes of 50 species (at least two individuals per species) by genome skimming, covering 71.4% of members of the genus Cymbidium. Sequence-based analyses (barcoding gaps and automatic barcode gap discovery) and tree-based analyses (maximum likelihood, Bayesian inference and multirate Poisson tree processes model) were conducted for species identification of Cymbidium. Our work provides a comprehensive DNA barcode reference library for Cymbidium species identification. The results show that compared with standard DNA barcodes (rbcL + matK) as well as the plastid trnH-psbA, the species identification rate of the plastome increased moderately from 58% to 68%. At the same time, we propose an optimized identification strategy for Cymbidium species. The plastome cannot completely resolve the species identification of Cymbidium, the main reasons being incomplete lineage sorting, artificial cultivation, natural hybridization and chloroplast capture. To further explore the potential use of nuclear data in identifying species, the Skmer method was adopted and the identification rate increased to 72%. It appears that nuclear genome data have a vital role in species identification and are expected to be used as next-generation nuclear barcodes.

Complete plastid genome of Litsea honghoensis Liou 1933, an endangered evergreen species in China

Litsea honghoensis Liou is an endangered tree endemic to south China. In this study, the first complete plastid genome of L. honghoensis was presented, which had a length of 152,605 base pairs (bp) with a GC content of 39.20%. The genome consisted of a large single-copy (LSC) region of 93,560 bp, a small single-copy (SSC) region of 18,905 bp, and two inverted repeat regions (IRa and IRb) of 20,070 bp. There were 125 genes in the plastid genome, including 81 protein-coding genes, 36 transfer RNA (tRNA) genes, and eight ribosomal RNA (rRNA) genes. Phylogenomic analysis based on 52 complete plastomes of Laureae in the family Lauraceae supports the close relationships among L. honghoensis, Lindera communis, Lindera nacusua, Lindera angustifolia, and Lindera glauca.

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.

The complete plastid genome of a medicinal tree Lindera chienii Cheng 1934 (Lauraceae: Laureae)

Lindera chienii Cheng 1934 is an important medicine plant. The first complete plastid genome sequence of L. chienii was assembled and analyzed in this study. The plastid genome is 152,744 bp in length with a GC content of 39.15%, contains a large single-copy region of 93,767 bp and a small single-copy region of 18,843 bp, which were separated by a pair of inverted repeat regions of 20,067 bp. A total of 128 genes were detected in the plastid genome, including eight ribosomal RNA genes, 36 transfer RNA genes, and 81 protein-coding genes. The phylogenomic analysis based on plastid genomes supports the close relationships among Lindera chienii, L. megaphylla and Litsea acutivena.

Plastid genome evolution of a monophyletic group in the subtribe Lauriineae (Laureae, Lauraceae)

Litsea, a non-monophyletic group of the tribe Laureae (Lauraceae), plays important roles in the tropical and subtropical forests of Asia, Australia, Central and North America, and the islands of the Pacific. However, intergeneric relationships between Litsea and Laurus, Lindera, Parasassafras and Sinosassafras of the tribe Laureae remain unresolved. In this study, we present phylogenetic analyses of seven newly sequenced Litsea plastomes, together with 47 Laureae plastomes obtained from public databases, representing six genera of the Laureae. Our results highlight two highly supported monophyletic groups of Litsea taxa. One is composed of 16 Litsea taxa and two Lindera taxa. The 18 plastomes of these taxa were further compared for their gene structure, codon usage, contraction and expansion of inverted repeats, sequence repeats, divergence hotspots, and gene evolution. The complete plastome size of newly sequenced taxa varied between 152,377 bp (Litsea auriculata) and 154,117 bp (Litsea pierrei). Seven of the 16 Litsea plastomes have a pair of insertions in the IRa (trnL-trnH) and IRb (ycf2) regions. The 18 plastomes of Litsea and Lindera taxa exhibit similar gene features, codon usage, oligonucleotide repeats, and inverted repeat dynamics. The codons with the highest frequency among these taxa favored A/T endings and each of these plastomes had nine divergence hotspots, which are located in the same regions. We also identified six protein coding genes (accD, ndhJ, rbcL, rpoC2, ycf1 and ycf2) under positive selection in Litsea; these genes may play important roles in adaptation of Litsea species to various environments.

Chloroplast Phylogenomic Analyses Reveal a Maternal Hybridization Event Leading to the Formation of Cultivated Peanuts

Peanuts (Arachis hypogaea L.) offer numerous healthy benefits, and the production of peanuts has a prominent role in global food security. As a result, it is in the interest of society to improve the productivity and quality of peanuts with transgenic means. However, the lack of a robust phylogeny of cultivated and wild peanut species has limited the utilization of genetic resources in peanut molecular breeding. In this study, a total of 33 complete peanut plastomes were sequenced, analyzed and used for phylogenetic analyses. Our results suggest that sect. Arachis can be subdivided into two lineages. All the cultivated species are contained in Lineage I with AABB and AA are the two predominant genome types present, while species in Lineage II possess diverse genome types, including BB, KK, GG, etc. Phylogenetic studies also indicate that all allotetraploid cultivated peanut species have been derived from a possible maternal hybridization event with one of the diploid Arachis duranensis accessions being a potential AA sub-genome ancestor. In addition, Arachis monticola, a tetraploid wild species, is placed in the same group with all the cultivated peanuts, and it may represent a transitional species, which has been through the recent hybridization event. This research could facilitate a better understanding of the taxonomic status of various Arachis species/accessions and the evolutionary relationship among them, and assists in the correct and efficient use of germplasm resources in breeding efforts to improve peanuts for the benefit of human beings.

Distinct late Pleistocene subtropical-tropical divergence revealed by fifteen low-copy nuclear genes in a dominant species in South-East China

In East Asia, genetic divergence is usually considered to be correlated to different floristic regions, however, subtropical-tropical divergence is largely ignored, compared to widely explored temperate-subtropical divergence. Lindera aggregata (Lauraceae), a dominant species in South-East China was selected to address this issue. Fifteen low-copy nuclear genes (LCGs) and four chloroplast DNA (cpDNA) fragments were used to detect its evolutionary history. In LCGs, STRUCTURE and dated Bayesian phylogeny analyses detect distinct subtropical-tropical divergence since late Pleistocene. Approximate Bayesian calculation (ABC) further supports the distinct subtropical-tropical divergence, and close related Taiwan and South China populations are diverged at the last interglacial. Isolation by distance, isolation by environment and isolation by resistance analyses suggest the current climatic difference rather than geographical distance contributes to the genetic differentiation. Principle component analysis shows populations of tropical cluster occur in warmer area with higher precipitation. Ancestral area reconstruction based on Bayesian phylogeny indicates that ancestral L. aggregata populations are distributed in tropical region. In cpDNA, although unique haplotypes are found in tropical region, distinct subtropical-tropical divergence is absent. In conclusion, distinct late Pleistocene subtropical-tropical divergence of L. aggregata is triggered by climate. It is likely that L. aggregata is originated in Southwest-South China and experienced hierarchical dispersal from south to north. The South China Sea land bridge has dual role in connecting or isolating Taiwan and mainland populations since the last glaciation.

Conflicting phylogenetic signals in plastomes of the tribe Laureae (Lauraceae)

Background

Gene tree discordance is common in phylogenetic analyses. Many phylogenetic studies have excluded non-coding regions of the plastome without evaluating their impact on tree topology. In general, plastid loci have often been treated as a single unit, and tree discordance among these loci has seldom been examined. Using samples of Laureae (Lauraceae) plastomes, we explored plastome variation among the tribe, examined the influence of non-coding regions on tree topology, and quantified intra-plastome conflict.

Results

We found that the plastomes of Laureae have low inter-specific variation and are highly similar in structure, size, and gene content. Laureae was divided into three groups, subclades I, II and III. The inclusion of non-coding regions changed the phylogenetic relationship among the three subclades. Topologies based on coding and non-coding regions were largely congruent except for the relationship among subclades I, II and III. By measuring the distribution of phylogenetic signal across loci that supported different topologies, we found that nine loci (two coding regions, two introns and five intergenic spacers) played a critical role at the contentious node.

Conclusions

Our results suggest that subclade III and subclade II are successively sister to subclade I. Conflicting phylogenetic signals exist between coding and non-coding regions of Laureae plastomes. Our study highlights the importance of evaluating the influence of non-coding regions on tree topology and emphasizes the necessity of examining discordance among different plastid loci in phylogenetic studies.

The complete chloroplast genome of Lindera pulcherrima var. hemsleyana (Lauraceae)

Lindera pulcherrima var. hemsleyana is an evergreen tree of the genus Lindera with important medicinal value. The complete chloroplast genome of L. pulcherrima var. hemsleyana is 152,710 bp in length, containing a LSC region of 93,751 bp, a SSC region of 18,813 bp, and a pair of inverted repeats (IRA and IRB) of 20,073 bp each. The genome encoded 128 genes, including 83 protein-coding genes, 8 ribosomal RNA genes, 36 transfer RNA genes, and one pseudogene (ycf1). Most of these genes occurred in a single copy, whereas 15 genes occurred in double copies, including all rRNA, 6 tRNA, and 5 protein-coding genes. The GC content of the whole genome, LSC, SSC, and IR regions is 39.2%, 38.0%, 33.9%, and 44.4%, respectively. A total of 90 SSRs were discovered, the numbers of mono-, di-, tri-, tetra-, penta-, and hexa-nucleotides SSRs were 66, 10, 4, 8, 1, and 1, respectively. Phylogenetic analysis of cp genomes from 24 species of Lauraceae revealed that cp genomes of L. pulcherrima var. hemsleyana, L. pulcherrima var. attenuata, and L. pulcherrima formed a monophyletic clade with 100% bootstrap value.

The complete plastid genome of an evergreen tree Litsea elongata (Lauraceae: Laureae)

Litsea elongata (Nees) J. D. Hooker is an economically important timber and medicine tree. In this study, the complete plastid genome of L. elongata was assembled and analyzed. The plastid genome mapped a 154,027 bp circular DNA molecule with a GC content of 39.2%, consisting of a large single-copy region (LSC) of 93,688 bp, a small single-copy region (SSC) of 18,851 bp, and two inverted repeat regions (IRa and Irb) of 20,744 bp. A total of 127 genes were detected in the plastid genome, including eight ribosomal RNA (rRNA) genes, 36 transfer RNA (tRNA) genes, and 83 protein-coding genes. Phylogenomic analysis based on 39 complete plastomes of Laureae in the family Lauraceae supports the close relationships among L. coreana, L. elongata, L. japonica, and L. pierrei.