Evolution of short inverted repeat in cupressophytes, transfer of accD to nucleus in Sciadopitys verticillata and phylogenetic position of Sciadopityaceae

A Large Intergenic Spacer Leads to the Increase in Genome Size and Sequential Gene Movement around IR/SC Boundaries in the Chloroplast Genome of Adiantum malesianum (Pteridaceae)

Expansion and contraction (ebb and flow events) of inverted repeat (IR) boundaries occur and are generally considered to be major factors affecting chloroplast (cp) genome size changes. Nonetheless, the Adiantum malesianum cp genome does not seem to follow this pattern. We sequenced, assembled and corrected the A. flabellulatum and A. malesianum cp genomes using the Illumina NovaSeq6000 platform, and we performed a comparative genome analysis of six Adiantum species. The results revealed differences in the IR/SC boundaries of A. malesianum caused by a 6876 bp long rpoB-trnD-GUC intergenic spacer (IGS) in the LSC. This IGS may create topological tension towards the LSC/IRb boundary in the cp genome, resulting in a sequential movement of the LSC genes. Consequently, this leads to changes of the IR/SC boundaries and may even destroy the integrity of trnT-UGU, which is located in IRs. This study provides evidence showing that it is the large rpoB-trnD-GUC IGS that leads to A. malesianum cp genome size change, rather than ebb and flow events. Then, the study provides a model to explain how the rpoB-trnD-GUC IGS in LSC affects A. malesianum IR/SC boundaries. Moreover, this study also provides useful data for dissecting the evolution of cp genomes of Adiantum. In future research, we can expand the sample to Pteridaceae to test whether this phenomenon is universal in Pteridaceae.

Comparative Plastid Genomics of Green-Colored Dinoflagellates Unveils Parallel Genome Compaction and RNA Editing

Dinoflagellates possess plastids that are diverse in both pigmentation and evolutionary background. One of the plastid types found in dinoflagellates is pigmented with chlorophylls a and b (Chl a + b) and originated from the endosymbionts belonging to a small group of green algae, Pedinophyceae. The Chl a + b-containing plastids have been found in three distantly related dinoflagellates Lepidodinium spp., strain MGD, and strain TGD, and were proposed to be derived from separate partnerships between a dinoflagellate (host) and a pedinophycean green alga (endosymbiont). Prior to this study, a plastid genome sequence was only available for L. chlorophorum, which was reported to bear the features that were not found in that of the pedinophycean green alga Pedinomonas minor, a putative close relative of the endosymbiont that gave rise to the current Chl a + b-containing plastid. In this study, we sequenced the plastid genomes of strains MGD and TGD to compare with those of L. chlorophorum as well as pedinophycean green algae. The mapping of the RNA-seq reads on the corresponding plastid genome identified RNA editing on plastid gene transcripts in the three dinoflagellates. Further, the comparative plastid genomics revealed that the plastid genomes of the three dinoflagellates achieved several features, which are not found in or much less obvious than the pedinophycean plastid genomes determined to date, in parallel.

The plastome of the arctic Oxytropis arctobia (Fabaceae) has large differences compared with that of O. splendens and those of related species

Anatomical and physiological specializations for plant adaptation to harsh climates are the results of molecular mechanisms that can be nuclear or organellar encoded. In this study, the complete plastomes of an arctic species, Oxytropis arctobia Bunge (Fabaceae,) and a closely related temperate species, O. splendens Douglas ex Hook., were assembled, annotated and analyzed to search for differences that might help explain their adaptation to different environments. Consistently with the previously sequenced O. bicolor DC. and O. glabra plastomes, the O. arctobia and O. splendens plastomes both have the common features of the inverted repeat-lacking clade (IRLC), as well as the atpF intron loss, which is unique to the genus. However, significant differences distinguishes the O. arctobia from O. splendens and other closely related plastomes (Oxytropis spp. and Astragalus spp.), including a 3 kb inversion, two large insertions (>1 kb), significant modifications of the accD gene, and an overall larger size.

Mahonia vs. Berberis Unloaded: Generic Delimitation and Infrafamilial Classification of Berberidaceae Based on Plastid Phylogenomics

The early-diverging eudicot family Berberidaceae is composed of a morphologically diverse assemblage of disjunctly distributed genera long praised for their great horticultural and medicinal values. However, despite century-long studies, generic delimitation of Berberidaceae remains controversial and its tribal classification has never been formally proposed under a rigorous phylogenetic context. Currently, the number of accepted genera in Berberidaceae ranges consecutively from 13 to 19, depending on whether to define Berberis, Jeffersonia, and Podophyllum broadly, or to segregate these three genera further and recognize Alloberberis, Mahonia, and Moranothamnus, Plagiorhegma, and Dysosma, Diphylleia, and Sinopodophyllum, respectively. To resolve Berberidaceae's taxonomic disputes, we newly assembled 23 plastomes and, together with 85 plastomes from the GenBank, completed the generic sampling of the family. With 4 problematic and 14 redundant plastome sequences excluded, robust phylogenomic relationships were reconstructed based on 93 plastomes representing all 19 genera of Berberidaceae and three outgroups. Maximum likelihood phylogenomic relationships corroborated with divergence time estimation support the recognition of three subfamilies Berberidoideae, Nandinoideae, and Podophylloideae, with tribes Berberideae and Ranzanieae, Leonticeae and Nandineae, and Podophylleae, Achlydeae, Bongardieae tr. nov., Epimedieae, and Jeffersonieae tr. nov. in the former three subfamilies, respectively. By applying specifically stated criteria, our phylogenomic data also support the classification of 19 genera, recognizing Alloberberis, Mahonia, and Moranothamnus, Plagiorhegma, and Diphylleia, Dysosma, and Sinopodophyllum that are morphologically and evolutionarily distinct from Berberis, Jeffersonia, and Podophyllum, respectively. Comparison of plastome structures across Berberidaceae confirms inverted repeat expansion in the tribe Berberideae and reveals substantial length variation in accD gene caused by repeated sequences in Berberidoideae. Comparison of plastome tree with previous studies and nuclear ribosomal DNA (nrDNA) phylogeny also reveals considerable conflicts at different phylogenetic levels, suggesting that incomplete lineage sorting and/or hybridization had occurred throughout the evolutionary history of Berberidaceae and that Alloberberis and Moranothamnus could have resulted from reciprocal hybridization between Berberis and Mahonia in ancient times prior to the radiations of the latter two genera.

Loss of the IR region in conifer plastomes: Changes in the selection pressure and substitution rate of protein-coding genes

Plastid genomes (plastomes) have a quadripartite structure, but some species have drastically reduced or lost inverted repeat (IR) regions. IR regions are important for genome stability and the evolution rate. In the evolutionary process of gymnosperms, the typical IRs of conifers were lost, possibly affecting the evolutionary rate and selection pressure of genomic protein-coding genes. In this study, we selected 78 gymnosperm species (51 genera, 13 families) for evolutionary analysis. The selection pressure analysis results showed that negative selection effects were detected in all 50 common genes. Among them, six genes in conifers had higher ω values than non-conifers, and 12 genes had lower ω values. The evolutionary rate analysis results showed that 9 of 50 common genes differed between conifers and non-conifers. It is more obvious that in non-conifers, the rates of psbA (trst, trsv, ratio, dN, dS, and ω) were 2.6- to 3.1-fold of conifers. In conifers, trsv, ratio, dN, dS, and ω of ycf2 were 1.2- to 3.6-fold of non-conifers. In addition, the evolution rate of ycf2 in the IR was significantly reduced. psbA is undergoing dynamic change, with an abnormally high evolution rate as a small portion of it enters the IR region. Although conifers have lost the typical IR regions, we detected no change in the substitution rate or selection pressure of most protein-coding genes due to gene function, plant habitat, or newly acquired IRs.

Molecular evolution and SSRs analysis based on the chloroplast genome of Callitropsis funebris

Chloroplast genome sequences have been used to understand evolutionary events and to infer efficiently phylogenetic relationships. Callitropsis funebris (Cupressaceae) is an endemic species in China. Its phylogenetic position is controversial due to morphological characters similar to those of Cupressus, Callitropsis, and Chamaecyparis. This study used next-generation sequencing technology to sequence the complete chloroplast genome of Ca. funebris and then constructed the phylogenetic relationship between Ca. funebris and its related species based on a variety of data sets and methods. Simple sequence repeats (SSRs) and adaptive evolution analysis were also conducted. Our results showed that the monophyletic branch consisting of Ca. funebris and Cupressus tonkinensis is a sister to Cupressus, while Callitropsis is not monophyletic; Ca. nootkatensis and Ca. vietnamensis are nested in turn at the base of the monophyletic group Hesperocyparis. The statistical results of SSRs supported the closest relationship between Ca. funebris and Cupressus. By performing adaptive evolution analysis under the phylogenetic background of Cupressales, the Branch model detected three genes and the Site model detected 10 genes under positive selection; and the Branch-Site model uncovered that rpoA has experienced positive selection in the Ca. funebries branch. Molecular analysis from the chloroplast genome highly supported that Ca. funebris is at the base of Cupressus. Of note, SSR features were found to be able to shed some light on phylogenetic relationships. In short, this chloroplast genomic study has provided new insights into the phylogeny of Ca. funebris and revealed multiple chloroplast genes possibly undergoing adaptive evolution.

Insights into phylogenetic relationships and genome evolution of subfamily Commelinoideae (Commelinaceae Mirb.) inferred from complete chloroplast genomes

Background

Commelinaceae (Commelinales) comprise 41 genera and are widely distributed in both the Old and New Worlds, except in Europe. The relationships among genera in this family have been suggested in several morphological and molecular studies. However, it is difficult to explain their relationships due to high morphological variations and low support values. Currently, many researchers have been using complete chloroplast genome data for inferring the evolution of land plants. In this study, we completed 15 new plastid genome sequences of subfamily Commelinoideae using the Mi-seq platform. We utilized genome data to reveal the structural variations and reconstruct the problematic positions of genera for the first time.

Results

All examined species of Commelinoideae have three pseudogenes (accD, rpoA, and ycf15), and the former two might be a synapomorphy within Commelinales. Only four species in tribe Commelineae presented IR expansion, which affected duplication of the rpl22 gene. We identified inversions that range from approximately 3 to 15 kb in four taxa (Amischotolype, Belosynapsis, Murdannia, and Streptolirion). The phylogenetic analysis using 77 chloroplast protein-coding genes with maximum parsimony, maximum likelihood, and Bayesian inference suggests that Palisota is most closely related to tribe Commelineae, supported by high support values. This result differs significantly from the current classification of Commelinaceae. Also, we resolved the unclear position of Streptoliriinae and the monophyly of Dichorisandrinae. Among the ten CDS (ndhH, rpoC2, ndhA, rps3, ndhG, ndhD, ccsA, ndhF, matK, and ycf1), which have high nucleotide diversity values (Pi > 0.045) and over 500 bp length, four CDS (ndhH, rpoC2, matK, and ycf1) show that they are congruent with the topology derived from 77 chloroplast protein-coding genes.

Conclusions

In this study, we provide detailed information on the 15 complete plastid genomes of Commelinoideae taxa. We identified characteristic pseudogenes and nucleotide diversity, which can be used to infer the family evolutionary history. Also, further research is needed to revise the position of Palisota in the current classification of Commelinaceae.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07541-1.

Comprehensive genomic analyses with 115 plastomes from algae to seed plants: structure, gene contents, GC contents, and introns

BACKGROUND

Chloroplasts are a common character in plants. The chloroplasts in each plant lineage have shaped their own genomes, plastomes, by structural changes and transferring many genes to nuclear genomes during plant evolution. Some plastid genes have introns that are mostly group II introns.

OBJECTIVE

This study aimed to get genomic and evolutionary insights on the plastomes from green algae to flowering plants.

METHODS

Plastomes of 115 species from green algae, bryophytes, pteridophytes (spore bearing vascular plants), gymnosperms, and angiosperms were mined from NCBI organelle genome database. Plastome structure, gene contents and GC contents were analyzed by the in-house developed Phyton code. Intronic features including presence/absence, length, intron phases were analyzed by manually in the annotated information in NCBI.

RESULTS

The canonical quadripartite structures were retained in most plastomes except of a few plastomes that had lost an invert repeat (IR). Expansion or reduction or deletion of IRs resulted in the length variation of the plastomes. The number of protein coding genes ranged from 40 to 92 with an average 79.43 ± 5.84 per plastome and gene losses were apparent in specific lineages. The number of trn genes ranged from 13 to 33 with an average 21.19 ± 2.42 per plastome. Ribosomal RNA genes, rrn, were located in the IRs so that they were present in a duplicate except of the species that had lost one of the IR. GC contents were variable from 24.9 to 51.0% with an average 38.21 ± 3.27%, indicating bias to high AT contents. Plastid introns were present in 18 protein coding genes, six trn genes, and one rrn gene. Intron losses occurred among the orthologous genes in different plant lineages. The plastid introns were long compared with the nuclear introns, which might be related with the spliceosome nuclear introns and self-splicing group II plastid introns. The trnK-UUU intron contained the maturase encoding matK gene except in the chlorophyte algae and monilophyte ferns in which the trnK-UUU was lost, but matK retained. There were many annotation artefacts in the intron positions in the NCBI database. In the analysis of intron phases, phase 0 introns were more frequent than those of phase 2 and 3 introns. Phase polymorphism was observed in the introns of clpP which was derived from nucleotide insertion. Plastid trn introns were long compared to the archaeal or eukaryotic nuclear tRNA introns. Of the six plastid trn introns, one was at the D loop and other five were at the anticodon loop. The insertion sites were conserved among the trn genes in archaea, eukaryotic nuclear and plastid tRNA genes.

CONCLUSIONS

Current study refurbrished the previous findings of structural variations, gene contents, and GC contents of the chloroplast genomes from green algae to flowering plants. The study also included some noble findings and discussions on the plastome introns including their length variations and phase variation. We also presented and corrected some false annotations on the introns in protein coding and tRNA genes in the genome database, which might be confirmed by the chloroplast transcriptome analysis in the future.

Two Independent Plastid accD Transfers to the Nuclear Genome of Gnetum and Other Insights on Acetyl-CoA Carboxylase Evolution in Gymnosperms

Acetyl-CoA carboxylase (ACCase) is the key regulator of fatty acid biosynthesis. In most plants, ACCase exists in two locations (cytosol and plastids) and in two forms (homomeric and heteromeric). Heteromeric ACCase comprises four subunits, three of them (ACCA-C) are nuclear encoded (nr) and the fourth (ACCD) is usually plastid encoded. Homomeric ACCase is encoded by a single nr-gene (ACC). We investigated the ACCase gene evolution in gymnosperms by examining the transcriptomes of newly sequenced Gnetum ula, combined with 75 transcriptomes and 110 plastomes of other gymnosperms. AccD-coding sequences are elongated through the insertion of repetitive DNA in four out of five cupressophyte families (except Sciadopityaceae) and were functionally transferred to the nucleus of gnetophytes and Sciadopitys. We discovered that, among the three genera of gnetophytes, only Gnetum has two copies of nr-accD. Furthermore, using protoplast transient expression assays, we experimentally verified that the nr-accD precursor proteins in Gnetum and Sciadopitys can be delivered to the plastids. Of the two nr-accD copies of Gnetum, one dually targets plastids and mitochondria, whereas the other potentially targets plastoglobuli. The distinct transit peptides, gene architectures, and flanking sequences between the two Gnetum accDs suggest that they have independent origins. Our findings are the first account of two distinctly targeted nr-accDs of any green plants and the most comprehensive analyses of ACCase evolution in gymnosperms to date.

Complete Chloroplast Genome of Pinus densiflora Siebold & Zucc. and Comparative Analysis with Five Pine Trees

Pinus densiflora (Korean red pine) is widely distributed in East Asia and considered one of the most important species in Korea. In this study, the complete chloroplast genome of P. densiflora was sequenced by combining the advantages of Oxford Nanopore MinION and Illumina MiSeq. The sequenced genome was then compared with that of a previously published conifer plastome. The chloroplast genome was found to be circular and comprised of a quadripartite structure, including 113 genes encoding 73 proteins, 36 tRNAs and 4 rRNAs. It had short inverted repeat regions and lacked ndh gene family genes, which is consistent with other Pinaceae species. The gene content of P. densiflora was found to be most similar to that of P. sylvestris. The newly attempted sequencing method could be considered an alternative method for obtaining accurate genetic information, and the chloroplast genome sequence of P. densiflora revealed in this study can be used in the phylogenetic analysis of Pinus species.

Prevalence of isomeric plastomes and effectiveness of plastome super-barcodes in yews (Taxus) worldwide

Taxus (yew) is both the most species-rich and taxonomically difficult genus in Taxaceae. To date, no study has elucidated the complexities of the plastid genome (plastome) or examined the possibility of whole plastomes as super-barcodes across yew species worldwide. In this study, we sequenced plastomes from two to three individuals for each of the 16 recognized yew species (including three potential cryptics) and Pseudotaxus chienii. Our comparative analyses uncovered several gene loss events that independently occurred in yews, resulting in a lower plastid gene number than other Taxaceous genera. In Pseudotaxus and Taxus, we found two isomeric arrangements that differ by the orientation of a 35 kb fragment flanked by "trnQ-IRs". These two arrangements exist in different ratios within each sampled individual, and intraspecific shifts in major isomeric arrangements are first reported here in Taxus. Moreover, we demonstrate that entire plastomes can be used to successfully discriminate all Taxus species with 100% support, suggesting that they are useful as super-barcodes for species identification. We also propose that accD and rrn16-rrn23 are promising special barcodes to discriminate yew species. Our newly developed Taxus plastomic sequences provide a resource for super-barcodes and conservation genetics of several endangered yews and serve as comprehensive data to improve models of plastome complexity in Taxaceae as a whole and authenticate Taxus species.

Novel genetic code and record-setting AT-richness in the highly reduced plastid genome of the holoparasitic plant Balanophora

Plastid genomes (plastomes) vary enormously in size and gene content among the many lineages of nonphotosynthetic plants, but key lineages remain unexplored. We therefore investigated plastome sequence and expression in the holoparasitic and morphologically bizarre Balanophoraceae. The two Balanophora plastomes examined are remarkable, exhibiting features rarely if ever seen before in plastomes or in any other genomes. At 15.5 kb in size and with only 19 genes, they are among the most reduced plastomes known. They have no tRNA genes for protein synthesis, a trait found in only three other plastid lineages, and thus Balanophora plastids must import all tRNAs needed for translation. Balanophora plastomes are exceptionally compact, with numerous overlapping genes, highly reduced spacers, loss of all cis-spliced introns, and shrunken protein genes. With A+T contents of 87.8% and 88.4%, the Balanophora genomes are the most AT-rich genomes known save for a single mitochondrial genome that is merely bloated with AT-rich spacer DNA. Most plastid protein genes in Balanophora consist of ≥90% AT, with several between 95% and 98% AT, resulting in the most biased codon usage in any genome described to date. A potential consequence of its radical compositional evolution is the novel genetic code used by Balanophora plastids, in which TAG has been reassigned from stop to tryptophan. Despite its many exceptional properties, the Balanophora plastome must be functional because all examined genes are transcribed, its only intron is correctly trans-spliced, and its protein genes, although highly divergent, are evolving under various degrees of selective constraint.

The Repeat Sequences and Elevated Substitution Rates of the Chloroplast accD Gene in Cupressophytes

The plastid accD gene encodes a subunit of the acetyl-CoA carboxylase (ACCase) enzyme. The length of accD gene has been supposed to expand in Cryptomeria japonica, Taiwania cryptomerioides, Cephalotaxus, Taxus chinensis, and Podocarpus lambertii, and the main reason for this phenomenon was the existence of tandemly repeated sequences. However, it is still unknown whether the accD gene length in other cupressophytes has expanded. Here, in order to investigate how widespread this phenomenon was, 18 accD sequences and its surrounding regions of cupressophyte were sequenced and analyzed. Together with 39 GenBank sequence data, our taxon sampling covered all the extant gymnosperm orders. The repetitive elements and substitution rates of accD among 57 gymnosperm species were analyzed, the results show: (1) Reading frame length of accD gene in 18 cupressophytes species has also expanded. (2) Many repetitive elements were identified in accD gene of cupressophyte lineages. (3) The synonymous and non-synonymous substitution rates of accD were accelerated in cupressophytes. (4) accD was located in rearrangement endpoints. These results suggested that repetitive elements may mediate the chloroplast genome rearrangement and accelerated the substitution rates.

Complete Chloroplast Genome of Pinus massoniana (Pinaceae): Gene Rearrangements, Loss of ndh Genes, and Short Inverted Repeats Contraction, Expansion

The chloroplast genome (CPG) of Pinus massoniana belonging to the genus Pinus (Pinaceae), which is a primary source of turpentine, was sequenced and analyzed in terms of gene rearrangements, ndh genes loss, and the contraction and expansion of short inverted repeats (IRs). P. massoniana CPG has a typical quadripartite structure that includes large single copy (LSC) (65,563 bp), small single copy (SSC) (53,230 bp) and two IRs (IRa and IRb, 485 bp). The 108 unique genes were identified, including 73 protein-coding genes, 31 tRNAs, and 4 rRNAs. Most of the 81 simple sequence repeats (SSRs) identified in CPG were mononucleotides motifs of A/T types and located in non-coding regions. Comparisons with related species revealed an inversion (21,556 bp) in the LSC region; P. massoniana CPG lacks all 11 intact ndh genes (four ndh genes lost completely; the five remained truncated as pseudogenes; and the other two ndh genes remain as pseudogenes because of short insertions or deletions). A pair of short IRs was found instead of large IRs, and size variations among pine species were observed, which resulted from short insertions or deletions and non-synchronized variations between "IRa" and "IRb". The results of phylogenetic analyses based on whole CPG sequences of 16 conifers indicated that the whole CPG sequences could be used as a powerful tool in phylogenetic analyses.

Contrasting Patterns of Nucleotide Substitution Rates Provide Insight into Dynamic Evolution of Plastid and Mitochondrial Genomes of Geranium

Geraniaceae have emerged as a model system for investigating the causes and consequences of variation in plastid and mitochondrial genomes. Incredible structural variation in plastid genomes (plastomes) and highly accelerated evolutionary rates have been reported in selected lineages and functional groups of genes in both plastomes and mitochondrial genomes (mitogenomes), and these phenomena have been implicated in cytonuclear incompatibility. Previous organelle genome studies have included limited sampling of Geranium, the largest genus in the family with over 400 species. This study reports on rates and patterns of nucleotide substitutions in plastomes and mitogenomes of 17 species of Geranium and representatives of other Geraniaceae. As detected across other angiosperms, substitution rates in the plastome are 3.5 times higher than the mitogenome in most Geranium. However, in the branch leading to Geranium brycei/Geranium incanum mitochondrial genes experienced significantly higher dN and dS than plastid genes, a pattern that has only been detected in one other angiosperm. Furthermore, rate accelerations differ in the two organelle genomes with plastomes having increased dN and mitogenomes with increased dS. In the Geranium phaeum/Geranium reflexum clade, duplicate copies of clpP and rpoA genes that experienced asymmetric rate divergence were detected in the single copy region of the plastome. In the case of rpoA, the branch leading to G. phaeum/G. reflexum experienced positive selection or relaxation of purifying selection. Finally, the evolution of acetyl-CoA carboxylase is unusual in Geraniaceae because it is only the second angiosperm family where both prokaryotic and eukaryotic ACCases functionally coexist in the plastid.

Large-Scale Comparative Analysis Reveals the Mechanisms Driving Plastomic Compaction, Reduction, and Inversions in Conifers II (Cupressophytes)

Conifers II (cupressophytes), comprising about 400 tree species in five families, are the most diverse group of living gymnosperms. Their plastid genomes (plastomes) are highly variable in size and organization, but such variation has never been systematically studied. In this study, we assessed the potential mechanisms underlying the evolution of cupressophyte plastomes. We analyzed the plastomes of 24 representative genera in all of the five cupressophyte families, focusing on their variation in size, noncoding DNA content, and nucleotide substitution rates. Using a tree-based method, we further inferred the ancestral plastomic organizations of internal nodes and evaluated the inversions across the evolutionary history of cupressophytes. Our data showed that variation in plastome size is statistically associated with the dynamics of noncoding DNA content, which results in different degrees of plastomic compactness among the cupressophyte families. The degrees of plastomic inversions also vary among the families, with the number of inversions per genus ranging from 0 in Araucariaceae to 1.27 in Cupressaceae. In addition, we demonstrated that synonymous substitution rates are significantly correlated with plastome size as well as degree of inversions. These data suggest that in cupressophytes, mutation rates play a critical role in driving the evolution of plastomic size while plastomic inversions evolve in a neutral manner.

Complete plastid genome sequence of Primula sinensis (Primulaceae): structure comparison, sequence variation and evidence for accD transfer to nucleus

Species-rich genus Primula L. is a typical plant group with which to understand genetic variance between species in different levels of relationships. Chloroplast genome sequences are used to be the information resource for quantifying this difference and reconstructing evolutionary history. In this study, we reported the complete chloroplast genome sequence of Primula sinensis and compared it with other related species. This genome of chloroplast showed a typical circular quadripartite structure with 150,859 bp in sequence length consisting of 37.2% GC base. Two inverted repeated regions (25,535 bp) were separated by a large single-copy region (82,064 bp) and a small single-copy region (17,725 bp). The genome consists of 112 genes, including 78 protein-coding genes, 30 tRNA genes and four rRNA genes. Among them, seven coding genes, seven tRNA genes and four rRNA genes have two copies due to their locations in the IR regions. The accD and infA genes lacking intact open reading frames (ORF) were identified as pseudogenes. SSR and sequence variation analyses were also performed on the plastome of Primula sinensis, comparing with another available plastome of P. poissonii. The four most variable regions, rpl36–rps8, rps16–trnQ, trnH–psbA and ndhC–trnV, were identified. Phylogenetic relationship estimates using three sub-datasets extracted from a matrix of 57 protein-coding gene sequences showed the identical result that was consistent with previous studies. A transcript found from P. sinensis transcriptome showed a high similarity to plastid accD functional region and was identified as a putative plastid transit peptide at the N-terminal region. The result strongly suggested that plastid accD has been functionally transferred to the nucleus in P. sinensis.

Birth of Four Chimeric Plastid Gene Clusters in Japanese Umbrella Pine

Many genes in the plastid genomes (plastomes) of plants are organized as gene clusters, in which genes are co-transcribed, resembling bacterial operons. These plastid operons are highly conserved, even among conifers, whose plastomes are highly rearranged relative to other seed plants. We have determined the complete plastome sequence of Sciadopitys verticillata (Japanese umbrella pine), the sole member of Sciadopityaceae. The Sciadopitys plastome is characterized by extensive inversions, pseudogenization of four tRNA genes after tandem duplications, and a unique pair of 370-bp inverted repeats involved in the formation of isomeric plastomes. We showed that plastomic inversions in Sciadopitys have led to shuffling of the remote conserved operons, resulting in the birth of four chimeric gene clusters. Our data also demonstrated that the relocated genes can be co-transcribed in these chimeric gene clusters. The plastome of Sciadopitys advances our current understanding of how the conifer plastomes have evolved toward increased diversity and complexity.