Complete Plastid Genome Sequence Research Articles

Analysis of Codon Usage Bias in the Plastid Genome of Diplandrorchis sinica (Orchidaceae).

In order to understand the bias and main affecting factors of codon usage in the plastid genome of Diplandrorchis sinica, which is a rare and endangered plant species in the Orchidaceae family, the complete plastid genome sequence of D. sinica was downloaded from the GenBank database and 20 protein-coding sequences that met the analysis requirements were finally selected. The GC content, length of the amino acid (Laa), relative synonymous codon usage (RSCU), and effective number of codon (ENC) of each gene and codon were calculated using the CodonW and EMBOSS online programs. Neutral plot analysis, ENC-plot analysis, PR2-plot analysis, and correspondence analysis were performed using Origin Pro 2024 software, and correlation analysis between various indicators was performed using SPSS 23.0 software. The results showed that the third base of the codon in the plastid genome of D. sinica was rich in A and T, with a GC3 content of 27%, which was lower than that of GC1 (45%) and GC2 (39%). The ENC value ranged from 35 to 57, with an average of 47. The codon usage bias was relatively low, and there was a significant positive correlation between ENC and GC3. There were a total of 32 codons with RSCU values greater than 1, of which 30 ended with either A or U. There were a total of nine optimal codons identified, namely, UCU, UCC, UCA, GCA, UUG, AUA, CGU, CGA, and GGU. This study indicated that the dominant factor affecting codon usage bias in the plastid genome of D. sinica was natural selection pressure, while the impact of base mutations was limited. The codon usage patterns were not closely related to gene types, and the distribution of photosynthetic system genes and ribosomal protein-coding gene loci was relatively scattered, indicating significant differences in the usage patterns of these gene codons. In addition, the codon usage patterns may not be related to whether the plant is a photosynthetic autotrophic or heterotrophic nutritional type. The results of this study could provide scientific references for the genomic evolution and phylogenetic research of plant species in the family Orchidaceae.

Comparative Analysis of Plastome Sequences of Seven Tulipa L. (Liliaceae Juss.) Species from Section Kolpakowskianae Raamsd. Ex Zonn and Veldk.

Tulipa L. is a genus of significant economic, environmental, and cultural importance in several parts of the world. The exact number of species in the genus remains uncertain due to inherent taxonomic challenges. We utilized next-generation sequencing technology to sequence and assemble the plastid genomes of seven Tulipa species collected in Kazakhstan and conducted a comparative analysis. The total number of annotated genes was 136 in all seven studied Tulipa species, 114 of which were unique, including 80 protein-coding, 30 tRNA, and 4 rRNA genes. Nine regions (petD, ndhH, ycf2-ycf3, ndhA, rpl16, clpP, ndhD-ndhF, rpoC2, and ycf1) demonstrated significant nucleotide variability, suggesting their potential as molecular markers. A total of 1388 SSRs were identified in the seven Tulipa plastomes, with mononucleotide repeats being the most abundant (60.09%), followed by dinucleotide (34.44%), tetranucleotide (3.90%), trinucleotide (1.08%), pentanucleotide (0.22%), and hexanucleotide (0.29%). The Ka/Ks values of the protein-coding genes ranged from 0 to 3.9286, with the majority showing values <1. Phylogenetic analysis based on a complete plastid genome and protein-coding gene sequences divided the species into three major clades corresponding to their subgenera. The results obtained in this study may contribute to understanding the phylogenetic relationships and molecular taxonomy of Tulipa species.

Skmer approach improves species discrimination in taxonomically problematic genus Schima (Theaceae)

Genome skimming has dramatically extended DNA barcoding from short DNA fragments to next generation barcodes in plants. However, conserved DNA barcoding markers, including complete plastid genome and nuclear ribosomal DNA (nrDNA) sequences, are inadequate for accurate species identification. Skmer, a recently proposed approach that estimates genetic distances among species based on unassembled genome skims, has been proposed to effectively improve species discrimination rate. In this study, we used Skmer to identify species based on genomic skims of 47 individuals representing 10 out of 13 species of Schima (Theaceae) from China. The unassembled reads identified six species, with a species identification rate of 60%, twice as high as previous efforts that used plastid genomes (27.27%). In addition, Skmer was able to identify Schima species with only 0.5× sequencing depth, as six species were well-supported with unassembled data sizes as small as 0.5 Gb. These findings demonstrate the potential for the Skmer approach in species identification, where nuclear genomic data plays a crucial role. For taxonomically difficult taxa such as Schima, which have diverged recently and have low levels of genetic variation, Skmer is a promising alternative to next generation barcodes.

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 114,618bp, whereas that of C. larsenii plastomes ranged from 114,900 to 114,988bp. 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.

Phylogenetic and comparative analyses of Hydnora abyssinica plastomes provide evidence for hidden diversity within Hydnoraceae

BackgroundTo date, plastid genomes have been published for all but two holoparasitic angiosperm families. However, only a single or a few plastomes represent most of these families. Of the approximately 40 genera of holoparasitic angiosperms, a complete plastid genome sequence is available for only about half. In addition, less than 15 species are currently represented with more than one published plastid genome, most of which belong to the Orobanchaceae. Therefore, a significant portion of the holoparasitic plant plastome diversity remains unexplored. This limited information could hinder potential evolutionary pattern recognition as well as the exploration of inter- and intra-species plastid genome diversity in the most extreme holoparasitic angiosperms.ResultsHere, we report the first plastomes of Kenyan Hydnora abyssinica accessions. The plastomes have a typical quadripartite structure and encode 24 unique genes. Phylogenetic tree reconstruction recovers the Kenyan accessions as monophyletic and together in a clade with the Namibian H. abyssinica accession and the recently published H. arabica from Oman. Hydnora abyssinica as a whole however is recovered as non-monophyletic, with H. arabica nested within. This result is supported by distinct structural plastome synapomorphies as well as pairwise distance estimates that reveal hidden diversity within the Hydnora species in Africa.ConclusionWe propose to increase efforts to sample widespread holoparasitic species for their plastid genomes, as is the case with H. abyssinica, which is widely distributed in Africa. Morphological reinvestigation and further molecular data are needed to fully investigate the diversity of H. abyssinica along the entire range of distribution, as well as the diversity of currently synonymized taxa.

A phylogenomic study of Iridaceae Juss. based on complete plastid genome sequences.

The plastid genome has proven to be an effective tool for examining deep correlations in plant phylogenetics, owing to its highly conserved structure, uniparental inheritance, and limited variation in evolutionary rates. Iridaceae, comprising more than 2,000 species, includes numerous economically significant taxa that are frequently utilized in food industries and medicines and for ornamental and horticulture purposes. Molecular studies on chloroplast DNA have confirmed the position of this family in the order Asparagales with non-asparagoids. The current subfamilial classification of Iridaceae recognizes seven subfamilies-Isophysioideae, Nivenioideae, Iridoideae, Crocoideae, Geosiridaceae, Aristeoideae, and Patersonioideae-which are supported by limited plastid DNA regions. To date, no comparative phylogenomic studies have been conducted on the family Iridaceae. We assembled and annotated (de novo) the plastid genomes of 24 taxa together with seven published species representing all the seven subfamilies of Iridaceae and performed comparative genomics using the Illumina MiSeq platform. The plastomes of the autotrophic Iridaceae represent 79 protein-coding, 30 tRNA, and four rRNA genes, with lengths ranging from 150,062 to 164,622 bp. The phylogenetic analysis of the plastome sequences based on maximum parsimony, maximum likelihood, and Bayesian inference analyses suggested that Watsonia and Gladiolus were closely related, supported by strong support values, which differed considerably from recent phylogenetic studies. In addition, we identified genomic events, such as sequence inversions, deletions, mutations, and pseudogenization, in some species. Furthermore, the largest nucleotide variability was found in the seven plastome regions, which can be used in future phylogenetic studies. Notably, three subfamilies-Crocoideae, Nivenioideae, and Aristeoideae-shared a common ycf2 gene locus deletion. Our study is a preliminary report of a comparative study of the complete plastid genomes of 7/7 subfamilies and 9/10 tribes, elucidating the structural characteristics and shedding light on plastome evolution and phylogenetic relationships within Iridaceae. Additionally, further research is required to update the relative position of Watsonia within the tribal classification of the subfamily Crocoideae.

Characterization of the complete plastid genome of Korean endemic, Ajuga spectabilis Nakai (Lamiaceae)

Ajuga spectabilis Nakai is a Korean endemic species in Lamiaceae. In spite of its importance, genomic studies are not performed on this species. Here, we report the complete plastid genome sequences of A. spectabilis, which will provide valuable information for its natural conservation and future studies for the plastid genome evolution. The plastid genome is 150,417 bp in length, containing a large single-copy region (LSC) of 82,140 bp and a small single-copy (SSC) region of 17,165 bp which are separated by a pair of inverted repeats (IR) of 25,556 bp. It encodes 113 genes, including 79 protein-coding genes, 30 tRNA genes, and four rRNA genes. The overall GC content is 38.3%, and those in the LSC, SSC, and IR regions are 36.4%, 32.2%, and 43.3%, respectively, which is consistent with other Ajuga species. Our phylogenetic analysis revealed that A. spectabilis formed a close relationship with A. ciliata and A. decumbens.

Plastid Phylogenomics Provide Evidence to Accept Two New Members of Ligusticopsis (Apiaceae, Angiosperms).

Peucedanum nanum and P. violaceum are recognized as members of the genus Peucedanum because of their dorsally compressed mericarps with slightly prominent dorsal ribs and narrowly winged lateral ribs. However, these species are not similar to other Peucedanum taxa but resemble Ligusticopsis in overall morphology. To check the taxonomic positions of P. nanum and P. violaceum, we sequenced their complete plastid genome (plastome) sequences and, together with eleven previously published Ligusticopsis plastomes, performed comprehensively comparative analyses. The thirteen plastomes were highly conserved and similar in structure, size, GC content, gene content and order, IR borders, and the patterns of codon bias, RNA editing, and simple sequence repeats (SSRs). Nevertheless, twelve mutation hotspots (matK, ndhC, rps15, rps8, ycf2, ccsA-ndhD, petN-psbM, psbA-trnK, rps2-rpoC2, rps4-trnT, trnH-psbA, and ycf2-trnL) were selected. Moreover, both the phylogenetic analyses based on plastomes and on nuclear ribosomal DNA internal transcribed spacer (ITS) sequences robustly supported that P. nanum and P. violaceum nested in Ligusticopsis, and this was further confirmed by the morphological evidence. Hence, transferring P. nanum and P. violaceum into Ligusticopsis genus is reasonable and convincing, and two new combinations are presented.

The complete plastome of Cynanchum rostellatum (Apocynaceae), an indigenous plant in Korea

The climbing plant Cynanchum rostellatum (Turcz.) Liede & Khanum is widely distributed throughout Korea and Northeast Asia as a member of the Apocynaceae family. Although this plant has a high value in medicinal and industrial purposes, genetic research on this plant is insufficient. This study announces the complete plastid genome (plastome) sequence of C. rostellatum with 663× mean coverage, which was assembled using 763 Mbp short-read data generated by the Illumina HiSeq X platform. The C. rostellatum plastome was 158,018 bp in length and displayed the typical quadripartite structure composed of the large single-copy (LSC) region (89,058 bp), the small single-copy (SSC) region (18,718 bp), and a pair of inverted repeat (IR) regions (25,116 bp). A total of 129 genes have been annotated, including 84 protein-coding genes, 37 transfer RNA genes, and eight ribosomal RNA genes. Phylogenetic analysis indicated the genus Cynanchum including 12 Cynanchum plastome sequences, was monophyletic and was located within the sub-family Asclepiadoideae. Two C. rostellatum plastomes, including the plastome assembled in this study, formed a subclade and were sister to the C. thesioides plastome, whereas the other C. rostellatum, which was previously reported one, was located within the clade of C. wilfordii and C. bungei.

Characteristics of the Complete Plastid Genome Sequences of the Monotypic Genus Dodecadenia (Family: Lauraceae) and Its Phylogenomic Implications

As one of a dozen monotypic genera in the family Lauraceae, the systematic position of Dodecadenia Nees remains controversial. Here, two complete plastomes of Dodecadenia grandiflora Nees were sequenced. The two plastid genomes, with the length of 152,659 bp and 152,773 bp, had similar quadripartite structure. Both consisted of one large single-copy (LSC) region with 93,740 bp and 93,791 bp, one small single-copy region (SSC) with 18,805 bp and 18,846 bp, and a pair of inverted repeats (IR) regions with 20,057 bp and 20,068 bp. A total of 128 genes were annotated for the D. grandiflora plastid genomes (plastomes), which included 84 protein-coding genes (PCGs), 36 tRNA genes and eight rRNA genes. Codon usage analysis of the D. grandiflora plastomes showed a bias toward A/U at the third codon. A total of 122 RNA editing events were predicted, and all codon conversions were cytosine to thymine. There were 30/36 oligonucleotide repeats and 89/94 simple sequence repeats in these two plastomes of D. grandiflora. Based on 71 plastomes, both Bayesian and maximum likelihood phylogenetic analyses showed that D. grandiflora are nested among the species of Litsea Lam. together with Litsea auriculata Chien et Cheng and suggested that the monotypic genus Dodecadenia Nees should be revised. In addition, the highly variable loci trnG intron and ycf3-trnS could be used as excellent candidate markers for population genetic and phylogenetic analyses of D. grandiflora.

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.

Comparative analysis of two Korean irises (Iris ruthenica and I. uniflora, Iridaceae) based on plastome sequencing and micromorphology.

Iris ruthenica Ker Gawl. and I. uniflora Pall. ex Link, which are rare and endangered species in Korea, possess considerable horticultural and medicinal value among Korean irises. However, discrimination of the species is hindered by extensive morphological similarity. Thus, the aim of the present study was to identify discriminating features by comparing the species’ complete plastid genome (i.e., plastome) sequences and micromorphological features, including leaf margins, stomatal complex distribution (hypostomatic vs. amphistomatic leaves), anther stomata density, and tepal epidermal cell patterns. Plastome comparison revealed slightly divergent regions within intergenic spacer regions, and the most variable sequences, which were distributed in non-coding regions, could be used as molecular markers for the discrimination of I. ruthenica and I. uniflora. Phylogenetic analysis of the Iris species revealed that I. ruthenica and I. uniflora formed a well-supported clade. The comparison of plastomes and micromorphological features performed in this study provides useful information for elucidating taxonomic, phylogenetic, and evolutionary relationships in Iridaceae. Further studies, including those based on molecular cytogenetic approaches using species specific markers, will offer insights into species delimitation of the two closely related Iris species.

The complete plastid genome sequence of Peucedanum hakuunense Nakai (Apiaceae), an endemic and rare species in Korea

Peucedanum hakuunense Nakai is one of the rare species in the Korean Peninsula. This study characterized the complete plastid genome (plastome) sequence of P. hakuunense by de novo assembly with next-generation sequencing data. The complete plastome of P. hakuunense is 147,426 bp in length with a typical quadripartite structure comprising a large single-copy region of 91,915 bp, a small single-copy region of 17,425 bp, and two inverted repeat regions of 19,043 bp in length. The plastome of P. hakuunense is composed of 85 protein-coding genes, 36 tRNA genes, and 8 rRNA genes. The phylogenetic analysis revealed that two Peucedanum species formed an independent subclade, sister to the subclade of Angelica species within the tribe Selineae.

Phylogenetic relationships of Aquilaria and Gyrinops (Thymelaeaceae) revisited: evidence from complete plastid genomes

Abstract The agarwood-producing genera, Aquilaria and Gyrinops, have been treated as sister genera in Thymelaeaceae (Aquilarieae, Malvales). However, there are differing opinions about their taxonomic positions. Using new evidence from complete plastid genome (plastome) sequences of 12 Aquilaria spp. and seven Gyrinops spp., corresponding to more than half the total number of species recognized in each genus, we reconstructed a phylogenetic tree of Aquilarieae. Members of both genera shared similar sequence variations at the plastome level. The phylogenetic relationships of the two genera were completely resolved using the complete plastome sequences. Gyrinops was paraphyletic to Aquilaria, i.e. Aquilaria nested in Gyrinops. Overlapping clustering patterns were observed from topology-clustering analysis using the complete plastome and the protein-coding gene sequences, which further reinforced the paraphyletic link. The maximum-likelihood-based RelTime analysis suggested that Aquilarieae and Daphneae diverged c. 44 Mya during the Eocene, followed by a rapid divergence event in Aquilarieae, with the split between G. vidalii and G. walla c. 1.24 Mya during the Pleistocene. Our new phylogenetic estimation demonstrates the relationship between the two genera, which will aid in future taxonomic revisions involving members of Aquilarieae.

The complete plastid genome of an Antarctic moss Chorisodontium aciphyllum (Hook. f. & Wilson) Broth (Dicranaceae, Dicranales)

Plastid genomes are useful markers in resolving plant phylogenetic relationships for various taxonomic groups. Here, we sequenced and de novo assembled the complete plastid genome sequence of an Antarctic moss Chorisodontium aciphyllum (Hook. f. & Wilson) Broth using genome skimming data. The newly generated plastid genome is conserved in structure and gene content compared with that of other Bryopsida. Plastid phylogenetic analysis of mosses recovered a robust phylogeny in which Chorisodontium aciphyllum clustered with Fissidens nobilis from the Dicranales. The plastid genome sequence of C. aciphyllum will aid future evolution and diversification studies of land plants.

Genome structure and diversity among Cynanchum wilfordii accessions

BackgroundCynanchum wilfordii (Cw) and Cynanchum auriculatum (Ca) have long been used in traditional medicine and as functional food in Korea and China, respectively. They have diverse medicinal functions, and many studies have been conducted, including pharmaceutical efficiency and metabolites. Especially, Cw is regarded as the most famous medicinal herb in Korea due to its menopausal symptoms relieving effect. Despite the high demand for Cw in the market, both species are cultivated using wild resources with rare genomic information.ResultsWe collected 160 Cw germplasm from local areas of Korea and analyzed their morphological diversity. Five Cw and one Ca of them, which were morphologically diverse, were sequenced, and nuclear ribosomal DNA (nrDNA) and complete plastid genome (plastome) sequences were assembled and annotated. We investigated the genomic characteristics of Cw as well as the genetic diversity of plastomes and nrDNA of Cw and Ca. The Cw haploid nuclear genome was approximately 178 Mbp. Karyotyping revealed the juxtaposition of 45S and 5S nrDNA on one of 11 chromosomes. Plastome sequences revealed 1226 interspecies polymorphisms and 11 Cw intraspecies polymorphisms. The 160 Cw accessions were grouped into 21 haplotypes based on seven plastome markers and into 108 haplotypes based on seven nuclear markers. Nuclear genotypes did not coincide with plastome haplotypes that reflect the frequent natural outcrossing events.ConclusionsCw germplasm had a huge morphological diversity, and their wide range of genetic diversity was revealed through the investigation with 14 molecular markers. The morphological and genomic diversity, chromosome structure, and genome size provide fundamental genomic information for breeding of undomesticated Cw plants.

The complete plastid genome of Cheniella didyma (H.Y.Chen) R.Clark & Mackinder (Leguminosae)

We report here for the first time the complete plastid genome of Cheniella didyma of the legume family. The plastid genome has a typical circular structure with a total length of 157,186 bp and contains two inverted repeat regions (IRs, 24,455 bp), a large single-copy region (LSC, 89,410 bp), and a small single-copy region (SSC, 18,866 bp). This is the first report of the complete plastid genome sequence of Cheniella, a genus recently segregated from Bauhinia s.l. The phylogenetic analysis based on 77 coding regions of the plastome of this species and those of the related species strongly suggested that C. didyma is sister to Lysiphyllum and is not directly related to Bauhinia s.s.

Structural and evolutive features of the Plinia phitrantha and P. cauliflora plastid genomes and evolutionary relationships within tribe Myrteae (Myrtaceae).

Plinia phitrantha and P. cauliflora are Myrtaceae species with recognized horticultural and pharmacological potential. Nevertheless, studies on molecular genetics and the evolution of these species are absent in the literature. In this study, we report the complete plastid genome sequence of these species and an analysis of structural and evolutive features of the plastid genome within the tribe Myrteae. The two plastid genomes present the conserved quadripartite structure and are similar to already reported plastid genomes of Myrteae species concerning the size, number, and order of the genes. A total of 69-70 SSR loci, 353 single nucleotide polymorphisms, and 574 indels were identified in P. phitrantha and P. caulifora. Observed evolutive features of the plastid genomes support the development of programs for the conservation and breeding of Plinia. The phylogenomic analysis based on the complete plastid genome sequence of 15 Myrteae species presented a robust phylogenetic signal and evolutive traits of the tribe. Ten hotspots of nucleotide diversity were identified, evidence of purifying selection was observed in 27 genes, and relative conservation of the plastid genomes was confirmed for Myrteae. Altogether, the outcomes of the present study provide support for planning conservation, breeding, and biotechnological programs for Plinia species.

The complete plastid genome of Kelloggia chinensis Franch. (Rubiaceae), an endemic species from East Asia

Kelloggia chinensis Franch. is an herbal plant species endemic to East Asia. Its complete plastid genome sequence is 155, 665 bp in length, with a large single-copy (LSC) region of 85, 788 bp, a small single-copy (SSC) region of 16, 977 bp, and a pair of inverted repeat regions (IRs) of 26, 450 bp. The whole plastid genome contains 132 genes, including 87 protein-coding genes, 37 tRNA genes, and 8 rRNA genes. The overall GC content of K. chinensis plastid genome is 37.1%. K. chinesis is evolutionarily close to tribe Rubieae according to the Maximum likelihood phylogenetic analysis based on 12 taxa.

The complete plastid genome sequence of the enigmatic moss, Takakia lepidozioides (Takakiopsida, Bryophyta): evolutionary perspectives on the largest collection of genes in mosses and the intensive RNA editing.

Complete chloroplast genome sequence of a moss, Takakia lepidozioides (Takakiopsida) is reported. The largest collection of genes in mosses and the intensive RNA editing were discussed from evolutionary perspectives. We assembled the entire plastid genome sequence of Takakia lepidozioides (Takakiopsida), emerging from the first phylogenetic split among extant mosses. The genome sequences were assembled into a circular molecule 149,016bp in length, with a quadripartite structure comprising a large and a small single-copy region separated by inverted repeats. It contained 88 genes coding for proteins, 32 for tRNA, four for rRNA, two open reading frames, and at least one pseudogene (tufA). This is the largest number of genes of all sequenced plastid genomes in mosses and Takakia is the only moss that retains the seven coding genes ccsA, cysA, cysT, petN rpoA, rps16 and trnPGGG. Parsimonious interpretation of gene loss suggests that the last common ancestor of bryophytes had all seven genes and that mosses lost at least three of them during their diversification. Analyses of the plastid transcriptome identified the extraordinary frequency of RNA editing with more than 1100 sites. We indicated a close correlation between the monoplastidy of vegetative tissue and the intensive RNA editing sites in the plastid genome in land plant lineages. Here, we proposed a hypothesis that the small population size of plastids in each vegetative cell of some early diverging land plants, including Takakia, might cause the frequent fixation of mutations in plastid genome through the intracellular genetic drift and that deleterious mutations might be continuously compensated by RNA editing during or following transcription.