The complete mitochondrial genome sequence of the hornwort Phaeoceros laevis: retention of many ancient pseudogenes and conservative evolution of mitochondrial genomes in hornworts

Recurrent evolutionary switches of mitochondrial cytochrome c maturation systems in Archaeplastida

Mitochondrial cytochrome c maturation (CCM) requires heme attachment via distinct pathways termed systems I and III. The mosaic distribution of these systems in Archaeplastida raises questions about the genetic mechanisms and evolutionary forces promoting repeated evolution. Here, we show a recurrent shift from ancestral system I to the eukaryotic-specific holocytochrome c synthase (HCCS) of system III in 11 archaeplastid lineages. Archaeplastid HCCS is sufficient to rescue mutants of yeast system III and Arabidopsis system I. Algal HCCS mutants exhibit impaired growth and respiration, and altered biochemical and metabolic profiles, likely resulting from deficient CCM and reduced cytochrome c-dependent respiratory activity. Our findings demonstrate that archaeplastid HCCS homologs function as system III components in the absence of system I. These results elucidate the evolutionary trajectory and functional divergence of CCM pathways in Archaeplastida, providing insight into the causes, mechanisms, and consequences of repeated cooption of an entire biological pathway.

Complete chloroplast genomes of Cerastium alpinum, C. arcticum and C. nigrescens: genome structures, comparative and phylogenetic analysis

The genus Cerastium includes about 200 species that are mostly found in the temperate climates of the Northern Hemisphere. Here we report the complete chloroplast genomes of Cerastium alpinum, C. arcticum and C. nigrescens. The length of cp genomes ranged from 147,940 to 148,722 bp. Their quadripartite circular structure had the same gene organization and content, containing 79 protein-coding genes, 30 tRNA genes, and four rRNA genes. Repeat sequences varied from 16 to 23 per species, with palindromic repeats being the most frequent. The number of identified SSRs ranged from 20 to 23 per species and they were mainly composed of mononucleotide repeats containing A/T units. Based on Ka/Ks ratio values, most genes were subjected to purifying selection. The newly sequenced chloroplast genomes were characterized by a high frequency of RNA editing, including both C to U and U to C conversion. The phylogenetic relationships within the genus Cerastium and family Caryophyllaceae were reconstructed based on the sequences of 71 protein-coding genes. The topology of the phylogenetic tree was consistent with the systematic position of the studied species. All representatives of the genus Cerastium were gathered in a single clade with C. glomeratum sharing the least similarity with the others.

Adaptive evolution of the enigmatic Takakia now facing climate change in Tibet

The most extreme environments are the most vulnerable to transformation under a rapidly changing climate. These ecosystems harbor some of the most specialized species, which will likely suffer the highest extinction rates. We document the steepest temperature increase (2010-2021) on record at altitudes of above 4,000 m, triggering a decline of the relictual and highly adapted moss Takakia lepidozioides. Its de-novo-sequenced genome with 27,467 protein-coding genes includes distinct adaptations to abiotic stresses and comprises the largest number of fast-evolving genes under positive selection. The uplift of the study site in the last 65 million years has resulted in life-threatening UV-B radiation and drastically reduced temperatures, and we detected several of the molecular adaptations of Takakia to these environmental changes. Surprisingly, specific morphological features likely occurred earlier than 165 mya in much warmer environments. Following nearly 400 million years of evolution and resilience, this species is now facing extinction.

Comparative analysis of mitochondrial genomes of two alpine medicinal plants of Gentiana (Gentianaceae)

Gentiana crassicaulis and G. straminea are alpine plants of Gentiana with important medicinal value and complex genetic backgrounds. In this study, the mitochondrial genomes (mtDNAs) of these two species were sequenced. The mtDNAs of G. crassicaulis and G. straminea are 368,808 and 410,086 bp long, respectively, 52 and 49 unique genes are annotated in the two species, and the gene arrangement varies widely. Compared to G. crassicaulis, G. straminea loses three effective genes, namely atp6, trnG-GCC and trnV-GAC. As a pseudogene, the atp6 gene of G. straminea is incomplete, which is rare in higher plants. We detected 1696 and 1858 pairs of long repeats and 213 SSRs and 250 SSs in the mtDNAs of G. crassicaulis and G. straminea, respectively. There are 392 SNPs and 18 InDels between the two genomes, and syntenic sequence and structural variation analysis show low collinearity between the two genomes. Chloroplast DNA transferring to mtDNA is observed in both species, and 46,511 and 55,043 bp transferred segments containing three tRNA genes are identified, respectively. Comparative analysis of mtDNAs of G. crassicaulis, G. straminea and four species of Gentianales determined 18 core genes, and there is no specific gene in G. crassicaulis and G. straminea. The phylogenetic tree based on mtDNAs places Gentianaceae in a branch of Gentianales. This study is the first to analyze the mtDNAs of Gentianaceae, which could provide information for analysis of the structure of mtDNAs of higher plants and phylogenetic research of Gentianaceae and Gentianales.

An empirical analysis of mtSSRs: could microsatellite distribution patterns explain the evolution of mitogenomes in plants?

Microsatellites (SSRs) are tandem repeat sequences in eukaryote genomes, including plant cytoplasmic genomes. The mitochondrial genome (mtDNA) has been shown to vary in size, number, and distribution of SSRs among different plant groups. Thus, SSRs contribute with genomic diversity in mtDNAs. However, the abundance, distribution, and evolutionary significance of SSRs in mtDNA from a wide range of algae and plants have not been explored. In this study, the mtDNAs of 204 plant and algal species were investigated related to the presence of SSRs. The number of SSRs was positively correlated with genome size. Its distribution is dependent on plant and algal groups analyzed, although the cluster analysis indicates the conservation of some common motifs in algal and terrestrial plants that reflect common ancestry of groups. Many SSRs in coding and non-coding regions can be useful for molecular markers. Moreover, mitochondrial SSRs are highly abundant, representing an important source for natural or induced genetic variation, i.e., for biotechnological approaches that can modulate mtDNA gene regulation. Thus, this comparative study increases the understanding of the plant and algal SSR evolution and brings perspectives for further studies.

The complete mitochondrial genome of Cycas debaoensis revealed unexpected static evolution in gymnosperm species

Mitochondrial genomes of vascular plants are well known for their liability in architecture evolution. However, the evolutionary features of mitogenomes at intra-generic level are seldom studied in vascular plants, especially among gymnosperms. Here we present the complete mitogenome of Cycas debaoensis, an endemic cycad species to the Guangxi region in southern China. In addition to assemblage of draft mitochondrial genome, we test the conservation of gene content and mitogenomic stability by comparing it to the previously published mitogenome of Cycas taitungensis. Furthermore, we explored the factors such as structural rearrangements and nuclear surveillance of double-strand break repair (DSBR) proteins in Cycas in comparison to other vascular plant groups. The C. debaoensis mitogenome is 413,715 bp in size and encodes 69 unique genes, including 40 protein coding genes, 26 tRNAs, and 3 rRNA genes, similar to that of C. taitungensis. Cycas mitogenomes maintained the ancestral intron content of seed plants (26 introns), which is reduced in other lineages of gymnosperms, such as Ginkgo biloba, Taxus cuspidata and Welwitschia mirabilis due to selective pressure or retroprocessing events. C. debaoensis mitogenome holds 1,569 repeated sequences (> 50 bp), which partially account for fairly large intron size (1200 bp in average) of Cycas mitogenome. The comparison of RNA-editing sites revealed 267 shared non-silent editing site among predicted vs. empirically observed editing events. Another 33 silent editing sites from empirical data increase the total number of editing sites in Cycas debaoensis mitochondrial protein coding genes to 300. Our study revealed unexpected conserved evolution between the two Cycas species. Furthermore, we found strict collinearity of the gene order along with the identical set of genomic content in Cycas mt genomes. The stability of Cycas mt genomes is surprising despite the existence of large number of repeats. This structural stability may be related to the relative expansion of three DSBR protein families (i.e., RecA, OSB, and RecG) in Cycas nuclear genome, which inhibit the homologous recombinations, by monitoring the accuracy of mitochondrial chromosome repair.

Gauging the trends of pseudogenes in plants

Pseudogenes, the debilitated parts of ancient genes, were previously scrapped off as junk or discarded genes with no functional significance. Pseudogenes have come under scrutiny for their functionality, since recent studies have unveiled their importance in the regulation of their corresponding parent genes and various biological mechanisms. Despite the enormous occurrence of pseudogenes in plants, the lack of experimental validation has contributed toward their unresolved roles in gene regulation. Contrarily, most of the studies associated with gene regulation have been mainly reported for humans, mice, and other mammalian genomes. Consequently, in order to present a cumulative report on plant-based pseudogenes research, an attempt has been made to assemble multiple studies presenting the pseudogene classification, the prediction and the determination of comparative accuracies of various computational pipelines, and recent trends in analyzing their biological functions, and regulatory mechanisms. This review represents the classical, as well as the recent advances on pseudogene identification and their potential roles in transcriptional regulation, which could possibly invigorate the quality of genome annotation, evolutionary analysis, and complexity surrounding the regulatory pathways in plants. Thus, when the ambiguous boundary girdling the pseudogenes eventually recedes on account of their explicit orchestration role, research in flora would no longer saunter compared to that on fauna.

Haslea silbo, A Novel Cosmopolitan Species of Blue Diatoms

Specimens of a new species of blue diatoms from the genus Haslea Simonsen were discovered in geographically distant sampling sites, first in the Canary Archipelago, then North Carolina, Gulf of Naples, the Croatian South Adriatic Sea, and Turkish coast of the Eastern Mediterranean Sea. An exhaustive characterization of these specimens, using a combined morphological and genomic approach led to the conclusion that they belong to a single new to science cosmopolitan species, Haslea silbo sp. nov. A preliminary characterization of its blue pigment shows similarities to marennine produced by Haslea ostrearia, as evidenced by UV-visible spectrophotometry and Raman spectrometry. Life cycle stages including auxosporulation were also observed, providing data on the cardinal points of this species. For the two most geographically distant populations (North Carolina and East Mediterranean), complete mitochondrial and plastid genomes were sequenced. The mitogenomes of both strains share a rare atp6 pseudogene, but the number, nature, and positions of the group II introns inside its cox1 gene differ between the two populations. There are also two pairs of genes fused in single ORFs. The plastid genomes are characterized by large regions of recombination with plasmid DNA, which are in both cases located between the ycf35 and psbA genes, but whose content differs between the strains. The two sequenced strains hosts three plasmids coding for putative serine recombinase protein whose sequences are compared, and four out of six of these plasmids were highly conserved.

The complete mitogenome sequence of the coral lily (Lilium pumilum) and the Lanzhou lily (Lilium davidii) in China

Background

The mitogenomes of higher plants are conserved. This study was performed to complete the mitogenome of two China Lilium species (Lilium pumilum Redouté and Lilium davidii var. unicolor (Hoog) cotton).

Methods

Genomic DNA was separately extracted from the leaves of L. pumilum and L. davidii in triplicate and used for sequencing. The mitogenome of Allium cepa was used as a reference. Genome assembly, annotation and phylogenetic tree were analyzed.

Results

The mitogenome of L. pumilum and L. davidii was 988,986 bp and 924,401 bp in length, respectively. There were 22 core protein-coding genes (including atp1, atp4, atp6, atp9, ccmB, ccmC, ccmFc, ccmFN1, ccmFN2, cob, cox3, matR, mttB, nad1, nad2, nad3, nad4, nad4L, nad5, nad6, nad7 and nad9), one open reading frame and one ribosomal protein-coding gene (rps12) in the mitogenomes. Compared with the A. cepa mitogenome, the coding sequence of the 24 genes and intergenic spacers in L. pumilum and L. davidii mitogenome contained 1,621 and 1,617 variable sites, respectively. In the phylogenetic tree, L. pumilum and L. davidii were distinct from A. cepa (NC_030100).

Conclusions

L. pumilum and L. davidii mitogenomes have far distances from other plants. This study provided additional information on the species resources of China Lilium.

Extensive Shifts from Cis- to Trans-splicing of Gymnosperm Mitochondrial Introns

Hundreds of plant mitogenomes have been sequenced from angiosperms, but relatively few mitogenomes are available from its sister lineage, gymnosperms. To examine mitogenomic diversity among extant gymnosperms, we generated draft mitogenomes from 11 diverse species and compared them with four previously published mitogenomes. Examined mitogenomes from Pinaceae and cycads retained all 41 protein genes and 26 introns present in the common ancestor of seed plants, whereas gnetophyte and cupressophyte mitogenomes experienced extensive gene and intron loss. In Pinaceae and cupressophyte mitogenomes, an unprecedented number of exons are distantly dispersed, requiring trans-splicing of 50-70% of mitochondrial introns to generate mature transcripts. RNAseq data confirm trans-splicing of these dispersed exons in Pinus. The prevalence of trans-splicing in vascular plant lineages with recombinogenic mitogenomes suggests that genomic rearrangement is the primary cause of shifts from cis- to trans-splicing in plant mitochondria.

Plastid genomes and phylogenomics of liverworts (Marchantiophyta): Conserved genome structure but highest relative plastid substitution rate in land plants

With some 7300 species of small nonvascular spore-producing plants, liverworts represent one of the major lineages of land plants. Although multi-locus molecular phylogenetic studies have elucidated relationships of liverworts at different taxonomic categories, the backbone phylogeny of liverworts is still to be fully resolved, especially for the placement of Ptilidiales and the relationships within Jungermanniales and Marchantiales. Here, we provided phylogenomic inferences of liverworts based on 42 newly sequenced and 24 published liverwort plastid genomes representing all but two orders of liverworts, and characterized the evolution of the plastome in liverworts. The structure of the plastid genome is overall conserved across the phylogeny of liverworts, with only two structural variants detected from simple thalloids, besides 18 out of 43 liverwort genera showing intron variations in their plastomes. Complex thalloid liverworts maintain the most plastid genes, and seem to undergo fewer gene deletions and pseudogenization events than other liverworts. Plastid phylogenetic inferences yielded mostly robustly supported relationships, and consistently resolved Ptilidiales as the sister to Porellales. The relative ratio of silent substitutions across the three genetic compartments (i.e., 1:15:10, for mitochondrial:plastid:nuclear) suggests that liverwort plastid genes have the potential to evolve faster than their nuclear counterparts, unlike in any other major land plant lineages where the mutation rate of nuclear genes overwhelm those of their plastid and mitochondrial counterparts.

Extensive variation in nucleotide substitution rate and gene/intron loss in mitochondrial genomes of Pelargonium

Geraniaceae organelle genomes have been shown to exhibit several highly unusual features compared to most other photosynthetic angiosperms. This includes massively rearranged plastomes with considerable size variation, extensive gene and intron loss, accelerated rates of nucleotide substitutions in both mitogenomes and plastomes, and biparental inheritance and cytonuclear incompatibility of the plastome. Most previous studies have focused on plastome evolution with mitogenome comparisons limited to only a few taxa or genes. In this study, mitogenomes and transcriptomes were examined for 27 species of Geraniales, including 13 species of Pelargonium. Extensive gene and intron losses were detected across the Geraniales with Pelargonium representing the most gene depauperate lineage in the family. Plotting these events on the Geraniaceae phylogenetic tree showed that gene losses occurred multiple times, whereas intron losses more closely reflected the relationships among taxa. In addition, P. australe acquired an intron by horizontal transfer. Comparisons of nucleotide substitution rates in Pelargonium showed that synonymous changes in nuclear genes were much lower than in mitochondrial genes. This is in contrast to the previously published studies that indicated that nuclear genes have 16 fold higher rates than mitochondrial genes across angiosperms. Elevated synonymous substitutions occurred for each mitochondrial gene in Pelargonium with the highest values 783 and 324 times higher than outgroups and other Geraniaceae, respectively. Pelargonium is one of four unrelated genera of angiosperms (Ajuga, Plantago and Silene) that have experienced highly accelerated nucleotide substitutions in mitogenomes. It is distinct from most angiosperms in also having elevated substitution rates in plastid genes but the cause of rate accelerations in Pelargonium plastomes and mitogenomes may be different.

Variation in protein gene and intron content among land plant mitogenomes

The functional content of the mitochondrial genome (mitogenome) is highly diverse across eukaryotes. Among land plants, our understanding of the variation in mitochondrial gene and intron content is improving from concerted efforts to densely sample mitogenomes from diverse land plants. Here I review the current state of knowledge regarding the diversity in content of protein genes and introns in the mitogenomes of all major land plant lineages. Mitochondrial protein gene content is largely conserved among mosses and liverworts, but it varies substantially among and within other land plant lineages due to convergent losses of genes encoding ribosomal proteins and, to a lesser extent, genes for proteins involved in cytochrome c maturation and oxidative phosphorylation. Mitochondrial intron content is fairly stable within each major land plant lineage, but highly variable among lineages, resulting from occasional gains and many convergent losses over time. Trans-splicing has evolved dozens of times in various vascular plant lineages, particularly those with relatively higher rates of mitogenomic rearrangement. Across eukaryotes, mitochondrial protein gene and intron content has been shaped massive convergent evolution.

A functional twintron, 'zombie' twintrons and a hypermobile group II intron invading itself in plant mitochondria

The occurrence of group II introns in plant mitochondrial genomes is strikingly different between the six major land plant clades, contrasting their highly conserved counterparts in chloroplast DNA. Their present distribution likely reflects numerous ancient intron gains and losses during early plant evolution before the emergence of seed plants. As a novelty for plant organelles, we here report on five cases of twintrons, introns-within-introns, in the mitogenomes of lycophytes and hornworts. An internal group II intron interrupts an intron-borne maturase of an atp9 intron in Lycopodiaceae, whose splicing precedes splicing of the external intron. An invasive, hypermobile group II intron in cox1, has conquered nine further locations including a previously overlooked sdh3 intron and, most surprisingly, also itself. In those cases, splicing of the external introns does not depend on splicing of the internal introns. Similar cases are identified in the mtDNAs of hornworts. Although disrupting a group I intron-encoded protein in one case, we could not detect splicing of the internal group II intron in this ‘mixed’ group I/II twintron. We suggest the name ‘zombie’ twintrons (half-dead, half-alive) for such cases where splicing of external introns does not depend any more on prior splicing of fossilized internal introns.

Why so Complex? The Intricacy of Genome Structure and Gene Expression, Associated with Angiosperm Mitochondria, May Relate to the Regulation of Embryo Quiescence or Dormancy-Intrinsic Blocks to Early Plant Life

Mitochondria play key roles in cellular-energy metabolism and are vital for plant-life, such as for successful germination and early-seedling establishment. Most mitochondria contain their own genetic system (mtDNA, mitogenome), with an intrinsic protein-synthesis machinery. Although the challenges of maintaining prokaryotic-type structures and functions are common to Eukarya, land plants possess some of the most complex organelle composition of all known organisms. Angiosperms mtDNAs are characteristically the largest and least gene-dense among the eukaryotes. They often contain highly-variable intergenic regions of endogenous or foreign origins and undergo frequent recombination events, which result in different mtDNA configurations, even between closely-related species. The expression of the mitogenome in angiosperms involves extensive mtRNA processing steps, including numerous editing and splicing events. Why do land-plant's mitochondria have to be so complex? The answer to this remains a matter of speculation. We propose that this complexity may have arisen throughout the terrestrialization of plants, as a means to control embryonic mitochondrial functions -a critical adaptive trait to optimize seed germination. The unique characteristics of plant mtDNA may play pivotal roles in the nuclear-regulation of organellar biogenesis and metabolism, possibly to control embryos quiescence or dormancy, essential determinants for the establishment of viable plantlets that can survive post-germination.

The draft mitochondrial genome of Magnolia biondii and mitochondrial phylogenomics of angiosperms

The mitochondrial genomes of flowering plants are well known for their large size, variable coding-gene set and fluid genome structure. The available mitochondrial genomes of the early angiosperms show extreme genetic diversity in genome size, structure, and sequences, such as rampant HGTs in Amborella mt genome, numerous repeated sequences in Nymphaea mt genome, and conserved gene evolution in Liriodendron mt genome. However, currently available early angiosperm mt genomes are still limited, hampering us from obtaining an overall picture of the mitogenomic evolution in angiosperms. Here we sequenced and assembled the draft mitochondrial genome of Magnolia biondii Pamp. from Magnoliaceae (magnoliids) using Oxford Nanopore sequencing technology. We recovered a single linear mitochondrial contig of 967,100 bp with an average read coverage of 122 × and a GC content of 46.6%. This draft mitochondrial genome contains a rich 64-gene set, similar to those of Liriodendron and Nymphaea, including 41 protein-coding genes, 20 tRNAs, and 3 rRNAs. Twenty cis-spliced and five trans-spliced introns break ten protein-coding genes in the Magnolia mt genome. Repeated sequences account for 27% of the draft genome, with 17 out of the 1,145 repeats showing recombination evidence. Although partially assembled, the approximately 1-Mb mt genome of Magnolia is still among the largest in angiosperms, which is possibly due to the expansion of repeated sequences, retention of ancestral mtDNAs, and the incorporation of nuclear genome sequences. Mitochondrial phylogenomic analysis of the concatenated datasets of 38 conserved protein-coding genes from 91 representatives of angiosperm species supports the sister relationship of magnoliids with monocots and eudicots, which is congruent with plastid evidence.

The Amount of RNA Editing Sites in Liverwort Organellar Genes Is Correlated with GC Content and Nuclear PPR Protein Diversity

RNA editing occurs in the organellar mRNAs of all land plants but the marchantioid liverworts, making liverworts a perfect group for studying the evolution of RNA editing. Here, we profiled the RNA editing of 42 exemplars spanning the ordinal phylogenetic diversity of liverworts, and screened for the nuclear-encoded pentatricopeptide repeat (PPR) proteins in the transcriptome assemblies of these taxa. We identified 7,428 RNA editing sites in 128 organellar genes from 31 non-marchantioid liverwort species, and characterized 25,059 PPR protein sequences. The abundance of organellar RNA editing sites varies greatly among liverwort lineages, genes, and codon positions, and shows strong positive correlations with the GC content of protein-coding genes, and the diversity of the PLS class of nuclear PPR proteins.

Interchangeable parts: The evolutionarily dynamic tRNA population in plant mitochondria

Transfer RNAs (tRNAs) remain one of the very few classes of genes still encoded in the mitochondrial genome. These key components of the protein translation system must interact with a large enzymatic network of nuclear-encoded gene products to maintain mitochondrial function. Plants have an evolutionarily dynamic mitochondrial tRNA population, including ongoing tRNA gene loss and replacement by both horizontal gene transfer from diverse sources and import of nuclear-expressed tRNAs from the cytosol. Thus, plant mitochondria represent an excellent model for understanding how anciently divergent genes can act as "interchangeable parts" during the evolution of complex molecular systems. In particular, understanding the integration of the mitochondrial translation system with elements of the corresponding machinery used in cytosolic protein synthesis is a key area for eukaryotic cellular evolution. Here, we review the increasingly detailed phylogenetic data about the evolutionary history of mitochondrial tRNA gene loss, transfer, and functional replacement that has created extreme variation in mitochondrial tRNA populations across plant species. We describe emerging tRNA-seq methods with promise for refining our understanding of the expression and subcellular localization of tRNAs. Finally, we summarize current evidence and identify open questions related to coevolutionary changes in nuclear-encoded enzymes that have accompanied turnover in mitochondrial tRNA populations.

Evolutionary Trends in the Mitochondrial Genome of Archaeplastida: How Does the GC Bias Affect the Transition from Water to Land?

Among the most intriguing mysteries in the evolutionary biology of photosynthetic organisms are the genesis and consequences of the dramatic increase in the mitochondrial and nuclear genome sizes, together with the concomitant evolution of the three genetic compartments, particularly during the transition from water to land. To clarify the evolutionary trends in the mitochondrial genome of Archaeplastida, we analyzed the sequences from 37 complete genomes. Therefore, we utilized mitochondrial, plastidial and nuclear ribosomal DNA molecular markers on 100 species of Streptophyta for each subunit. Hierarchical models of sequence evolution were fitted to test the heterogeneity in the base composition. The best resulting phylogenies were used for reconstructing the ancestral Guanine-Cytosine (GC) content and equilibrium GC frequency (GC*) using non-homogeneous and non-stationary models fitted with a maximum likelihood approach. The mitochondrial genome length was strongly related to repetitive sequences across Archaeplastida evolution; however, the length seemed not to be linked to the other studied variables, as different lineages showed diverse evolutionary patterns. In contrast, Streptophyta exhibited a powerful positive relationship between the GC content, non-coding DNA, and repetitive sequences, while the evolution of Chlorophyta reflected a strong positive linear relationship between the genome length and the number of genes.

Plant organellar RNA editing: what 30 years of research has revealed

The central dogma in biology defines the flow of genetic information from DNA to RNA to protein. Accordingly, RNA molecules generally accurately follow the sequences of the genes from which they are transcribed. This rule is transgressed by RNA editing, which creates RNA products that differ from their DNA templates. Analyses of the RNA landscapes of terrestrial plants have indicated that RNA editing (in the form of C-U base transitions) is highly prevalent within organelles (that is, mitochondria and chloroplasts). Numerous C→U conversions (and in some plants also U→C) alter the coding sequences of many of the organellar transcripts and can also produce translatable mRNAs by creating AUG start sites or eliminating premature stop codons, or affect the RNA structure, influence splicing and alter the stability of RNAs. RNA-binding proteins are at the heart of post-transcriptional RNA expression. The C-to-U RNA editing process in plant mitochondria involves numerous nuclear-encoded factors, many of which have been identified as pentatricopeptide repeat (PPR) proteins that target editing sites in a sequence-specific manner. In this review we report on major discoveries on RNA editing in plant organelles, since it was first documented 30 years ago.

Towards a plant model for enigmatic U-to-C RNA editing: the organelle genomes, transcriptomes, editomes and candidate RNA editing factors in the hornwort Anthoceros agrestis

Hornworts are crucial to understand the phylogeny of early land plants. The emergence of 'reverse' U-to-C RNA editing accompanying the widespread C-to-U RNA editing in plant chloroplasts and mitochondria may be a molecular synapomorphy of a hornwort-tracheophyte clade. C-to-U RNA editing is well understood after identification of many editing factors in models like Arabidopsis thaliana and Physcomitrella patens, but there is no plant model yet to investigate U-to-C RNA editing. The hornwort Anthoceros agrestis is now emerging as such a model system. We report on the assembly and analyses of the A. agrestis chloroplast and mitochondrial genomes, their transcriptomes and editomes, and a large nuclear gene family encoding pentatricopeptide repeat (PPR) proteins likely acting as RNA editing factors. Both organelles in A. agrestis feature high amounts of RNA editing, with altogether > 1100 sites of C-to-U and 1300 sites of U-to-C editing. The nuclear genome reveals > 1400 genes for PPR proteins with variable carboxyterminal DYW domains. We observe significant variants of the 'classic' DYW domain, in the meantime confirmed as the cytidine deaminase for C-to-U editing, and discuss the first attractive candidates for reverse editing factors given their excellent matches to U-to-C editing targets according to the PPR-RNA binding code.

Organellomic data sets confirm a cryptic consensus on (unrooted) land-plant relationships and provide new insights into bryophyte molecular evolution

PREMISE

Phylogenetic trees of bryophytes provide important evolutionary context for land plants. However, published inferences of overall embryophyte relationships vary considerably. We performed phylogenomic analyses of bryophytes and relatives using both mitochondrial and plastid gene sets, and investigated bryophyte plastome evolution.

METHODS

We employed diverse likelihood-based analyses to infer large-scale bryophyte phylogeny for mitochondrial and plastid data sets. We tested for changes in purifying selection in plastid genes of a mycoheterotrophic liverwort (Aneura mirabilis) and a putatively mycoheterotrophic moss (Buxbaumia), and compared 15 bryophyte plastomes for major structural rearrangements.

RESULTS

Overall land-plant relationships conflict across analyses, generally weakly. However, an underlying (unrooted) four-taxon tree is consistent across most analyses and published studies. Despite gene coverage patchiness, relationships within mosses, liverworts, and hornworts are largely congruent with previous studies, with plastid results generally better supported. Exclusion of RNA edit sites restores cases of unexpected non-monophyly to monophyly for Takakia and two hornwort genera. Relaxed purifying selection affects multiple plastid genes in mycoheterotrophic Aneura but not Buxbaumia. Plastid genome structure is nearly invariant across bryophytes, but the tufA locus, presumed lost in embryophytes, is unexpectedly retained in several mosses.

CONCLUSIONS

A common unrooted tree underlies embryophyte phylogeny, [(liverworts, mosses), (hornworts, vascular plants)]; rooting inconsistency across studies likely reflects substantial distance to algal outgroups. Analyses combining genomic and transcriptomic data may be misled locally for heavily RNA-edited taxa. The Buxbaumia plastome lacks hallmarks of relaxed selection found in mycoheterotrophic Aneura. Autotrophic bryophyte plastomes, including Buxbaumia, hardly vary in overall structure.

Maternal Inheritance of U's Triangle and Evolutionary Process of Brassica Mitochondrial Genomes

The sequences and genomic structures of plant mitochondrial (mt) genomes provide unique material for phylogenetic studies. The nature of uniparental inheritance renders an advantage when utilizing mt genomes for determining the parental sources of hybridized taxa. In this study, a concatenated matrix of mt genes was used to infer the phylogenetic relationships of six cultivated Brassica taxa and explore the maternal origins of three allotetraploids. The well-resolved sister relationships between two pairs of diploid and allotetraploid taxa suggest that Brassica carinata (car) possessed a maternal origin from Brassica nigra, while Brassica juncea (jun) was maternally derived from Brassica rapa (cam). Another allotetraploid taxon, Brassica napus (cv. Wester) may have been maternally derived from the common ancestor of B. rapa and Brassica oleracea (ole), and/or have undergone (an) extra hybridization event(s) along its evolutionary history. The characteristics of Brassica mt genomic structures also supported the phylogenetic results. Sinapis arvensis was nested inside the Brassica species, sister to the B. nigra-B. carinata lineage, and possessed an mt genome structure that mostly resembled B. nigra. Collectively, the evidence supported a systematic revision that placed S. arvensis within Brassica. Finally, ancestral mt genomes at each evolutionary node of Brassica were reconstructed, and the detailed and dynamic evolution of Brassica mt genomes was successfully reproduced. The mt genome of B. nigra structurally resembled that of the Brassica ancestor the most, with only one reversion of a block, and the Brassica oleracea underwent the most drastic changes. These findings suggested that repeat-mediated recombinations were largely responsible for the observed structural variations in the evolutionary history of Brassica mt genomes.

Mitochondrial genomes of the early land plant lineage liverworts (Marchantiophyta): conserved genome structure, and ongoing low frequency recombination

BACKGROUND

In contrast to the highly labile mitochondrial (mt) genomes of vascular plants, the architecture and composition of mt genomes within the main lineages of bryophytes appear stable and invariant. The available mt genomes of 18 liverwort accessions representing nine genera and five orders are syntenous except for Gymnomitrion concinnatum whose genome is characterized by two rearrangements. Here, we expanded the number of assembled liverwort mt genomes to 47, broadening the sampling to 31 genera and 10 orders spanning much of the phylogenetic breadth of liverworts to further test whether the evolution of the liverwort mitogenome is overall static.

RESULTS

Liverwort mt genomes range in size from 147 Kb in Jungermanniales (clade B) to 185 Kb in Marchantiopsida, mainly due to the size variation of intergenic spacers and number of introns. All newly assembled liverwort mt genomes hold a conserved set of genes, but vary considerably in their intron content. The loss of introns in liverwort mt genomes might be explained by localized retroprocessing events. Liverwort mt genomes are strictly syntenous in genome structure with no structural variant detected in our newly assembled mt genomes. However, by screening the paired-end reads, we do find rare cases of recombination, which means multiple concurrent genome structures may exist in the vegetative tissues of liverworts. Our phylogenetic analyses of the nuclear encoded double stand break repair protein families revealed liverwort-specific subfamilies expansions.

CONCLUSIONS

The low repeat recombination level, selection, along with the intensified nuclear surveillance, might together shape the structural evolution of liverwort mt genomes.

Complete sequence of kenaf (Hibiscus cannabinus) mitochondrial genome and comparative analysis with the mitochondrial genomes of other plants

Plant mitochondrial (mt) genomes are species specific due to the vast of foreign DNA migration and frequent recombination of repeated sequences. Sequencing of the mt genome of kenaf (Hibiscus cannabinus) is essential for elucidating its evolutionary characteristics. In the present study, single-molecule real-time sequencing technology (SMRT) was used to sequence the complete mt genome of kenaf. Results showed that the complete kenaf mt genome was 569,915 bp long and consisted of 62 genes, including 36 protein-coding, 3 rRNA and 23 tRNA genes. Twenty-five introns were found among nine of the 36 protein-coding genes, and five introns were trans-spliced. A comparative analysis with other plant mt genomes showed that four syntenic gene clusters were conserved in all plant mtDNAs. Fifteen chloroplast-derived fragments were strongly associated with mt genes, including the intact sequences of the chloroplast genes psaA, ndhB and rps7. According to the plant mt genome evolution analysis, some ribosomal protein genes and succinate dehydrogenase genes were frequently lost during the evolution of angiosperms. Our data suggest that the kenaf mt genome retained evolutionarily conserved characteristics. Overall, the complete sequencing of the kenaf mt genome provides additional information and enhances our better understanding of mt genomic evolution across angiosperms.

Assembly and comparative analysis of the complete mitochondrial genome sequence of Sophora japonica 'JinhuaiJ2'

Sophora japonica L. (Faboideae, Leguminosae) is an important traditional Chinese herb with a long history of cultivation. Its flower buds and fruits contain abundant flavonoids, and therefore, the plants are cultivated for the industrial extraction of rutin. Here, we determined the complete nucleotide sequence of the mitochondrial genome of S. japonica ‘JinhuaiJ2’, the most widely planted variety in Guangxi region of China. The total length of the mtDNA sequence is 484,916 bp, with a GC content of 45.4%. Sophora japonica mtDNA harbors 32 known protein-coding genes, 17 tRNA genes, and three rRNA genes with 17 cis-spliced and five trans-spliced introns disrupting eight protein-coding genes. The gene coding and intron regions, and intergenic spacers account for 7.5%, 5.8% and 86.7% of the genome, respectively. The gene profile of S. japonica mitogenome differs from that of the other Faboideae species by only one or two gene gains or losses. Four of the 17 cis-spliced introns showed distinct length variations in the Faboideae, which could be attributed to the homologous recombination of the short repeats measuring a few bases located precisely at the edges of the putative deletions. This reflects the importance of small repeats in the sequence evolution in Faboideae mitogenomes. Repeated sequences of S. japonica mitogenome are mainly composed of small repeats, with only 20 medium-sized repeats, and one large repeat, adding up to 4% of its mitogenome length. Among the 25 pseudogene fragments detected in the intergenic spacer regions, the two largest ones and their corresponding functional gene copies located in two different sets of medium-sized repeats, point to their origins from homologous recombinations. As we further observed the recombined reads associated with the longest repeats of 2,160 bp with the PacBio long read data set of just 15 × in depth, repeat mediated homologous recombinations may play important role in the mitogenomic evolution of S. japonica. Our study provides insightful knowledge to the genetic background of this important herb species and the mitogenomic evolution in the Faboideae species.

The complete mitochondrial genome of the early flowering plant Nymphaea colorata is highly repetitive with low recombination

BACKGROUND

Mitochondrial genomes of flowering plants (angiosperms) are highly dynamic in genome structure. The mitogenome of the earliest angiosperm Amborella is remarkable in carrying rampant foreign DNAs, in contrast to Liriodendron, the other only known early angiosperm mitogenome that is described as 'fossilized'. The distinctive features observed in the two early flowering plant mitogenomes add to the current confusions of what early flowering plants look like. Expanded sampling would provide more details in understanding the mitogenomic evolution of early angiosperms. Here we report the complete mitochondrial genome of water lily Nymphaea colorata from Nymphaeales, one of the three orders of the earliest angiosperms.

RESULTS

Assembly of data from Pac-Bio long-read sequencing yielded a circular mitochondria chromosome of 617,195 bp with an average depth of 601×. The genome encoded 41 protein coding genes, 20 tRNA and three rRNA genes with 25 group II introns disrupting 10 protein coding genes. Nearly half of the genome is composed of repeated sequences, which contributed substantially to the intron size expansion, making the gross intron length of the Nymphaea mitochondrial genome one of the longest among angiosperms, including an 11.4-Kb intron in cox2, which is the longest organellar intron reported to date in plants. Nevertheless, repeat mediated homologous recombination is unexpectedly low in Nymphaea evidenced by 74 recombined reads detected from ten recombinationally active repeat pairs among 886,982 repeat pairs examined. Extensive gene order changes were detected in the three early angiosperm mitogenomes, i.e. 38 or 44 events of inversions and translocations are needed to reconcile the mitogenome of Nymphaea with Amborella or Liriodendron, respectively. In contrast to Amborella with six genome equivalents of foreign mitochondrial DNA, not a single horizontal gene transfer event was observed in the Nymphaea mitogenome.

CONCLUSIONS

The Nymphaea mitogenome resembles the other available early angiosperm mitogenomes by a similarly rich 64-coding gene set, and many conserved gene clusters, whereas stands out by its highly repetitive nature and resultant remarkable intron expansions. The low recombination level in Nymphaea provides evidence for the predominant master conformation in vivo with a highly substoichiometric set of rearranged molecules.

Genome-wide organellar analyses from the hornwort Leiosporoceros dussii show low frequency of RNA editing

Because hornworts occupy a pivotal position in early land colonization as sister to other bryophytes, sister to tracheophytes, or sister to all other land plants, a renewed interest has arisen in their phylogenetic diversity, morphology, and genomes. To date, only five organellar genome sequences are available for hornworts. We sequenced the plastome (155,956 bp) and mitogenome (212,153 bp) of the hornwort Leiosporoceros dussii, the sister taxon to all hornworts. The Leiosporoceros organellar genomes show conserved gene structure and order with respect to the other hornworts and other bryophytes. Additionally, using RNA-seq data we quantified the frequency of RNA-editing events (the canonical C-to-U and the reverse editing U-to-C) in both organellar genomes. In total, 109 sites were found in the plastome and 108 in the mitogenome, respectively. The proportion of edited sites corresponds to 0.06% of the plastome and 0.05% of the mitogenome (in reference to the total genome size), in contrast to 0.58% of edited sites in the plastome of Anthoceros angustus (161,162 bp). All edited sites in the plastome and 88 of 108 sites in the mitogenome are C-to-U conversions. Twenty reverse edited sites (U-to-C conversions) were found in the mitogenome (17.8%) and none in the plastome. The low frequency of RNA editing in Leiosporoceros, which is nearly 88% less than in the plastome of Anthoceros and the mitogenome of Nothoceros, indicates that the frequency of RNA editing has fluctuated during hornwort diversification. Hornworts are a pivotal land plant group to unravel the genomic implications of RNA editing and its maintenance despite the evident evolutionary disadvantages.

Complete mitochondrial genome sequence of the "copper moss" Mielichhoferia elongata reveals independent nad7 gene functionality loss

The mitochondrial genome of moss Mielichhoferia elongata has been sequenced and assembled with Spades genome assembler. It consists of 100,342 base pairs and has practically the same gene set and order as in other known bryophyte chondriomes. The genome contains 66 genes including three rRNAs, 24 tRNAs, and 40 conserved mitochondrial proteins genes. Unlike the majority of previously sequenced bryophyte mitogenomes, it lacks the functional nad7 gene. The phylogenetic reconstruction and scrutiny analysis of the primary structure of nad7 gene carried out in this study suggest its independent pseudogenization in different bryophyte lineages. Evaluation of the microsatellite (simple sequence repeat) content of the M. elongata mitochondrial genome indicates that it could be used as a tool in further studies as a phylogenetic marker. The strongly supported phylogenetic tree presented here, derived from 33 protein coding sequences of 40 bryophyte species, is consistent with other reconstructions based on a number of different data sets.

Comparative Analysis of Four Calypogeia Species Revealed Unexpected Change in Evolutionarily-Stable Liverwort Mitogenomes

Liverwort mitogenomes are considered to be evolutionarily stable. A comparative analysis of four Calypogeia species revealed differences compared to previously sequenced liverwort mitogenomes. Such differences involve unexpected structural changes in the two genes, cox1 and atp1, which have lost three and two introns, respectively. The group I introns in the cox1 gene are proposed to have been lost by two-step localized retroprocessing, whereas one-step retroprocessing could be responsible for the disappearance of the group II introns in the atp1 gene. These cases represent the first identified losses of introns in mitogenomes of leafy liverworts (Jungermanniopsida) contrasting the stability of mitochondrial gene order with certain changes in the gene content and intron set in liverworts.

The extraordinary variation of the organellar genomes of the Aneura pinguis revealed advanced cryptic speciation of the early land plants

Aneura pinguis is known as a species complex with several morphologically indiscernible species, which are often reproductively isolated from each other and show distinguishable genetic differences. Genetic dissimilarity of cryptic species may be detected by genomes comparison. This study presents the first complete sequences of chloroplast and mitochondrial genomes of six cryptic species of A. pinguis complex: A. pinguis A, B, C, E, F, J. These genomes have been compared to each other in order to reconstruct phylogenetic relationships and to gain better understanding of the evolutionary process of cryptic speciation in this complex. The chloroplast genome with the nucleotide diversity 0.05111 and 1537 indels is by far more variable than mitogenome with π value 0.00233 and number of indels 1526. Tests of selection evidenced that on about 36% of chloroplast genes and on 10% of mitochondrial genes of A. pinguis acts positive selection. It suggests an advanced speciation of species. The phylogenetic analyses based on genomes show that A. pinguis is differentiated and forms three distinct clades. Moreover, on the cpDNA trees, Aneura mirabilis is nested among the cryptic species of A. pinguis. This indicates that the A. pinguis cryptic species do not derive directly from one common ancestor.

Complete mitochondrial genomes from the ferns Ophioglossum californicum and Psilotum nudum are highly repetitive with the largest organellar introns

Currently, complete mitochondrial genomes (mitogenomes) are available from all major land plant lineages except ferns. Sequencing of fern mitogenomes could shed light on the major evolutionary transitions that established mitogenomic diversity among extant lineages. In this study, we generated complete mitogenomes from the adder's tongue fern (Ophioglossum californicum) and the whisk fern (Psilotum nudum). The Psilotum mitogenome (628 kb) contains a rich complement of genes and introns, some of which are the largest of any green plant organellar genome. In the Ophioglossum mitogenome (372 kb), gene and intron content is slightly reduced, including the loss of all four mitochondrial ccm genes. Transcripts of nuclear Ccm genes also were not detected, suggesting loss of the entire mitochondrial cytochrome c maturation pathway from Ophioglossum. Both fern mitogenomes are highly repetitive, yet they show extremely low levels of active recombination. Transcriptomic sequencing uncovered ˜1000 sites of C-to-U RNA editing in both species, plus a small number (< 60) of U-to-C edit sites. Overall, the first mitochondrial genomes of ferns show a mix of features shared with lycophytes and/or seed plants and several novel genomic features, enabling a robust reconstruction of the mitogenome in the common ancestor of vascular plants.

RNA Editing and Its Molecular Mechanism in Plant Organelles

RNA editing by cytidine (C) to uridine (U) conversions is widespread in plant mitochondria and chloroplasts. In some plant taxa, “reverse” U-to-C editing also occurs. However, to date, no instance of RNA editing has yet been reported in green algae and the complex thalloid liverworts. RNA editing may have evolved in early land plants 450 million years ago. However, in some plant species, including the liverwort, Marchantia polymorpha, editing may have been lost during evolution. Most RNA editing events can restore the evolutionarily conserved amino acid residues in mRNAs or create translation start and stop codons. Therefore, RNA editing is an essential process to maintain genetic information at the RNA level. Individual RNA editing sites are recognized by plant-specific pentatricopeptide repeat (PPR) proteins that are encoded in the nuclear genome. These PPR proteins are characterized by repeat elements that bind specifically to RNA sequences upstream of target editing sites. In flowering plants, non-PPR proteins also participate in multiple RNA editing events as auxiliary factors. C-to-U editing can be explained by cytidine deamination. The proteins discovered to date are important factors for RNA editing but a bona fide RNA editing enzyme has yet to be identified.

Monilophyte mitochondrial rps1 genes carry a unique group II intron that likely originated from an ancient paralog in rpl2

Intron patterns in plant mitochondrial genomes differ significantly between the major land plant clades. We here report on a new, clade-specific group II intron in the rps1 gene of monilophytes (ferns). This intron, rps1i25g2, is strikingly similar to rpl2i846g2 previously identified in the mitochondrial rpl2 gene of seed plants, ferns, and the lycophyte Phlegmariurus squarrosus Although mitochondrial ribosomal protein genes are frequently subject to endosymbiotic gene transfer among plants, we could retrieve the mitochondrial rps1 gene in a taxonomically wide sampling of 44 monilophyte taxa including basal lineages such as the Ophioglossales, Psilotales, and Marattiales with the only exception being the Equisetales (horsetails). Introns rps1i25g2 and rpl2i846g2 were likewise consistently present with only two exceptions: Intron rps1i25g2 is lost in the genus Ophioglossum and intron rpl2i846g2 is lost in Equisetum bogotense Both intron sequences are moderately affected by RNA editing. The unprecedented primary and secondary structure similarity of rps1i25g2 and rpl2i846g2 suggests an ancient retrotransposition event copying rpl2i846g2 into rps1, for which we suggest a model. Our phylogenetic analysis adding the new rps1 locus to a previous data set is fully congruent with recent insights on monilophyte phylogeny and further supports a sister relationship of Gleicheniales and Hymenophyllales.

Organellar phylogenomics of an emerging model system: Sphagnum (peatmoss)

BACKGROUND AND AIMS

Sphagnum-dominated peatlands contain approx. 30 % of the terrestrial carbon pool in the form of partially decomposed plant material (peat), and, as a consequence, Sphagnum is currently a focus of studies on biogeochemistry and control of global climate. Sphagnum species differ in ecologically important traits that scale up to impact ecosystem function, and sequencing of the genome from selected Sphagnum species is currently underway. As an emerging model system, these resources for Sphagnum will facilitate linking nucleotide variation to plant functional traits, and through those traits to ecosystem processes. A solid phylogenetic framework for Sphagnum is crucial to comparative analyses of species-specific traits, but relationships among major clades within Sphagnum have been recalcitrant to resolution because the genus underwent a rapid radiation. Herein a well-supported hypothesis for phylogenetic relationships among major clades within Sphagnum based on organellar genome sequences (plastid, mitochondrial) is provided.

METHODS

We obtained nucleotide sequences (273 753 nucleotides in total) from the two organellar genomes from 38 species (including three outgroups). Phylogenetic analyses were conducted using a variety of methods applied to nucleotide and amino acid sequences. The Sphagnum phylogeny was rooted with sequences from the related Sphagnopsida genera, Eosphagnum and Flatbergium

KEY RESULTS

Phylogenetic analyses of the data converge on the following subgeneric relationships: (Rigida (((Subsecunda) (Cuspidata)) ((Sphagnum) (Acutifolia))). All relationships were strongly supported. Species in the two major clades (i.e. Subsecunda + Cuspidata and Sphagnum + Acutifolia), which include >90 % of all Sphagnum species, differ in ecological niches and these differences correlate with other functional traits that impact biogeochemical cycling. Mitochondrial intron presence/absence are variable among species and genera of the Sphagnopsida. Two new nomenclatural combinations are made, in the genera Eosphagnum and Flatbergium

CONCLUSIONS

Newly resolved relationships now permit phylogenetic analyses of morphological, biochemical and ecological traits among Sphagnum species. The results clarify long-standing disagreements about subgeneric relationships and intrageneric classification.

Reverse U-to-C editing exceeds C-to-U RNA editing in some ferns - a monilophyte-wide comparison of chloroplast and mitochondrial RNA editing suggests independent evolution of the two processes in both organelles

BACKGROUND

RNA editing by C-to-U conversions is nearly omnipresent in land plant chloroplasts and mitochondria, where it mainly serves to reconstitute conserved codon identities in the organelle mRNAs. Reverse U-to-C RNA editing in contrast appears to be restricted to hornworts, some lycophytes, and ferns (monilophytes). A well-resolved monilophyte phylogeny has recently emerged and now allows to trace the side-by-side evolution of both types of pyrimidine exchange editing in the two endosymbiotic organelles.

RESULTS

Our study of RNA editing in four selected mitochondrial genes show a wide spectrum of divergent RNA editing frequencies including a dominance of U-to-C over the canonical C-to-U editing in some taxa like the order Schizaeales. We find that silent RNA editing leaving encoded amino acids unchanged is highly biased with more than ten-fold amounts of silent C-to-U over U-to-C edits. In full contrast to flowering plants, RNA editing frequencies are low in early-branching monilophyte lineages but increase in later emerging clades. Moreover, while editing rates in the two organelles are usually correlated, we observe uncoupled evolution of editing frequencies in fern mitochondria and chloroplasts. Most mitochondrial RNA editing sites are shared between the recently emerging fern orders whereas chloroplast editing sites are mostly clade-specific. Finally, we observe that chloroplast RNA editing appears to be completely absent in horsetails (Equisetales), the sister clade of all other monilophytes.

CONCLUSIONS

C-to-U and U-to-C RNA editing in fern chloroplasts and mitochondria follow disinct evolutionary pathways that are surprisingly different from what has previously been found in flowering plants. The results call for careful differentiation of the two types of RNA editing in the two endosymbiotic organelles in comparative evolutionary studies.

Rapid evolutionary divergence of Gossypium barbadense and G. hirsutum mitochondrial genomes

Background

The mitochondrial genome from upland cotton, G. hirsutum, was previously sequenced. To elucidate the evolution of mitochondrial genomic diversity within a single genus, we sequenced the mitochondrial genome from Sea Island cotton (Gossypium barbadense L.).

Methods

Mitochondrial DNA from week-old etiolated seedlings was extracted from isolated organelles using discontinuous sucrose density gradient method. Mitochondrial genome was sequenced with Solexa using paired-end, 90 bp read. The clean reads were assembled into contigs using ABySS and finished via additional fosmid and BAC sequencing. Finally, the genome was annotated and analyzed using different softwares.

Results

The G. barbadense (Sea Island cotton) mitochondrial genome was fully sequenced (677,434-bp) and compared to the mitogenome of upland cotton. The G. barbadense mitochondrial DNA contains seven more genes than that of upland cotton, with a total of 40 protein coding genes (excluding possible pseudogenes), 6 rRNA genes, and 29 tRNA genes. Of these 75 genes, atp1, mttB, nad4, nad9, rrn5, rrn18, and trnD(GTC)-cp were each represented by two identical copies. A single 64 kb repeat was largely responsible for the 9 % difference in genome size between the two mtDNAs. Comparison of genome structures between the two mitochondrial genomes revealed 8 rearranged syntenic regions and several large repeats. The largest repeat was missing from the master chromosome in G. hirsutum. Both mitochondrial genomes contain a duplicated copy of rps3 (rps3-2) in conjunction with a duplication of repeated sequences. Phylogenetic and divergence considerations suggest that a 544-bp fragment of rps3 was transferred to the nuclear genome shortly after divergence of the A- and D- genome diploid cottons.

Conclusion

These results highlight the insights to the evolution of structural variation between Sea Island and upland cotton mitochondrial genomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1988-0) contains supplementary material, which is available to authorized users.

Variable frequency of plastid RNA editing among ferns and repeated loss of uridine-to-cytidine editing from vascular plants

The distinct distribution and abundance of C-to-U and U-to-C RNA editing among land plants suggest that these two processes originated and evolve independently, but the paucity of information from several key lineages limits our understanding of their evolution. To examine the evolutionary diversity of RNA editing among ferns, we sequenced the plastid transcriptomes from two early diverging species, Ophioglossum californicum and Psilotum nudum. Using a relaxed automated approach to minimize false negatives combined with manual inspection to eliminate false positives, we identified 297 C-to-U and three U-to-C edit sites in the O. californicum plastid transcriptome but only 27 C-to-U and no U-to-C edit sites in the P. nudum plastid transcriptome. A broader comparison of editing content with the leptosporangiate fern Adiantum capillus-veneris and the hornwort Anthoceros formosae uncovered large variance in the abundance of plastid editing, indicating that the frequency and type of RNA editing is highly labile in ferns. Edit sites that increase protein conservation among species are more abundant and more efficiently edited than silent and non-conservative sites, suggesting that selection maintains functionally important editing. The absence of U-to-C editing from P. nudum plastid transcripts and other vascular plants demonstrates that U-to-C editing loss is a recurrent phenomenon in vascular plant evolution.

350 my of mitochondrial genome stasis in mosses, an early land plant lineage

Among land plants, angiosperms have the structurally most labile mitochondrial (mt) genomes. In contrast, the so-called early land plants (e.g., mosses) seem to have completely static mt chromosomes. We assembled the complete mt genomes from 12 mosses spanning the moss tree of life, to assess 1) the phylogenetic depth of the conserved mt gene content and order and 2) the correlation between scattered sequence repeats and gene order lability in land plants. The mt genome of most mosses is approximately 100 kb in size, and thereby the smallest among land plants. Based on divergence time estimates, moss mt genome structure has remained virtually frozen for 350 My, with only two independent gene losses and a single gene relocation detected across the macroevolutionary tree. This is the longest period of mt genome stasis demonstrated to date in a plant lineage. The complete lack of intergenic repeat sequences, considered to be essential for intragenomic recombinations, likely accounts for the evolutionary stability of moss mt genomes.

Comparative analysis of 11 Brassicales mitochondrial genomes and the mitochondrial transcriptome of Brassica oleracea

To elucidate the evolution of mitochondrial genomic diversity within a single order of angiosperms, we sequenced seven Brassicales genomes and the transcriptome of Brassica oleracea. In the common ancestor of Brassicaceae, several genes of known function were lost and the ccmFN gene was split into two independent genes, which also coincides with a trend of genome reduction towards the smallest sequenced angiosperm genomes of Brassica. For most ORFs of unknown function, the lack of conservation throughout Brassicales and the generally low expression and absence of RNA editing in B. oleracea argue against functionality. However, two chimeric ORFs were expressed and edited in B. oleracea, suggesting a potential role in cytoplasmic male sterility in certain nuclear backgrounds. These results demonstrate how frequent shifts in size, structure, and content of plant mitochondrial genomes can occur over short evolutionary time scales.

Organellar genomes of the four-toothed moss, Tetraphis pellucida

BACKGROUND

Mosses are the largest of the three extant clades of gametophyte-dominant land plants and remain poorly studied using comparative genomic methods. Major monophyletic moss lineages are characterised by different types of a spore dehiscence apparatus called the peristome, and the most important unsolved problem in higher-level moss systematics is the branching order of these peristomate clades. Organellar genome sequencing offers the potential to resolve this issue through the provision of both genomic structural characters and a greatly increased quantity of nucleotide substitution characters, as well as to elucidate organellar evolution in mosses. We publish and describe the chloroplast and mitochondrial genomes of Tetraphis pellucida, representative of the most phylogenetically intractable and morphologically isolated peristomate lineage.

RESULTS

Assembly of reads from Illumina SBS and Pacific Biosciences RS sequencing reveals that the Tetraphis chloroplast genome comprises 127,489 bp and the mitochondrial genome 107,730 bp. Although genomic structures are similar to those of the small number of other known moss organellar genomes, the chloroplast lacks the petN gene (in common with Tortula ruralis) and the mitochondrion has only a non-functional pseudogenised remnant of nad7 (uniquely amongst known moss chondromes).

CONCLUSIONS

Structural genomic features exist with the potential to be informative for phylogenetic relationships amongst the peristomate moss lineages, and thus organellar genome sequences are urgently required for exemplars from other clades. The unique genomic and morphological features of Tetraphis confirm its importance for resolving one of the major questions in land plant phylogeny and for understanding the evolution of the peristome, a likely key innovation underlying the diversity of mosses. The functional loss of nad7 from the chondrome is now shown to have occurred independently in all three bryophyte clades as well as in the early-diverging tracheophyte Huperzia squarrosa.

Tracing the evolution of streptophyte algae and their mitochondrial genome

Six monophyletic groups of charophycean green algae are recognized within the Streptophyta. Although incongruent with earlier studies based on genes from three cellular compartments, chloroplast and nuclear phylogenomic analyses have resolved identical relationships among these groups, placing the Zygnematales or the Zygnematales + Coleochaetales as sister to land plants. The present investigation aimed at determining whether this consensus view is supported by the mitochondrial genome and at gaining insight into mitochondrial DNA (mtDNA) evolution within and across streptophyte algal lineages and during the transition toward the first land plants. We present here the newly sequenced mtDNAs of representatives of the Klebsormidiales (Entransia fimbriata and Klebsormidium spec.) and Zygnematales (Closterium baillyanum and Roya obtusa) and compare them with their homologs in other charophycean lineages as well as in selected embryophyte and chlorophyte lineages. Our results indicate that important changes occurred at the levels of genome size, gene order, and intron content within the Zygnematales. Although the representatives of the Klebsormidiales display more similarity in genome size and intron content, gene order seems more fluid and gene losses more frequent than in other charophycean lineages. In contrast, the two members of the Charales display an extremely conservative pattern of mtDNA evolution. Collectively, our analyses of gene order and gene content and the phylogenies we inferred from 40 mtDNA-encoded proteins failed to resolve the relationships among the Zygnematales, Coleochaetales, and Charales; however, they are consistent with previous phylogenomic studies in favoring that the morphologically complex Charales are not sister to land plants.

Simple sequence repeats in bryophyte mitochondrial genomes

Simple sequence repeats (SSRs) are thought to be common in plant mitochondrial (mt) genomes, but have yet to be fully described for bryophytes. We screened the mt genomes of two liverworts (Marchantia polymorpha and Pleurozia purpurea), two mosses (Physcomitrella patens and Anomodon rugelii) and two hornworts (Phaeoceros laevis and Nothoceros aenigmaticus), and detected 475 SSRs. Some SSRs are found conserved during the evolution, among which except one exists in both liverworts and mosses, all others are shared only by the two liverworts, mosses or hornworts. SSRs are known as DNA tracts having high mutation rates; however, according to our observations, they still can evolve slowly. The conservativeness of these SSRs suggests that they are under strong selection and could play critical roles in maintaining the gene functions.

The mitochondrial genome of Malus domestica and the import-driven hypothesis of mitochondrial genome expansion in seed plants

Mitochondrial genomes of spermatophytes are the largest of all organellar genomes. Their large size has been attributed to various factors; however, the relative contribution of these factors to mitochondrial DNA (mtDNA) expansion remains undetermined. We estimated their relative contribution in Malus domestica (apple). The mitochondrial genome of apple has a size of 396 947 bp and a one to nine ratio of coding to non-coding DNA, close to the corresponding average values for angiosperms. We determined that 71.5% of the apple mtDNA sequence was highly similar to sequences of its nuclear DNA. Using nuclear gene exons, nuclear transposable elements and chloroplast DNA as markers of promiscuous DNA content in mtDNA, we estimated that approximately 20% of the apple mtDNA consisted of DNA sequences imported from other cell compartments, mostly from the nucleus. Similar marker-based estimates of promiscuous DNA content in the mitochondrial genomes of other species ranged between 21.2 and 25.3% of the total mtDNA length for grape, between 23.1 and 38.6% for rice, and between 47.1 and 78.4% for maize. All these estimates are conservative, because they underestimate the import of non-functional DNA. We propose that the import of promiscuous DNA is a core mechanism for mtDNA size expansion in seed plants. In apple, maize and grape this mechanism contributed far more to genome expansion than did homologous recombination. In rice the estimated contribution of both mechanisms was found to be similar.

The organelle genomes of Hassawi rice (Oryza sativa L.) and its hybrid in saudi arabia: genome variation, rearrangement, and origins

Hassawi rice (Oryza sativa L.) is a landrace adapted to the climate of Saudi Arabia, characterized by its strong resistance to soil salinity and drought. Using high quality sequencing reads extracted from raw data of a whole genome sequencing project, we assembled both chloroplast (cp) and mitochondrial (mt) genomes of the wild-type Hassawi rice (Hassawi-1) and its dwarf hybrid (Hassawi-2). We discovered 16 InDels (insertions and deletions) but no SNP (single nucleotide polymorphism) is present between the two Hassawi cp genomes. We identified 48 InDels and 26 SNPs in the two Hassawi mt genomes and a new type of sequence variation, termed reverse complementary variation (RCV) in the rice cp genomes. There are two and four RCVs identified in Hassawi-1 when compared to 93–11 (indica) and Nipponbare (japonica), respectively. Microsatellite sequence analysis showed there are more SSRs in the genic regions of both cp and mt genomes in the Hassawi rice than in the other rice varieties. There are also large repeats in the Hassawi mt genomes, with the longest length of 96,168 bp and 96,165 bp in Hassawi-1 and Hassawi-2, respectively. We believe that frequent DNA rearrangement in the Hassawi mt and cp genomes indicate ongoing dynamic processes to reach genetic stability under strong environmental pressures. Based on sequence variation analysis and the breeding history, we suggest that both Hassawi-1 and Hassawi-2 originated from the Indonesian variety Peta since genetic diversity between the two Hassawi cultivars is very low albeit an unknown historic origin of the wild-type Hassawi rice.