The complete chloroplast DNA sequences of the charophycean green algae Staurastrum and Zygnema reveal that the chloroplast genome underwent extensive changes during the evolution of the Zygnematales

MatK impacts differential chloroplast translation by limiting spliced tRNA-K(UUU) abundance

The protein levels of chloroplast photosynthetic genes and genes related to the chloroplast genetic apparatus vary to adapt to different conditions. However, the underlying mechanisms governing these variations remain unclear. The chloroplast intron Maturase K is encoded within the trnK intron and has been suggested to be required for splicing several group IIA introns, including the trnK intron. In this study, we used RNA immunoprecipitation followed by high-throughput sequencing (RIP-Seq) to identify MatK's preference for binding to group IIA intron domains I and VI within target transcripts. Importantly, these domains are crucial for splice site selection, and we discovered alternative 5'-splice sites in three MatK target introns. The resulting alternative trnK lariat structure showed increased accumulation during heat acclimation. The cognate codon of tRNA-K(UUU) is highly enriched in mRNAs encoding ribosomal proteins and a trnK-matK over-expressor exhibited elevated levels of the spliced tRNA-K(UUU). Ribosome profiling analysis of the overexpressor revealed a significant up-shift in the translation of ribosomal proteins compared to photosynthetic genes. Our findings suggest the existence of a novel regulatory mechanism linked to the abundance of tRNA-K(UUU), enabling the differential expression of functional chloroplast gene groups.

Genomes of multicellular algal sisters to land plants illuminate signaling network evolution

Zygnematophyceae are the algal sisters of land plants. Here we sequenced four genomes of filamentous Zygnematophyceae, including chromosome-scale assemblies for three strains of Zygnema circumcarinatum. We inferred traits in the ancestor of Zygnematophyceae and land plants that might have ushered in the conquest of land by plants: expanded genes for signaling cascades, environmental response, and multicellular growth. Zygnematophyceae and land plants share all the major enzymes for cell wall synthesis and remodifications, and gene gains shaped this toolkit. Co-expression network analyses uncover gene cohorts that unite environmental signaling with multicellular developmental programs. Our data shed light on a molecular chassis that balances environmental response and growth modulation across more than 600 million years of streptophyte evolution.

Chloroplast genome evolution and phylogeny of the early-diverging charophycean green algae with a focus on the Klebsormidiophyceae and Streptofilum

The Klebsormidiophyceae are a class of green microalgae observed globally in both freshwater and terrestrial habitats. Morphology-based classification schemes of this class have been shown to be inadequate due to the simple morphology of these algae, the tendency of morphology to vary in culture versus field conditions, and rampant morphological homoplasy. Molecular studies revealing cryptic diversity have renewed interest in this group. We sequenced the complete chloroplast genomes of a broad series of taxa spanning the known taxonomic breadth of this class. We also sequenced the chloroplast genomes of three strains of Streptofilum, a recently discovered green algal lineage with close affinity to the Klebsormidiophyceae. Our results affirm the previously hypothesized polyphyly of the genus Klebsormidium as well as the polyphyly of the nominal species in this genus, K. flaccidum. Furthermore, plastome sequences strongly support the status of Streptofilum as a distinct, early-diverging lineage of charophytic algae sister to a clade comprising Klebsormidiophyceae plus Phragmoplastophyta. We also uncovered major structural alterations in the chloroplast genomes of species in Klebsormidium that have broad implications regarding the underlying mechanisms of chloroplast genome evolution.

The Chloroplast Genome of the Lichen Photobiont Trebouxiophyceae sp. DW1 and Its Phylogenetic Implications

Lichens are symbiotic associations of algae and fungi. The genetic mechanism of the symbiosis of lichens and the influence of symbiosis on the size and composition of the genomes of symbiotic algae have always been intriguing scientific questions explored by lichenologists. However, there were limited data on lichen genomes. Therefore, we isolated and purified a lichen symbiotic alga to obtain a single strain (Trebouxiophyceae sp. DW1), and then obtained its chloroplast genome information by next-generation sequencing (NGS). The chloroplast genome is 129,447 bp in length, and the GC content is 35.2%. Repetitive sequences with the length of 30–35 bp account for 1.27% of the total chloroplast genome. The simple sequence repeats are all mononucleotide repeats. Codon usage analysis showed that the genome tended to use codon ending in A/U. By comparing the length of different regions of Trebouxiophyceae genomes, we found that the changes in the length of exons, introns, and intergenic sequences affect the size of genomes. Trebouxiophyceae had an unstable chloroplast genome structure, with IRs repeatedly losing during evolution. Phylogenetic analysis showed that Trebouxiophyceae is paraphyletic, and Trebouxiophyceae sp. DW1 is sister to the clade of Koliella longiseta and Pabia signiensis.

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 149,016 bp 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 trnP^GGG. 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.

Organellar Introns in Fungi, Algae, and Plants

Introns are ubiquitous in eukaryotic genomes and have long been considered as ‘junk RNA’ but the huge energy expenditure in their transcription, removal, and degradation indicate that they may have functional significance and can offer evolutionary advantages. In fungi, plants and algae introns make a significant contribution to the size of the organellar genomes. Organellar introns are classified as catalytic self-splicing introns that can be categorized as either Group I or Group II introns. There are some biases, with Group I introns being more frequently encountered in fungal mitochondrial genomes, whereas among plants Group II introns dominate within the mitochondrial and chloroplast genomes. Organellar introns can encode a variety of proteins, such as maturases, homing endonucleases, reverse transcriptases, and, in some cases, ribosomal proteins, along with other novel open reading frames. Although organellar introns are viewed to be ribozymes, they do interact with various intron- or nuclear genome-encoded protein factors that assist in the intron RNA to fold into competent splicing structures, or facilitate the turn-over of intron RNAs to prevent reverse splicing. Organellar introns are also known to be involved in non-canonical splicing, such as backsplicing and trans-splicing which can result in novel splicing products or, in some instances, compensate for the fragmentation of genes by recombination events. In organellar genomes, Group I and II introns may exist in nested intronic arrangements, such as introns within introns, referred to as twintrons, where splicing of the external intron may be dependent on splicing of the internal intron. These nested or complex introns, with two or three-component intron modules, are being explored as platforms for alternative splicing and their possible function as molecular switches for modulating gene expression which could be potentially applied towards heterologous gene expression. This review explores recent findings on organellar Group I and II introns, focusing on splicing and mobility mechanisms aided by associated intron/nuclear encoded proteins and their potential roles in organellar gene expression and cross talk between nuclear and organellar genomes. Potential application for these types of elements in biotechnology are also discussed.

Characterization of Two Zygnema Strains (Zygnema circumcarinatum SAG 698-1a and SAG 698-1b) and a Rapid Method to Estimate Nuclear Genome Size of Zygnematophycean Green Algae

Zygnematophyceae green algae (ZGA) have been shown to be the closest relatives of land plants. Three nuclear genomes (Spirogloea muscicola, Mesotaenium endlicherianum, and Penium margaritaceum) of ZGA have been recently published, and more genomes are underway. Here we analyzed two Zygnema circumcarinatum strains SAG 698-1a (mating +) and SAG 698-1b (mating -) and found distinct cell sizes and other morphological differences. The molecular identities of the two strains were further investigated by sequencing their 18S rRNA, psaA and rbcL genes. These marker genes of SAG 698-1a were surprisingly much more similar to Z. cylindricum (SAG 698-2) than to SAG 698-1b. Phylogenies of these marker genes also showed that SAG 698-1a and SAG 698-1b were well separated into two different Zygnema clades, where SAG 698-1a was clustered with Z. cylindricum, while SAG 698-1b was clustered with Z. tunetanum. Additionally, physiological parameters like ETRmax values differed between SAG 698-1a and SAG 698-1b after 2 months of cultivation. The de-epoxidation state (DEPS) of the xanthophyll cycle pigments also showed significant differences. Surprisingly, the two strains could not conjugate, and significantly differed in the thickness of the mucilage layer. Additionally, ZGA cell walls are highly enriched with sticky and acidic polysaccharides, and therefore the widely used plant nuclear extraction protocols do not work well in ZGA. Here, we also report a fast and simple method, by mechanical chopping, for efficient nuclear extraction in the two SAG strains. More importantly, the extracted nuclei were further used for nuclear genome size estimation of the two SAG strains by flow cytometry (FC). To confirm the FC result, we have also used other experimental methods for nuclear genome size estimation of the two strains. Interestingly, the two strains were found to have very distinct nuclear genome sizes (313.2 ± 2.0 Mb in SAG 698-1a vs. 63.5 ± 0.5 Mb in SAG 698-1b). Our multiple lines of evidence strongly indicate that SAG 698-1a possibly had been confused with SAG 698-2 prior to 2005, and most likely represents Z. cylindricum or a closely related species.

The Chloroplast Trans-Splicing RNA-Protein Supercomplex from the Green Alga Chlamydomonas reinhardtii

In eukaryotes, RNA trans-splicing is a significant RNA modification process for the end-to-end ligation of exons from separately transcribed primary transcripts to generate mature mRNA. So far, three different categories of RNA trans-splicing have been found in organisms within a diverse range. Here, we review trans-splicing of discontinuous group II introns, which occurs in chloroplasts and mitochondria of lower eukaryotes and plants. We discuss the origin of intronic sequences and the evolutionary relationship between chloroplast ribonucleoprotein complexes and the nuclear spliceosome. Finally, we focus on the ribonucleoprotein supercomplex involved in trans-splicing of chloroplast group II introns from the green alga Chlamydomonas reinhardtii. This complex has been well characterized genetically and biochemically, resulting in a detailed picture of the chloroplast ribonucleoprotein supercomplex. This information contributes substantially to our understanding of the function of RNA-processing machineries and might provide a blueprint for other splicing complexes involved in trans- as well as cis-splicing of organellar intron RNAs.

Taxonomic scheme of the order Chaetophorales (Chlorophyceae, Chlorophyta) based on chloroplast genomes

Background

Order Chaetophorales currently includes six families, namely Schizomeridaceae, Aphanochaetaceae, Barrancaceae, Uronemataceae, Fritschiellaceae, and Chaetophoraceae. The phylogenetic relationships of Chaetophorales have been inferred primarily based on short and less informative rDNA sequences. This study aimed to phylogenetically reconstruct order Chaetophorales and determine the taxonomic scheme, and to further understand the evolution of order Chaetophorales.

Results

In the present study, seven complete and five fragmentary chloroplast genomes were harvested. Phylogenomic and comparative genomic analysis were performed to determine the taxonomic scheme within Chaetophorales. Consequently, Oedogoniales was found to be a sister to a clade linking Chaetophorales and Chaetopeltidales. Schizomeriaceae, and Aphanochaetaceae clustered into a well-resolved basal clade in Chaetophorales, inconsistent with the results of phylogenetic analysis based on rDNA sequences. Comparative genomic analyses revealed that the chloroplast genomes of Schizomeriaceae and Aphanochaetaceae were highly conserved and homologous, highlighting the closest relationship in this order. Germination types of zoospores precisely correlated with the phylogenetic relationships.

Conclusions

chloroplast genome structure analyses, synteny analyses, and zoospore germination analyses were concurrent with phylogenetic analyses based on the chloroplast genome, and all of them robustly determined the unique taxonomic scheme of Chaetophorales and the relationships of Oedogoniales, Chaetophorales, and Chaetopeltidales.

Zygnema circumcarinatum UTEX 1559 chloroplast and mitochondrial genomes provide insight into land plant evolution

The complete chloroplast and mitochondrial genomes of Charophyta have shed new light on land plant terrestrialization. Here, we report the organellar genomes of the Zygnema circumcarinatum strain UTEX 1559, and a comparative genomics investigation of 33 plastomes and 18 mitogenomes of Chlorophyta, Charophyta (including UTEX 1559 and its conspecific relative SAG 698-1a), and Embryophyta. Gene presence/absence was determined across these plastomes and mitogenomes. A comparison between the plastomes of UTEX 1559 (157 548 bp) and SAG 698-1a (165 372 bp) revealed very similar gene contents, but substantial genome rearrangements. Surprisingly, the two plastomes share only 85.69% nucleotide sequence identity. The UTEX 1559 mitogenome size is 215 954 bp, the largest among all sequenced Charophyta. Interestingly, this large mitogenome contains a 50 kb region without homology to any other organellar genomes, which is flanked by two 86 bp direct repeats and contains 15 ORFs. These ORFs have significant homology to proteins from bacteria and plants with functions such as primase, RNA polymerase, and DNA polymerase. We conclude that (i) the previously published SAG 698-1a plastome is probably from a different Zygnema species, and (ii) the 50 kb region in the UTEX 1559 mitogenome might be recently acquired as a mobile element.

The chloroplast genome of the lichen-symbiont microalga Trebouxia sp. Tr9 (Trebouxiophyceae, Chlorophyta) shows short inverted repeats with a single gene and loss of the rps4 gene, which is encoded by the nucleus

The Trebouxiophyceae is the class of Chlorophyta algae from which the highest number of chloroplast genome (cpDNA) sequences has been obtained. Several species in this class participate in symbioses with fungi to form lichens. However, no cpDNA has been obtained from any Trebouxia lichen-symbiont microalgae, which are present in approximately half of all lichens. Here, we report the sequence of the completely assembled cpDNA from Trebouxia sp. TR9 and a comparative study with other Trebouxio-phyceae. The organization of the chloroplast genome of Trebouxia sp. TR9 has certain features that are unusual in the Trebouxiophyceae and other green algae. The most remarkable characteristics are the presence of long intergenic spacers, a quadripartite structure with short inverted repeated sequences (IRs), and the loss of the rps4 gene. The presence of long intergenic spacers accounts for a larger cpDNA size in comparison to other closely related Trebouxiophyceae. The IRs, which were thought to be lost in the Trebouxiales, are distinct from most of cpDNAs since they lack the rRNA operon and uniquely includes the rbcL gene. The functional transfer of the rps4 gene to the nuclear genome has been confirmed by sequencing and examination of the gene architecture, which includes three spliceosomal introns as well as the verification of the presence of the corresponding transcript. This is the first documented transfer of the rps4 gene from the chloroplast to the nucleus among Viridiplantae. Additionally, a fairly well-resolved phylogenetic reconstruction, including Trebouxia sp. TR9 along with other Trebouxiophyceae, was obtained based on a set of conserved chloroplast genes.

Large Diversity of Nonstandard Genes and Dynamic Evolution of Chloroplast Genomes in Siphonous Green Algae (Bryopsidales, Chlorophyta)

Chloroplast genomes have undergone tremendous alterations through the evolutionary history of the green algae (Chloroplastida). This study focuses on the evolution of chloroplast genomes in the siphonous green algae (order Bryopsidales). We present five new chloroplast genomes, which along with existing sequences, yield a data set representing all but one families of the order. Using comparative phylogenetic methods, we investigated the evolutionary dynamics of genomic features in the order. Our results show extensive variation in chloroplast genome architecture and intron content. Variation in genome size is accounted for by the amount of intergenic space and freestanding open reading frames that do not show significant homology to standard plastid genes. We show the diversity of these nonstandard genes based on their conserved protein domains, which are often associated with mobile functions (reverse transcriptase/intron maturase, integrases, phage- or plasmid-DNA primases, transposases, integrases, ligases). Investigation of the introns showed proliferation of group II introns in the early evolution of the order and their subsequent loss in the core Halimedineae, possibly through RT-mediated intron loss.

The Carboxy Terminus of YCF1 Contains a Motif Conserved throughout >500 Myr of Streptophyte Evolution

Plastids evolved from cyanobacteria by endosymbiosis. During the course of evolution, the coding capacity of plastid genomes shrinks due to gene loss or transfer to the nucleus. In the green lineage, however, there were apparent gene gains including that of ycf1. Although its function is still debated, YCF1 has proven to be a useful marker for plastid evolution. YCF1 sequence and predicted structural features unite the plastid genomes of land plants with those of their closest algal relatives, the higher streptophyte algae; YCF1 appears to have undergone pronounced changes during the course of streptophyte algal evolution. Using new data, we show that YCF1 underwent divergent evolution in the common ancestor of higher streptophyte algae and Klebsormidiophycae. This divergence resulted in the origin of an extreme, klebsormidiophycean-specific YCF1 and the higher streptophyte Ste-YCF1. Most importantly, our analysis uncovers a conserved carboxy-terminal sequence stretch within YCF1 that is unique to higher streptophytes and hints at an important, yet unexplored function.

Ectopic Transplastomic Expression of a Synthetic MatK Gene Leads to Cotyledon-Specific Leaf Variegation

Chloroplasts (and other plastids) harbor their own genetic material, with a bacterial-like gene-expression systems. Chloroplast RNA metabolism is complex and is predominantly mediated by nuclear-encoded RNA-binding proteins. In addition to these nuclear factors, the chloroplast-encoded intron maturase MatK has been suggested to perform as a splicing factor for a subset of chloroplast introns. MatK is essential for plant cell survival in tobacco, and thus null mutants have not yet been isolated. We therefore attempted to over-express MatK from a neutral site in the chloroplast, placing it under the control of a theophylline-inducible riboswitch. This ectopic insertion of MatK lead to a variegated cotyledons phenotype. The addition of the inducer theophylline exacerbated the phenotype in a concentration-dependent manner. The extent of variegation was further modulated by light, sucrose and spectinomycin, suggesting that the function of MatK is intertwined with photosynthesis and plastid translation. Inhibiting translation in the transplastomic lines has a profound effect on the accumulation of several chloroplast mRNAs, including the accumulation of an RNA antisense to rpl33, a gene coding for an essential chloroplast ribosomal protein. Our study further supports the idea that MatK expression needs to be tightly regulated to prevent detrimental effects and establishes another link between leaf variegation and chloroplast translation.

Comparative Chloroplast Genome Analyses of Streptophyte Green Algae Uncover Major Structural Alterations in the Klebsormidiophyceae, Coleochaetophyceae and Zygnematophyceae

The Streptophyta comprises all land plants and six main lineages of freshwater green algae: Mesostigmatophyceae, Chlorokybophyceae, Klebsormidiophyceae, Charophyceae, Coleochaetophyceae and Zygnematophyceae. Previous comparisons of the chloroplast genome from nine streptophyte algae (including four zygnematophyceans) revealed that, although land plant chloroplast DNAs (cpDNAs) inherited most of their highly conserved structural features from green algal ancestors, considerable cpDNA changes took place during the evolution of the Zygnematophyceae, the sister group of land plants. To gain deeper insights into the evolutionary dynamics of the chloroplast genome in streptophyte algae, we sequenced the cpDNAs of nine additional taxa: two klebsormidiophyceans (Entransia fimbriata and Klebsormidium sp. SAG 51.86), one coleocheatophycean (Coleochaete scutata) and six zygnematophyceans (Cylindrocystis brebissonii, Netrium digitus, Roya obtusa, Spirogyra maxima, Cosmarium botrytis and Closterium baillyanum). Our comparative analyses of these genomes with their streptophyte algal counterparts indicate that the large inverted repeat (IR) encoding the rDNA operon experienced loss or expansion/contraction in all three sampled classes and that genes were extensively shuffled in both the Klebsormidiophyceae and Zygnematophyceae. The klebsormidiophycean genomes boast greatly expanded IRs, with the Entransia 60,590-bp IR being the largest known among green algae. The 206,025-bp Entransia cpDNA, which is one of the largest genome among streptophytes, encodes 118 standard genes, i.e., four additional genes compared to its Klebsormidium flaccidum homolog. We inferred that seven of the 21 group II introns usually found in land plants were already present in the common ancestor of the Klebsormidiophyceae and its sister lineages. At 107,236 bp and with 117 standard genes, the Coleochaete IR-less genome is both the smallest and most compact among the streptophyte algal cpDNAs analyzed thus far; it lacks eight genes relative to its Chaetosphaeridium globosum homolog, four of which represent unique events in the evolutionary scenario of gene losses we reconstructed for streptophyte algae. The 10 compared zygnematophycean cpDNAs display tremendous variations at all levels, except gene content. During zygnematophycean evolution, the IR disappeared a minimum of five times, the rDNA operon was broken at four distinct sites, group II introns were lost on at least 43 occasions, and putative foreign genes, mainly of phage/viral origin, were gained.

Dynamic Evolution of the Chloroplast Genome in the Green Algal Classes Pedinophyceae and Trebouxiophyceae

Previous studies of trebouxiophycean chloroplast genomes revealed little information regarding the evolutionary dynamics of this genome because taxon sampling was too sparse and the relationships between the sampled taxa were unknown. We recently sequenced the chloroplast genomes of 27 trebouxiophycean and 2 pedinophycean green algae to resolve the relationships among the main lineages recognized for the Trebouxiophyceae. These taxa and the previously sampled members of the Pedinophyceae and Trebouxiophyceae are included in the comparative chloroplast genome analysis we report here. The 38 genomes examined display considerable variability at all levels, except gene content. Our results highlight the high propensity of the rDNA-containing large inverted repeat (IR) to vary in size, gene content and gene order as well as the repeated losses it experienced during trebouxiophycean evolution. Of the seven predicted IR losses, one event demarcates a superclade of 11 taxa representing 5 late-diverging lineages. IR expansions/contractions account not only for changes in gene content in this region but also for changes in gene order and gene duplications. Inversions also led to gene rearrangements within the IR, including the reversal or disruption of the rDNA operon in some lineages. Most of the 20 IR-less genomes are more rearranged compared with their IR-containing homologs and tend to show an accelerated rate of sequence evolution. In the IR-less superclade, several ancestral operons were disrupted, a few genes were fragmented, and a subgroup of taxa features a G+C-biased nucleotide composition. Our analyses also unveiled putative cases of gene acquisitions through horizontal transfer.

The chloroplast genomes of Bryopsis plumosa and Tydemania expeditiones (Bryopsidales, Chlorophyta): compact genomes and genes of bacterial origin

Background

Species of Bryopsidales form ecologically important components of seaweed communities worldwide. These siphonous macroalgae are composed of a single giant tubular cell containing millions of nuclei and chloroplasts, and harbor diverse bacterial communities. Little is known about the diversity of chloroplast genomes (cpDNAs) in this group, and about the possible consequences of intracellular bacteria on genome composition of the host. We present the complete cpDNAs of Bryopsis plumosa and Tydemania expeditiones, as well as a re-annotated cpDNA of B. hypnoides, which was shown to contain a higher number of genes than originally published. Chloroplast genomic data were also used to evaluate phylogenetic hypotheses in the Chlorophyta, such as monophyly of the Ulvophyceae (the class in which the order Bryopsidales is currently classified).

Results

Both DNAs are circular and lack a large inverted repeat. The cpDNA of B. plumosa is 106,859 bp long and contains 115 unique genes. A 13 kb region was identified with several freestanding open reading frames (ORFs) of putative bacterial origin, including a large ORF (>8 kb) closely related to bacterial rhs-family genes. The cpDNA of T. expeditiones is 105,200 bp long and contains 125 unique genes. As in B. plumosa, several regions were identified with ORFs of possible bacterial origin, including genes involved in mobile functions (transposases, integrases, phage/plasmid DNA primases), and ORFs showing close similarity with bacterial DNA methyltransferases. The cpDNA of B. hypnoides differs from that of B. plumosa mainly in the presence of long intergenic spacers, and a large tRNA region. Chloroplast phylogenomic analyses were largely inconclusive with respect to monophyly of the Ulvophyceae, and the relationship of the Bryopsidales within the Chlorophyta.

Conclusions

The cpDNAs of B. plumosa and T. expeditiones are amongst the smallest and most gene dense chloroplast genomes in the core Chlorophyta. The presence of bacterial genes, including genes typically found in mobile elements, suggest that these have been acquired through horizontal gene transfer, which may have been facilitated by the occurrence of obligate intracellular bacteria in these siphonous algae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1418-3) contains supplementary material, which is available to authorized users.

Organellar maturases: A window into the evolution of the spliceosome

During the evolution of eukaryotic genomes, many genes have been interrupted by intervening sequences (introns) that must be removed post-transcriptionally from RNA precursors to form mRNAs ready for translation. The origin of nuclear introns is still under debate, but one hypothesis is that the spliceosome and the intron-exon structure of genes have evolved from bacterial-type group II introns that invaded the eukaryotic genomes. The group II introns were most likely introduced into the eukaryotic genome from an α-proteobacterial predecessor of mitochondria early during the endosymbiosis event. These self-splicing and mobile introns spread through the eukaryotic genome and later degenerated. Pieces of introns became part of the general splicing machinery we know today as the spliceosome. In addition, group II introns likely brought intron maturases with them to the nucleus. Maturases are found in most bacterial introns, where they act as highly specific splicing factors for group II introns. In the spliceosome, the core protein Prp8 shows homology to group II intron-encoded maturases. While maturases are entirely intron specific, their descendant of the spliceosomal machinery, the Prp8 protein, is an extremely versatile splicing factor with multiple interacting proteins and RNAs. How could such a general player in spliceosomal splicing evolve from the monospecific bacterial maturases? Analysis of the organellar splicing machinery in plants may give clues on the evolution of nuclear splicing. Plants encode various proteins which are closely related to bacterial maturases. The organellar genomes contain one maturase each, named MatK in chloroplasts and MatR in mitochondria. In addition, several maturase genes have been found in the nucleus as well, which are acting on mitochondrial pre-RNAs. All plant maturases show sequence deviation from their progenitor bacterial maturases, and interestingly are all acting on multiple organellar group II intron targets. Moreover, they seem to function in the splicing of group II introns together with a number of additional nuclear-encoded splicing factors, possibly acting as an organellar proto-spliceosome. Together, this makes them interesting models for the early evolution of nuclear spliceosomal splicing. In this review, we summarize recent advances in our understanding of the role of plant maturases and their accessory factors in plants. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Six newly sequenced chloroplast genomes from prasinophyte green algae provide insights into the relationships among prasinophyte lineages and the diversity of streamlined genome architecture in picoplanktonic species

Background

Because they represent the earliest divergences of the Chlorophyta, the morphologically diverse unicellular green algae making up the prasinophytes hold the key to understanding the nature of the first viridiplants and the evolutionary patterns that accompanied the radiation of chlorophytes. Nuclear-encoded 18S rDNA phylogenies unveiled nine prasinophyte clades (clades I through IX) but their branching order is still uncertain. We present here the newly sequenced chloroplast genomes of Nephroselmis astigmatica (clade III) and of five picoplanktonic species from clade VI (Prasinococcus sp. CCMP 1194, Prasinophyceae sp. MBIC 106222 and Prasinoderma coloniale) and clade VII (Picocystis salinarum and Prasinophyceae sp. CCMP 1205). These chloroplast DNAs (cpDNAs) were compared with those of the six previously sampled prasinophytes (clades I, II, III and V) in order to gain information both on the relationships among prasinophyte lineages and on chloroplast genome evolution.

Results

Varying from 64.3 to 85.6 kb in size and encoding 100 to 115 conserved genes, the cpDNAs of the newly investigated picoplanktonic species are substantially smaller than those observed for larger-size prasinophytes, are economically packed and contain a reduced gene content. Although the Nephroselmis and Picocystis cpDNAs feature a large inverted repeat encoding the rRNA operon, gene partitioning among the single copy regions is remarkably different. Unexpectedly, we found that all three species from clade VI (Prasinococcales) harbor chloroplast genes not previously documented for chlorophytes (ndhJ, rbcR, rpl21, rps15, rps16 and ycf66) and that Picocystis contains a trans-spliced group II intron. The phylogenies inferred from cpDNA-encoded proteins are essentially congruent with 18S rDNA trees, resolving with robust support all six examined prasinophyte lineages, with the exception of the Pycnococcaceae.

Conclusions

Our results underscore the high variability in genome architecture among prasinophyte lineages, highlighting the strong pressure to maintain a small and compact chloroplast genome in picoplanktonic species. The unique set of six chloroplast genes found in the Prasinococcales supports the ancestral status of this lineage within the prasinophytes. The widely diverging traits uncovered for the clade-VII members (Picocystis and Prasinophyceae sp. CCMP 1205) are consistent with their resolution as separate lineages in the chloroplast phylogeny.

Evolutionary and biotechnology implications of plastid genome variation in the inverted-repeat-lacking clade of legumes

Land plant plastid genomes (plastomes) provide a tractable model for evolutionary study in that they are relatively compact and gene dense. Among the groups that display an appropriate level of variation for structural features, the inverted-repeat-lacking clade (IRLC) of papilionoid legumes presents the potential to advance general understanding of the mechanisms of genomic evolution. Here, are presented six complete plastome sequences from economically important species of the IRLC, a lineage previously represented by only five completed plastomes. A number of characters are compared across the IRLC including gene retention and divergence, synteny, repeat structure and functional gene transfer to the nucleus. The loss of clpP intron 2 was identified in one newly sequenced member of IRLC, Glycyrrhiza glabra. Using deeply sequenced nuclear transcriptomes from two species helped clarify the nature of the functional transfer of accD to the nucleus in Trifolium, which likely occurred in the lineage leading to subgenus Trifolium. Legumes are second only to cereal crops in agricultural importance based on area harvested and total production. Genetic improvement via plastid transformation of IRLC crop species is an appealing proposition. Comparative analyses of intergenic spacer regions emphasize the need for complete genome sequences for developing transformation vectors for plastid genetic engineering of legume crops.

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.

Analyses of charophyte chloroplast genomes help characterize the ancestral chloroplast genome of land plants

Despite the significance of the relationships between embryophytes and their charophyte algal ancestors in deciphering the origin and evolutionary success of land plants, few chloroplast genomes of the charophyte algae have been reconstructed to date. Here, we present new data for three chloroplast genomes of the freshwater charophytes Klebsormidium flaccidum (Klebsormidiophyceae), Mesotaenium endlicherianum (Zygnematophyceae), and Roya anglica (Zygnematophyceae). The chloroplast genome of Klebsormidium has a quadripartite organization with exceptionally large inverted repeat (IR) regions and, uniquely among streptophytes, has lost the rrn5 and rrn4.5 genes from the ribosomal RNA (rRNA) gene cluster operon. The chloroplast genome of Roya differs from other zygnematophycean chloroplasts, including the newly sequenced Mesotaenium, by having a quadripartite structure that is typical of other streptophytes. On the basis of the improbability of the novel gain of IR regions, we infer that the quadripartite structure has likely been lost independently in at least three zygnematophycean lineages, although the absence of the usual rRNA operonic synteny in the IR regions of Roya may indicate their de novo origin. Significantly, all zygnematophycean chloroplast genomes have undergone substantial genomic rearrangement, which may be the result of ancient retroelement activity evidenced by the presence of integrase-like and reverse transcriptase-like elements in the Roya chloroplast genome. Our results corroborate the close phylogenetic relationship between Zygnematophyceae and land plants and identify 89 protein-coding genes and 22 introns present in the chloroplast genome at the time of the evolutionary transition of plants to land, all of which can be found in the chloroplast genomes of extant charophytes.

Complex RNA metabolism in the chloroplast: an update on the psbB operon

Expression of most plastid genes involves multiple post-transcriptional processing events, such as splicing, editing, and intercistronic processing. The latter involves the formation of mono-, di-, and multicistronic transcripts, which can further be regulated by differential stability and expression. The plastid pentacistronic psbB transcription unit has been well characterized in vascular plants. It encodes the subunits CP47 (psbB), T (psbT), and H (psbH) of photosystem II as well as cytochrome b (6) (petB) and subunit IV (petD) of the cytochrome b (6) f complex. Each of the petB and petD genes contains a group II intron, which is spliced during post-transcriptional modification. The small subunit of photosystem II, PsbN, is encoded in the intercistronic region between psbH and psbT but is transcribed in the opposite direction. Expression of the psbB gene cluster necessitates different processing events along with numerous newly evolved specificity factors conferring stability to many of the processed RNA transcripts, and thus exemplarily shows the complexity of RNA metabolism in the chloroplast.

SYSTEMATICS OF THE GENUS ZYGNEMA (ZYGNEMATOPHYCEAE, CHAROPHYTA) FROM CALIFORNIAN WATERSHEDS(1)

Natural populations of Zygnema were collected from 80 stream sites across California, and eight species were identified and characterized morphologically. Generic and infrageneric concepts of Zygnema and Zygogonium were tested with cox3 and rbcL gene sequence analysis. Strains of Zygnema were positioned in a single monophyletic clade sister to Zygogonium tunetanum Gauth.-Lièvre. In both the rbcL and cox3 phylogenies, strains of Zygnema formed two major clades. The first clade contained species that have zygospores with a blue-colored mesospore or akinetes with a colorless mesospore. The second clade contained species that have a yellow or brown mesospore. The existing taxonomic concepts for Zygnema classification are not consistent with our molecular phylogeny and do not correspond to natural groups. We propose that mesospore color may be useful in the infrageneric classification of Zygnema. Newly described Zygnema aplanosporum sp. nov. and Zygnema californicum sp. nov. have zygospores with a blue mesospore formed in the conjugation tube and separated by a cellulosic sporangial wall. Z. aplanosporum also possessed a combination of vegetative and reproductive features characteristic of Zygogonium, such as presence of short branches, rhizoidal outgrowths, thickened vegetative cell walls, purple-colored cell content, small compressed-globular chloroplasts as well as predominant asexual reproduction. Z. aplanosporum and Z. californicum were deeply embedded in a larger clade of Zygnema both in rbcL and cox3 analyses. Based on our observations, there are no features or combination of features that separate Zygnema and Zygogonium. Therefore, we conclude that Zygogonium is probably a synonym of Zygnema.

Frequent pseudogenization and loss of the plastid-encoded sulfate-transport gene cysA throughout the evolution of liverworts

PREMISE OF THE STUDY

The presence or absence of a functional copy of a plastid gene may reflect relaxed selection, and may be phylogenetically significant, reflecting shared ancestry. In some liverworts, the plastid gene cysA is a pseudogene (inferred to be nonfunctional). We surveyed 63 liverworts from all major clades to determine whether the loss of cysA is phylogenetically significant, whether intact copies of cysA are under selective constraints, and whether rates of nucleotide substitution differ in other plastid genes from taxa with and without a functional copy of cysA.

METHODS

Primers annealing to flanking and internal regions were used to amplify and sequence cysA from 61 liverworts. Two additional cysA sequences were downloaded from NCBI. The ancestral states of cysA were reconstructed on a phylogenetic hypothesis inferred from seven markers. Rates of nucleotide substitution were estimated for three plastid loci to reflect the intrinsic mutation rate in the plastid genome. The ratio of nonsynonymous to synonymous substitutions was estimated for intact copies of cysA to infer selective constraints.

KEY RESULTS

Throughout liverworts, cysA has been lost up to 29 times, yet intact copies of cysA are evolving under selective constraints. Gene loss is more frequent in groups with an increased substitution rate in the plastid genome.

CONCLUSIONS

The number of inferred losses of cysA in liverworts exceeds any other reported plastid gene. Despite frequent losses, cysA is evolving under purifying selection in liverworts that retain the gene. It appears that cysA is lost more frequently in lineages characterized by a higher rate of nucleotide substitutions in the plastid.

The chloroplast genome of the green alga Schizomeris leibleinii (Chlorophyceae) provides evidence for bidirectional DNA replication from a single origin in the chaetophorales

In the Chlorophyceae, the chloroplast genome is extraordinarily fluid in architecture and displays unique features relative to other groups of green algae. For the Chaetophorales, 1 of the 5 major lineages of the Chlorophyceae, it has been shown that the distinctive architecture of the 223,902-bp genome of Stigeoclonium helveticum is consistent with bidirectional DNA replication from a single origin. Here, we report the 182,759-bp chloroplast genome sequence of Schizomeris leibleinii, a member of the earliest diverging lineage of the Chaetophorales. Like its Stigeoclonium homolog, the Schizomeris genome lacks a large inverted repeat encoding the rRNA operon and displays a striking bias in coding regions that is associated with a bias in base composition along each strand. Our results support the notion that these two chaetophoralean genomes replicate bidirectionally from a putative origin located in the vicinity of the small subunit ribosomal RNA gene. Their shared structural characteristics were most probably inherited from the common ancestor of all chaetophoralean algae. Short dispersed repeats account for most of the 41-kb size variation between the Schizomeris and Stigeoclonium genomes, and there is no indication that homologous recombination between these repeated elements led to the observed gene rearrangements. A comparison of the extent of variation sustained by the Stigeoclonium and Schizomeris chloroplast DNAs (cpDNAs) with that observed for the cpDNAs of the chlamydomonadalean Chlamydomonas and Volvox suggests that gene rearrangements as well as changes in the abundance of intergenic and intron sequences occurred at a slower pace in the Chaetophorales than in the Chlamydomonadales.

Multigene phylogeny of the green lineage reveals the origin and diversification of land plants

The Viridiplantae (green plants) include land plants as well as the two distinct lineages of green algae, chlorophytes and charophytes. Despite their critical importance for identifying the closest living relatives of land plants, phylogenetic studies of charophytes have provided equivocal results [1-5]. In addition, many relationships remain unresolved among the land plants, such as the position of mosses, liverworts, and the enigmatic Gnetales. Phylogenomics has proven to be an insightful approach for resolving challenging phylogenetic issues, particularly concerning deep nodes [6-8]. Here we extend this approach to the green lineage by assembling a multilocus data set of 77 nuclear genes (12,149 unambiguously aligned amino acid positions) from 77 taxa of plants. We therefore provide the first multigene phylogenetic evidence that Coleochaetales represent the closest living relatives of land plants. Moreover, our data reinforce the early divergence of liverworts and the close relationship between Gnetales and Pinaceae. These results provide a new phylogenetic framework and represent a key step in the evolutionary interpretation of developmental and genomic characters in green plants.

Complete plastome sequences of Equisetum arvense and Isoetes flaccida: implications for phylogeny and plastid genome evolution of early land plant lineages

Background

Despite considerable progress in our understanding of land plant phylogeny, several nodes in the green tree of life remain poorly resolved. Furthermore, the bulk of currently available data come from only a subset of major land plant clades. Here we examine early land plant evolution using complete plastome sequences including two previously unexamined and phylogenetically critical lineages. To better understand the evolution of land plants and their plastomes, we examined aligned nucleotide sequences, indels, gene and nucleotide composition, inversions, and gene order at the boundaries of the inverted repeats.

Results

We present the plastome sequences of Equisetum arvense, a horsetail, and of Isoetes flaccida, a heterosporous lycophyte. Phylogenetic analysis of aligned nucleotides from 49 plastome genes from 43 taxa supported monophyly for the following clades: embryophytes (land plants), lycophytes, monilophytes (leptosporangiate ferns + Angiopteris evecta + Psilotum nudum + Equisetum arvense), and seed plants. Resolution among the four monilophyte lineages remained moderate, although nucleotide analyses suggested that P. nudum and E. arvense form a clade sister to A. evecta + leptosporangiate ferns. Results from phylogenetic analyses of nucleotides were consistent with the distribution of plastome gene rearrangements and with analysis of sequence gaps resulting from insertions and deletions (indels). We found one new indel and an inversion of a block of genes that unites the monilophytes.

Conclusions

Monophyly of monilophytes has been disputed on the basis of morphological and fossil evidence. In the context of a broad sampling of land plant data we find several new pieces of evidence for monilophyte monophyly. Results from this study demonstrate resolution among the four monilophytes lineages, albeit with moderate support; we posit a clade consisting of Equisetaceae and Psilotaceae that is sister to the "true ferns," including Marattiaceae.

Extreme reconfiguration of plastid genomes in the angiosperm family Geraniaceae: rearrangements, repeats, and codon usage

Geraniaceae plastid genomes (plastomes) have experienced a remarkable number of genomic changes. The plastomes of Erodium texanum, Geranium palmatum, and Monsonia speciosa were sequenced and compared with other rosids and the previously published Pelargonium hortorum plastome. Geraniaceae plastomes were found to be highly variable in size, gene content and order, repetitive DNA, and codon usage. Several unique plastome rearrangements include the disruption of two highly conserved operons (S10 and rps2-atpA), and the inverted repeat (IR) region in M. speciosa does not contain all genes in the ribosomal RNA operon. The sequence of M. speciosa is unusually small (128,787 bp); among angiosperm plastomes sequenced to date, only those of nonphotosynthetic species and those that have lost one IR copy are smaller. In contrast, the plastome of P. hortorum is the largest, at 217,942 bp. These genomes have experienced numerous gene and intron losses and partial and complete gene duplications. Some of the losses are shared throughout the family (e.g., trnT-GGU and the introns of rps16 and rpl16); however, other losses are homoplasious (e.g., trnG-UCC intron in G. palmatum and M. speciosa). IR length is also highly variable. The IR in P. hortorum was previously shown to be greatly expanded to 76 kb, and the IR is lost in E. texanum and reduced in G. palmatum (11 kb) and M. speciosa (7 kb). Geraniaceae plastomes contain a high frequency of large repeats (>100 bp) relative to other rosids. Within each plastome, repeats are often located at rearrangement end points and many repeats shared among the four Geraniaceae flank rearrangement end points. GC content is elevated in the genomes and also in coding regions relative to other rosids. Codon usage per amino acid and GC content at third position sites are significantly different for Geraniaceae protein-coding sequences relative to other rosids. Our findings suggest that relaxed selection and/or mutational biases lead to increased GC content, and this in turn altered codon usage. We propose that increases in genomic rearrangements, repetitive DNA, nucleotide substitutions, and GC content may be caused by relaxed selection resulting from improper DNA repair.

The exceptionally large chloroplast genome of the green alga Floydiella terrestris illuminates the evolutionary history of the Chlorophyceae

The Chlorophyceae, an advanced class of chlorophyte green algae, comprises five lineages that form two major clades (Chlamydomonadales + Sphaeropleales and Oedogoniales + Chaetopeltidales + Chaetophorales). The four complete chloroplast DNA (cpDNA) sequences currently available for chlorophyceans uncovered an extraordinarily fluid genome architecture as well as many structural features distinguishing this group from other green algae. We report here the 521,168-bp cpDNA sequence from a member of the Chaetopeltidales (Floydiella terrestris), the sole chlorophycean lineage not previously sampled for chloroplast genome analysis. This genome, which contains 97 conserved genes and 26 introns (19 group I and 7 group II introns), is the largest chloroplast genome ever sequenced. Intergenic regions account for 77.8% of the genome size and are populated by short repeats. Numerous genomic features are shared with the cpDNA of the chaetophoralean Stigeoclonium helveticum, notably the absence of a large inverted repeat and the presence of unique gene clusters and trans-spliced group II introns. Although only one of the Floydiella group I introns encodes a homing endonuclease gene, our finding of five free-standing reading frames having similarity with such genes suggests that chloroplast group I introns endowed with mobility were once more abundant in the Floydiella lineage. Parsimony analysis of structural genomic features and phylogenetic analysis of chloroplast sequence data unambiguously resolved the Oedogoniales as sister to the Chaetopeltidales and Chaetophorales. An evolutionary scenario of the molecular events that shaped the chloroplast genome in the Chlorophyceae is presented.

Chloroplast RNA metabolism

The chloroplast genome encodes proteins required for photosynthesis, gene expression, and other essential organellar functions. Derived from a cyanobacterial ancestor, the chloroplast combines prokaryotic and eukaryotic features of gene expression and is regulated by many nucleus-encoded proteins. This review covers four major chloroplast posttranscriptional processes: RNA processing, editing, splicing, and turnover. RNA processing includes the generation of transcript 5' and 3' termini, as well as the cleavage of polycistronic transcripts. Editing converts specific C residues to U and often changes the amino acid that is specified by the edited codon. Chloroplasts feature introns of groups I and II, which undergo protein-facilitated cis- or trans-splicing in vivo. Each of these RNA-based processes involves proteins of the pentatricopeptide motif-containing family, which does not occur in prokaryotes. Plant-specific RNA-binding proteins may underpin the adaptation of the chloroplast to the eukaryotic context.

Trans-splicing of organelle introns--a detour to continuous RNAs

In eukaryotes, RNA trans-splicing is an important RNA-processing form for the end-to-end ligation of primary transcripts that are derived from separately transcribed exons. So far, three different categories of RNA trans-splicing have been found in organisms as diverse as algae to man. Here, we review one of these categories: the trans-splicing of discontinuous group II introns, which occurs in chloroplasts and mitochondria of lower eukaryotes and plants. Trans-spliced exons can be predicted from DNA sequences derived from a large number of sequenced organelle genomes. Further molecular genetic analysis of mutants has unravelled proteins, some of which being part of high-molecular-weight complexes that promote the splicing process. Based on data derived from the alga Chlamydomonas reinhardtii, a model is provided which defines the composition of an organelle spliceosome. This will have a general relevance for understanding the function of RNA-processing machineries in eukaryotic organelles.

The chloroplast genomes of the green algae Pedinomonas minor, Parachlorella kessleri, and Oocystis solitaria reveal a shared ancestry between the Pedinomonadales and Chlorellales

The green algae belonging to the Chlorophyta-the lineage sister to that comprising the land plants and their charophycean green algal relatives (Streptophyta)-have been subdivided into four classes (Prasinophyceae, Ulvophyceae, Trebouxiophyceae, and Chlorophyceae). Yet the Pedinomonadales, an assemblage consisting of tiny, naked uniflagellates with a second basal body, has no clear affiliation with these classes and the branching order of the crown chlorophytes remains unknown. To gain an insight into the phylogenetic position of the Pedinomonadales and the relationships among the recognized chlorophyte classes, we have sequenced the chloroplast genomes of Pedinomonas minor (Pedinomonadales) and of two trebouxiophyceans belonging to the Chlorellales, Parachlorella kessleri (Chlorellaceae) and Oocystis solitaria (Oocystaceae), and compared these genomes with those of previously examined streptophytes and chlorophytes, including Chlorella vulgaris (Chlorellaceae). Unlike their Chlorella homolog, the three newly investigated chloroplast DNAs (cpDNAs) carry a large rRNA-encoding inverted repeat (IR) that divides the genome into large and small single-copy regions. In contrast to the situation found for ulvophycean and chlorophycean cpDNAs, the gene contents of the IR and single-copy regions are strikingly similar to that inferred for the common ancestor of chlorophytes and streptophytes. The intronless 98,340-bp Pedinomonas genome is among the chlorophyte cpDNAs featuring the smallest size and most ancestral gene organization. All 105 conserved genes encoded by this genome are included in the gene repertoires of Oocystis (111 genes) and Chlorella (113 genes), with just trnR(ccg) missing from Parachlorella cpDNA. Trees inferred from 71 cpDNA-encoded genes/proteins of 16 chlorophytes and nine streptophytes showed that Pedinomonas is nested in the Chlorellales, a group of algae lacking flagella. This phylogenetic conclusion is independently supported by uniquely shared gene linkages. We hypothesize that chlorellalean and pedinomonadalean green algae are reduced forms of a distant biflagellate ancestor that might have also given rise to the other known trebouxiophycean lineages. Our structural cpDNA data suggest that the Chlorellales and Pedinomonadales represent a deep branch of core chlorophytes, strengthening the notion that the Trebouxiophyceae emerged before the Ulvophyceae and Chlorophyceae. Our results further emphasize the importance of secondary reduction at both the cellular and genome levels during chlorophyte evolution.

Streptophyte algae and the origin of embryophytes

BACKGROUND

Land plants (embryophytes) evolved from streptophyte green algae, a small group of freshwater algae ranging from scaly, unicellular flagellates (Mesostigma) to complex, filamentous thalli with branching, cell differentiation and apical growth (Charales). Streptophyte algae and embryophytes form the division Streptophyta, whereas the remaining green algae are classified as Chlorophyta. The Charales (stoneworts) are often considered to be sister to land plants, suggesting progressive evolution towards cellular complexity within streptophyte green algae. Many cellular (e.g. phragmoplast, plasmodesmata, hexameric cellulose synthase, structure of flagellated cells, oogamous sexual reproduction with zygote retention) and physiological characters (e.g. type of photorespiration, phytochrome system) originated within streptophyte algae.

RECENT PROGRESS

Phylogenetic studies have demonstrated that Mesostigma (flagellate) and Chlorokybus (sarcinoid) form the earliest divergence within streptophytes, as sister to all other Streptophyta including embryophytes. The question whether Charales, Coleochaetales or Zygnematales are the sister to embryophytes is still (or, again) hotly debated. Projects to study genome evolution within streptophytes including protein families and polyadenylation signals have been initiated. In agreement with morphological and physiological features, many molecular traits believed to be specific for embryophytes have been shown to predate the Chlorophyta/Streptophyta split, or to have originated within streptophyte algae. Molecular phylogenies and the fossil record allow a detailed reconstruction of the early evolutionary events that led to the origin of true land plants, and shaped the current diversity and ecology of streptophyte green algae and their embryophyte descendants.

CONCLUSIONS

The Streptophyta/Chlorophyta divergence correlates with a remarkably conservative preference for freshwater/marine habitats, and the early freshwater adaptation of streptophyte algae was a major advantage for the earliest land plants, even before the origin of the embryo and the sporophyte generation. The complete genomes of a few key streptophyte algae taxa will be required for a better understanding of the colonization of terrestrial habitats by streptophytes.

The chloroplast genomes of the green algae Pyramimonas, Monomastix, and Pycnococcus shed new light on the evolutionary history of prasinophytes and the origin of the secondary chloroplasts of euglenids

Because they represent the earliest divergences of the Chlorophyta and include the smallest known eukaryotes (e.g., the coccoid Ostreococcus), the morphologically diverse unicellular green algae making up the Prasinophyceae are central to our understanding of the evolutionary patterns that accompanied the radiation of chlorophytes and the reduction of cell size in some lineages. Seven prasinophyte lineages, four of which exhibit a coccoid cell organization (no flagella nor scales), were uncovered from analysis of nuclear-encoded 18S rDNA data; however, their order of divergence remains unknown. In this study, the chloroplast genome sequences of the scaly quadriflagellate Pyramimonas parkeae (clade I), the coccoid Pycnococcus provasolii (clade V), and the scaly uniflagellate Monomastix (unknown affiliation) were determined, annotated, and compared with those previously reported for green algae/land plants, including two prasinophytes (Nephroselmis olivacea, clade III and Ostreococcus tauri, clade II). The chlorarachniophyte Bigelowiella natans and the euglenid Euglena gracilis, whose chloroplasts originate presumably from distinct green algal endosymbionts, were also included in our comparisons. The three newly sequenced prasinophyte genomes differ considerably from one another and from their homologs in overall structure, gene content, and gene order, with the 80,211-bp Pycnococcus and 114,528-bp Monomastix genomes (98 and 94 conserved genes, respectively) resembling the 71,666-bp Ostreococcus genome (88 genes) in featuring a significantly reduced gene content. The 101,605-bp Pyramimonas genome (110 genes) features two conserved genes (rpl22 and ycf65) and ancestral gene linkages previously unrecognized in chlorophytes as well as a DNA primase gene putatively acquired from a virus. The Pyramimonas and Euglena cpDNAs revealed uniquely shared derived gene clusters. Besides providing unequivocal evidence that the green algal ancestor of the euglenid chloroplasts belonged to the Pyramimonadales, phylogenetic analyses of concatenated chloroplast genes and proteins elucidated the position of Monomastix and showed that the Mamiellales, a clade comprising Ostreococcus and Monomastix, are sister to the Pyramimonadales + Euglena clade. Our results also revealed that major reduction in gene content and restructuring of the chloroplast genome occurred in conjunction with important changes in cell organization in at least two independent prasinophyte lineages, the Mamiellales and the Pycnococcaceae.

The chloroplast sulfate transport system in the green alga Chlamydomonas reinhardtii

The genome of the model unicellular green alga Chlamydomonas reinhardtii contains four distinct genes, SulP, SulP2, Sbp and Sabc, which together are postulated to encode a chloroplast envelope-localized sulfate transporter holocomplex. In this work, evidence is presented that regulation of expression of SulP2, Sbp and Sabc is specifically modulated by sulfur availability to the cells. Induction of transcription and higher steady-state levels of the respective mRNAs are reported under S-deprivation conditions. No such induction could be observed under N or P deprivation conditions. Expression, localization, and complex-association of the Sabc protein was specifically investigated using cellular and chloroplast fractionations, BN-PAGE, SDS-PAGE and Western blot analyses. It is shown that Sabc protein levels in the cells increased under S-deprivation conditions, consistent with the observed induction of Sabc gene transcription. It is further shown that the Sabc protein co-localizes with SulP to the chloroplast envelope. Blue-native PAGE followed by Western blot analysis revealed the presence of an apparent 380 kDa complex in C. reinhardtii, specifically recognized by polyclonal antibodies against SulP and Sabc. These results suggest the presence and function in C. reinhardtii of a Sbp-SulP-SulP2-Sabc chloroplast envelope sulfate transporter holocomplex.

DEEP DIVISION IN THE CHLOROPHYCEAE (CHLOROPHYTA) REVEALED BY CHLOROPLAST PHYLOGENOMIC ANALYSES(1)

The Chlorophyceae (sensu Mattox and Stewart) is a morphologically diverse class of the Chlorophyta displaying biflagellate and quadriflagellate motile cells with varying configurations of the flagellar apparatus. Phylogenetic analyses of 18S rDNA data and combined 18S and 26S rDNA data from a broad range of chlorophycean taxa uncovered five major monophyletic groups (Chlamydomonadales, Sphaeropleales, Oedogoniales, Chaetophorales, and Chaetopeltidales) but could not resolve their branching order. To gain insight into the interrelationships of these groups, we analyzed multiple genes encoded by the chloroplast genomes of Chlamydomonas reinhardtii P. A. Dang. and Chlamydomonas moewusii Gerloff (Chlamydomonadales), Scenedesmus obliquus (Turpin) Kütz. (Sphaeropleales), Oedogonium cardiacum Wittr. (Oedogoniales), Stigeoclonium helveticum Vischer (Chaetophorales), and Floydiella terrestris (Groover et Hofstetter) Friedl et O'Kelly (Chaetopeltidales). The C. moewusii, Oedogonium, and Floydiella chloroplast DNAs were partly sequenced using a random strategy. Trees were reconstructed from nucleotide and amino acid data sets derived from 44 protein-coding genes of 11 chlorophytes and nine streptophytes as well as from 57 protein-coding genes of the six chlorophycean taxa. All best trees identified two robustly supported major lineages within the Chlorophyceae: a clade uniting the Chlamydomonadales and Sphaeropleales, and a clade uniting the Oedogoniales, Chaetophorales, and Chaetopeltidales (OCC clade). This dichotomy is independently supported by molecular signatures in chloroplast genes, such as insertions/deletions and the distribution of trans-spliced group II introns. Within the OCC clade, the sister relationship observed for the Chaetophorales and Chaetopeltidales is also strengthened by independent data. Character state reconstruction of basal body orientation allowed us to refine hypotheses regarding the evolution of the flagellar apparatus.

Chloroplast genome sequencing analysis of Heterosigma akashiwo CCMP452 (West Atlantic) and NIES293 (West Pacific) strains

BACKGROUND

Heterokont algae form a monophyletic group within the stramenopile branch of the tree of life. These organisms display wide morphological diversity, ranging from minute unicells to massive, bladed forms. Surprisingly, chloroplast genome sequences are available only for diatoms, representing two (Coscinodiscophyceae and Bacillariophyceae) of approximately 18 classes of algae that comprise this taxonomic cluster. A universal challenge to chloroplast genome sequencing studies is the retrieval of highly purified DNA in quantities sufficient for analytical processing. To circumvent this problem, we have developed a simplified method for sequencing chloroplast genomes, using fosmids selected from a total cellular DNA library. The technique has been used to sequence chloroplast DNA of two Heterosigma akashiwo strains. This raphidophyte has served as a model system for studies of stramenopile chloroplast biogenesis and evolution.

RESULTS

H. akashiwo strain CCMP452 (West Atlantic) chloroplast DNA is 160,149 bp in size with a 21,822-bp inverted repeat, whereas NIES293 (West Pacific) chloroplast DNA is 159,370 bp in size and has an inverted repeat of 21,665 bp. The fosmid cloning technique reveals that both strains contain an isomeric chloroplast DNA population resulting from an inversion of their single copy domains. Both strains contain multiple small inverted and tandem repeats, non-randomly distributed within the genomes. Although both CCMP452 and NIES293 chloroplast DNAs contains 197 genes, multiple nucleotide polymorphisms are present in both coding and intergenic regions. Several protein-coding genes contain large, in-frame inserts relative to orthologous genes in other plastids. These inserts are maintained in mRNA products. Two genes of interest in H. akashiwo, not previously reported in any chloroplast genome, include tyrC, a tyrosine recombinase, which we hypothesize may be a result of a lateral gene transfer event, and an unidentified 456 amino acid protein, which we hypothesize serves as a G-protein-coupled receptor. The H. akashiwo chloroplast genomes share little synteny with other algal chloroplast genomes sequenced to date.

CONCLUSION

The fosmid cloning technique eliminates chloroplast isolation, does not require chloroplast DNA purification, and reduces sequencing processing time. Application of this method has provided new insights into chloroplast genome architecture, gene content and evolution within the stramenopile cluster.

The complete nucleotide sequence of the cassava (Manihot esculenta) chloroplast genome and the evolution of atpF in Malpighiales: RNA editing and multiple losses of a group II intron

The complete sequence of the chloroplast genome of cassava (Manihot esculenta, Euphorbiaceae) has been determined. The genome is 161,453 bp in length and includes a pair of inverted repeats (IR) of 26,954 bp. The genome includes 128 genes; 96 are single copy and 16 are duplicated in the IR. There are four rRNA genes and 30 distinct tRNAs, seven of which are duplicated in the IR. The infA gene is absent; expansion of IRb has duplicated 62 amino acids at the 3' end of rps19 and a number of coding regions have large insertions or deletions, including insertions within the 23S rRNA gene. There are 17 intron-containing genes in cassava, 15 of which have a single intron while two (clpP, ycf3) have two introns. The usually conserved atpF group II intron is absent and this is the first report of its loss from land plant chloroplast genomes. The phylogenetic distribution of the atpF intron loss was determined by a PCR survey of 251 taxa representing 34 families of Malpighiales and 16 taxa from closely related rosids. The atpF intron is not only missing in cassava but also from closely related Euphorbiaceae and other Malpighiales, suggesting that there have been at least seven independent losses. In cassava and all other sequenced Malphigiales, atpF gene sequences showed a strong association between C-to-T substitutions at nucleotide position 92 and the loss of the intron, suggesting that recombination between an edited mRNA and the atpF gene may be a possible mechanism for the intron loss.

Common intervals and symmetric difference in a model-free phylogenomics, with an application to streptophyte evolution

The common intervals of two permutations on n elements are the subsets of terms contiguous in both permutations. They constitute the most basic representation of conserved local order. We use d, the size of the symmetric difference (the complement of the common intervals) of the two subsets of 2({1,n}) thus determined by two permutations, as an evolutionary distance between the gene orders represented by the permutations. We consider the Steiner Tree problem in the space (2({1,n}), d) as the basis for constructing phylogenetic trees, including ancestral gene orders. We extend this to genomes with unequal gene content and to genomes containing gene families. Applied to streptophyte phylogeny, our method does not support the positioning of the complex algae Charales as a sister group to the land plants. Simulations show that the method, though unmotivated by any specific model of genome rearrangement, accurately reconstructs a tree from artificial genome data generated by random inversions deriving each genome from its ancestor on this tree.

The chloroplast genome sequence of the green alga Leptosira terrestris: multiple losses of the inverted repeat and extensive genome rearrangements within the Trebouxiophyceae

Background

In the Chlorophyta – the green algal phylum comprising the classes Prasinophyceae, Ulvophyceae, Trebouxiophyceae and Chlorophyceae – the chloroplast genome displays a highly variable architecture. While chlorophycean chloroplast DNAs (cpDNAs) deviate considerably from the ancestral pattern described for the prasinophyte Nephroselmis olivacea, the degree of remodelling sustained by the two ulvophyte cpDNAs completely sequenced to date is intermediate relative to those observed for chlorophycean and trebouxiophyte cpDNAs. Chlorella vulgaris (Chlorellales) is currently the only photosynthetic trebouxiophyte whose complete cpDNA sequence has been reported. To gain insights into the evolutionary trends of the chloroplast genome in the Trebouxiophyceae, we sequenced cpDNA from the filamentous alga Leptosira terrestris (Ctenocladales).

Results

The 195,081-bp Leptosira chloroplast genome resembles the 150,613-bp Chlorella genome in lacking a large inverted repeat (IR) but differs greatly in gene order. Six of the conserved genes present in Chlorella cpDNA are missing from the Leptosira gene repertoire. The 106 conserved genes, four introns and 11 free standing open reading frames (ORFs) account for 48.3% of the genome sequence. This is the lowest gene density yet observed among chlorophyte cpDNAs. Contrary to the situation in Chlorella but similar to that in the chlorophycean Scenedesmus obliquus, the gene distribution is highly biased over the two DNA strands in Leptosira. Nine genes, compared to only three in Chlorella, have significantly expanded coding regions relative to their homologues in ancestral-type green algal cpDNAs. As observed in chlorophycean genomes, the rpoB gene is fragmented into two ORFs. Short repeats account for 5.1% of the Leptosira genome sequence and are present mainly in intergenic regions.

Conclusion

Our results highlight the great plasticity of the chloroplast genome in the Trebouxiophyceae and indicate that the IR was lost on at least two separate occasions. The intriguing similarities of the derived features exhibited by Leptosira cpDNA and its chlorophycean counterparts suggest that the same evolutionary forces shaped the IR-lacking chloroplast genomes in these two algal lineages.

An unexpectedly large and loosely packed mitochondrial genome in the charophycean green alga Chlorokybus atmophyticus

Background

The Streptophyta comprises all land plants and six groups of charophycean green algae. The scaly biflagellate Mesostigma viride (Mesostigmatales) and the sarcinoid Chlorokybus atmophyticus (Chlorokybales) represent the earliest diverging lineages of this phylum. In trees based on chloroplast genome data, these two charophycean green algae are nested in the same clade. To validate this relationship and gain insight into the ancestral state of the mitochondrial genome in the Charophyceae, we sequenced the mitochondrial DNA (mtDNA) of Chlorokybus and compared this genome sequence with those of three other charophycean green algae and the bryophytes Marchantia polymorpha and Physcomitrella patens.

Results

The Chlorokybus genome differs radically from its 42,424-bp Mesostigma counterpart in size, gene order, intron content and density of repeated elements. At 201,763-bp, it is the largest mtDNA yet reported for a green alga. The 70 conserved genes represent 41.4% of the genome sequence and include nad10 and trnL(gag), two genes reported for the first time in a streptophyte mtDNA. At the gene order level, the Chlorokybus genome shares with its Chara, Chaetosphaeridium and bryophyte homologues eight to ten gene clusters including about 20 genes. Notably, some of these clusters exhibit gene linkages not previously found outside the Streptophyta, suggesting that they originated early during streptophyte evolution. In addition to six group I and 14 group II introns, short repeated sequences accounting for 7.5% of the genome were identified. Mitochondrial trees were unable to resolve the correct position of Mesostigma, due to analytical problems arising from accelerated sequence evolution in this lineage.

Conclusion

The Chlorokybus and Mesostigma mtDNAs exemplify the marked fluidity of the mitochondrial genome in charophycean green algae. The notion that the mitochondrial genome was constrained to remain compact during charophycean evolution is no longer tenable. Our data raise the possibility that the emergence of land plants was not associated with a substantial gain of intergenic sequences by the mitochondrial genome.

A clade uniting the green algae Mesostigma viride and Chlorokybus atmophyticus represents the deepest branch of the Streptophyta in chloroplast genome-based phylogenies

BACKGROUND

The Viridiplantae comprise two major phyla: the Streptophyta, containing the charophycean green algae and all land plants, and the Chlorophyta, containing the remaining green algae. Despite recent progress in unravelling phylogenetic relationships among major green plant lineages, problematic nodes still remain in the green tree of life. One of the major issues concerns the scaly biflagellate Mesostigma viride, which is either regarded as representing the earliest divergence of the Streptophyta or a separate lineage that diverged before the Chlorophyta and Streptophyta. Phylogenies based on chloroplast and mitochondrial genomes support the latter view. Because some green plant lineages are not represented in these phylogenies, sparse taxon sampling has been suspected to yield misleading topologies. Here, we describe the complete chloroplast DNA (cpDNA) sequence of the early-diverging charophycean alga Chlorokybus atmophyticus and present chloroplast genome-based phylogenies with an expanded taxon sampling.

RESULTS

The 152,254 bp Chlorokybus cpDNA closely resembles its Mesostigma homologue at the gene content and gene order levels. Using various methods of phylogenetic inference, we analyzed amino acid and nucleotide data sets that were derived from 45 protein-coding genes common to the cpDNAs of 37 green algal/land plant taxa and eight non-green algae. Unexpectedly, all best trees recovered a robust clade uniting Chlorokybus and Mesostigma. In protein trees, this clade was sister to all streptophytes and chlorophytes and this placement received moderate support. In contrast, gene trees provided unequivocal support to the notion that the Mesostigma + Chlorokybus clade represents the earliest-diverging branch of the Streptophyta. Independent analyses of structural data (gene content and/or gene order) and of subsets of amino acid data progressively enriched in slow-evolving sites led us to conclude that the latter topology reflects the true organismal relationships.

CONCLUSION

In disclosing a sister relationship between the Mesostigmatales and Chlorokybales, our study resolves the long-standing debate about the nature of the unicellular flagellated ancestors of land plants and alters significantly our concepts regarding the evolution of streptophyte algae. Moreover, in predicting a richer chloroplast gene repertoire than previously inferred for the common ancestor of all streptophytes, our study has contributed to a better understanding of chloroplast genome evolution in the Viridiplantae.