The complete chloroplast genome sequence of the chlorophycean green alga Scenedesmus obliquus reveals a compact gene organization and a biased distribution of genes on the two DNA strands

Taxonomic reinvestigation of the genus Tetradesmus (Scenedesmaceae; Sphaeropleales) based on morphological characteristics and chloroplast genomes

The genus Tetradesmus (Scenedesmaceae; Sphaeropleales) comprises one of the most abundant green algae in freshwater environments. It includes morphologically diverse species that exhibit bundle-like, plane-arranged coenobia, and unicells, because several different Scenedesmus-like groups were integrated into this genus based on phylogenetic analysis. Nevertheless, there is no clear information regarding the phylogenetic relationship of Tetradesmus species, determined using several marker genes, because of low phylogenetic support and insufficient molecular data. Currently, genome information is available from diverse taxa, which could provide high-resolution evolutionary relationships. In particular, phylogenetic studies using chloroplast genomes demonstrated the potential to establish high-resolution phylogenetic relationships. However, only three chloroplast genomes are available from the genus Tetradesmus. In this study, we newly generated 9 chloroplast genomes from Tetradesmus and constructed a high-resolution phylogeny using a concatenated alignment of 69 chloroplast protein sequences. We also report one novel species (T. lancea), one novel variety (T. obliquus var. spiraformis), and two novel formae (T. dissociatus f. oviformis, T. obliquus f. rectilineare) within the genus Tetradesmus based on morphological characteristics (e.g., cellular arrangements and coenobial types) and genomic features (e.g., different exon-intron structures in chloroplast genomes). Moreover, we taxonomically reinvestigated the genus Tetradesmus based on these results. Altogether, our study can provide a comprehensive understanding of the taxonomic approaches for investigating this genus.

Complete plastome of Coelastrum microporum Nägeli (Scenedesmaceae, Sphaeropleales)

The genus Coelastrum Nägeli (Sphaeropleales; Scenedesmaceae) is a diverse genus of green algae with potential biotechnical applications. A sound understanding of its phylogeny will be a useful tool for predicting the distribution of traits that may enhance its utility, and may lead to a better understanding of its evolution and ecology. Here we present the plastome of Coelastrum microporum. Our exemplar was isolated from Gull Lake, Michigan and the complete plastome as assembled was 169,961 bp in length. The plastome contained 104 genes of which 68 were protein-coding genes (CDSs), 27 tRNA genes and three rRNA genes. The GC content of the plastome was 31.2%. The maximum likelihood phylogeny suggested that C. microporum was the sister group to a clade of single exemplars of three other genera in the Scenedesmaceae (Tetradesmus, Pectinodesmus and Coelastrella).

Characterization of the mitochondrial genome of Chlorolobion braunii ITBB-AG6, an azolla-associated green alga isolated from sanitary sewage

Sphaeropleales have the characteristics of rapid growth, high oil content, and efficient removal rates of nitrogen and phosphorus in sewage waters, and is potentially valuable in biodiesel production and environmental remediation. In this study, we isolated a strain of Sphaeropleales, Chlorolobion braunii strain ITBB-AG6 from an azolla community in a sewage pond. Its mitochondrial genome contains 110,124 bp and harbors at least 40 genes, including 15 protein-coding genes, 20 tRNA genes, and three rRNA genes. The protein-coding genes include two for ATP synthases, seven for NAD(P)H-quinone oxidoreductases (nad), three for cytochrome c oxidase subunits (coxs), and one for cytochrome b (cob). Transfer RNA genes for 18 amino acids were identified, in which the tRNA genes for leucine and serine are doubled, but the tRNA genes for threonine and valine are not annotated. Phylogenetic analysis using the mitochondrial genomes of seven families of Sphaeropleales indicated that ITBB-AG6 is closely related to Monoraphidium neglectum, and falls in the family Selenastraceae with 100% bootstrap support. Two species in the family Neochloridaceae are separated by a species in Hydrodictyaceae, indicating a polyphyletic nature. These findings revealed the complicated phylogenetic relationships of the Sphaeropleales and the necessity of genome sequences in the taxonomy of microalgae.

Structure and Phylogeny of Chloroplast and Mitochondrial Genomes of a Chlorophycean Algae Pectinodesmus pectinatus (Scenedesmaceae, Sphaeropleales)

Pectinodesmus pectinatus is a green alga of commercial interest in sewage purification. Clarification of its organelle genomes is helpful for genetic manipulation, taxonomic revisions and evolutionary research. Here, de novo sequencing was used to determine chloroplast genome and mitochondrial genome of P. pectinatus strain F34. The chloroplast genome was composed of a large single-copy (LSC) region of 99,156 bp, a small single-copy (SSC) region of 70,665 bp, and a pair of inverted repeats (IRs) with a length of 13,494 bp each separated by LSC and SSC. The chloroplast genome contained 69 protein-coding genes, 25 transfer-RNA (tRNA) genes, 3 ribosomal RNA (rRNA) genes. The mitochondrial genome was 32,195 bp in length and consisted of 46 unique genes, including 16 protein-coding genes, 27 tRNA genes and 3 rRNA genes. The predominant mutations in organelle genomes were T/A to G/C transitions. Phylogenic analysis indicated P. pectinatus was a sister species to Tetradesmus obliquus and Hariotina sp. within the Pectinodesmus genus. In analysis with CGView Comparison Tool, P. pectinatus organelle genomes displayed the highest sequence similarity with that of T. obliquus. These findings advanced research on the taxonomy and phylogeny of Chlorophyceae algae and particularly revealed the role of P. pectinatus in microalgae evolution.

Towards green biomanufacturing of high-value recombinant proteins using promising cell factory: Chlamydomonas reinhardtii chloroplast

Microalgae are cosmopolitan organisms in nature with short life cycles, playing a tremendous role in reducing the pressure of industrial carbon emissions. Besides, microalgae have the unique advantages of being photoautotrophic and harboring both prokaryotic and eukaryotic expression systems, becoming a popular host for recombinant proteins. Currently, numerous advanced molecular tools related to microalgal transgenesis have been explored and established, especially for the model species Chlamydomonas reinhardtii (C. reinhardtii hereafter). The development of genetic tools and the emergence of new strategies further increase the feasibility of developing C. reinhardtii chloroplasts as green factories, and the strong genetic operability of C. reinhardtii endows it with enormous potential as a synthetic biology platform. At present, C. reinhardtii chloroplasts could successfully produce plenty of recombinant proteins, including antigens, antibodies, antimicrobial peptides, protein hormones and enzymes. However, additional techniques and toolkits for chloroplasts need to be developed to achieve efficient and markerless editing of plastid genomes. Mining novel genetic elements and selectable markers will be more intensively studied in the future, and more factors affecting protein expression are urged to be explored. This review focuses on the latest technological progress of selectable markers for Chlamydomonas chloroplast genetic engineering and the factors that affect the efficiency of chloroplast protein expression. Furthermore, urgent challenges and prospects for future development are pointed out.

Uncovering New Diversity of Photosynthetic Microorganisms from the Mediterranean Region

In the large and morphologically diverse phylum of Chlorophyta, new taxa are discovered every year and their phylogenetic relationships are reconstructed by the incorporation of molecular phylogenetic methods into traditional taxonomy. Herein, we aim to contribute to the photosynthetic microorganisms’ diversity knowledge in the Mediterranean area, a relatively unexplored ecoregion with high diversity. Based on a polyphasic approach, 18 Chlorophyta isolates were investigated and characterized. Morphological characteristics and ultrastructure, the phylogeny based on 18S rRNA gene (small subunit ribosomal RNA), 18S–28S internal transcribed spacer (ITS region), and the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit region (rbcL gene), support establishing four new genera (Nomia, Ava, Akraea, Lilaea) and five new species (Spongiosarcinopsis limneus, N. picochloropsia, Av. limnothalassea, Ak. chliaropsychia, and L. pamvotia) belonging to orders Sphaeropleales, Chlorellales, and Chlamydomonadales. For some of them, this is the first report of their occurrence in specific aquatic environments.

A Comparative Analysis of the Chloroplast Genomes of Four Polygonum Medicinal Plants

Polygonum is a generalized genus of the Polygonaceae family that includes various herbaceous plants. In order to provide aid in understanding the evolutionary and phylogenetic relationship in Polygonum at the chloroplast (cp) genome-scale level, we sequenced and annotated the complete chloroplast genomes of four Polygonum species using next-generation sequencing technology and CpGAVAS. Then, repeat sequences, IR contractions, and expansion and transformation sites of chloroplast genomes of four Polygonum species were studied, and a phylogenetic tree was built using the chloroplast genomes of Polygonum. The results indicated that the chloroplast genome construction of Polygonum also displayed characteristic four types of results, comparable to the published chloroplast genome of recorded angiosperms. The chloroplast genomes of the four Polygonum plants are highly consistent in genome size (159,015 bp-163,461 bp), number of genes (112 genes, including 78 protein-coding genes, 30 tRNA genes, and 4 rRNA genes), gene types, gene order, codon usage, and repeat sequence distribution, which identifies the high preservation among the Polygonum chloroplast genomes. The Polygonum phylogenetic tree was recreated by a full sequence of the chloroplast genome, which illustrates that the P. bistorta, P. orientale, and P. perfoliatum are divided into the same branch, and P. aviculare belongs to Fallopia. The precise system site of lots base parts requires further verification, but the study would provide a basis for developing the available genetic resources and evolutionary relationships of Polygonum.

Characterization of the Complete Chloroplast Genome of Buddleja Lindleyana

BACKGROUND

Buddleja lindleyana Fort., which belongs to the Loganiaceae with a distribution throughout the tropics, is widely used as an ornamental plant in China. Buddleja contains several morphologically similar species, which need to be identified by molecular identification. But there is little molecular research on the genus Buddleja.

OBJECTIVE

Using molecular biology techniques to sequence and analyze the complete chloroplast (cp) genome of B. lindleyana.

METHODS

According to next-generation sequencing to sequence the genome data, a series of bioinformatics software were used to assembly and analysis the molecular structure of cp genome of B. lindleyana.

RESULTS

The complete cp genome of B. lindleyana is a circular 154,487-bp-long molecule with a GC content of 38.1%. It has a familiar quadripartite structure, including a large single-copy region (LSC; 85,489 bp), a small single-copy region (SSC; 17,898bp) and a pair of inverted repeats (IRs; 25,550 bp). A total of 133 genes were identified in the genome, including 86 protein-coding genes, 37 tRNA genes, 8 rRNA genes and 2 pseudogenes.

CONCLUSIONS

These results suggested that B. lindelyana cp genome could be used as a potential genomic resource to resolve the phylogenetic positions and relationships of Loganiaceae, and will offer valuable information for future research in the identification of Buddleja species and will conduce to genomic investigations of these species.

Comparing and phylogenetic analysis chloroplast genome of three Achyranthes species

In this study, the chloroplast genome sequencing of the Achyranthes longifolia, Achyranthes bidentata and Achyranthes aspera were performed by Next-generation sequencing technology. The results revealed that there were a length of 151,520 bp (A. longifolia), 151,284 bp (A. bidentata), 151,486 bp (A. aspera), respectively. These chloroplast genome have a highly conserved structure with a pair of inverted repeat (IR) regions (25,150 bp; 25,145 bp; 25,150 bp), a large single copy (LSC) regions (83,732 bp; 83,933 bp; 83,966 bp) and a small single copy (SSC) regions (17,252 bp; 17,263 bp; 17,254 bp) in A. bidentate, A. aspera and A. longifolia. There were 127 genes were annotated, which including 8 rRNA genes, 37 tRNA genes and 82 functional genes. The phylogenetic analysis strongly revealed that Achyranthes is monophyletic, and A. bidentata was the closest relationship with A. aspera and A. longifolia. A. bidentata and A. longifolia were clustered together, the three Achyranthes species had the same origin, then the gunes of Achyranthes is the closest relative to Alternanthera, and that forms a group with Alternanthera philoxeroides. The research laid a foundation and provided relevant basis for the identification of germplasm resources in the future.

Ecotoxicological and biochemical effects of environmental concentrations of the plastic-bond pollutant dibutyl phthalate on Scenedesmus sp

Phthalate esters are highly present in aquatic plastic litter, which can interfere with the biological processes in the wildlife. In this work, the commonly found freshwater microalga Scenedesmus sp. was exposed to environmental concentrations (0.02, 1 and 100 μg L^-1) and to a higher concentration (500 μg L^-1) of dibutyl phthalate (DBP), which is an environmental pollutant. The growth, pH variation, production of photosynthetic pigments, proteins and carbohydrates were evaluated. The main inhibition effect of DBP on the microalgal growth was observed in the first 48 h of the exposure (EC₅₀: 41.88 μg L^-1). A reduction in the photosynthetic pigment concentration was observed for the 0.02, 1 and 100 μg L^-1 conditions indicating that the DBP downregulated the growth rate and affected the photosynthetic process. A significant increase in protein production was only observed under 500 μg L^-1 DBP exposure. The extracellular carbohydrates production slightly decreased with the presence of DBP, with a stronger decrease occurring in the 500 μg L^-1 condition. These results highlight the environmental risk evaluation and ecotoxicological effects of DBP on the production of biovaluable compounds by microalgae. The results also emphasize the importance of assessing the consequences of the environmental concentrations exposure as a result of the DBP dose-dependent correlation effects.

Next-Generation Genome Sequencing of Sedum plumbizincicola Sheds Light on the Structural Evolution of Plastid rRNA Operon and Phylogenetic Implications within Saxifragales

The genus Sedum, with about 470 recognized species, is classified in the family Crassulaceae of the order Saxifragales. Phylogenetic relationships within the Saxifragales are still unresolved and controversial. In this study, the plastome of S. plumbizincicola was firstly presented, with a focus on the structural analysis of rrn operon and phylogenetic implications within the order Saxifragaceae. The assembled complete plastome of S. plumbizincicola is 149,397 bp in size, with a typical circular, double-stranded, and quadripartite structure of angiosperms. It contains 133 genes, including 85 protein-coding genes (PCGs), 36 tRNA genes, 8 rRNA genes, and four pseudogenes (one ycf1, one rps19, and two ycf15). The predicted secondary structure of S. plumbizincicola 16S rRNA includes three main domains organized in 74 helices. Further, our results confirm that 4.5S rRNA of higher plants is associated with fragmentation of 23S rRNA progenitor. Notably, we also found the sequence of putative rrn5 promoter has some evolutionary implications within the order Saxifragales. Moreover, our phylogenetic analyses suggested that S. plumbizincicola had a closer relationship with S. sarmentosum than S. oryzifolium, and supported the taxonomic revision of Phedimus. Our findings of the present study will be useful for further investigation of the evolution of plastid rRNA operon and phylogenetic relationships within Saxifragales.

Complete Chloroplast Genome Sequence and Phylogenetic Analysis of Quercus bawanglingensis Huang, Li et Xing, a Vulnerable Oak Tree in China

Quercus bawanglingensis Huang, Li et Xing, an endemic evergreen oak of the genus Quercus (Fagaceae) in China, is currently listed in the Red List of Chinese Plants as a vulnerable (VU) plant. No chloroplast (cp) genome information is currently available for Q. bawanglingensis, which would be essential for the establishment of guidelines for its conservation and breeding. In the present study, the cp genome of Q. bawanglingensis was sequenced and assembled into double-stranded circular DNA with a length of 161,394 bp. Two inverted repeats (IRs) with a total of 51,730 bp were identified, and the rest of the sequence was separated into two single-copy regions, namely, a large single-copy (LSC) region (90,628 bp) and a small single-copy (SSC) region (19,036 bp). The genome of Q. bawanglingensis contains 134 genes (86 protein-coding genes, 40 tRNAs and eight rRNAs). More forward (29) than inverted long repeats (21) are distributed in the cp genome. A simple sequence repeat (SSR) analysis showed that the genome contains 82 SSR loci, involving 84.15% A/T mononucleotides. Sequence comparisons among the nine complete cp genomes, including the genomes of Q. bawanglingensis, Q. tarokoensis Hayata (NC036370), Q. aliena var. acutiserrata Maxim. ex Wenz. (KU240009), Q. baronii Skan (KT963087), Q. aquifolioides Rehd. et Wils. (KX911971), Q. variabilis Bl. (KU240009), Fagus engleriana Seem. (KX852398), Lithocarpus balansae (Drake) A. Camus (KP299291) and Castanea mollissima Bl. (HQ336406), demonstrated that the diversity of SC regions was higher than that of IR regions, which might facilitate identification of the relationships within this extremely complex family. A phylogenetic analysis showed that Fagus engleriana and Trigonobalanus doichangensis form the basis of the produced evolutionary tree. Q. bawanglingensis and Q. tarokoensis, which belong to the group Ilex, share the closest relationship. The analysis of the cp genome of Q. bawanglingensis provides crucial genetic information for further studies of this vulnerable species and the taxonomy, phylogenetics and evolution of Quercus.

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.

Complete Chloroplast Genomes from Sanguisorba: Identity and Variation Among Four Species

The genus Sanguisorba, which contains about 30 species around the world and seven species in China, is the source of the medicinal plant Sanguisorba officinalis, which is commonly used as a hemostatic agent as well as to treat burns and scalds. Here we report the complete chloroplast (cp) genome sequences of four Sanguisorba species (S. officinalis, S. filiformis, S. stipulata, and S. tenuifolia var. alba). These four Sanguisorba cp genomes exhibit typical quadripartite and circular structures, and are 154,282 to 155,479 bp in length, consisting of large single-copy regions (LSC; 84,405⁻85,557 bp), small single-copy regions (SSC; 18,550⁻18,768 bp), and a pair of inverted repeats (IRs; 25,576⁻25,615 bp). The average GC content was ~37.24%. The four Sanguisorba cp genomes harbored 112 different genes arranged in the same order; these identical sections include 78 protein-coding genes, 30 tRNA genes, and four rRNA genes, if duplicated genes in IR regions are counted only once. A total of 39⁻53 long repeats and 79⁻91 simple sequence repeats (SSRs) were identified in the four Sanguisorba cp genomes, which provides opportunities for future studies of the population genetics of Sanguisorba medicinal plants. A phylogenetic analysis using the maximum parsimony (MP) method strongly supports a close relationship between S. officinalis and S. tenuifolia var. alba, followed by S. stipulata, and finally S. filiformis. The availability of these cp genomes provides valuable genetic information for future studies of Sanguisorba identification and provides insights into the evolution of the genus Sanguisorba.

Raphidocelis subcapitata (=Pseudokirchneriella subcapitata) provides an insight into genome evolution and environmental adaptations in the Sphaeropleales

The Sphaeropleales are a dominant group of green algae, which contain species important to freshwater ecosystems and those that have potential applied usages. In particular, Raphidocelis subcapitata is widely used worldwide for bioassays in toxicological risk assessments. However, there are few comparative genome analyses of the Sphaeropleales. To reveal genome evolution in the Sphaeropleales based on well-resolved phylogenetic relationships, nuclear, mitochondrial, and plastid genomes were sequenced in this study. The plastid genome provides insights into the phylogenetic relationships of R. subcapitata, which is located in the most basal lineage of the four species in the family Selenastraceae. The mitochondrial genome shows dynamic evolutionary histories with intron expansion in the Selenastraceae. The 51.2 Mbp nuclear genome of R. subcapitata, encoding 13,383 protein-coding genes, is more compact than the genome of its closely related oil-rich species, Monoraphidium neglectum (Selenastraceae), Tetradesmus obliquus (Scenedesmaceae), and Chromochloris zofingiensis (Chromochloridaceae); however, the four species share most of their genes. The Sphaeropleales possess a large number of genes for glycerolipid metabolism and sugar assimilation, which suggests that this order is capable of both heterotrophic and mixotrophic lifestyles in nature. Comparison of transporter genes suggests that the Sphaeropleales can adapt to different natural environmental conditions, such as salinity and low metal concentrations.

The Complete Amomum kravanh Chloroplast Genome Sequence and Phylogenetic Analysis of the Commelinids

Amomum kravanh is an important edible and medicinal herb, the dried fruits of which are widely used in traditional herbal medicine as cardamom. We sequenced and analyzed the complete chloroplast (cp) genome of A. kravanh with herbgenomics technologies. The size of the A. kravanh cp genome was 162,766 bp, which consisted of long (LSC; 87,728 bp) and short (SSC; 15,390 bp) single-copy regions, separated by a pair of inverted repeats (IRs; 29,824 bp). The genome encoded 114 unique genes, including 80 protein-coding genes, 30 tRNAs and four rRNAs. A total of 299 simple sequence repeats (SSRs) were identified in the A. kravanh cp genome, which provides an effective method to study species identification and population genetics of the medicinal plant. Moreover, one complement, 12 forward, 12 palindrome and two reverse repeats were detected. Comparative cp genome sequence analysis of four Zingiberaceae species indicated that their intergenic spacers are highly divergent, although the gene order, gene content and genome structure differed only minimally. In particular, there was a remarkable expansion of the IR regions in the A. kravanh cp genome. Phylogenetic analysis strongly supported a sister relationship between A. kravanh and Alpinia zerumbet. This study identified the unique characteristics of the A. kravanh cp genome and might provide valuable information for future studies aiming for Amomum identification, and provide insights into the taxonomy of the commelinids.

Complete Chloroplast Genome Sequence and Phylogenetic Analysis of the Medicinal Plant Artemisia annua

The complete chloroplast genome of Artemisia annua (Asteraceae), the primary source of artemisinin, was sequenced and analyzed. The A. annua cp genome is 150,995 bp, and harbors a pair of inverted repeat regions (IRa and IRb), of 24,850 bp each that separate large (LSC, 82,988 bp) and small (SSC, 18,267 bp) single-copy regions. Our annotation revealed that the A. annua cp genome contains 113 genes and 18 duplicated genes. The gene order in the SSC region of A. annua is inverted; this fact is consistent with the sequences of chloroplast genomes from three other Artemisia species. Fifteen (15) forward and seventeen (17) inverted repeats were detected in the genome. The existence of rich SSR loci in the genome suggests opportunities for future population genetics work on this anti-malarial medicinal plant. In A. annua cpDNA, the rps19 gene was found in the LSC region rather than the IR region, and the rps19 pseudogene was absent in the IR region. Sequence divergence analysis of five Asteraceae species indicated that the most highly divergent regions were found in the intergenic spacers, and that the differences between A. annua and A. fukudo were very slight. A phylogenetic analysis revealed a sister relationship between A. annua and A. fukudo. This study identified the unique characteristics of the A. annua cp genome. These results offer valuable information for future research on Artemisia species identification and for the selective breeding of A. annua with high pharmaceutical efficacy.

Divergent copies of the large inverted repeat in the chloroplast genomes of ulvophycean green algae

The chloroplast genomes of many algae and almost all land plants carry two identical copies of a large inverted repeat (IR) sequence that can pair for flip-flop recombination and undergo expansion/contraction. Although the IR has been lost multiple times during the evolution of the green algae, the underlying mechanisms are still largely unknown. A recent comparison of IR-lacking and IR-containing chloroplast genomes of chlorophytes from the Ulvophyceae (Ulotrichales) suggested that differential elimination of genes from the IR copies might lead to IR loss. To gain deeper insights into the evolutionary history of the chloroplast genome in the Ulvophyceae, we analyzed the genomes of Ignatius tetrasporus and Pseudocharacium americanum (Ignatiales, an order not previously sampled), Dangemannia microcystis (Oltmannsiellopsidales), Pseudoneochloris marina (Ulvales) and also Chamaetrichon capsulatum and Trichosarcina mucosa (Ulotrichales). Our comparison of these six chloroplast genomes with those previously reported for nine ulvophyceans revealed unsuspected variability. All newly examined genomes feature an IR, but remarkably, the copies of the IR present in the Ignatiales, Pseudoneochloris, and Chamaetrichon diverge in sequence, with the tRNA genes from the rRNA operon missing in one IR copy. The implications of this unprecedented finding for the mechanism of IR loss and flip-flop recombination are discussed.

Draft Genome Sequence of the Oleaginous Green Alga Tetradesmus obliquus UTEX 393

The microalgae Tetradesmus obliquus is able to maintain a high photosynthetic efficiency under nitrogen limitation and is considered a promising green microalgae for sustainable production of diverse compounds, including biofuels. Here, we report the first draft whole-genome shotgun sequencing of T. obliquus. The final assembly comprises 108,715,903 bp with over 1,368 scaffolds.

Proliferation of group II introns in the chloroplast genome of the green alga Oedocladium carolinianum (Chlorophyceae)

Background

The chloroplast genome sustained extensive changes in architecture during the evolution of the Chlorophyceae, a morphologically and ecologically diverse class of green algae belonging to the Chlorophyta; however, the forces driving these changes are poorly understood. The five orders recognized in the Chlorophyceae form two major clades: the CS clade consisting of the Chlamydomonadales and Sphaeropleales, and the OCC clade consisting of the Oedogoniales, Chaetophorales, and Chaetopeltidales. In the OCC clade, considerable variations in chloroplast DNA (cpDNA) structure, size, gene order, and intron content have been observed. The large inverted repeat (IR), an ancestral feature characteristic of most green plants, is present in Oedogonium cardiacum (Oedogoniales) but is lacking in the examined members of the Chaetophorales and Chaetopeltidales. Remarkably, the Oedogonium 35.5-kb IR houses genes that were putatively acquired through horizontal DNA transfer. To better understand the dynamics of chloroplast genome evolution in the Oedogoniales, we analyzed the cpDNA of a second representative of this order, Oedocladium carolinianum.

Methods

The Oedocladium cpDNA was sequenced and annotated. The evolutionary distances separating Oedocladium and Oedogonium cpDNAs and two other pairs of chlorophycean cpDNAs were estimated using a 61-gene data set. Phylogenetic analysis of an alignment of group IIA introns from members of the OCC clade was performed. Secondary structures and insertion sites of oedogonialean group IIA introns were analyzed.

Results

The 204,438-bp Oedocladium genome is 7.9 kb larger than the Oedogonium genome, but its repertoire of conserved genes is remarkably similar and gene order differs by only one reversal. Although the 23.7-kb IR is missing the putative foreign genes found in Oedogonium, it contains sequences coding for a putative phage or bacterial DNA primase and a hypothetical protein. Intergenic sequences are 1.5-fold longer and dispersed repeats are more abundant, but a smaller fraction of the Oedocladium genome is occupied by introns. Six additional group II introns are present, five of which lack ORFs and carry highly similar sequences to that of the ORF-less IIA intron shared with Oedogonium. Secondary structure analysis of the group IIA introns disclosed marked differences in the exon-binding sites; however, each intron showed perfect or nearly perfect base pairing interactions with its target site.

Discussion

Our results suggest that chloroplast genes rearrange more slowly in the Oedogoniales than in the Chaetophorales and raise questions as to what was the nature of the foreign coding sequences in the IR of the common ancestor of the Oedogoniales. They provide the first evidence for intragenomic proliferation of group IIA introns in the Viridiplantae, revealing that intron spread in the Oedocladium lineage likely occurred by retrohoming after sequence divergence of the exon-binding sites.

A Ribonucleoprotein Supercomplex Involved in trans-Splicing of Organelle Group II Introns

In the chloroplast of the green alga Chlamydomonas reinhardtii, two discontinuous group II introns, psaA-i1 and psaA-i2, splice in trans, and thus their excision process resembles the nuclear spliceosomal splicing pathway. Here, we address the question whether fragmentation of trans-acting RNAs is accompanied by the formation of a chloroplast spliceosome-like machinery. Using a combination of liquid chromatography-mass spectrometry (LC-MS), size exclusion chromatography, and quantitative RT-PCR, we provide the first characterization of a high molecular weight ribonucleoprotein apparatus participating in psaA mRNA splicing. This supercomplex contains two subcomplexes (I and II) that are responsible for trans-splicing of either psaA-i1 or psaA-i2. We further demonstrate that both subcomplexes are associated with intron RNA, which is a prerequisite for the correct assembly of subcomplex I. This study contributes further to our view of how the eukaryotic nuclear spliceosome evolved after bacterial endosymbiosis through fragmentation of self-splicing group II introns into a dynamic, protein-rich RNP machinery.

Mitochondrion-to-Chloroplast DNA Transfers and Intragenomic Proliferation of Chloroplast Group II Introns in Gloeotilopsis Green Algae (Ulotrichales, Ulvophyceae)

To probe organelle genome evolution in the Ulvales/Ulotrichales clade, the newly sequenced chloroplast and mitochondrial genomes of Gloeotilopsis planctonica and Gloeotilopsis sarcinoidea (Ulotrichales) were compared with those of Pseudendoclonium akinetum (Ulotrichales) and of the few other green algae previously sampled in the Ulvophyceae. At 105,236 bp, the G planctonica mitochondrial DNA (mtDNA) is the largest mitochondrial genome reported so far among chlorophytes, whereas the 221,431-bp G planctonica and 262,888-bp G sarcinoidea chloroplast DNAs (cpDNAs) are the largest chloroplast genomes analyzed among the Ulvophyceae. Gains of non-coding sequences largely account for the expansion of these genomes. Both Gloeotilopsis cpDNAs lack the inverted repeat (IR) typically found in green plants, indicating that two independent IR losses occurred in the Ulvales/Ulotrichales. Our comparison of the Pseudendoclonium and Gloeotilopsis cpDNAs offered clues regarding the mechanism of IR loss in the Ulotrichales, suggesting that internal sequences from the rDNA operon were differentially lost from the two original IR copies during this process. Our analyses also unveiled a number of genetic novelties. Short mtDNA fragments were discovered in two distinct regions of the G sarcinoidea cpDNA, providing the first evidence for intracellular inter-organelle gene migration in green algae. We identified for the first time in green algal organelles, group II introns with LAGLIDADG ORFs as well as group II introns inserted into untranslated gene regions. We discovered many group II introns occupying sites not previously documented for the chloroplast genome and demonstrated that a number of them arose by intragenomic proliferation, most likely through retrohoming.

Codon Adaptation of Plastid Genes

Codon adaptation is codon usage bias that results from selective pressure to increase the translation efficiency of a gene. Codon adaptation has been studied across a wide range of genomes and some early analyses of plastids have shown evidence for codon adaptation in a limited set of highly expressed plastid genes. Here we study codon usage bias across all fully sequenced plastid genomes which includes representatives of the Rhodophyta, Alveolata, Cryptophyta, Euglenozoa, Glaucocystophyceae, Rhizaria, Stramenopiles and numerous lineages within the Viridiplantae, including Chlorophyta and Embryophyta. We show evidence that codon adaptation occurs in all genomes except for two, Theileria parva and Heicosporidium sp., both of which have highly reduced gene contents and no photosynthesis genes. We also show evidence that selection for codon adaptation increases the representation of the same set of codons, which we refer to as the adaptive codons, across this wide range of taxa, which is probably due to common features descended from the initial endosymbiont. We use various measures to estimate the relative strength of selection in the different lineages and show that it appears to be fairly strong in certain Stramenopiles and Chlorophyta lineages but relatively weak in many members of the Rhodophyta, Euglenozoa and Embryophyta. Given these results we propose that codon adaptation in plastids is widespread and displays the same general features as adaptation in eubacterial genomes.

Distinctive Architecture of the Chloroplast Genome in the Chlorodendrophycean Green Algae Scherffelia dubia and Tetraselmis sp. CCMP 881

The Chlorodendrophyceae is a small class of green algae belonging to the core Chlorophyta, an assemblage that also comprises the Pedinophyceae, Trebouxiophyceae, Ulvophyceae and Chlorophyceae. Here we describe for the first time the chloroplast genomes of chlorodendrophycean algae (Scherffelia dubia, 137,161 bp; Tetraselmis sp. CCMP 881, 100,264 bp). Characterized by a very small single-copy (SSC) region devoid of any gene and an unusually large inverted repeat (IR), the quadripartite structures of the Scherffelia and Tetraselmis genomes are unique among all core chlorophytes examined thus far. The lack of genes in the SSC region is offset by the rich and atypical gene complement of the IR, which includes genes from the SSC and large single-copy regions of prasinophyte and streptophyte chloroplast genomes having retained an ancestral quadripartite structure. Remarkably, seven of the atypical IR-encoded genes have also been observed in the IRs of pedinophycean and trebouxiophycean chloroplast genomes, suggesting that they were already present in the IR of the common ancestor of all core chlorophytes. Considering that the relationships among the main lineages of the core Chlorophyta are still unresolved, we evaluated the impact of including the Chlorodendrophyceae in chloroplast phylogenomic analyses. The trees we inferred using data sets of 79 and 108 genes from 71 chlorophytes indicate that the Chlorodendrophyceae is a deep-diverging lineage of the core Chlorophyta, although the placement of this class relative to the Pedinophyceae remains ambiguous. Interestingly, some of our phylogenomic trees together with our comparative analysis of gene order data support the monophyly of the Trebouxiophyceae, thus offering further evidence that the previously observed affiliation between the Chlorellales and Pedinophyceae is the result of systematic errors in phylogenetic reconstruction.

Chloroplast phylogenomic analysis of chlorophyte green algae identifies a novel lineage sister to the Sphaeropleales (Chlorophyceae)

Background

The class Chlorophyceae (Chlorophyta) includes morphologically and ecologically diverse green algae. Most of the documented species belong to the clade formed by the Chlamydomonadales (also called Volvocales) and Sphaeropleales. Although studies based on the nuclear 18S rRNA gene or a few combined genes have shed light on the diversity and phylogenetic structure of the Chlamydomonadales, the positions of many of the monophyletic groups identified remain uncertain. Here, we used a chloroplast phylogenomic approach to delineate the relationships among these lineages.

Results

To generate the analyzed amino acid and nucleotide data sets, we sequenced the chloroplast DNAs (cpDNAs) of 24 chlorophycean taxa; these included representatives from 16 of the 21 primary clades previously recognized in the Chlamydomonadales, two taxa from a coccoid lineage (Jenufa) that was suspected to be sister to the Golenkiniaceae, and two sphaeroplealeans. Using Bayesian and/or maximum likelihood inference methods, we analyzed an amino acid data set that was assembled from 69 cpDNA-encoded proteins of 73 core chlorophyte (including 33 chlorophyceans), as well as two nucleotide data sets that were generated from the 69 genes coding for these proteins and 29 RNA-coding genes. The protein and gene phylogenies were congruent and robustly resolved the branching order of most of the investigated lineages. Within the Chlamydomonadales, 22 taxa formed an assemblage of five major clades/lineages. The earliest-diverging clade displayed Hafniomonas laevis and the Crucicarteria, and was followed by the Radicarteria and then by the Chloromonadinia. The latter lineage was sister to two superclades, one consisting of the Oogamochlamydinia and Reinhardtinia and the other of the Caudivolvoxa and Xenovolvoxa. To our surprise, the Jenufa species and the two spine-bearing green algae belonging to the Golenkinia and Treubaria genera were recovered in a highly supported monophyletic group that also included three taxa representing distinct families of the Sphaeropleales (Bracteacoccaceae, Mychonastaceae, and Scenedesmaceae).

Conclusions

Our phylogenomic study advances our knowledge regarding the circumscription and internal structure of the Chlamydomonadales, suggesting that a previously unrecognized lineage is sister to the Sphaeropleales. In addition, it offers new insights into the flagellar structures of the founding members of both the Chlamydomonadales and Sphaeropleales.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0544-5) contains supplementary material, which is available to authorized users.

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.

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.

Gene arrangement convergence, diverse intron content, and genetic code modifications in mitochondrial genomes of sphaeropleales (chlorophyta)

The majority of our knowledge about mitochondrial genomes of Viridiplantae comes from land plants, but much less is known about their green algal relatives. In the green algal order Sphaeropleales (Chlorophyta), only one representative mitochondrial genome is currently available—that of Acutodesmus obliquus. Our study adds nine completely sequenced and three partially sequenced mitochondrial genomes spanning the phylogenetic diversity of Sphaeropleales. We show not only a size range of 25–53 kb and variation in intron content (0–11) and gene order but also conservation of 13 core respiratory genes and fragmented ribosomal RNA genes. We also report an unusual case of gene arrangement convergence in Neochloris aquatica, where the two rns fragments were secondarily placed in close proximity. Finally, we report the unprecedented usage of UCG as stop codon in Pseudomuriella schumacherensis. In addition, phylogenetic analyses of the mitochondrial protein-coding genes yield a fully resolved, well-supported phylogeny, showing promise for addressing systematic challenges in green algae.

Inclusion of chloroplast genes that have undergone expansion misleads phylogenetic reconstruction in the Chlorophyta

PREMISE OF THE STUDY

Chlorophytes comprise a substantial proportion of green plant diversity. However, sister-group relationships and circumscription of the classes Chlorophyceae, Trebouxiophyceae, and Ulvophyceae have been problematic to resolve. Some analyses support a sister relationship between the trebouxiophycean Leptosira and chlorophyceans, potentially altering the circumscription of two classes, also supported by a shared fragmentation in the chloroplast gene rpoB. We sought to determine whether the latter is a synapomorphy or whether the supporting analyses are vulnerable to systematic bias.

METHODS

We sequenced a portion of rpoB spanning the fragmented region in strains for which it had not previously been sampled: four Chlorophyceae, six counterclockwise (CCW) group (ulvophyceans and trebouxiophyceans) and one streptophyte. We then explored the effect of subsampling proteins and taxa on phylogenetic reconstruction from a data set of 41 chloroplast proteins.

KEY RESULTS

None of the CCW or streptophyte strains possessed the split in rpoB, including inferred near relatives of Leptosira, but it was found in all chlorophycean strains. We reconstructed alternative phylogenies (Leptosira + Chlorophyceae and Leptosira + Chlorellales) using two different protein groups (Rpo and Rps), both subject to coding-region expansion. A conserved region of RpoB remained suitable for analysis of more recent divergences.

CONCLUSIONS

The Rps sequences can explain earlier findings linking Leptosira with the Chlorophyceae and should be excluded from phylogenetic analyses attempting to resolve deep nodes because their expansion violates the assumptions of substitution models. We reaffirm that Leptosira is a trebouxiophycean and that fragmentation of rpoB has occurred at least twice in chlorophyte evolution.

Split photosystem protein, linear-mapping topology, and growth of structural complexity in the plastid genome of Chromera velia

The canonical photosynthetic plastid genomes consist of a single circular-mapping chromosome that encodes a highly conserved protein core, involved in photosynthesis and ATP generation. Here, we demonstrate that the plastid genome of the photosynthetic relative of apicomplexans, Chromera velia, departs from this view in several unique ways. Core photosynthesis proteins PsaA and AtpB have been broken into two fragments, which we show are independently transcribed, oligoU-tailed, translated, and assembled into functional photosystem I and ATP synthase complexes. Genome-wide transcription profiles support expression of many other highly modified proteins, including several that contain extensions amounting to hundreds of amino acids in length. Canonical gene clusters and operons have been fragmented and reshuffled into novel putative transcriptional units. Massive genomic coverage by paired-end reads, coupled with pulsed-field gel electrophoresis and polymerase chain reaction, consistently indicate that the C. velia plastid genome is linear-mapping, a unique state among all plastids. Abundant intragenomic duplication probably mediated by recombination can explain protein splits, extensions, and genome linearization and is perhaps the key driving force behind the many features that defy the conventional ways of plastid genome architecture and function.

An exogenous chloroplast genome for complex sequence manipulation in algae

We demonstrate a system for cloning and modifying the chloroplast genome from the green alga, Chlamydomonas reinhardtii. Through extensive use of sequence stabilization strategies, the ex vivo genome is assembled in yeast from a collection of overlapping fragments. The assembled genome is then moved into bacteria for large-scale preparations and transformed into C. reinhardtii cells. This system also allows for the generation of simultaneous, systematic and complex genetic modifications at multiple loci in vivo. We use this system to substitute genes encoding core subunits of the photosynthetic apparatus with orthologs from a related alga, Scenedesmus obliquus. Once transformed into algae, the substituted genome recombines with the endogenous genome, resulting in a hybrid plastome comprising modifications in disparate loci. The in vivo function of the genomes described herein demonstrates that simultaneous engineering of multiple sites within the chloroplast genome is now possible. This work represents the first steps toward a novel approach for creating genetic diversity in any or all regions of a chloroplast genome.

Composition and stability of the microbial community inside the digestive tract of the aquatic crustacean Daphnia magna

Small filter-feeding zooplankton organisms like the cladoceran Daphnia spp. are key members of freshwater food webs. Although several interactions between Daphnia and bacteria have been investigated, the importance of the microbial communities inside Daphnia guts has been studied only poorly so far. In the present study, we characterised the bacterial community composition inside the digestive tract of a laboratory-reared clonal culture of Daphnia magna using 16S rRNA gene libraries and terminal-restriction length polymorphism fingerprint analyses. In addition, the diversity and stability of the intestinal microbial community were investigated over time, with different food sources as well as under starvation stress and death, and were compared to the community in the cultivation water. The diversity of the Daphnia gut microbiota was low. The bacterial community consisted mainly of Betaproteobacteria (e.g. Limnohabitans sp.), few Gammaproteobacteria (e.g. Pseudomonas sp.) and Bacteroidetes that were related to facultatively anaerobic bacteria, but did not contain typical fermentative or obligately anaerobic gut bacteria. Rather, the microbiota was constantly dominated by Limnohabitans sp. which belongs to the Lhab-A1 tribe (previously called R-BT065 cluster) that is abundant in various freshwaters. Other bacterial groups varied distinctly even under constant cultivation conditions. Overall, the intestinal microbial community did not reflect the community in the surrounding cultivation water and clustered separately when analysed via the Additive Main Effects and Multiplicative Interaction model. In addition, the microbiota proved to be stable also when Daphnia were exposed to bacteria associated with a different food alga. After starvation, the community in the digestive tract was reduced to stable members. After death of the host animals, the community composition in the gut changed distinctly, and formerly undetected bacteria were activated. Our results suggest that the Daphnia microbiota consists mainly of an aerobic resident bacterial community which is indigenous to this habitat.

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.

The Bryopsis hypnoides plastid genome: multimeric forms and complete nucleotide sequence

Background

Bryopsis hypnoides Lamouroux is a siphonous green alga, and its extruded protoplasm can aggregate spontaneously in seawater and develop into mature individuals. The chloroplast of B. hypnoides is the biggest organelle in the cell and shows strong autonomy. To better understand this organelle, we sequenced and analyzed the chloroplast genome of this green alga.

Principal Findings

A total of 111 functional genes, including 69 potential protein-coding genes, 5 ribosomal RNA genes, and 37 tRNA genes were identified. The genome size (153,429 bp), arrangement, and inverted-repeat (IR)-lacking structure of the B. hypnoides chloroplast DNA (cpDNA) closely resembles that of Chlorella vulgaris. Furthermore, our cytogenomic investigations using pulsed-field gel electrophoresis (PFGE) and southern blotting methods showed that the B. hypnoides cpDNA had multimeric forms, including monomer, dimer, trimer, tetramer, and even higher multimers, which is similar to the higher order organization observed previously for higher plant cpDNA. The relative amounts of the four multimeric cpDNA forms were estimated to be about 1, 1/2, 1/4, and 1/8 based on molecular hybridization analysis. Phylogenetic analyses based on a concatenated alignment of chloroplast protein sequences suggested that B. hypnoides is sister to all Chlorophyceae and this placement received moderate support.

Conclusion

All of the results suggest that the autonomy of the chloroplasts of B. hypnoides has little to do with the size and gene content of the cpDNA, and the IR-lacking structure of the chloroplasts indirectly demonstrated that the multimeric molecules might result from the random cleavage and fusion of replication intermediates instead of recombinational events.

Auxiliary electron transport pathways in chloroplasts of microalgae

Microalgae are photosynthetic organisms which cover an extraordinary phylogenic diversity and have colonized extremely diverse habitats. Adaptation to contrasted environments in terms of light and nutrient's availabilities has been possible through a high flexibility of the photosynthetic machinery. Indeed, optimal functioning of photosynthesis in changing environments requires a fine tuning between the conversion of light energy by photosystems and its use by metabolic reaction, a particularly important parameter being the balance between phosphorylating (ATP) and reducing (NADPH) power supplies. In addition to the main route of electrons operating during oxygenic photosynthesis, called linear electron flow or Z scheme, auxiliary routes of electron transfer in interaction with the main pathway have been described. These reactions which include non-photochemical reduction of intersystem electron carriers, cyclic electron flow around PSI, oxidation by molecular O(2) of the PQ pool or of the PSI electron acceptors, participate in the flexibility of photosynthesis by avoiding over-reduction of electron carriers and modulating the NADPH/ATP ratio depending on the metabolic demand. Forward or reverse genetic approaches performed in model organisms such as Arabidopsis thaliana for higher plants, Chlamydomonas reinhardtii for green algae and Synechocystis for cyanobacteria allowed identifying molecular components involved in these auxiliary electron transport pathways, including Ndh-1, Ndh-2, PGR5, PGRL1, PTOX and flavodiiron proteins. In this article, we discuss the diversity of auxiliary routes of electron transport in microalgae, with particular focus in the presence of these components in the microalgal genomes recently sequenced. We discuss how these auxiliary mechanisms of electron transport may have contributed to the adaptation of microalgal photosynthesis to diverse and changing environments.

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.

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.

Identification and characterization of freshwater algae from a pollution gradient using rbcL sequencing and toxicity testing

One approach in toxicity testing using microalgae is to assess the modulation of light energy absorbed as a result of exposure to contaminants. In this study, four strains of microalgae were isolated to obtain a variety of taxa for testing from sites receiving various levels of environmental stressors around Christchurch, New Zealand. The strains were characterized by partially sequencing rbcL, a routinely used gene in plant phylogenetics with a large existing database of strains. Based on morphological observation and gene sequences, the strains were identified as Chlorella sp., Neochloris sp., and Choricystis minor. The isolates were exposed to the herbicide glyphosate and the metal zinc, and their responses were measured using the ToxY-PAM system. Chlorella sp. was the most sensitive. Two strains of Choricystis minor were isolated from different ponds in an effluent gradient at a sewage treatment plant. Analysis of variance indicated that the isolate from the least contaminated pond was more sensitive to zinc (although regression analysis did not show this result). This suggests that the selective pressure exerted on algal strains by a contamination gradient over short a distance is detectable by both genetic and physiological methods, with implications for sourcing appropriate indicator organisms from the environment.

Gene duplication, transfer, and evolution in the chloroplast genome

In addition to the nuclear genome, organisms have organelle genomes. Most of the DNA present in eukaryotic organisms is located in the cell nucleus. Chloroplasts have independent genomes which are inherited from the mother. Duplicated genes are common in the genomes of all organisms. It is believed that gene duplication is the most important step for the origin of genetic variation, leading to the creation of new genes and new gene functions. Despite the fact that extensive gene duplications are rare among the chloroplast genome, gene duplication in the chloroplast genome is an essential source of new genetic functions and a mechanism of neo-evolution. The events of gene transfer between the chloroplast genome and nuclear genome via duplication and subsequent recombination are important processes in evolution. The duplicated gene or genome in the nucleus has been the subject of several recent reviews. In this review, we will briefly summarize gene duplication and evolution in the chloroplast genome. Also, we will provide an overview of gene transfer events between chloroplast and nuclear genomes.

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.

Multistep processing of an insertion sequence in an essential subunit of the chloroplast ClpP complex

In Chlamydomonas reinhardtii, the clpP1 chloroplast gene encoding one of the catalytic subunits of the ClpP protease complex contains a large in-frame insertion sequence (IS1). Based on the Escherichia coli ClpP structure, IS1 is predicted to protrude at the apical surface of the complex, likely influencing the interaction of the catalytic core with ClpC/HSP100 chaperones. Immunoblotting with an anti-ClpP1 antibody detected two immunoreactive forms of ClpP1: ClpP1(H) (59 kDa) and ClpP1(L) (25 kDa). It has been proposed that IS1 is a new type of protein intron (different from inteins). By studying transformants harboring mutations at the predicted borders of IS1 and tags at the C terminus of ClpP1 (tandem affinity purification tag, His tag, Strep.Tag) or within the IS1 sequence (3-hemagglutinin tag), we show that IS1 is not a protein intron and that ClpP1(L) results from endoproteolytic cleavage inside IS1. Processing sites have been identified in the middle of IS1 and near its C terminus. The sites can be mutated without abolishing processing.

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.