Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Dynamic evolution of inverted repeats in Euglenophyta plastid genomes


Photosynthetic euglenids (Euglenophyta) are a monophyletic group of unicellular eukaryotes characterized by the presence of plastids, which arose as the result of the secondary endosymbiosis. Many Euglenophyta plastid (pt) genomes have been characterized recently, but they represented mainly one family – Euglenaceae. Here, we report a comparative analysis of plastid genomes from eight representatives of the family Phacaceae. Newly sequenced plastid genomes share a number of features including synteny and gene content, except for genes mat2 and mat5 encoding maturases. The observed diversity of intron number and presence/absence of maturases corroborated previously suggested correlation between the number of maturases in the pt genome and intron proliferation. Surprisingly, pt genomes of taxa belonging to Discoplastis and Lepocinclis encode two inverted repeat (IR) regions containing the rDNA operon, which are absent from the Euglenaceae. By mapping the presence/absence of IR region on the obtained phylogenomic tree, we reconstructed the most probable events in the evolution of IRs in the Euglenophyta. Our study highlights the dynamic nature of the Euglenophyta plastid genome, in particular with regards to the IR regions that underwent losses repeatedly.


Plastids derived from a single endosymbiotic event between a cyanobacterium and the common ancestor of the green algae (including land plants), red algae and glaucophytes - an event called primary endosymbiosis1. The plastids of both green algae and red algae were subsequently transferred to other eukaryotic lineages, which gave rise to secondary plastids. Green algal plastids were taken up by two groups of microbial eukaryotes (protists): euglenophytes and chlorarachniophytes1. Plastids from red algae were inherited to several algal lineages such as ochrophytes, haptophytes, and others. Although the origin and evolutionary history of plastids in various eukaryotic lineages are very different, almost all plastids contain their own genomes, which share some common features. Most sequenced plastid genomes are from land plants and green algae, but the number of other plastid genomes has increased substantially over the last few years. The vast majority of plastids have a similar structure, genes are dispersed among the inverted repeats (IR) and large and small single-copy (LSC and SSC)2,3 regions. The IR regions that contain genes for rRNAs and a variable number of tRNAs and proteins are broadly distributed in all primary and secondary plastids as well as in cyanobacterial genomes3. The IR has been suggested to play a role in the replication initiation4, genome stabilization5, and gene conservation5,6. Gene content varies greatly when we consider all photosynthetic eukaryotes, but within each of the evolutionary lineages, gene content and order are conserved features. In some plastid genomes, most notably those of euglenids and land plants, there are also self-splicing introns, which are thought to have invaded the plastid genome on multiple occasions7,8. Plastid-encoded group I introns, found in rRNA, tRNA or protein-coding genes, have been reported in apicomplexans, glaucophytes, stramenopiles and Viridiplantae3. Group II introns are found in the plastids of cryptophytes, euglenids, and Viridiplantae and usually exist within tRNA or protein-coding genes3. The most extreme examples of the group II intron derivatives are characteristic for plastid genomes of euglenids. These include “group III” introns reduced to 73–119 nucleotides, and “twintrons” which are group II introns nested within another group II intron9,10,11. Secondary origin of euglenid plastids and discovery of their unusual introns in plastid genomes triggered further studies on their plastid genomes evolution.

Photosynthetic euglenids (Euglenophyta) constitute a single subclade within euglenids. Their plastids, enclosed by three membranes, arose as the result of the secondary endosymbiosis between phagotrophic eukaryovorous euglenid and the Pyramimonas-related green alga12. Within photosynthetic euglenids, three evolutionary lineages are distinguished. A single mixotrophic species Rapaza viridis forms the most basal lineage13. Other photosynthetic euglenids are split into two groups: predominantly marine Eutreptiales and freshwater Euglenales. Euglenales are divided into two families: Phacaceae and Euglenaceae14,15. Genomic features of the secondary plastid of euglenids have been studied very intensively in the recent years as more plastid genomes have been sequenced. The first plastid genome of Euglena gracilis was sequenced more than two decades ago16 but the number of plastid genome sequences rapidly increased since 2012 resulting in 18 euglenid plastid genomes sequenced so far. Three Eutreptiales plastid genomes have been characterized12,17,18 and 15 from the Euglenales, including 14 of Euglenaceae19 and only one from Phacaceae20. The euglenid plastid genome has undergone dynamic changes, including genome reduction due to the gene loss or transfer to the nucleus12, the proliferation of introns11,21, and genome rearrangements. Intron proliferation has been proposed to be correlated with the number of maturases in the pt genome20. The majority of genes among previously sequenced euglenid pt genomes have had the same basic complement of protein-coding genes. However, there have been significant changes in the gene arrangement. More closely related taxa tend to have greater synteny21 than more divergent organisms, for which extensive rearrangement have been shown22.

Phacaceae comprises three monophyletic genera: Phacus, Lepocinclis, and Discoplastis. The family Phacaceae has been proposed quite recently based on phylogenetic relationships14,15,23 and morphological synapomorphy – the presence of numerous, small plastids without pyrenoids. The genus Phacus was erected in mid-XIX century24 to accommodate taxa from the genus Euglena that were rigid and did not undergo metaboly. The genus Lepocinclis was established a few years later by Perty25 to incorporate taxa from the genus Phacus that were not flattened. With the advent of molecular sequencing, the division of taxa between Phacus and Lepocinclis have been validated and a new genus Discoplastis was erected to accommodate several species formerly belonging to the genus Euglena and characterized by numerous small plastids without pyrenoids and strong metaboly of the cell26. Genus Phacus and Lepocinclis are species-rich and have been intensively studied on the species-level in the last decade, and several new species have been described27,28,29,30,31. In contrast, genus Discoplastis comprises only two species26.

Here, we report the structural features of the eight newly sequenced plastid genomes of the representatives of the family Phacaceae, including five taxa from Lepocinclis, two taxa from Phacus, and one Discoplastis taxon. We sought to identify the main genomic changes that occurred in the investigated lineages. The examined genomes display considerable variability at all levels except gene content. We also present the phylogenomic trees inferred from the plastid genome sequences. Our results highlight the repeated losses of IR region during the evolution of Euglenophyta.

Results and Discussion

We investigated eight taxa representing all three genera encompassed in the family Phacaceae and found that the plastid genome experienced important alterations at the level of genome organization and intron prevalence during the evolution of the Phacaceae. Before our comparative analyses, relatively little was known about the extent of chloroplast genomic changes throughout Phacaceae and only one species, Phacus orbicularis, has been sequenced20. When compared to the characteristics of Euglenaceae pt genomes19,21,32, they were very similar in their gene content. However, a few physical traits present within the Phacaceae were remarkable when compared to the rest of the Euglenales and the Euglenophyta.

General features

Previously, the pt genome of Eutreptia viridis contained the smallest genome among Euglenophyta at 65,513 bp17, followed by that of the fellow member of Eutreptiales – Eutreptiella gymnastica (67, 622 bp)12, leading to the assumption that there was an expansion in the genome size in the freshwater euglenids (Euglenales). Among members of the family Euglenaceae, Monomorphina aenigmatica11 was identified to contain both the smallest genome (74,746 bp) and the smallest number of introns (53 + 1 unidentified ORF). Sequencing of the first representative of the Phacaceae family – Phacus orbicularis20 revealed that its pt genome (66, 418 bp) is smaller than any of the known Euglenaceae genomes and comparable in size with the pt genomes of Eutreptiella gymnastica. All the strains sequenced in this study possess relatively small pt genomes (below 91 Kb) (Fig. 1, Table 1, Supplementary Table 1) and the P. inflexus pt genome (~58 Kb) was identified as the smallest Euglenophyceae pt genome sequenced to date, even smaller than the genome of Eutreptiella gymnastica. That might suggest that the pt genome of P. inflexus underwent reduction, which could be explained by the loss of mat2 and mat5 and subsequently a smaller number of introns, along with the low average length of genes (531 bp) (Table 1), lower than those of Eutreptia viridis (587 bp).

Figure 1
figure 1

Gene maps of plastid genomes. Boxes of different colors represent genes of similar functional groups. Genes on the outside of the circle are considered on the positive strand, genes inside the circle on the negative strand. (a) Plastid genome map of D. spathirhyncha, (b) L. ovum, (c) L. playfairiana, (d) L. steinii, (e) L. tripteris MI 101, (f) L. tripteris UTEX 1311, (g) P. inflexus, (h) P. pleuronectes. The vertical line at the top of the circle indicates that there is a gap and the genomes are not circularized.

Table 1 Physical characteristics of Phacaceae pt genomes. New pt genomes in Bold. PCG = Protein-Coding Gene(s); Avg. = Average, ≥ = at least. Intron space included when calculating average protein-coding gene length. P. orbicularis data from Kasiborski et al.20.

Phylogenomic analyses

Before comparing the gene content and gene organization of the examined genomes, here we present the phylogenetic context required to interpret those results. Our chloroplast phylogenomic analyses were carried out using amino acid and nucleotide data sets that included representatives of all genera of Euglenophyta (23 taxa in total). The amino acid data set was generated using 57 protein-coding genes (11,499 sites), whereas the nucleotide data set contained two rRNA genes (3,959 sites). The obtained phylogenetic tree (Fig. 2) confirmed sister relationship of Phacaceae and Euglenaceae15,23,33. The inferred relationships among genera of Euglenaceae are not well supported, and only those between closely related taxa are in accordance with multigene analyses on nuclear-encoded genes14,15. In contrast relationships among Phacaceae species were well resolved and highly supported. Phacus and Lepocinclis form sister clades with Discoplastis represented by D. spathirhyncha branching off first. That topology was previously recovered in phylogenetic analyses based on nuclear-encoded genes14,15.

Figure 2
figure 2

Phylogenetic relationship among the Euglenophyta, with number of introns, presence of maturases and inverted repeat (IR) losses, indicated. The best-scoring maximum likelihood (ML) tree inferred from 57 cpDNA-encoded proteins and two rRNA genes is presented. The numbers on the nodes indicate the ML bootstrap support (bs). An asterisks (*) indicates that the corresponding branch received a BS value of >95%; values below <70% are not presented. The scale bar denotes the estimated number of amino acid substitutions per site. The plastid structure and number of introns are depicted next to the name of the taxa. Number and type (mat1, mat2, mat5) of maturases are denoted on the branches by dots. Species names in bold indicate that their plastid genome sequence is complete.

IR presence/absence

The most prominent structural difference observed among analyzed pt genomes is the presence/absence of inverted repeat (IR) regions encoding the rRNA operon. Two identical IR copies containing all five genes (5S, 16S, 23S, trnI, trnA) making up the standard rRNA operon were present in the pt genome of Discoplastis, and all analyzed Lepocinclis (except the 5S gene in L. playfairiana, L. ovum, and L. steinii) but absent from the representatives of Phacus. Initial assemblies of the plastid genomes of Discoplastis and Lepocinclis were not circularized and the resulting contigs differed in the coverage, suggesting duplication. The contigs were joined via PCR reactions and additional PCR reactions were used to extend the pt genome to the reported genomic sequences. The resulting assembly confirmed that the circularization was impossible due to the presence of an inverted repeat of the ribosomal operon. The arrangement of the plastid genomes in Discoplastis and Lepocinclis was very similar, IRs are separated by a very short SSC, which doesn’t contain any genes, only tandem repeats.

The plastid genomes of land plants and green algae often contain two copies of an IR encoding the rRNA operon34, among them Pyramimonas, a prasinophyte alga most closely related to the ancestor of the plastid of Euglenophyta12,35. Among previously sequenced pt genomes of euglenids the quadripartite arrangement has been not shown for Euglenaceae, Phacus orbicularis, and Eutreptia viridis (Eutreptiales). Only the operons of two species of Eutreptiales, Eutreptiella gymnastica12, and Etl. pomquetensis18, resemble the operons and their strand orientation in prasinophytes18,35,36, whereas one of the copies was most probably lost during the divergence of Euglenales. Although all known pt genomes of Euglenales lack one copy of IR, in E. gracilis and Stromobomonas acuminata37 they have been shown to contain tandemly repeated copies of the rRNA operon16,38.

By mapping the presence/absence of IR on the obtained phylogenomic tree, we reconstructed the most probable events in the evolution of IRs in the Euglenophyta (Fig. 2). Our results suggest that the IR regions were most likely lost three times in the course of evolution of the Euglenophyta (if we assume that the plastid genome acquired from a prasinophyte alga was characterized by the typical quadripartite arrangement of the plastid genome). The first possible loss occurred in the genus Eutreptia, however we have to take into account that only one representative of that genus was analyzed and its sequence is incomplete17. The second loss occurred during the evolution of the genus Phacus, as none of the three analyzed species carried two copies of IR. Finally, the third loss most probably appeared in the common ancestor of all taxa belonging to the family Euglenaceae, which were lacking the quadripartite arrangement of the plastid genome. That scenario is more likely than the creation of an IR de novo from an IR-less plastid genome39. However, some of the Euglenaceae pt genomes are incomplete, and the number of losses might increase with the completion of those genomes. Specifically, pt genome of Colacium vesiculosum and Strombomonas acuminata have traits of the possible remnant of the IR; they possess an additional piece of 16S rRNA or 23S rRNA, respectively37.

The plastid genomes of almost all land plants and algae carry two identical copies of a large IR sequence40. It has been shown that the substitution rates are several times lower for the IR relative to the single-copy (SC) regions among several angiosperms41 suggesting a major impact of the IR on the rate of plastome sequence evolution. The omnipresence of the quadripartite structure of plastid genome suggest its important role, however loss of the IR regions, although rare, is known from land plants42 and several lineages of the green algae, such as prasinophyceans43, trebouxiophyceans40, and streptophytes44. The molecular mechanisms underlying these events remains unclear, although several hypotheses have been proposed based on studies on plants and green algae39. It might be a consequence of repeated events of IR contraction, the complete excision of one of the IR sequence or through the differential elimination of the gene sequences from one IR copy. Among the plastid genomes of the Phacaceae, we didn’t observe any intermediate types of IR copies, which might suggest the step-wise loss of one of the IRs. In contrast, in Etl. gymnastica two copies of the IR differ, which might indicate that one of them is on its way to be lost12. More plastid genomes sequences form the early branching lineages of Euglenophyta is needed to support either of the aforementioned hypotheses.

Gene content and introns

The rRNA operon was present in all investigated strains and Discoplastis and Lepocinclis possess two copies. We did not identify any traces of 5S rDNA in two species, namely L. ovum and L. playfairiana, and only the core of 5S rDNA (27 nucleotides long) in L. steinii. The 5S rRNA was also not identified in Etl. gymnastica pt genome12 and in some green algae, like Pyramimonas parkeae and Pycnococcus provasolii35. However, the absence of 5S rRNA in plastid genomes, in particular among protists, have been previously shown to be the result of difficulties with identification, not the lack of 5S rRNA in those genomes45.

The majority of the protein-coding genes and tRNAs were shared among all investigated taxa, except roaA, which was absent in P. inflexus and D. spathirhyncha, and tRNA L-(UAG) which was absent in L. ovum. The number of genes encoding proteins and tRNAs was also consistent with other Euglenophyceae taxa21. In several taxa, roaA (P. pleuronectes) and atpF (P. pleuronectes, D. spathirhyncha, L. steinii, and L. playfairiana) began with alternative start codons, which was also previously observed for other taxa21.

We found notable differences among the maturase-encoding genes. The mat5 gene has patchy distribution along the tree of photosynthetic euglenids, and it was present in two out of seven analyzed species (both strains of L. tripteris and P. pleuronectes) (Fig. 2). Bennett and Triemer21 proposed that mat5 was gained after the split of Euglenales from the Eutreptiales while identifying three instances in which mat5 was independently lost within Euglenaceae. However, they also recognized mat5 in L. spirogyroides, which demonstrated that it was gained before the Euglenaceae/Phacaceae split. Kasiborski et al.20 did not identify mat5 in the first pt genome of Phacus and proposed that this gene was lost in P. orbicularis after the split of Phacus and Lepocinclis. Our analyses of eight additional strains from the Phacaceae rejected the previous hypothesis since in both Lepocinclis, and Phacus mat5 was absent in some taxa but present in others (Fig. 2). It was also absent in Discoplastis but based only on one strain we couldn’t draw definite conclusions. Most likely, mat5 was present in the ancestor of all Euglenales. The presence of a maturase-like protein in the second intron in psbC of Etl. gymnastica12, which showed a weak similarity to mat5 that is usually located in the psbA gene in Euglenales19, supports this scenario.

More surprisingly, another maturase gene, mat2 was absent in two out of the three analyzed Phacus species (P. inflexus and P. orbicularis) and one Lepocinclis species (L. tripteris) (Fig. 2). Previously, it was shown to be missing in P. orbicularis which suggested the acquisition of mat2 in the Euglenaceae lineage after the split from the Phacaceae20. Our results demonstrate that the majority of the Phacaceae possess mat2, which would imply the independent loss of that gene in some Phacaceae taxa.

It was proposed that proliferation of introns might be related to the number of maturases. Eutreptiales, with one maturase, has the lowest number of introns and Euglenacae with two or three maturases, tend to have more introns20. Our results confirmed the previously observed relation between the number of introns and number of maturases (Fig. 2). While P. pleuronectes was the only one of the Phacaceae that contained three maturases and had the highest number of introns (104) in this family. Discoplastis and all analyzed Lepocinclis strains contained two maturases and average (50–59) to a high (79–95) number of introns (Table 1) but never as high as P. pleuronectes (104 introns) (Table 1). Moreover, P. inflexus with one maturase contained only 29 introns, which is the lowest number of introns among Euglenales reported so far and only slightly higher number than in Eutreptia viridis (23 introns)17. The observation of the greater number of introns in strains with more maturases seems to be consistent but doesn’t explain the mechanism of intron proliferation.

An intron of psbC, which carries an intron-encoded maturase ycf13, is considered the ancestral intron because it is the only homologous intron in euglenophyte cpDNAs46. Surprisingly, intron1 in psbC of both strains of L. tripteris was not homologous with E. gracilis psbC intron2, though L. tripteris psbC intron1 still contained ycf13.

Two previous findings and conclusions about twintrons hold true with the newly analyzed genomes. We confirmed that twintrons present in psbF and psbD genes in E. gracilis16 are unique to that species and absent not only in all Euglenaceae11,17,19,21,22 but also in Phacaceae20 (Table 2). We also confirmed the presence of a twintron in psbC (intron1/Egra.2) (Table 2) in Phacaceae, which strongly supports the previously proposed ancestral origin of this intron20. Our data contradicted the suggestion of petB twintron (intron1) as a synapomorphy for all Euglenophyta20,21 as we identified the petB twintron in taxa representing all genera of Phacaceae except L. ovum and P. pleuronectes (Table 2). That might suggest that indeed the petB twintron is ancestral for the Euglenaceae but has been lost in at least two lineages of Phacaceae.

Table 2 Phacaceae pt genomes twintron analysis.

Synteny, intrageneric and intraspecific variation

Synteny analyses revealed the same order of clusters within Phacus and Lepocinclis and some rearrangements among genera (Fig. 3). Those findings agreed with the observations within Euglenaceae. Taxa within a genus usually have the same arrangement of clusters and differences occur among genera11,19,21,47.

Figure 3
figure 3

Synteny alignment of plastid genomes between Phacaceae representatives. Alignment performed in Mauve showing plastomes with one copy of the IR taken out. The order of plastomes, from top to bottom, is D. spathirhyncha, L. ovum, L. playfairiana, L. steinii, L. tripteris MI, L. tripteris UTEX, P. inflexus, and P. pleuronectes. Each colored block is a region of collinear sequence among all eight plastomes. Blocks on the top row are in the same orientation, while blocks on the bottom row are in inverse orientation. White and red boxes represent annotated CDS (protein-coding sequence) and rRNAs in the genomes respectively.

Intrageneric variability within the Euglenaceae was previously explored in Euglena21,47 and Mononorphina11,21, but not in Phacaceae. Newly sequenced pt genomes allowed to explore intrageneric variability within Phacus and Lepocinclis. In all previously analyzed cases, intrageneric evolution was limited, and significant changes occurred before the separation of those genera. Comparative studies of Monomorphina pt genomes revealed some differences, namely the presence of mat5 and a higher number of introns in M. parapyrum resulting in the bigger size of the pt genome11,21. Finally, a comparison of the potential twintrons contained in the Monomorphina species revealed that the presence of twintrons was not conserved between taxa, suggesting that presence or absence of a twintron in a given intron is taxon-specific21. We observed similar differences within Phacus and Lepocinclis. In both genera, the number of introns varied greatly (50–95 for Lepocinclis and 29–104 for Phacus) (Table 1), and the main difference in the gene content is related to the presence/absence of maturases mat5 and mat2. Analyses of twintrons (Table 2) also revealed some differences within genera and further supported earlier conclusions21 that twintrons are taxon-specific.

Plastid genomes of two strains of the Lepocinclis tripteris (MI 101 and UTEX 1311) were very similar, and ~90% of nucleotide sites were identical when compared across the entire genomes. This was a much higher identity than was observed in the two strains of Euglena viridis, which were only 70.5% identical21. The identity was even higher within coding space – 95.3%, which was also higher than between two Euglena viridis strains (91.5%). The pt genomes of the L. tripteris strains were also very similar in size but a comparison between the two genomes revealed that the area between tRNAs-S(GCU) and Y(GUA) were different (Fig. 1e,f): while both genomes contained two unidentified ORFs in this area, the two ORFs were different, and this region of the genome was 1,338 bp longer in UTEX 1311, which accounted for almost the entire difference in size between the genomes. Moreover, in both strains of L. tripteris, we observed the presence of a stop codon UAG in the 7th amino acid of the psbC gene. Since psbC encodes a protein essential in photosynthesis, it is clear that this gene is still functional. Most likely the UAG codon is read-through48. Otherwise, it can be an example of RNA editing, observed so far in plastid transcripts of land plants and peridinin and fucoxanthin dinoflagellates49,50.


Despite the fact that many Euglenaceae pt genomes have been characterized recently, surprisingly little was known about the plastid genomes of its sister family Phacaceae. To fill this gap, we have sequenced plastid genomes of eight taxa from all three genera classified in the Phacaceae. Gene content was highly conserved within the family, and the main differences were related to the presence/absence of maturases mat2 and mat5. Genes were arranged into seven clusters, and their order was conserved within genera. No pattern of intron number was present in the Phacaceae, although we confirmed the correlation between the number of maturases and the number of introns. We also confirmed that twintrons present in psbF and psbD genes in E. gracilis16 are unique to that species and that the twintron in psbC is of ancestral origin. We rejected, however, the idea of the twintron in petB as a synapomorphy for all Euglenophyta, because it was not present in some Phacaceae.

Our study highlights the highly dynamic nature of the Euglenophyta plastid genomes, in particular with regards to the large IR sequence that experienced repeated losses, most probably at least once within the Phacaceae, once before the branching off of the Euglenaceae and once in the genus Eutreptia. It is necessary to analyze additional taxa from the basal lineages, such as Eutreptiales and Rapaza, to fully understand the dynamic history of the plastid genome in the Euglenophyta and decipher the mechanisms underlying the observed IR losses.

Materials and Methods

Culturing, sequencing, and annotation

The following eight cultures were used in this research: Discoplastis spathirhyncha, SAG 1224-42 (Experimental Phycology and Culture Collection of Algae at the University of Göttingen (EPSAG), Germany); Lepocinclis ovum, SAG 1244-8; Lepocinclis playfairiana, MI102 (strain isolated at Michigan State University); Lepocinclis steinii, UTEX 523 (The Culture Collection of Algae at The University of Texas at Austin, USA); Lepocinclis tripteris, UTEX 1331; Lepocinclis tripteris, (MI101); Phacus inflexus, ACOI 1336 (Coimbra Collection of Algae, Portugal); Phacus pleuronectes SAG 1261-3b. Cultures were maintained as described in21. Lepocinclis playfairiana and L. tripteris (MI) cells were identified in field samples collected from the East Lansing, MI, USA area and brought into culture in the following manner: single cells were picked from field samples using a sterile Pasteur pipette under a Leica MZ16 dissecting microscope (Leica Microsystems, Wetzlar, Germany) and transferred through a series of sterile drops of growth media, modified AF-6 medium51 with 150 mL L−1 of Soil-Water Medium (Carolina Biological Supply Company, Burlington, NC, USA), to ensure the presence of only one cell. The isolated cells were then placed into one well of a 96-well plate containing sterile growth media, one cell per well, and allowed to divide for ~2 weeks. Following this, well contents were moved into 12 mL culture tubes that contained sterile growth media and were allowed to grow for another ~2 weeks. Finally, the culture tubes were subsampled and viewed with a Zeiss Axioscope 2 plus microscope (Carl Zeiss Ing., Hallbergmoos, Germany) to ensure that they were uni-algal and verify the identity of the culture. These two cultures were also maintained as described previously21.

Cultures of L. ovum, L. playfairiana, L. steinii, L. tripteris (UTEX), L. tripteris (MI), P. inflexus, and P. pleuronectes were concentrated, washed, and had their DNA extracted with following protocols52, with the following alterations: Percoll (Research Organics, Cleveland, OH, USA) was substituted for Centricoll (Sigma Inc., St. Louis, MO, USA) in the P. inflexus process; L. ovum, L. playfairiana, L. steinii, and P. pleuronectes cultures were not run through the gradient step. DNA of D. spathirhyncha was extracted using phenol/chloroform, and DNA separation, using a cesium chloride gradient, was performed as described before17. All DNA was sequenced with Illumina paired-end reads at the Michigan State University Research Technology Support Facility: D. spathirhyncha with HiSeq 2 × 100 bp reads; all other taxa with MiSeq 2 × 150 bp reads. Phacus inflexus raw sequence data were assembled into contigs with the ‘De Novo Assemble…’ program in Geneious Pro version 6.8.1 (Biomatters Ltd, Auckland, New Zealand) as previously described21. All other raw sequence data were assembled into contigs with the ‘De Novo Assembly’ program in CLC Genomics Workbench version 5.5.1 (CLC Bio, Cambridge, Massachusetts, USA) as previously described21. The number of contigs and their lengths, number of reads per contig, and average coverage per contig are provided in the Supplementary Table 1.

All other aspects of genome discovery, including: identification of pt genome-containing contigs, joining of contigs (as necessary), sequence cleanup (as necessary), PCR primer creation, annotation, arrangement, and genome map creation were as performed as previously described21. The primers designed to confirm the presence of IRs are listed in the Supplementary Table S2. Additionally, when we were unable to identify the 5S in this pt genome through Rfam53, RNAmmer54, or by manual aligning, we used MFannot tool55 and confirmed its annotation by homology searches with 5S rRNA Database56. The search for potential twintrons was performed as previously described21, with the exception that instead of a manual search, a Python script was created (Supplementary File 1; the source code is available on GitHub to find the 3′ conserved motif for Group III twintrons9 within the homologous external introns. Genome maps were drawn using OGDRAW57. Newly generated organelle genomes were deposited in GenBank (Table 1).

Synteny between the pt genomes of Phacaceae was determined and visualized with progressive Mauve 2.3.158. Mauve performs syntenic comparisons between multiple genomes and displays these syntenic regions graphically. In the Mauve alignment the repeat regions of rRNA were not included because Mauve will not align repeat regions which have multiple matches in both genomes.

Phylogenomic analyses

The 57 orthologous protein sequences and two rRNA nucleotide sequences encoded by 26 analyzed plastid genomes (Supplementary Table 3) were individually aligned with MAFFT ver. 7.27159, trimmed with BMGE ver. 1.1260, and concatenated with SequenceMatrix ver. 1.861, leaving 11,499 reliably aligned amino acid (aa) positions and 3,959 reliably aligned nucleotide positions (rRNA). The protein data set was assembled from protein-coding genes: atpA, B, E, F, H, I, ccsA, petB, G, psaA, B, C, I, J, psbA, B, C, D, E, F, H, I, J, K, L, N, T, rbcL, rpl2, 5, 12, 14, 16, 20, 22, 23, 32, 36, rpoB, C1, C2, rps2, 3, 4, 7, 8, 9, 11, 12, 14, 18, 19, tufA, ycf4, 9, 12, 65. We excluded from the dataset sequences from the strains of the same species (E. gracilis var. bacillaris KP686076 and E. viridis KP686075). We excluded also Colacium vesiculosum (JN674636), which has been previously shown to have the unstable position at phylogenomic trees, and its elimination doesn’t lead to different topology but could reinforce the support for the relationships within the tree33. As an outgroup, we used three species representing Chlorophyta, previously shown to be the close relatives of Euglenophyta33. For Maximum Likelihood (ML) analyses each protein-coding gene was divided into a separate partition, and one rRNA partition was applied, resulting in 58 partitions. We determined the best choice of model for each partition with ModelFinder62 implemented in IQ-TREE ver. 1.6.163. ML analyses were performed with IQ-TREE ver. 1.6.163, using a partitioned analysis for multigene alignments under the recommended models64 and 1,000 ultrafast bootstrap65.

Data Availability

All the newly obtained sequences are deposited in GenBank under accession numbers MH898667-MH898674. All the sequence data set and analysis results obtained in this work are available from the corresponding author on reasonable request.


  1. Keeling, P. J. The endosymbiotic origin, diversification and fate of plastids. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 365, 729–748 (2010).

    CAS  Article  Google Scholar 

  2. Oudot-Le Secq, M.-P. et al. Chloroplast genomes of the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana: comparison with other plastid genomes of the red lineage. Mol. Genet. Genomics 277, 427–439 (2007).

    CAS  Article  Google Scholar 

  3. Kim, E. & Archibald, J. M. Diversity and Evolution of Plastids and Their Genomes. In The Chloroplast (eds Sandelius, A. S. & Aronsson, H.) 1–39 (Springer Berlin Heidelberg, 2009).

  4. Heinhorst, S. & Cannon, G. C. DNA replication in chloroplasts. J. Cell Sci. 1–9 (1993).

  5. Palmer, J. D. & Thompson, W. F. Chloroplast DNA rearrangements are more frequent when a large inverted repeat sequence is lost. Cell 29, 537–550 (1982).

    CAS  Article  Google Scholar 

  6. Wolfe, K. H., Li, W. H. & Sharp, P. M. Rates of nucleotide substitution vary greatly among plant mitochondrial, chloroplast, and nuclear DNAs. Proc. Natl. Acad. Sci. USA 84, 9054–9058 (1987).

    ADS  CAS  Article  Google Scholar 

  7. Hausner, G. et al. Origin and Evolution of the Chloroplast trnK (matK) Intron: A Model for Evolution of Group II Intron RNA Structures. Mol. Biol. Evol. 23, 380–391 (2005).

    Article  Google Scholar 

  8. Haugen, P. et al. Cyanobacterial ribosomal RNA genes with multiple, endonuclease-encoding group I introns. BMC Evol. Biol. 7, 159 (2007).

    Article  Google Scholar 

  9. Copertino, D. W. & Hallick, R. B. Group II and group III introns of twintrons: potential relationships with nuclear pre-mRNA introns. Trends Biochem. Sci. 18, 467–471 (1993).

    CAS  Article  Google Scholar 

  10. Doetsch, N. A., Thompson, M. D. & Hallick, R. B. A maturase-encoding group III twintron is conserved in deeply rooted euglenoid species: are group III introns the chicken or the egg? Mol. Biol. Evol. 15, 76–86 (1998).

    CAS  Article  Google Scholar 

  11. Pombert, J.-F., James, E. R., Janouškovec, J. & Keeling, P. J. Evidence for transitional stages in the evolution of euglenid group II introns and twintrons in the Monomorphina aenigmatica plastid genome. PLoS One 7, e53433 (2012).

    ADS  CAS  Article  Google Scholar 

  12. Hrdá, Š., Fousek, J., Szabová, J., Hampl, V. & Vlček, Č. The plastid genome of Eutreptiella provides a window into the process of secondary endosymbiosis of plastid in euglenids. PLoS One 7, e33746 (2012).

    ADS  Article  Google Scholar 

  13. Yamaguchi, A., Yubuki, N. & Leander, B. S. Morphostasis in a novel eukaryote illuminates the evolutionary transition from phagotrophy to phototrophy: description of Rapaza viridis n. gen. et sp. (Euglenozoa, Euglenida). BMC Evol. Biol. 12, 29 (2012).

    Article  Google Scholar 

  14. Kim, J. I., Linton, E. W. & Shin, W. Taxon-rich multigene phylogeny of the photosynthetic euglenoids (Euglenophyceae). Front. Ecol. Evol. 3, 1–11 (2015).

    Article  Google Scholar 

  15. Karnkowska, A. et al. Phylogenetic Relationships and Morphological Character Evolution of Photosynthetic Euglenids (Excavata) Inferred from Taxon-rich Analyses of Five Genes. J. Eukaryot. Microbiol. 62, 362–373 (2015).

    CAS  Article  Google Scholar 

  16. Hallick, R. B. et al. Complete sequence of Euglena gracilis chloroplast DNA. Nucleic Acids Res. 21, 3537–3544 (1993).

    CAS  Article  Google Scholar 

  17. Wiegert, K. E., Bennett, M. S. & Triemer, R. E. Evolution of the Chloroplast Genome in Photosynthetic Euglenoids: A Comparison of Eutreptia viridis and Euglena gracilis (Euglenophyta). Protist 163, 832–843 (2012).

    CAS  Article  Google Scholar 

  18. Dabbagh, N., Bennett, M. S., Triemer, R. E. & Preisfeld, A. Chloroplast genome expansion by intron multiplication in the basal psychrophilic euglenoid Eutreptiella pomquetensis. PeerJ 5, e3725 (2017).

    Article  Google Scholar 

  19. Dabbagh, N. & Preisfeld, A. The Chloroplast Genome of Euglena mutabilis-Cluster Arrangement, Intron Analysis, and Intrageneric Trends. J. Eukaryot. Microbiol. 64, 31–44 (2017).

    CAS  Article  Google Scholar 

  20. Kasiborski, B. A., Bennett, M. S. & Linton, E. W. The chloroplast genome of Phacus orbicularis (Euglenophyceae): an initial datum point for the Phacaceae. J. Phycol. 52, 404–411 (2016).

    CAS  Article  Google Scholar 

  21. Bennett, M. S. & Triemer, R. E. Chloroplast Genome Evolution in the Euglenaceae. J. Eukaryot. Microbiol. 62, 773–785 (2015).

    CAS  Article  Google Scholar 

  22. Bennett, M. S., Wiegert, K. E. & Triemer, R. E. Characterization of Euglenaformis gen. nov. and the chloroplast genome of Euglenaformis [Euglena] proxima (Euglenophyta). Phycologia 53, 66–73 (2014).

    CAS  Article  Google Scholar 

  23. Kim, J. I., Shin, W. & Triemer, R. E. Multigene analyses of photosynthetic euglenoids and new family, Phacaceae (Euglenales). J. Phycol. 46, 1278–1287 (2010).

    Article  Google Scholar 

  24. Dujardin, F. Histoire Naturelle des Zoophytes: Infusoires, Comprenant La Physiologie et la Classification de ces Animaux et la Manière de les étudior Étudier a l’aide du Microscope. (Libraire encyclope dique de Roret., 1841).

  25. Perty, M. Über verticale Verbreitung mikroskopischer Lebensformen. Mitth. der Naturforschenden Gesellschaft Bern 17–45 (1849).

  26. Triemer, R. E. et al. Phylogeny of the Euglenales based upon combined SSU and LSU rDNA sequence comparisons and description of Discoplastis gen. nov. (Euglenophyta). J. Phycol. 42, 731–740 (2006).

    Article  Google Scholar 

  27. Kosmala, S., Karnkowska, A., Milanowski, R., Kwiatowski, J. & Zakryś, B. Phylogenetic and taxonomic position of Lepocinclis fusca comb. nov. (=Euglena fusca) (Euglenaceae): morphological and molecular justification. J. Phycol. 41, 1258–1267 (2005).

    CAS  Article  Google Scholar 

  28. Kosmala, S., Bereza, M., Milanowski, R., Kwiatowski, J. & Zakryś, B. Morphological and molecular examination of relationships and epitype establishment of Phacus pleuronectes, Phacus orbicularis, and Phacus hamelii. J. Phycol. 43, 1071–1082 (2007).

    CAS  Article  Google Scholar 

  29. Karnkowska-Ishikawa, A., Milanowski, R., Kwiatowski, J. & Zakryś, B. Taxonomy of the Phacus oscillans (Euglenaceae) and its close relatives - balancing morphological and molecular features. J. Phycol. 46, 172–182 (2010).

    CAS  Article  Google Scholar 

  30. Łukomska-Kowalczyk, M., Karnkowska, A., Milanowski, R., Łach, Ł. & Zakryś, B. Delimiting species in the Phacus longicauda complex (Euglenida) through morphological and molecular analyses. J. Phycol. 51, 1147–1157 (2015).

    Article  Google Scholar 

  31. Kim, J. I. & Shin, W. Molecular phylogeny and cryptic diversity of the genus Phacus (Phacaceae, Euglenophyceae) and the descriptions of seven new species. J. Phycol. 50, 948–959 (2014).

    CAS  Article  Google Scholar 

  32. Bennett, M. S., Shiu, S.-H. & Triemer, R. E. A rare case of plastid protein-coding gene duplication in the chloroplast genome of Euglena archaeoplastidiata (Euglenophyta). J. Phycol. 53, 493–502 (2017).

    CAS  Article  Google Scholar 

  33. Dabbagh, N. & Preisfeld, A. Intrageneric variability between the chloroplast genomes of Trachelomonas grandis and Trachelomonas volvocina and phylogenomic analysis of phototrophic Euglenoids. J. Eukaryot. Microbiol. 65, 648–660 (2018).

    CAS  Article  Google Scholar 

  34. Green, B. R. Chloroplast genomes of photosynthetic eukaryotes. Plant J. 66, 34–44 (2011).

    CAS  Article  Google Scholar 

  35. Turmel, M., Gagnon, M.-C., O’Kelly, C. J., Otis, C. & Lemieux, C. 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. Mol. Biol. Evol. 26, 631–648 (2009).

    CAS  Article  Google Scholar 

  36. Robbens, S. et al. The Complete Chloroplast and Mitochondrial DNA Sequence of Ostreococcus tauri: Organelle Genomes of the Smallest Eukaryote Are Examples of Compaction. Mol. Biol. Evol. 24, 956–968 (2007).

    CAS  Article  Google Scholar 

  37. Wiegert, K. E., Bennett, M. S. & Triemer, R. E. Tracing Patterns of Chloroplast Evolution in Euglenoids: Contributions from Colacium vesiculosum and Strombomonas acuminata (Euglenophyta). J. Eukaryot. Microbiol. 60, 214–221 (2013).

    CAS  Article  Google Scholar 

  38. Gockel, G., Hachtel, W. & Melkonian, M. Complete gene map of the plastid genome of the nonphotosynthetic euglenoid flagellate Astasia longa. Protist 151, 347–351 (2000).

    CAS  Article  Google Scholar 

  39. Turmel, M., Otis, C. & Lemieux, C. Divergent copies of the large inverted repeat in the chloroplast genomes of ulvophycean green algae. Sci. Rep. 7, 994 (2017).

    ADS  Article  Google Scholar 

  40. Turmel, M., Otis, C. & Lemieux, C. Dynamic Evolution of the Chloroplast Genome in the Green Algal Classes Pedinophyceae and Trebouxiophyceae. Genome Biol. Evol. 7, 2062–2082 (2015).

    CAS  Article  Google Scholar 

  41. Zhu, A., Guo, W., Gupta, S., Fan, W. & Mower, J. P. Evolutionary dynamics of the plastid inverted repeat: the effects of expansion, contraction, and loss on substitution rates. New Phytol. 209, 1747–1756 (2016).

    CAS  Article  Google Scholar 

  42. Jansen, R. K. & Ruhlman, T. A. Plastid genomes of seed plants. In Genomics of Chloroplasts and Mitochondria. Advances in Photosynthesis and Respiration (eds Bock, R. & Knoop, V.) 103–126 (Springer Netherlands, 2012).

  43. Lemieux, C., Otis, C. & Turmel, M. 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. BMC Genomics 15, 857 (2014).

    Article  Google Scholar 

  44. Lemieux, C., Otis, C. & Turmel, M. Comparative Chloroplast Genome Analyses of Streptophyte Green Algae Uncover Major Structural Alterations in the Klebsormidiophyceae, Coleochaetophyceae and Zygnematophyceae. Front. Plant Sci. 7 (2016).

  45. Valach, M., Burger, G., Gray, M. W. & Lang, B. F. Widespread occurrence of organelle genome-encoded 5S rRNAs including permuted molecules. Nucleic Acids Res. 42, 13764–13777 (2014).

    CAS  Article  Google Scholar 

  46. Vanclová, A. M. G., Hadariová, L., Hrdá, Š. & Hampl, V. Secondary Plastids of Euglenophytes. Adv. Bot. Res. 84, 321–358 (2017).

    Article  Google Scholar 

  47. Bennett, M. S., Wiegert, K. E. & Triemer, R. E. Comparative chloroplast genomics between Euglena viridis and Euglena gracilis (Euglenophyta). Phycologia 51, 711–718 (2012).

    CAS  Article  Google Scholar 

  48. Beier, H. & Grimm, M. Misreading of termination codons in eukaryotes by natural nonsense suppressor tRNAs. Nucleic Acids Res. 29, 4767–4782 (2001).

    CAS  Article  Google Scholar 

  49. Gray, M. W. RNA editing in plant organelles: a fertile field. Proc. Natl. Acad. Sci. USA 93, 8157–8159 (1996).

    ADS  CAS  Article  Google Scholar 

  50. Jackson, C. J., Gornik, S. G. & Waller, R. F. A Tertiary Plastid Gains RNA Editing in Its New Host. Mol. Biol. Evol. 30, 788–792 (2013).

    CAS  Article  Google Scholar 

  51. Watanabe, M., Kawachi, M., Hiroki, M. & Kasai, F. NIES Collection List of Strains. Sixth Edition, 2000, Microalgae and Protozoa. (Microbial CultureCollections, National Institute for Environmental Studies., 2000).

  52. Bennett, M. S. & Triemer, R. E. A new method for obtaining nuclear gene sequences from field samples and taxonomic revisions of the photosynthetic euglenoids Lepocinclis (Euglena) helicoideus and Lepocinclis (Phacus) horridus (Euglenophyta). J. Phycol. 48, 254–260 (2012).

    CAS  Article  Google Scholar 

  53. Burge, S. W. et al. Rfam 11.0: 10 years of RNA families. Nucleic Acids Res. 41, D226–D232 (2013).

    CAS  Article  Google Scholar 

  54. Lagesen, K. et al. RNAmmer: consistent and rapid annotation of ribosomal RNA genes. Nucleic Acids Res. 35, 3100–3108 (2007).

    ADS  CAS  Article  Google Scholar 

  55. Burger, G., Lavrov, D. V., Forget, L. & Lang, B. F. Sequencing complete mitochondrial and plastid genomes. Nat Protoc 2, 603–614 (2007).

    CAS  Article  Google Scholar 

  56. Szymanski, M., Zielezinski, A., Barciszewski, J., Erdmann, V. A. & Karlowski, W. M. 5SRNAdb: an information resource for 5S ribosomal RNAs. Nucleic Acids Res. 44 (2016).

    Article  Google Scholar 

  57. Lohse, M., Drechsel, O., Kahlau, S. & Bock, R. OrganellarGenomeDRAW—a suite of tools for generating physical maps of plastid and mitochondrial genomes and visualizing expression data sets. Nucleic Acids Res. 41, W575–W581 (2013).

    Article  Google Scholar 

  58. Darling, A. E., Mau, B. & Perna, N. T. progressiveMauve: multiple genome alignment with gene gain, loss and rearrangement. PLoS One 5, e11147 (2010).

    ADS  Article  Google Scholar 

  59. Katoh, K. & Standley, D. M. MAFFT multiple sequence alignment software version 7: improvements in performance and usability. Mol. Biol. Evol. 30, 772–780 (2013).

    CAS  Article  Google Scholar 

  60. Criscuolo, A. & Gribaldo, S. BMGE (Block Mapping and Gathering with Entropy): a new software for selection of phylogenetic informative regions from multiple sequence alignments. BMC Evol. Biol. 10, 210 (2010).

    Article  Google Scholar 

  61. Vaidya, G., Lohman, D. J. & Meier, R. SequenceMatrix: concatenation software for the fast assembly of multi-gene datasets with character set and codon information. Cladistics 27, 171–180 (2011).

    Article  Google Scholar 

  62. Kalyaanamoorthy, S., Minh, B. Q., Wong, T. K. F., Von Haeseler, A. & Jermiin, L. S. ModelFinder: Fast model selection for accurate phylogenetic estimates. Nat. Methods 14, 587–589 (2017).

    CAS  Article  Google Scholar 

  63. Nguyen, L.-T., Schmidt, H. A., von Haeseler, A. & Minh, B. Q. IQ-TREE: a fast and effective stochastic algorithm for estimating maximum-likelihood phylogenies. Mol. Biol. Evol. 32, 268–274 (2015).

    CAS  Article  Google Scholar 

  64. Chernomor, O., von Haeseler, A. & Minh, B. Q. Terrace Aware Data Structure for Phylogenomic Inference from Supermatrices. Syst. Biol. 65, 997–1008 (2016).

    Article  Google Scholar 

  65. Hoang, D. T., Chernomor, O., Von Haeseler, A., Minh, B. Q. & Vinh, L. S. UFBoot2: Improving the ultrafast bootstrap approximation. Mol. Biol. Evol. 35 (2018).

Download references


This work was supported by Polish National Science Centre (2016/21/D/NZ8/01288 to A.K.) and Ministry of Science and Higher Education scholarship for outstanding young researchers to A.K.

Author information

Authors and Affiliations



A.K. and R.E.T. designed the study. M.S.B. isolated DNA and performed additional PCR reactions. A.K. and M.S.B. carried out the assembly and annotation. A.K. performed the phylogenomic analyses. A.K. and R.E.T. contributed reagents, materials and analysis tools. A.K. interpreted the data, prepared figures and wrote the manuscript. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Anna Karnkowska.

Ethics declarations

Competing Interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Electronic supplementary material

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Karnkowska, A., Bennett, M.S. & Triemer, R.E. Dynamic evolution of inverted repeats in Euglenophyta plastid genomes. Sci Rep 8, 16071 (2018).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI:


  • Plastid Genome
  • Lepocinclis
  • Maturase
  • Photosynthetic Euglenids
  • Twintrons

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing