Phylogenetic position of Rhynchopus sp. and Diplonema ambulator as indicated by analyses of euglenozoan small subunit ribosomal DNA

Recent expansion of metabolic versatility in Diplonema papillatum, the model species of a highly speciose group of marine eukaryotes

BACKGROUND

Diplonemid flagellates are among the most abundant and species-rich of known marine microeukaryotes, colonizing all habitats, depths, and geographic regions of the world ocean. However, little is known about their genomes, biology, and ecological role.

RESULTS

We present the first nuclear genome sequence from a diplonemid, the type species Diplonema papillatum. The ~ 280-Mb genome assembly contains about 32,000 protein-coding genes, likely co-transcribed in groups of up to 100. Gene clusters are separated by long repetitive regions that include numerous transposable elements, which also reside within introns. Analysis of gene-family evolution reveals that the last common diplonemid ancestor underwent considerable metabolic expansion. D. papillatum-specific gains of carbohydrate-degradation capability were apparently acquired via horizontal gene transfer. The predicted breakdown of polysaccharides including pectin and xylan is at odds with reports of peptides being the predominant carbon source of this organism. Secretome analysis together with feeding experiments suggest that D. papillatum is predatory, able to degrade cell walls of live microeukaryotes, macroalgae, and water plants, not only for protoplast feeding but also for metabolizing cell-wall carbohydrates as an energy source. The analysis of environmental barcode samples shows that D. papillatum is confined to temperate coastal waters, presumably acting in bioremediation of eutrophication.

CONCLUSIONS

Nuclear genome information will allow systematic functional and cell-biology studies in D. papillatum. It will also serve as a reference for the highly diverse diplonemids and provide a point of comparison for studying gene complement evolution in the sister group of Kinetoplastida, including human-pathogenic taxa.

Perfection of eccentricity: Mitochondrial genomes of diplonemids

Mitochondria are the sandbox of evolution as exemplified most particularly by the diplonemids, a group of marine microeukaryotes. These protists are uniquely characterized by their highly multipartite mitochondrial genome and systematically fragmented genes whose pieces are spread out over several dozens of chromosomes. The type species Diplonema papillatum was the first member of this group in which the expression of fragmented mitochondrial genes was investigated experimentally. We now know that gene expression involves separate transcription of gene pieces (modules), RNA editing of module transcripts, and module joining to mature mRNAs and rRNAs. The mechanism of cognate module recognition and ligation is distinct from known intron splicing and remains to be uncovered. Here, we review the current status of research on mitochondrial genome architecture, as well as gene complement, structure, and expression modes in diplonemids. Further, we discuss the potential molecular mechanisms of posttranscriptional processing, and finally reflect on the evolutionary trajectories and trends of mtDNA evolution as seen in this protist group. © 2018 IUBMB Life, 70(12):1197-1206, 2018.

Life Cycle, Ultrastructure, and Phylogeny of New Diplonemids and Their Endosymbiotic Bacteria

Diplonemids represent a hyperdiverse and abundant yet poorly studied group of marine protists. Here we describe two new members of the genus Diplonema (Diplonemea, Euglenozoa), Diplonema japonicum sp. nov. and Diplonema aggregatum sp. nov., based on life cycle, morphology, and 18S rRNA gene sequences. Along with euglenozoan apomorphies, they contain several unique features. Their life cycle is complex, consisting of a trophic stage that is, following the depletion of nutrients, transformed into a sessile stage and subsequently into a swimming stage. The latter two stages are characterized by the presence of tubular extrusomes and the emergence of a paraflagellar rod, the supportive structure of the flagellum, which is prominently lacking in the trophic stage. These two stages also differ dramatically in motility and flagellar size. Both diplonemid species host endosymbiotic bacteria that are closely related to each other and constitute a novel branch within Holosporales, for which a new genus, "Candidatus Cytomitobacter" gen. nov., has been established. Remarkably, the number of endosymbionts in the cytoplasm varies significantly, as does their localization within the cell, where they seem to penetrate the mitochondrion, a rare occurrence.IMPORTANCE We describe the morphology, behavior, and life cycle of two new Diplonema species that established a relationship with two Holospora-like bacteria in the first report of an endosymbiosis in diplonemids. Both endosymbionts reside in the cytoplasm and the mitochondrion, which establishes an extremely rare case. Within their life cycle, the diplonemids undergo transformation from a trophic to a sessile and eventually a highly motile swimming stage. These stages differ in several features, such as the presence or absence of tubular extrusomes and a paraflagellar rod, along with the length of the flagella. These morphological and behavioral interstage differences possibly reflect distinct functions in dispersion and invasion of the host and/or prey and may provide novel insight into the virtually unknown function of diplonemids in the oceanic ecosystem.

Phylogeny and Morphology of New Diplonemids from Japan

Diplonemids were recently found to be the most species-rich group of marine planktonic protists. Based on phylogenetic analysis of 18S rRNA gene sequences and morphological observations, we report the description of new members of the genus Rhynchopus - R. humris sp. n. and R. serpens sp. n., and the establishment of two new genera - Lacrimia gen. n. and Sulcionema gen. n., represented by L. lanifica sp. n. and S. specki sp. n., respectively. In addition, we describe the organism formerly designated as Diplonema sp. 2 (ATCC 50224) as Flectonema neradi gen. n., sp. n. The newly described diplonemids share a common set of traits. Cells are sac-like but variable in shape and size, highly metabolic, and surrounded by a naked cell membrane, which is supported by a tightly packed corset of microtubules. They carry a single highly reticulated peripheral mitochondrion containing a large amount of mitochondrial DNA, with lamellar cristae. The cytopharyngeal complex and flagellar pocket are contiguous and have separate openings. Two parallel flagella are inserted sub-apically into a pronounced flagellar pocket. Rhynchopus species have their flagella concealed in trophic stages and fully developed in swimming stages, while they permanently protrude in all other known diplonemid species.

Evolutionarily conserved cox1 trans-splicing without cis-motifs

In the protist Diplonema papillatum (Diplonemea, Euglenozoa), mitochondrial genes are systematically fragmented with each nonoverlapping piece (module) encoded individually on a distinct circular chromosome. Gene modules are transcribed separately, and precursor transcripts are assembled to mature mRNA by a trans-splicing process of yet unknown mechanism. Expression of the cox1 gene that consists of nine modules, also involves RNA editing by which six uridines are added between Modules 4 and 5. Here, we investigate whether the unusual features of cox1 are shared by all Diplonemea and what the mechanism of trans-splicing might be. We examine three additional species representing both Diplonemea genera, namely D. papillatum described before, and D. ambulator, Diplonema sp.2, and Rhynchopus euleeides and discover that in all Diplonemea, the cox1 gene is discontinuous and split up into nine modules that each reside on a distinct chromosome. Positions of gene breakpoints vary by up to two nucleotides. Further, all taxa have six nonencoded uridines inserted in cox1 mRNA at exactly the same position as D. papillatum. In silico searches do not detect signatures of introns known to engage in trans-splicing, in particular Group I, Group II, spliceosomal, and transfer RNA introns. Nor did we find statistically significant reverse-complementary motifs between adjacent modules and their flanking regions, or residues conserved within or across species. This provides compelling evidence that trans-splicing in Diplonemea mitochondria does not rely on sequence elements in cis but rather proceeds by a mechanism employing matchmaking trans factors, such as RNAs or proteins.

Reconstructing euglenoid evolutionary relationships using three genes: nuclear SSU and LSU, and chloroplast SSU rDNA sequences and the description of Euglenaria gen. nov. (Euglenophyta)

Using Maximum Likelihood and Bayesian analyses of three genes, nuclear SSU (nSSU) and LSU (nLSU) rDNA, and chloroplast SSU (cpSSU) rDNA, the relationships among 82 plastid-containing strains of euglenophytes were clarified. The resulting tree split into two major clades: clade one contained Euglena, Trachelomonas, Strombomonas, Colacium, Monomorphina, Cryptoglena and Euglenaria; clade two contained Lepocinclis, Phacus and Discoplastis. The majority of the members of Euglena were contained in clade A, but seven members were outside of this clade. Euglena limnophila grouped with, and was thus transferred to Phacus. Euglena proxima was a single taxon at the base of clade one and is unassociated with any subclade. Five members of Euglena grouped together within clade one and were transferred into the newly erected genus Euglenaria. The monophyly of the remaining genera was supported by Bayesian and Maximum Likelihood analyses. Combining datasets resolved the relationships among ten genera of photosynthetic euglenoids.

Description of Rhynchopus euleeides n. sp. (Diplonemea), a free-living marine euglenozoan

We describe Rhynchopus euleeides n. sp., using light and electron microscopy. This free-living flagellate, which was isolated earlier from a marine habitat, can be grown axenically in a rich medium based on modified seawater. In the trophic stage, cells are predominantly elliptical and laterally flattened, but frequently change their shape (metaboly). Gliding is the predominant manner of locomotion. The two flagella, which are typically concealed in their pocket, are short stubs of unequal length, have conventional axonemes, but apparently lack a paraxonemal rod. Swarmer cells, which form only occasionally, are smaller in size and carry two conspicuous flagella of more than 2 times the body length. Cells are decorated with a prominent apical papillum. Both the flagellar pocket and the adjacent feeding apparatus seem to merge together into a single sub-apical opening. The mitochondrion, which is most likely single, is located peripherally. It is reticulated in shape and contains only a few lamellar cristae. Mitochondrial DNA is abundant and evenly distributed throughout the organelle. Morphological synapomorphies confirm the affiliation of the species with the genus Rhynchopus (Diplonemea, Euglenozoa). We discuss the characters that distinguish Rhynchopus from Diplonema corroborating the validity of the two genera.

A multiple PCR-primer approach to access the microeukaryotic diversity in environmental samples

The Cariaco Basin off the Venezuelan coast in the Caribbean Sea is the world's largest truly marine body of anoxic water. The first rRNA survey of microbial eukaryotes in this environment revealed a number of novel lineages, but sampled only a fraction of the entire diversity. The goal of this study was to significantly improve recovery of protistan rRNA from the Basin. This was achieved by a systematic application of multiple PCR primer sets and substantially larger sequencing efforts. We focused on the most diverse habitat in the basin, anoxic waters approximately 100m below the oxic-anoxic interface, and detected novel lineages that escaped the single PCR primer approach. All clones obtained proved unique. A 99% sequence similarity cut-off value combined these clones into operational taxonomic units (OTUs), over 75% of which proved novel. Some of these OTUs form deep branches within established protistan groups. Others signify discovery of novel protistan lineages that appear unrelated to any known microeukaryote. Surprisingly, even this large-scale multi-primer rRNA approach still missed a substantial part of the samples' rRNA diversity. The overlap between the species lists obtained with different primers is low, with only 4% of OTUs shared by all three libraries, and the number of species detected only once is large (55%). This strongly indicates that, at least in anoxic environments, protistan diversity may be much larger than is commonly thought. A single sample appears to contain thousands of largely novel protistan species. Multiple PCR primer combinations may be needed to capture these species.

An updated view of kinetoplastid phylogeny using environmental sequences and a closer outgroup: proposal for a new classification of the class Kinetoplastea

Given their ecological and medical importance, the classification of the kinetoplastid protists (class Kinetoplastea) has attracted much scientific attention for a long time. Morphology-based taxonomic schemes distinguished two major kinetoplastid groups: the strictly parasitic, uniflagellate trypanosomatids and the biflagellate bodonids. Molecular phylogenetic analyses based on 18S rRNA sequence comparison suggested that the trypanosomatids emerged from within the bodonids. However, these analyses revealed a huge evolutionary distance between the kinetoplastids and their closest relatives (euglenids and diplonemids) that makes very difficult the correct inference of the phylogenetic relationships between the different kinetoplastid groups. Using direct PCR amplification of 18S rRNA genes from hydrothermal vent samples, several new kinetoplastid-like sequences have been reported recently. Three of them emerge robustly at the base of the kinetoplastids, breaking the long branch leading to the euglenids and diplonemids. One of these sequences belongs to a close relative of Ichthyobodo necator (a fish parasite) and of the 'Perkinsiella amoebae'-like endosymbiont of Neoparamoeba spp. amoebae. The authors have studied the reliability of their basal position and used all these slow-evolving basal-emerging sequences as a close outgroup to analyse the phylogeny of the apical kinetoplastids. They thus find a much more stable and resolved kinetoplastid phylogeny, which supports the monophyly of groups that very often emerged as polyphyletic in the trees rooted using the traditional, distant outgroup sequences. A new classification of the class Kinetoplastea is proposed based on the results of the phylogenetic analysis presented. This class is now subdivided into two new subclasses, Prokinetoplastina (accommodating the basal species I. necator and 'Perkinsiella amoebae') and Metakinetoplastina (containing the Trypanosomatida together with three additional new orders: Eubodonida, Parabodonida and Neobodonida). The classification of the species formerly included in the genus Bodo is also revised, with the amendment of this genus and the genus Parabodo and the creation of a new genus, Neobodo.

Ribosomal RNA phylogeny of bodonid and diplonemid flagellates and the evolution of euglenozoa

Euglenozoa is a major phylum of excavate protozoa (comprising euglenoids, kinetoplastids, and diplonemids) with highly unusual nuclear, mitochondrial, and chloroplast genomes. To improve understanding of euglenozoan evolution, we sequenced nuclear small-subunit rRNA genes from 34 bodonids (Bodo, Neobodo, Parabodo, Dimastigella-like, Rhynchobodo, Rhynchomonas, and unidentified strains), nine diplonemids (Diplonema, Rhynchopus), and a euglenoid (Entosiphon). Phylogenetic analysis reveals that diplonemids and bodonids are more diverse than previously recognised, but does not clearly establish the branching order of kinetoplastids, euglenoids, and diplonemids. Rhynchopus is holophyletic; parasitic species arose from within free-living species. Kinetoplastea (bodonids and trypanosomatids) are robustly holophyletic and comprise a major clade including all trypanosomatids and most bodonids ('core bodonids') and a very divergent minor one including Ichthyobodo. The root of the major kinetoplastid clade is probably between trypanosomatids and core bodonids. Core bodonids have three distinct subclades. Clade 1 has two distinct Rhynchobodo-like lineages; a lineage comprising Dimastigella and Rhynchomonas; and another including Cruzella and Neobodo. Clade 2 comprises Cryptobia/ Trypanoplasma, Procryptobia, and Parabodo. Clade 3 is an extensive Bodo saltans species complex. Neobodo designis is a vast genetically divergent species complex with mutually exclusive marine and freshwater subclades. Our analysis supports three phagotrophic euglenoid orders: Petalomonadida (holophyletic), Ploeotiida (probably holophyletic), Peranemida (paraphyletic).

Protein phylogenies robustly resolve the deep-level relationships within Euglenozoa

The deepest-level relationships amongst Euglenozoa remain poorly resolved, despite a rich history of morphological examination and numerous molecular phylogenetic studies of small subunit ribosomal RNA (SSU rRNA) data. We address this question using two nuclear-encoded proteins, the cytosolic isoforms of heat shock protein 90 (hsp90) and heat shock protein 70 (hsp70). For both proteins we examined sequences from the three primary groups within Euglenozoa (euglenids, diplonemids, and kinetoplastids), and from their close relatives, Heterolobosea. Maximum likelihood (ML) and ML distance analyses of these proteins support a close relationship between diplonemids and kinetoplastids to the exclusion of the euglenid Euglena gracilis. In hsp90 and combined protein analyses bootstrap support is very strong and alternative topologies are generally rejected by 'approximately unbiased' (AU) tests. This result is consistent with recent molecular biological and morphological data, but contradicts early structural accounts and many SSU rRNA analyses that favour a closer relationship between diplonemids and euglenids. However, a re-examination of an important SSU rRNA data set highlights the instability of the inferences from this marker. The protein analyses also suggest that bodonids are paraphyletic, with trypanosomatids grouping with 'clade 2' and 'clade 3' bodonids to the exclusion of 'clade 1' bodonids.

Discovery of a group I intron in the SSU rDNA of Ploeotia costata (Euglenozoa)

The gene coding for the small ribosomal subunit RNA of Ploeotia costata contains an actively splicing group I intron (Pco.S516) which is unique among euglenozoans. Secondary structure predictions indicate that paired segments P1-P10 as well as several conserved elements typical of group I introns and of subclass IC1 in particular are present. Phylogenetic analyses of SSU rDNA sequences demonstrate a well-supported placement of Ploeotia costata within the Euglenozoa; whereas, analyses of intron data sets uncover a close phylogenetic relation of Pco.S516 to S-516 introns from Acanthamoeba, Aureoumbra lagunensis (Stramenopila) and red algae of the order Bangiales. Discrepancies between SSU rDNA and intron phylogenies suggest horizontal spread of the group I intron. Monophyly of IC1 516 introns from Ploeotia costata, A. lagunensis and rhodophytes is supported by a unique secondary structure element: helix P5b possesses an insertion of 19 nt length with a highly conserved tetraloop which is supposed to take part in tertiary interactions. Neither functional nor degenerated ORFs coding for homing endonucleases can be identified in Pco.S516. Nevertheless, degenerated ORFs with His-Cys box motifs in closely related intron sequences indicate that homing may have occurred during evolution of the investigated intron group.

Phylogeny and taxonomic revision of plastid-containing euglenophytes based on SSU rDNA sequence comparisons and synapomorphic signatures in the SSU rRNA secondary structure

Sequence comparisons and a revised classification of the Euglenophyceae were based on 92 new SSU rDNA sequences obtained from strains of Euglena, Astasia, Phacus, Trachelomonas, Colacium, Cryptoglena, Lepocinclis, Eutreptia, Eutreptiella and Tetreutreptia. Sequence data also provided molecular signatures for taxa from genus to class level in the SSU rRNA secondary structure, revealed by a novel approach (search for non-homoplasious synapomorphies) and used for taxonomic diagnoses. Photosynthetic euglenoids and secondary heterotrophs formed a clade, designated as Euglenophyceae (emend.) with two orders: Euglenales and Eutreptiales. The mostly marine Eutreptiales (Eutreptia, Eutreptiella; not Distigma) comprised taxa with two or four emergent flagella (the quadriflagellate Tetreutreptia was integrated within Eutreptiella). The Euglenales (freshwater genera with < or = one emergent flagellum) formed nine clades and two individual branches (single strains); however, only two clades were congruent with traditional genera: Trachelomonas (incl. Strombomonas) and Colacium. Euglena was polyphyletic and diverged into four independent clades (intermixed with Astasia, Khawkinea and Lepocinclis) and two individual branches (e.g. E. polymorpha). Phacus was also subdivided into Phacus s. str. and two combined lineages (mixed with Lepocinclis spp. or Cryptoglena). In consequence, Euglena (s. str.), Phacus and other genera were emended and one lineage (mixed Phacus/Lepocinclis-clade) was recognized as the previously neglected genus Monomorphina Mereschkowsky (1877). The sister clade of Phacus s. str. (mixed Euglena/Lepocinclis-clade) was identified as Lepocinclis Perty (emended).

Systematics of primary osmotrophic euglenids: a molecular approach to the phylogeny of Distigma and Astasia (Euglenozoa)

Nuclear-encoded SSU rRNA genes from nine strains of Distigma and three strains of Astasia were sequenced and analysed phylogenetically with maximum-likelihood and maximum-parsimony methods. It could be demonstrated that the genus Distigma is paraphyletic, consisting of two distinct clades: one comprises four strains of the type species, Distigma proteus, and the other includes four strains of Distigma curvatum, Distigma gracile, Distigma sennii and Distigma elegans. These findings are well corroborated by morphological characteristics. The investigated species of Astasia are closely related to members of the Rhabdomonadida, thus rendering the genus Astasia polyphyletic, with Astasia longa branching within the phototrophs. All of the species investigated cluster in a well-supported group of primary osmotrophic euglenids that are not derived from photosynthetic ancestors. The recovered clades are characterized by their sequence diversity. After different evolutionary rates among lineages had been determined, a modified slow-fast approach was used to differentiate phylogenetic signal from noise. Finally, a revised systematic scheme based on phylogenetic relationships is suggested to render euglenid taxonomy more transparent: primary osmotrophic euglenids are classified as Aphagea, and members of the D. curvatum group are transferred into the new subgenus Parvonema.