The revised classification of eukaryotes

The cell biology of the endocytic system from an evolutionary perspective

Evolutionary cell biology can afford an interdisciplinary comparative view that gives insights into both the functioning of modern cells and the origins of cellular systems, including the endocytic organelles. Here, we explore several recent evolutionary cell biology studies, highlighting investigations into the origin and diversity of endocytic systems in eukaryotes. Beginning with a brief overview of the eukaryote tree of life, we show how understanding the endocytic machinery in a select, but diverse, array of organisms provides insights into endocytic system origins and predicts the likely configuration in the last eukaryotic common ancestor (LECA). Next, we consider three examples in which a comparative approach yielded insight into the function of modern cellular systems. First, using ESCRT-0 as an example, we show how comparative cell biology can discover both lineage-specific novelties (ESCRT-0) as well as previously ignored ancient proteins (Tom1), likely of both evolutionary and functional importance. Second, we highlight the power of comparative cell biology for discovery of previously ignored but potentially ancient complexes (AP5). Finally, using examples from ciliates and trypanosomes, we show that not all organisms possess canonical endocytic pathways, but instead likely evolved lineage-specific mechanisms. Drawing from these case studies, we conclude that a comparative approach is a powerful strategy for advancing knowledge about the general mechanisms and functions of endocytic systems.

Haplomyxa saranae gen. nov. et sp. nov., a new naked freshwater foraminifer

A new naked foraminifer, Haplomyxa saranae gen. nov. sp. nov., is described from an established cell line made from a single cell isolated from a freshwater garden pond. The new species was morphologically close to Reticulomyxa filosa, the only valid naked freshwater foraminifer species. However the two species differed when it came to the morphology of the cell body, the number of cysts, and the nutrition. The 18S rRNA gene had one of the longest sequences to date (4863 nucleotides), and it contained many insertions that are typical of Foraminifera. The size of this gene was 45% longer than the one of R. filosa due to the elongation of A+T rich regions, but molecular phylogeny based on conserved regions of the 3'-end placed the new species in the same morphological clade K. This report includes both morphological and genetic data which undoubtedly show that the new species is a new naked freshwater foraminifer and the second species of the clade K.

Applications of next-generation sequencing to unravelling the evolutionary history of algae

First-generation Sanger DNA sequencing revolutionized science over the past three decades and the current next-generation sequencing (NGS) technology has opened the doors to the next phase in the sequencing revolution. Using NGS, scientists are able to sequence entire genomes and to generate extensive transcriptome data from diverse photosynthetic eukaryotes in a timely and cost-effective manner. Genome data in particular shed light on the complicated evolutionary history of algae that form the basis of the food chain in many environments. In the Eukaryotic Tree of Life, the fact that photosynthetic lineages are positioned in four supergroups has important evolutionary consequences. We now know that the story of eukaryotic photosynthesis unfolds with a primary endosymbiosis between an ancestral heterotrophic protist and a captured cyanobacterium that gave rise to the glaucophytes, red algae and Viridiplantae (green algae and land plants). These primary plastids were then transferred to other eukaryotic groups through secondary endosymbiosis. A red alga was captured by the ancestor(s) of the stramenopiles, alveolates (dinoflagellates, apicomplexa, chromeridae), cryptophytes and haptophytes, whereas green algae were captured independently by the common ancestors of the euglenophytes and chlorarachniophytes. A separate case of primary endosymbiosis is found in the filose amoeba Paulinella chromatophora, which has at least nine heterotrophic sister species. Paulinella genome data provide detailed insights into the early stages of plastid establishment. Therefore, genome data produced by NGS have provided many novel insights into the taxonomy, phylogeny and evolutionary history of photosynthetic eukaryotes.

The rise and fall of Picobiliphytes: how assumed autotrophs turned out to be heterotrophs

Algae are significant members of Earth's biodiversity. Having been studied for a long time, the discovery of new algal phyla is extremely unusual. Recently, the enigmatic "Picobiliphyta," a group of uncultured eukaryotes unveiled using molecular tools, were claimed to represent an unrecognized early branching algal lineage with a nucleomorph (remnant nucleus of a secondary algal endosymbiont) in their plastids. However, subsequent studies rejected the presence of a nucleomorph, and single-cell genomic studies failed to detect any plastid-related genes, ruling out the possibility of plastid occurrence. The isolation of the first "picobiliphyte," Picomonas judraskeda, a tiny organism that feeds on very small (<150 nm) organic particles, came as final proof of their non-photosynthetic lifestyle. Consequently, the group has been renamed Picozoa. The passage from "picobiliphytes" to "picozoa" illustrates the crucial role that classical protistology should play to provide sound biological context for the wealth of data produced by modern molecular techniques.

Nuclear pore complex evolution: a trypanosome Mlp analogue functions in chromosomal segregation but lacks transcriptional barrier activity

The nuclear face of the nuclear pore complex (NPC) interfaces with chromatin, transcription, and transport intermediates. A novel architecture for the nuclear face of the trypanosome NPC provides insights into NPC function and evolution.

The nuclear pore complex (NPC) has dual roles in nucleocytoplasmic transport and chromatin organization. In many eukaryotes the coiled-coil Mlp/Tpr proteins of the NPC nuclear basket have specific functions in interactions with chromatin and defining specialized regions of active transcription, whereas Mlp2 associates with the mitotic spindle/NPC in a cell cycle–dependent manner. We previously identified two putative Mlp-related proteins in African trypanosomes, TbNup110 and TbNup92, the latter of which associates with the spindle. We now provide evidence for independent ancestry for TbNup92/TbNup110 and Mlp/Tpr proteins. However, TbNup92 is required for correct chromosome segregation, with knockout cells exhibiting microaneuploidy and lowered fidelity of telomere segregation. Further, TbNup92 is intimately associated with the mitotic spindle and spindle anchor site but apparently has minimal roles in control of gene transcription, indicating that TbNup92 lacks major barrier activity. TbNup92 therefore acts as a functional analogue of Mlp/Tpr proteins, and, together with the lamina analogue NUP-1, represents a cohort of novel proteins operating at the nuclear periphery of trypanosomes, uncovering complex evolutionary trajectories for the NPC and nuclear lamina.

Molecular diversity of endosymbiotic Nephroselmis (Nephroselmidophyceae) in Hatena arenicola (Katablepharidophycota)

Hatena arenicola (Katablepharidophycota) is a single-celled eukaryote that temporarily possesses a chlorophyte alga of the genus Nephroselmis as an intracellular symbiont. In the present study, we investigated the molecular diversity of the endosymbiont Nephroselmis in a natural population of the host H. arenicola. We sequenced the host's 18S rRNA gene and the endosymbiont's plastid-encoded 16S rRNA gene. The results indicated that almost identical strains of the host harbored at least three distinct strains of the algal endosymbiont affiliated to the clade Nephroselmis rotunda. This finding supports our previous hypothesis that H. arenicola and its symbiotic alga are in an early stage of secondary endosymbiosis.

Phylogeny of the poorly known ciliates, microthoracida, a systematically confused taxon (ciliophora), with morphological reports of three species

Three species of Microthoracids, Lopezoterenia paratorpens n. sp., Trochiliopsis australis Foissner et al., 1988 and Discotricha papillifera Tuffrau, 1954, collected from Chinese coastal waters, were investigated using live observation and protargol staining methods. Lopezoterenia paratorpens n. sp. was characterized by its squarely shaped cortical papillae and by dorsal kineties which contained loosely distributed basal bodies. Trochiliopsis australis was revealed to have two oral membranelles, which was not recorded in the original report. Phylogenetic analyses were carried out based on SSU rRNA gene sequence data from each of the three species, and on other available data for microthoracids. The results showed that the order Microthoracida is not monophyletic because the family Discotrichidae, which contains L. paratorpens and D. papillifera, forms a clade separated from the "core" Microthoracids clade. The topologies of the maximum likelihood and Bayesian inference trees, along with the distinct morphological characteristics found previously, suggest that the family Discotrichidae should not be assigned to the order Microthoracida. We propose to designate a new order, Discotrichida n. ord. which diagnosed as: flattened ciliates with conspicuous cortical papillae on both dorsal and ventral faces, rod-shaped mucocysts, and an asymmetric cytopharyngeal basket. Also, the fact that Leptopharynx clusters with the assemblage including T. australis, and Pseudomicrothorax is located distantly from Leptopharynx indicates that the classification of Pseudomicrothoracidae and Microthoracidae by Foissner (1985) is justified.

The predicted secretomes of Monosiga brevicollis and Capsaspora owczarzaki, close unicellular relatives of metazoans, reveal new insights into the evolution of the metazoan extracellular matrix

The extracellular matrix (ECM) is a major mediator of multi-cellularity in the metazoa. Multiple ECM proteins are conserved from sponges to human, raising questions about the evolutionary origin of ECM. Choanoflagellates are the closest unicellular relatives of the metazoa and proteins with domains characteristic of metazoan ECM proteins have been identified from the genome-predicted proteome of the choanoflagellate Monosiga brevicollis. However, a systematic analysis of M. brevicollis secretory signal peptide-containing proteins with ECM domains has been lacking. We analysed all predicted secretory signal-peptide-containing proteins of M. brevicollis for ECM domains. Nine domains that are widespread in metazoan ECM proteins are represented, with EGF, fibronectin III, laminin G, and von Willebrand Factor_A domains being the most numerous. Three proteins contain more than one category of ECM domain, however, no proteins correspond to the domain architecture of metazoan ECM proteins. The fibronectin III domains are all present within glycoside hydrolases and none contain an integrin-binding motif. Glycosaminoglycan-binding motifs identified in animal thrombospondin type 1 domains are conserved in some M. brevicollis representatives of this domain, whereas there is little evidence of conservation of glycosaminoglycan-binding motifs in the laminin G domains. The identified proteins were compared with the predicted secretory ECM domain-containing proteins of the integrin-expressing filasterean, Capsaspora owczarzaki. C. owczarzaki encodes a smaller number of secretory, ECM domain-containing proteins and only EGF, fibronectin type III and laminin G domains are represented. The M. brevicollis and C. owczarzaki proteins have distinct domain architectures and all proteins differ in their domain architecture to metazoan ECM proteins. These identifications provide a basis for future experiments to validate the extracellular location of these proteins and uncover their functions in choanoflagellates and C. owczarzaki. The data strengthen the model that ECM proteins are metazoan-specific and evolved as innovations in the last common metazoan ancestor.

Analysis of EST data of the marine protist Oxyrrhis marina, an emerging model for alveolate biology and evolution

BACKGROUND

The alveolates include a large number of important lineages of protists and algae, among which are three major eukaryotic groups: ciliates, apicomplexans and dinoflagellates. Collectively alveolates are present in virtually every environment and include a vast diversity of cell shapes, molecular and cellular features and feeding modes including lifestyles such as phototrophy, phagotrophy/predation and intracellular parasitism, in addition to a variety of symbiotic associations. Oxyrrhis marina is a well-known model for heterotrophic protist biology, and is now emerging as a useful organism to explore the many changes that occurred during the origin and diversification of dinoflagellates by virtue of its phylogenetic position at the base of the dinoflagellate tree.

RESULTS

We have generated and analysed expressed sequence tag (EST) sequences from the alveolate Oxyrrhis marina in order to shed light on the evolution of a number of dinoflagellate characteristics, especially regarding the emergence of highly unusual genomic features. We found that O. marina harbours extensive gene redundancy, indicating high rates of gene duplication and transcription from multiple genomic loci. In addition, we observed a correlation between expression level and copy number in several genes, suggesting that copy number may contribute to determining transcript levels for some genes. Finally, we analyze the genes and predicted products of the recently discovered Dinoflagellate Viral Nuclear Protein, and several cases of horizontally acquired genes.

CONCLUSION

The dataset presented here has proven very valuable for studying this important group of protists. Our analysis indicates that gene redundancy is a pervasive feature of dinoflagellate genomes, thus the mechanisms involved in its generation must have arisen early in the evolution of the group.

The genome of Spironucleus salmonicida highlights a fish pathogen adapted to fluctuating environments

Spironucleus salmonicida causes systemic infections in salmonid fish. It belongs to the group diplomonads, binucleated heterotrophic flagellates adapted to micro-aerobic environments. Recently we identified energy-producing hydrogenosomes in S. salmonicida. Here we present a genome analysis of the fish parasite with a focus on the comparison to the more studied diplomonad Giardia intestinalis. We annotated 8067 protein coding genes in the ∼12.9 Mbp S. salmonicida genome. Unlike G. intestinalis, promoter-like motifs were found upstream of genes which are correlated with gene expression, suggesting a more elaborate transcriptional regulation. S. salmonicida can utilise more carbohydrates as energy sources, has an extended amino acid and sulfur metabolism, and more enzymes involved in scavenging of reactive oxygen species compared to G. intestinalis. Both genomes have large families of cysteine-rich membrane proteins. A cluster analysis indicated large divergence of these families in the two diplomonads. Nevertheless, one of S. salmonicida cysteine-rich proteins was localised to the plasma membrane similar to G. intestinalis variant-surface proteins. We identified S. salmonicida homologs to cyst wall proteins and showed that one of these is functional when expressed in Giardia. This suggests that the fish parasite is transmitted as a cyst between hosts. The extended metabolic repertoire and more extensive gene regulation compared to G. intestinalis suggest that the fish parasite is more adapted to cope with environmental fluctuations. Our genome analyses indicate that S. salmonicida is a well-adapted pathogen that can colonize different sites in the host.

Studies of model organisms are very powerful. However, to appreciate the enormous diversity of genetic and cell biological processes we need to extend the number of available model organisms. For example, there are very few model organisms for diverse microbial eukaryotes, a group of organisms which indeed represents the vast majority of the eukaryotic diversity. To this end, we have developed a system to do genetic modification on the Atlantic salmon pathogen Spironucleus salmonicida. Using this system we could show that the organism is capable of producing hydrogen within specialised compartments. Here we present the genome sequence of S. salmonicida together with a thorough annotation. We compare the results with the closest available model organism, the human intestinal parasite Giardia intestinalis. The fish parasite has a more elaborate system for regulation of gene expression, as well as a larger metabolic capacity. This indicates that S. salmonicida is a well-adapted pathogen that can deal with fluctuating environments, an important trait to be able to establish systemic infections in the host. The development of S. salmonicida into a model system will benefit the studies of fish infections, as well as cell biological processes.

Evolution of the nucleus

•

The nuclear pore complex is well conserved, with some regions of divergence.

•

The nuclear lamina appears quite variable between major supergroups.

•

Centrosomes are ancient structures, but with complex evolutionary history.

•

There is evidence for prokaryotic ancestors of some nuclear components.

•

Analysis of divergent organisms is essential to fully understand nuclear biology and its origins.

The nucleus represents a major evolutionary transition. As a consequence of separating translation from transcription many new functions arose, which likely contributed to the remarkable success of eukaryotic cells. Here we will consider what has recently emerged on the evolutionary histories of several key aspects of nuclear biology; the nuclear pore complex, the lamina, centrosomes and evidence for prokaryotic origins of relevant players.

An alternative root for the eukaryote tree of life

The root of the eukaryote tree of life defines some of the most fundamental relationships among species. It is also critical for defining the last eukaryote common ancestor (LECA), the shared heritage of all extant species. The unikont-bikont root has been the reigning paradigm for eukaryotes for more than 10 years but is becoming increasingly controversial. We developed a carefully vetted data set, consisting of 37 nuclear-encoded proteins of close bacterial ancestry (euBacs) and their closest bacterial relatives, augmented by deep sequencing of the Acrasis kona (Heterolobosea, Discoba) transcriptome. Phylogenetic analysis of these data produces a highly robust, fully resolved global phylogeny of eukaryotes. The tree sorts all examined eukaryotes into three megagroups and identifies the Discoba, and potentially its parent taxon Excavata, as the sister group to the bulk of known eukaryote diversity, the proposed Neozoa (Amorphea + Stramenopila+Alveolata+Rhizaria+Plantae [SARP]). All major alternative hypotheses are rejected with as little as ∼50% of the data, and this resolution is unaffected by the presence of fast-evolving alignment positions or distant outgroup sequences. This "neozoan-excavate" root revises hypotheses of early eukaryote evolution and highlights the importance of the poorly studied Discoba for understanding the evolution of eukaryotic diversity and basic cellular processes.

Kinase signalling in Plasmodium sexual stages and interventions to stop malaria transmission

The symptoms of malaria, one of the infectious diseases with the highest mortality and morbidity world-wide, are caused by asexual parasites replicating inside red blood cells. Disease transmission, however, is effected by non-replicating cells which have differentiated into male or female gametocytes. These are the forms infectious to mosquito vectors and the insects are the only hosts where parasite sexual reproduction can take place. Malaria is thus a complex infection in which pharmacological treatment of symptoms may still allow transmission for long periods, while pharmacological blockage of infectivity may not cure symptoms. The process of parasite sexual differentiation and development is still being revealed but it is clear that kinase-mediated signalling mechanisms play a significant role. This review attempts to summarise our limited current knowledge on the signalling mechanisms involved in the transition from asexual replication to sexual differentiation and reproduction, with a brief mention to the effects of current treatments on the sexual stages and to some of the difficulties inherent in developing pharmacological interventions to curtail disease transmission.

Ancient signals: comparative genomics of green plant CDPKs

Calcium-dependent protein kinases (CDPKs) are multifunctional proteins that combine calcium-binding and signaling capabilities within a single gene product. This unique versatility enables multiple plant biological processes to be controlled, including developmental programs and stress responses. The genome of flowering plants typically encodes around 30 CDPK homologs that cluster in four conserved clades. In this review, we take advantage of the recent availability of genome sequences from green algae and early land plants to examine how well the previously described CDPK family from angiosperms compares to the broader evolutionary states associated with early diverging green plant lineages. Our analysis suggests that the current architecture of the CDPK family was shaped during the colonization of the land by plants, whereas CDPKs from ancestor green algae have continued to evolve independently.

Placing environmental next-generation sequencing amplicons from microbial eukaryotes into a phylogenetic context

Nucleotide positions in the hypervariable V4 and V9 regions of the small subunit (SSU)-rDNA locus are normally difficult to align and are usually removed before standard phylogenetic analyses. Yet, with next-generation sequencing data, amplicons of these regions are all that are available to answer ecological and evolutionary questions that rely on phylogenetic inferences. With ciliates, we asked how inclusion of the V4 or V9 regions, regardless of alignment quality, affects tree topologies using distinct phylogenetic methods (including PairDist that is introduced here). Results show that the best approach is to place V4 amplicons into an alignment of full-length Sanger SSU-rDNA sequences and to infer the phylogenetic tree with RAxML. A sliding window algorithm as implemented in RAxML shows, though, that not all nucleotide positions in the V4 region are better than V9 at inferring the ciliate tree. With this approach and an ancestral-state reconstruction, we use V4 amplicons from European nearshore sampling sites to infer that rather than being primarily terrestrial and freshwater, colpodean ciliates may have repeatedly transitioned from terrestrial/freshwater to marine environments.

Osmotolerance in the Cryptophyceae: jacks-of-all-trades in the Chroomonas Clade

No detailed studies have been performed to date on osmotolerance in cryptophytes, although one species, Chroomonas africana, had previously been reported to grow in freshwater as well as seawater. This study focused on osmotolerance in Chroomonas. Growth at different osmolalities and parameters of contractile vacuole function were examined and compared across a high-resolution phylogeny. Two evolutionary lineages in the Chroomonas clade proved to be euryhaline. Ranges of osmotolerance depended not only on osmolality, but also on culture medium. All cryptophytes contained contractile vacuoles. In the euryhaline strain CCAP 978/08 contractile vacuoles could be observed even at an osmolality beyond that of seawater. In addition the cells accumulated floridoside, an osmoprotectant likely originating from the red algal carbohydrate metabolism of the complex rhodoplast. Further evidence for functional contractile vacuoles also in marine cryptophytes was provided by identification of contractile vacuole-specific genes in the genome of Guillardia theta.

Horizontal gene transfer in eukaryotic plant pathogens

Gene transfer has been identified as a prevalent and pervasive phenomenon and an important source of genomic innovation in bacteria. The role of gene transfer in microbial eukaryotes seems to be of a reduced magnitude but in some cases can drive important evolutionary innovations, such as new functions that underpin the colonization of different niches. The aim of this review is to summarize published cases that support the hypothesis that horizontal gene transfer (HGT) has played a role in the evolution of phytopathogenic traits in fungi and oomycetes. Our survey of the literature identifies 46 proposed cases of transfer of genes that have a putative or experimentally demonstrable phytopathogenic function. When considering the life-cycle steps through which a pathogen must progress, the majority of the HGTs identified are associated with invading, degrading, and manipulating the host. Taken together, these data suggest HGT has played a role in shaping how fungi and oomycetes colonize plant hosts.

Paleobiological perspectives on early eukaryotic evolution

Eukaryotic organisms radiated in Proterozoic oceans with oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than 1200 million years ago, but older (up to 1600-1800 million years) microfossils could record stem group eukaryotes. Major eukaryotic diversification ~800 million years ago is documented by the increase in the taxonomic richness of complex, organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and simple foraminiferans and diverse scales comparable to organic and siliceous scales formed today by protists in several clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.

Mitochondrial genome sequences and comparative genomics of Achlya hypogyna and Thraustotheca clavata

As a lineage, oomycetes have adapted to a wide range of lifestyles. Although the common ancestor of the group was likely a marine pathogen, extant members inhabit a spectrum from free-living saprobes to obligate biotrophs. The mitochondrial genomes of Achlya hypogyna and Thraustotheca clavata were sequenced to directly compare a facultative parasitic species (A. hypogyna) to a closely related free living saprobe (T. clavata). Both sequenced mitochondrial genomes are circular, with sizes of 46,869 bp for A. hypogyna and 47,381 bp for T. clavata. They share 63 common genes, indicating little influence of lifestyle on gene content, but small differences in total number and order of genes. Achlya hypogyna has a single copy of nad2, whereas T. clavata has one pseudogene (rps7) and two duplicated genes (nad5 and nad2), each with one full and one truncated copy. The genomes encode a total of 29 or 30 tRNAs (A. hypogyna and T. clavata, respectively) for 19 amino acids. Three unidentified open reading frames are conserved, and one is unique to T. clavata. Comparisons of these genomes with published sequences of the closely related Saprolegnia ferax mitochondrial genome, and four other more distantly related oomycetes, reveals no correlation in genome content or architecture with lifestyle.

The genome of the foraminiferan Reticulomyxa filosa

BACKGROUND

Rhizaria are a major branch of eukaryote evolution with an extensive microfossil record, but only scarce molecular data are available. The rhizarian species Reticulomyxa filosa, belonging to the Foraminifera, is free-living in freshwater environments. In culture, it thrives only as a plasmodium with thousands of haploid nuclei in one cell. The R. filosa genome is the first foraminiferal genome to be deciphered.

RESULTS

The genome is extremely repetitive, and the large amounts of identical sequences hint at frequent amplifications and homologous recombination events. Presumably, these mechanisms are employed to provide more gene copies for higher transcriptional activity and to build up a reservoir of gene diversification in certain gene families, such as the kinesin family. The gene repertoire indicates that it is able to switch to a single-celled, flagellated sexual state never observed in culture. Comparison to another rhizarian, the chlorarachniophyte alga Bigelowiella natans, reveals that proteins involved in signaling were likely drivers in establishing the Rhizaria lineage. Compared to some other protists, horizontal gene transfer is limited, but we found evidence of bacterial-to-eukaryote and eukaryote-to-eukaryote transfer events.

CONCLUSIONS

The R. filosa genome exhibits a unique architecture with extensive repeat homogenization and gene amplification, which highlights its potential for diverse life-cycle stages. The ability of R. filosa to rapidly transport matter from the pseudopodia to the cell body may be supported by the high diversification of actin and kinesin gene family members.

Distribution of conventional and nonconventional introns in tubulin (α and β) genes of euglenids

The nuclear genomes of euglenids contain three types of introns: conventional spliceosomal introns, nonconventional introns for which a splicing mechanism is unknown (variable noncanonical borders, RNA secondary structure bringing together intron ends), and so-called intermediate introns, which combine features of conventional and nonconventional introns. Analysis of two genes, tubA and tubB, from 20 species of euglenids reveals contrasting distribution patterns of conventional and nonconventional introns--positions of conventional introns are conserved, whereas those of the nonconventional ones are unique to individual species or small groups of closely related taxa. Moreover, in the group of phototrophic euglenids, 11 events of conventional intron loss versus 15 events of nonconventional intron gain were identified. A comparison of all nonconventional intron sequences highlighted the most conserved elements in their sequence and secondary structure. Our results led us to put forward two hypotheses. 1) The first one posits that mutational changes in intron sequence could lead to a change in their excision mechanism--intermediate introns would then be a transitional form between the conventional and nonconventional introns. 2) The second hypothesis concerns the origin of nonconventional introns--because of the presence of inverted repeats near their ends, insertion of MITE-like transposon elements is proposed as a possible source of new introns.

The SILVA and "All-species Living Tree Project (LTP)" taxonomic frameworks

SILVA (from Latin silva, forest, http://www.arb-silva.de) is a comprehensive resource for up-to-date quality-controlled databases of aligned ribosomal RNA (rRNA) gene sequences from the Bacteria, Archaea and Eukaryota domains and supplementary online services. SILVA provides a manually curated taxonomy for all three domains of life, based on representative phylogenetic trees for the small- and large-subunit rRNA genes. This article describes the improvements the SILVA taxonomy has undergone in the last 3 years. Specifically we are focusing on the curation process, the various resources used for curation and the comparison of the SILVA taxonomy with Greengenes and RDP-II taxonomies. Our comparisons not only revealed a reasonable overlap between the taxa names, but also points to significant differences in both names and numbers of taxa between the three resources.

Nuclear DNA replication initiation in kinetoplastid parasites: new insights into an ancient process

Nuclear DNA replication is, arguably, the central cellular process in eukaryotes, because it drives propagation of life and intersects with many other genome reactions. Perhaps surprisingly, our understanding of nuclear DNA replication in kinetoplastids was limited until a clutch of studies emerged recently, revealing new insight into both the machinery and genome-wide coordination of the reaction. Here, we discuss how these studies suggest that the earliest acting components of the kinetoplastid nuclear DNA replication machinery - the factors that demarcate sites of the replication initiation, termed origins - are diverged from model eukaryotes. In addition, we discuss how origin usage and replication dynamics relate to the highly unusual organisation of transcription in the genome of Trypanosoma brucei.

A comparative analysis of trypanosomatid SNARE proteins

The Kinetoplastida are flagellated protozoa evolutionary distant and divergent from yeast and humans. Kinetoplastida include trypanosomatids, and a number of important pathogens. Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. inflict significant morbidity and mortality on humans and livestock as the etiological agents of human African trypanosomiasis, Chagas' disease and leishmaniasis respectively. For all of these organisms, intracellular trafficking is vital for maintenance of the host–pathogen interface, modulation/evasion of host immune system responses and nutrient uptake. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are critical components of the intracellular trafficking machinery in eukaryotes, mediating membrane fusion and contributing to organelle specificity. We asked how the SNARE complement evolved across the trypanosomatids. An in silico search of the predicted proteomes of T. b. brucei and T. cruzi was used to identify candidate SNARE sequences. Phylogenetic analysis, including comparisons with yeast and human SNAREs, allowed assignment of trypanosomatid SNAREs to the Q or R subclass, as well as identification of several SNAREs orthologous with those of opisthokonts. Only limited variation in number and identity of SNAREs was found, with Leishmania major having 27 and T. brucei 26, suggesting a stable SNARE complement post-speciation. Expression analysis of T. brucei SNAREs revealed significant differential expression between mammalian and insect infective forms, especially within R and Qb-SNARE subclasses, suggesting possible roles in adaptation to different environments. For trypanosome SNAREs with clear orthologs in opisthokonts, the subcellular localization of TbVAMP7C is endosomal while both TbSyn5 and TbSyn16B are at the Golgi complex, which suggests conservation of localization and possibly also function. Despite highly distinct life styles, the complement of trypanosomatid SNAREs is quite stable between the three pathogenic lineages, suggesting establishment in the last common ancestor of trypanosomes and Leishmania. Developmental changes to SNARE mRNA levels between blood steam and procyclic life stages suggest that trypanosomes modulate SNARE functions via expression. Finally, the locations of some conserved SNAREs have been retained across the eukaryotic lineage.

•

SNARE proteins are essential components of intracellular transport.

•

These proteins exhibit considerable conservation across pathogenic trypanosomes.

•

Some trypanosome SNARE families are expanded or lost.

•

Developmental changes in trypanosome SNARE expression are apparent.

•

Orthologous SNAREs demonstrate conserved locations and hence function.

Evolution of the cytosolic iron-sulfur cluster assembly machinery in Blastocystis species and other microbial eukaryotes

The cytosolic iron/sulfur cluster assembly (CIA) machinery is responsible for the assembly of cytosolic and nuclear iron/sulfur clusters, cofactors that are vital for all living cells. This machinery is uniquely found in eukaryotes and consists of at least eight proteins in opisthokont lineages, such as animals and fungi. We sought to identify and characterize homologues of the CIA system proteins in the anaerobic stramenopile parasite Blastocystis sp. strain NandII. We identified transcripts encoding six of the components-Cia1, Cia2, MMS19, Nbp35, Nar1, and a putative Tah18-and showed using immunofluorescence microscopy, immunoelectron microscopy, and subcellular fractionation that the last three of them localized to the cytoplasm of the cell. We then used comparative genomic and phylogenetic approaches to investigate the evolutionary history of these proteins. While most Blastocystis homologues branch with their eukaryotic counterparts, the putative Blastocystis Tah18 seems to have a separate evolutionary origin and therefore possibly a different function. Furthermore, our phylogenomic analyses revealed that all eight CIA components described in opisthokonts originated before the diversification of extant eukaryotic lineages and were likely already present in the last eukaryotic common ancestor (LECA). The Nbp35, Nar1 Cia1, and Cia2 proteins have been conserved during the subsequent evolutionary diversification of eukaryotes and are present in virtually all extant lineages, whereas the other CIA proteins have patchy phylogenetic distributions. Cia2 appears to be homologous to SufT, a component of the prokaryotic sulfur utilization factors (SUF) system, making this the first reported evolutionary link between the CIA and any other Fe/S biogenesis pathway. All of our results suggest that the CIA machinery is an ubiquitous biosynthetic pathway in eukaryotes, but its apparent plasticity in composition raises questions regarding how it functions in nonmodel organisms and how it interfaces with various iron/sulfur cluster systems (i.e., the iron/sulfur cluster, nitrogen fixation, and/or SUF system) found in eukaryotic cells.

Exploring the uncultured microeukaryote majority in the oceans: reevaluation of ribogroups within stramenopiles

Molecular surveys in planktonic marine systems have unveiled a large novel diversity of small protists. A large part of this diversity belongs to basal heterotrophic stramenopiles and is distributed in a set of polyphyletic ribogroups (described from rDNA sequences) collectively named as MAST (MArine STramenopiles). In the few groups investigated, MAST cells are globally distributed and abundant bacterial grazers, therefore having a putatively large impact on marine ecosystem functioning. The main aim of this study is to reevaluate the MAST ribogroups described so far and to determine whether additional groups can be found. For this purpose, we used traditional and state-of-the-art molecular tools, combining 18S rDNA sequences from publicly available clone libraries, single amplified genomes (SAGs) of planktonic protists, and a pyrosequencing survey from coastal waters and sediments. Our analysis indicated a final set of 18 MAST groups plus 5 new ribogroups within Ochrophyta (named as MOCH). The MAST ribogroups were then analyzed in more detail. Seven were typical of anoxic systems and one of oxic sediments. The rest were clearly members of oxic marine picoplankton. We characterized the genetic diversity within each MAST group and defined subclades for the more diverse (46 subclades in 8 groups). The analyses of sequences within subclades revealed further ecological specializations. Our data provide a renovated framework for phylogenetic classification of the numerous MAST ribogroups and support the notion of a tight link between phylogeny and ecological distribution. These diverse and largely uncultured protists are widespread and ecologically relevant members of marine microbial assemblages.

Detection of the kinetoplastid Azumiobodo hoyamushi, the causative agent of soft tunic syndrome, in wild ascidians Halocynthia roretzi

The occurrence of soft tunic syndrome in wild populations of the ascidian Halocynthia roretzi was monitored by diving at 5 to 6 sites in Miyagi Prefecture in Japan in summer 2010 and 2011. These sites were located at varying distances from farming sites at which the disease had previously been detected. All dead ascidians were collected, and their tunics were examined for Azumiobodo hoyamushi, the causative agent of soft tunic syndrome, using 18S rRNA PCR. In both years, <1% of wild ascidians we observed (18 out of 2100 in 2010, and 30 out of 3100 in 2011) were dead. The flagellates were only detected in 8 out of 18 dead ascidians from 3 sites in 2010, and 4 out of 30 from 2 sites in 2011. Healthy ascidians were successfully experimentally infected with the disease by immersing tunic samples from diseased wild ascidians into the rearing water. When apparently healthy ascidians collected from the wild population were reared for 40 d using pathogen-free water, the tunics of some ascidians became softened. The flagellates were detected in these individuals, which were diagnosed with soft tunic syndrome. Our results suggest that soft tunic syndrome affects the wild population of ascidians in Japan.

Ribosomal gene polymorphism in small genomes: analysis of different 16S rRNA sequences expressed in the honeybee parasite Nosema ceranae (Microsporidia)

To date, few organisms have been shown to possess variable ribosomal RNA, otherwise considered a classic example of uniformity by concerted evolution. The polymorphism for the 16S rRNA in Nosema ceranae analysed here is striking as Microsporidia are intracellular parasites which have suffered a strong reduction in their genomes and cellular organization. Moreover, N. ceranae infects the honeybee Apis mellifera, and has been associated with the colony-loss phenomenon during the last decade. The variants of 16S rRNA include single nucleotide substitutions, one base insertion-deletion, plus a tetranucleotide indel. We show that different gene variants are expressed. The polymorphic sites tend to be located in particular regions of the rRNA molecule, and the comparison to the Escherichia coli 16S rRNA secondary structure indicates that most variations probably do not preclude ribosomal activity. The fact that the polymorphisms in such a minimal organism as N. ceranae are maintained in samples collected worldwide suggest that the existence of differently expressed rRNA may play an adaptive role in the microsporidian.

A genome-wide analysis of annexins from parasitic organisms and their vectors

In this study, we conduct an in-depth analysis of annexin proteins from a diverse range of invertebrate taxa, including the major groups that contain the parasites and vector organisms that are harmful to humans and domestic animals. Using structure-based amino acid sequence alignments and phylogenetic analyses, we present a classification for this protein group and assign names to sequences with ambiguous annotations in public databases. Our analyses reveal six distinct annexin clades, and the mapping of genes encoding annexins to the genome of the human blood fluke Schistosoma mansoni supports the hypothesis of gene duplication as a major evolutionary event in annexin genesis. This study illuminates annexin diversity from a novel perspective using contemporary phylogenetic hypotheses of eukaryote evolution, and will aid the consolidation of annexin protein identities in public databases and provide a foundation for future functional analysis and characterisation of these proteins in parasites of socioeconomic importance.

Phylogenomic analyses support the bifurcation of ciliates into two major clades that differ in properties of nuclear division

Ciliates are a diverse assemblage of eukaryotes that have been the source of many discoveries including self-splicing RNAs, telomeres and trans-splicing. While analyses of ciliate morphology have given rise to robust hypotheses on relatively shallow level relationships, the deeper evolutionary history of ciliates is largely unknown. This is in part because studies to date have focused on only a single locus, small subunit ribosomal DNA (SSU-rDNA). In the present study, we use a taxon-rich strategy based on multiple loci from GenBank and recently completed transcriptomes to assess deep phylogenetic relationships among ciliates. Our phylogenomic data set includes up to 537 taxa, all of which have been sampled for SSU-rDNA and a subset of which have LSU-rDNA and up to 7 protein-coding sequences. Analyses of these data support the bifurcation of ciliates as suggested by SSU-rDNA, with one major clade defined by having somatic macronuclei that divide with intranuclear microtubules (Intramacronucleata) and the other clade containing lineages that either divide their macronuclei with microtubules external to the macronucleus or are unable to divide their macronuclei (Postciliodesmatophora). These multigene phylogenies provide a robust framework for interpreting the evolution of innovations across the ciliate tree of life.

The importance of the 45 S ribosomal small subunit-related complex for mitochondrial translation in Trypanosoma brucei

The mitochondrial 45 S SSU* complex in Trypanosoma brucei contains the 9 S SSU ribosomal RNA, a set of SSU ribosomal proteins, several pentatricopeptide repeat (PPR) proteins, and proteins not typically found in ribosomes, including rhodanese domain protein (Rhod) and a 200-kDa coiled-coil protein. To investigate the function of this complex, PPR29, Rhod, 200-kDa protein, and mitochondrial ribosomal protein S17 were knocked down by RNAi in procyclic T. brucei. A growth retardation phenotype, a reduction in the amount of the 45 S SSU* complexes, and the preferential inhibition of synthesis of the cytochrome c oxidase subunit I over apocytochrome b were observed as early as day 2 postinduction of RNAi. On the contrary, the down-regulation of mitochondrial ribosomal protein L3 drastically reduced the amount of the large subunit and indiscriminately inhibited mitochondrial translation. The relative amounts of translation-competent, long poly(AU)-tailed cytochrome c oxidase subunit I and edited apocytochrome b mRNAs were selectively reduced by ablation of the 45 S SSU* complex. The formation of the 80 S translation complexes, identified by association of the long-tailed mRNAs with the mitoribosomes, was also disrupted. On the other hand, the relative amount of long-tailed edited RPS12 mRNA was not substantially affected, and there was no noticeable effect on the RPS12 translation complexes. In bloodstream trypanosomes, the amount of the 45 S complexes was drastically reduced compared with procyclics. We propose that the 45 S SSU* complex represents a factor required for normal mitochondrial translation that may have selective effects on different mRNAs.

Phylogenomics demonstrates that breviate flagellates are related to opisthokonts and apusomonads

Most eukaryotic lineages belong to one of a few major groups. However, several protistan lineages have not yet been robustly placed in any of these groups. Both the breviates and apusomonads are two such lineages that appear to be related to the Amoebozoa and Opisthokonta (i.e. the 'unikonts' or Amorphea); however, their precise phylogenetic positions remain unclear. Here, we describe a novel microaerophilic breviate, Pygsuia biforma gen. nov. sp. nov., isolated from a hypoxic estuarine sediment. Ultrastructurally, this species resembles the breviate genera Breviata and Subulatomonas but has two cell morphologies, adherent and swimming. Phylogenetic analyses of the small sub-unit rRNA gene show that Pygsuia is the sister to the other breviates. We constructed a 159-protein supermatrix, including orthologues identified in RNA-seq data from Pygsuia. Phylogenomic analyses of this dataset show that breviates, apusomonads and Opisthokonta form a strongly supported major eukaryotic grouping we name the Obazoa. Although some phylogenetic methods disagree, the balance of evidence suggests that the breviate lineage forms the deepest branch within Obazoa. We also found transcripts encoding a nearly complete integrin adhesome from Pygsuia, indicating that this protein complex involved in metazoan multicellularity may have evolved earlier in eukaryote evolution than previously thought.

'Candidatus Megaira polyxenophila' gen. nov., sp. nov.: considerations on evolutionary history, host range and shift of early divergent rickettsiae

"Neglected Rickettsiaceae" (i.e. those harboured by non-hematophagous eukaryotic hosts) display greater phylogenetic variability and more widespread dispersal than pathogenic ones; yet, the knowledge about their actual host range and host shift mechanism is scarce. The present work reports the characterization following the full-cycle rRNA approach (SSU rRNA sequence, specific in situ hybridization, and ultrastructure) of a novel rickettsial bacterium, herewith proposed as 'Candidatus Megaira polyxenophila' gen. nov., sp. nov. We found it in association with four different free-living ciliates (Diophrys oligothrix, Euplotes octocarinatus, Paramecium caudatum, and Spirostomum sp., all belonging to Alveolata, Ciliophora); furthermore it was recently observed as intracellular occurring in Carteria cerasiformis and Pleodorina japonica (Chlorophyceae, Chlorophyta). Phylogenetic analyses demonstrated the belonging of the candidate new genus to the family Rickettsiaceae (Alphaproteobacteria, Rickettsiales) as a sister group of the genus Rickettsia. In situ observations revealed the ability of the candidate new species to colonize either nuclear or cytoplasmic compartments, depending on the host organism. The presence of the same bacterial species within different, evolutionary distant, hosts indicates that 'Candidatus Megaira polyxenophila' recently underwent several distinct host shifts, thus suggesting the existence of horizontal transmission pathways. We consider these findings as indicative of an unexpected spread of rickettsial infections in aquatic communities, possibly by means of trophic interactions, and hence propose a new interpretation of the origin and phylogenetic diversification of rickettsial bacteria.

Evolution and distribution of saxitoxin biosynthesis in dinoflagellates

Numerous species of marine dinoflagellates synthesize the potent environmental neurotoxic alkaloid, saxitoxin, the agent of the human illness, paralytic shellfish poisoning. In addition, certain freshwater species of cyanobacteria also synthesize the same toxic compound, with the biosynthetic pathway and genes responsible being recently reported. Three theories have been postulated to explain the origin of saxitoxin in dinoflagellates: The production of saxitoxin by co-cultured bacteria rather than the dinoflagellates themselves, convergent evolution within both dinoflagellates and bacteria and horizontal gene transfer between dinoflagellates and bacteria. The discovery of cyanobacterial saxitoxin homologs in dinoflagellates has enabled us for the first time to evaluate these theories. Here, we review the distribution of saxitoxin within the dinoflagellates and our knowledge of its genetic basis to determine the likely evolutionary origins of this potent neurotoxin.

A second rhodopsin-like protein in Cyanophora paradoxa: gene sequence and protein expression in a cell-free system

Here we report the identification and expression of a second rhodopsin-like protein in the alga Cyanophora paradoxa (Glaucophyta), named Cyanophopsin_2. This new protein was identified due to a serendipity event, since the RACE reaction performed to complete the sequence of Cyanophopsin_1, (the first rhodopsin-like protein of C. paradoxa identified in 2009 by our group), amplified a 619 bp sequence corresponding to a portion of a new gene of the same protein family. The full sequence consists of 1175 bp consisting of 849 bp coding DNA sequence and 4 introns of 326 bp. The protein is characterized by an N-terminal region of 47 amino acids, followed by a region with 7 α-helices of 213 amino acids and a C-terminal region of 22 amino acids. This protein showed high identity with Cyanophopsin_1 and other rhodopsin-like proteins of Archea, Bacteria, Fungi and Algae. Cyanophosin_2 (CpR2) was expressed in a cell-free expression system, and characterized by means of absorption spectroscopy.