Plantae (sensu Cavalier-Smith 1981)  plastids evolved via primary endosymbiosis whereby a heterotrophic protist enslaved a photosynthetic cyanobacterium. This 'primary' plastid spread into other eukaryotes via secondary endosymbiosis. An important but contentious theory in algal evolution is the chromalveolate hypothesis that posits chromists (cryptophytes, haptophytes, and stramenopiles) and alveolates (ciliates, apicomplexans, and dinoflagellates) share a common ancestor that contained a red algal derived 'secondary' plastid . Under this view, the existence of several later-diverging plastid-lacking chromalveolates such as ciliates and oomycetes would be explained by plastid loss in these lineages. To test the idea of a photosynthetic ancestry for ciliates we used the 27,446 predicted proteins from the macronuclear genome of Tetrahymena thermophila to query prokaryotic and eukaryotic genomes. We identified 16 proteins of possible algal origin in the ciliates Tetrahymena and Paramecium tetraurelia. Fourteen of these are present in other chromalveolates. Here we compare and contrast the likely scenarios for algal gene origin in ciliates either via multiple rounds of horizontal gene transfer (HGT) from algal prey or symbionts, or through endosymbiotic gene transfer (EGT) during a putative photosynthetic phase in their evolution.
Diatoms and other chlorophyll-c containing, or chromalveolate, algae are among the most productive and diverse phytoplankton in the ocean. Evolutionarily, chlorophyll-c algae are linked through common, although not necessarily monophyletic, acquisition of plastid endosymbionts of red as well as most likely green algal origin. There is also strong evidence for a relatively high level of lineage-specific bacterial gene acquisition within chromalveolates. Therefore, analyses of gene content and derivation in chromalveolate taxa have indicated particularly diverse origins of their overall gene repertoire. As a single group of functionally related enzymes spanning two distinct gene families, fructose 1,6-bisphosphate aldolases (FBAs) illustrate the influence on core biochemical pathways of specific evolutionary associations among diatoms and other chromalveolates with various plastid-bearing and bacterial endosymbionts. Protein localization and activity, gene expression, and phylogenetic analyses indicate that the pennate diatom Phaeodactylum tricornutum contains five FBA genes with very little overall functional overlap. Three P. tricornutum FBAs, one class I and two class II, are plastid localized, and each appears to have a distinct evolutionary origin as well as function. Class I plastid FBA appears to have been acquired by chromalveolates from a red algal endosymbiont, whereas one copy of class II plastid FBA is likely to have originated from an ancient green algal endosymbiont. The other copy appears to be the result of a chromalveolate-specific gene duplication. Plastid FBA I and chromalveolate-specific class II plastid FBA are localized in the pyrenoid region of the chloroplast where they are associated with β-carbonic anhydrase, which is known to play a significant role in regulation of the diatom carbon concentrating mechanism. The two pyrenoid-associated FBAs are distinguished by contrasting gene expression profiles under nutrient limiting compared with optimal CO2 fixation conditions, suggestive of a distinct specialized function for each. Cytosolically localized FBAs in P. tricornutum likely play a role in glycolysis and cytoskeleton function and seem to have originated from the stramenopile host cell and from diatom-specific bacterial gene transfer, respectively.
carbon concentrating mechanism (CCM); carbon metabolism; carbonic anhydrase; diatom; fructose bisphosphate aldolase; pyrenoid
The evolution of microbial eukaryotes, in particular of photosynthetic lineages, is complicated by multiple instances of endosymbiotic and horizontal gene transfer (E/HGT) resulting from plastid origin(s). Our recent analysis of diatom membrane transporters provides evidence of red and/or green algal origins of 172 of the genes encoding these proteins (ca. 25% of the examined phylogenies), with the majority putatively derived from green algae. These data suggest that E/HGT has been an important driver of evolutionary innovation among diatoms (and likely other stramenopiles), and lend further support to the hypothesis of an ancient, cryptic green algal endosymbiosis in “chromalveolate” lineages. Here, we discuss the implications of our findings on the understanding of eukaryote evolution and inference of the tree of life.
horizontal gene transfer; endosymbiotic gene transfer; diatoms; membrane transporters; eukaryote evolution; tree of life
The endosymbiotic birth of organelles is accompanied by massive transfer of endosymbiont genes to the eukaryotic host nucleus. In the centric diatom Thalassiosira pseudonana the Psb28 protein is encoded in the plastid genome while a second version is nuclear-encoded and possesses a bipartite N-terminal presequence necessary to target the protein into the diatom complex plastid. Thus it can represent a gene captured during endosymbiotic gene transfer.
To specify the origin of nuclear- and plastid-encoded Psb28 in T. pseudonana we have performed extensive phylogenetic analyses of both mentioned genes. We have also experimentally tested the intracellular location of the nuclear-encoded Psb28 protein (nuPsb28) through transformation of the diatom Phaeodactylum tricornutum with the gene in question fused to EYFP.
We show here that both versions of the psb28 gene in T. pseudonana are transcribed. We also provide experimental evidence for successful targeting of the nuPsb28 fused with EYFP to the diatom complex plastid. Extensive phylogenetic analyses demonstrate that nucleotide composition of the analyzed genes deeply influences the tree topology and that appropriate methods designed to deal with a compositional bias of the sequences and the long branch attraction artefact (LBA) need to be used to overcome this obstacle. We propose that nuclear psb28 in T. pseudonana is a duplicate of a plastid localized version, and that it has been transferred from its endosymbiont.
Photosynthetic eukaryotes with a secondary plastid of red algal origin (cryptophytes, haptophytes, stramenopiles, dinoflagellates, and apicomplexans) are hypothesized to share a single origin of plastid acquisition according to Chromalveolate hypothesis. Recent phylogenomic analyses suggest that photosynthetic “chromalveolates” form a large clade with inclusion of several non-photosynthetic protist lineages. Katablepharids are one such non-photosynthetic lineage closely related to cryptophytes. Despite their evolutionary and ecological importance, katablepharids are poorly investigated.
Here, we report a newly discovered flagellate, Roombia truncata gen. et sp. nov., that is related to katablepharids, but is morphologically distinct from othermembers of the group in the following ways: (1) two flagella emerge from a papilla-like subapical protrusion, (2) conspicuous ejectisomes are aligned in multiple (5–11) rows, (3) each ejectisome increases in size towards the posterior end of the rows, and (4) upon feeding, a part of cytoplasm elastically stretch to engulf whole prey cell. Molecular phylogenies inferred from Hsp90, SSU rDNA, and LSU rDNA sequences consistently and strongly show R. truncata as the sister lineage to all other katablepharids, including lineages known only from environmental sequence surveys. A close association between katablepharids and cryptophytes was also recovered in most analyses. Katablepharids and cryptophytes are together part of a larger, more inclusive, group that also contains haptophytes, telonemids, centrohelids and perhaps biliphytes. The monophyly of this group is supported by several different molecular phylogenetic datasets and one shared lateral gene transfer; therefore, we formally establish this diverse clade as the “Hacrobia.”
Our discovery of R. truncata not only expands our knowledge in the less studied flagellate group, but provide a better understanding of phylogenetic relationship and evolutionary view of plastid acquisition/losses of Hacrobia. Being an ancestral to all katablepharids, and readily cultivable, R. truncata is a good candidate for multiple gene analyses that will contribute to future phylogenetic studies of Hacrobia.
In photosynthetic eukaryotes, many genes were transferred from plastids or algal endosymbionts to nuclear genomes of host cells. These transferred genes are often considered genetic footprints of plastids. However, genes of algal origin have also been detected in some plastid-lacking eukaryotes, and these genes are often cited as evidence of historical plastids. In this paper, we discuss two recent publications about algal genes in plastid-lacking eukaryotes. Both studies highlight the point that algal genes are not exclusively derived from historical plastids. Instead, the findings show that gene acquisition through feeding activities is a plausible explanation.
endosymbiosis; gene transfer; phagotroph; photosynthetic eukaryotes
Plastid replacements through secondary endosymbioses include massive transfer of genes from the endosymbiont to the host nucleus and require a new targeting system to enable transport of the plastid-targeted proteins across 3-4 plastid membranes. The dinoflagellates are the only eukaryotic lineage that has been shown to have undergone several plastid replacement events, and this group is thus highly relevant for studying the processes involved in plastid evolution. In this study, we analyzed the phylogenetic origin and N-terminal extensions of plastid-targeted proteins from Lepidodinium chlorophorum, a member of the only dinoflagellate genus that harbors a green secondary plastid rather than the red algal-derived, peridinin-containing plastid usually found in photosynthetic dinoflagellates.
We sequenced 4,746 randomly picked clones from a L. chlorophorum cDNA library. 22 of the assembled genes were identified as genes encoding proteins functioning in plastids. Some of these were of green algal origin. This confirms that genes have been transferred from the plastid to the host nucleus of L. chlorophorum and indicates that the plastid is fully integrated as an organelle in the host. Other nuclear-encoded plastid-targeted protein genes, however, are clearly not of green algal origin, but have been derived from a number of different algal groups, including dinoflagellates, streptophytes, heterokonts, and red algae. The characteristics of N-terminal plastid-targeting peptides of all of these genes are substantially different from those found in peridinin-containing dinoflagellates and green algae.
L. chlorophorum expresses plastid-targeted proteins with a range of different origins, which probably arose through endosymbiotic gene transfer (EGT) and horizontal gene transfer (HGT). The N-terminal extension of the genes is different from the extensions found in green alga and other dinoflagellates (peridinin- and haptophyte plastids). These modifications have likely enabled the mosaic proteome of L. chlorophorum.
Plastids (photosynthetic organelles of plants and algae) are known to have spread between eukaryotic lineages by secondary endosymbiosis, that is, by the uptake of a eukaryotic alga by another eukaryote. But the number of times this has taken place is controversial. This is particularly so in the case of eukaryotes with plastids derived from red algae, which are numerous and diverse. Despite their diversity, it has been suggested that all these eukaryotes share a recent common ancestor and that their plastids originated in a single endosymbiosis, the so-called “chromalveolate hypothesis.” Here we describe a novel molecular character that supports the chromalveolate hypothesis. Fructose-1,6-bisphosphate aldolase (FBA) is a glycolytic and Calvin cycle enzyme that exists as two nonhomologous types, class I and class II. Red algal plastid-targeted FBA is a class I enzyme related to homologues from plants and green algae, and it would be predicted that the plastid-targeted FBA from algae with red algal secondary endosymbionts should be related to this class I enzyme. However, we show that plastid-targeted FBA of heterokonts, cryptomonads, haptophytes, and dinoflagellates (all photosynthetic chromalveolates) are class II plastid-targeted enzymes, completely unlike those of red algal plastids. The chromalveolate enzymes form a strongly supported group in FBA phylogeny, and their common possession of this unexpected plastid characteristic provides new evidence for their close relationship and a common origin for their plastids.
Diatoms are unicellular algae responsible for approximately 20% of global carbon fixation. Their evolution by secondary endocytobiosis resulted in a complex cellular structure and metabolism compared to algae with primary plastids.
The whole genome sequence of the diatom Phaeodactylum tricornutum has recently been completed. We identified and annotated genes for enzymes involved in carbohydrate pathways based on extensive EST support and comparison to the whole genome sequence of a second diatom, Thalassiosira pseudonana. Protein localization to mitochondria was predicted based on identified similarities to mitochondrial localization motifs in other eukaryotes, whereas protein localization to plastids was based on the presence of signal peptide motifs in combination with plastid localization motifs previously shown to be required in diatoms. We identified genes potentially involved in a C4-like photosynthesis in P. tricornutum and, on the basis of sequence-based putative localization of relevant proteins, discuss possible differences in carbon concentrating mechanisms and CO2 fixation between the two diatoms. We also identified genes encoding enzymes involved in photorespiration with one interesting exception: glycerate kinase was not found in either P. tricornutum or T. pseudonana. Various Calvin cycle enzymes were found in up to five different isoforms, distributed between plastids, mitochondria and the cytosol. Diatoms store energy either as lipids or as chrysolaminaran (a β-1,3-glucan) outside of the plastids. We identified various β-glucanases and large membrane-bound glucan synthases. Interestingly most of the glucanases appear to contain C-terminal anchor domains that may attach the enzymes to membranes.
Here we present a detailed synthesis of carbohydrate metabolism in diatoms based on the genome sequences of Thalassiosira pseudonana and Phaeodactylum tricornutum. This model provides novel insights into acquisition of dissolved inorganic carbon and primary metabolic pathways of carbon in two different diatoms, which is of significance for an improved understanding of global carbon cycles.
The chromalveolate “supergroup” is of key interest in contemporary phycology, as it contains the overwhelming majority of extant algal species, including several phyla of key importance to oceanic net primary productivity such as diatoms, kelps, and dinoflagellates. There is also intense current interest in the exploitation of these algae for industrial purposes, such as biodiesel production. However, the evolution of the constituent species, and in particular the origin and radiation of the chloroplast genomes, remains poorly understood. In this review, we discuss current theories of the origins of the extant red alga-derived chloroplast lineages in the chromalveolates and the potential ramifications of the recent discovery of large numbers of green algal genes in chromalveolate genomes. We consider that the best explanation for this is that chromalveolates historically possessed a cryptic green algal endosymbiont that was subsequently replaced by a red algal chloroplast. We consider how changing selective pressures acting on ancient chromalveolate lineages may have selectively favored the serial endosymbioses of green and red algae and whether a complex endosymbiotic history facilitated the rise of chromalveolates to their current position of ecological prominence.
In organisms with complex plastids acquired by secondary endosymbiosis from a photosynthetic eukaryote, the majority of plastid proteins are nuclear-encoded, translated on cytoplasmic ribosomes, and guided across four membranes by a bipartite targeting sequence. In-depth understanding of this vital import process has been impeded by a lack of information about the transit peptide part of this sequence, which mediates transport across the inner three membranes. We determined the mature N-termini of hundreds of proteins from the model diatom Thalassiosira pseudonana, revealing extensive N-terminal modification by acetylation and proteolytic processing in both cytosol and plastid. We identified 63 mature N-termini of nucleus-encoded plastid proteins, deduced their complete transit peptide sequences, determined a consensus motif for their cleavage by the stromal processing peptidase, and found evidence for subsequent processing by a plastid methionine aminopeptidase. The cleavage motif differs from that of higher plants, but is shared with other eukaryotes with complex plastids.
Apicomplexa are protist parasites that include Plasmodium spp., the causative agents of malaria, and Toxoplasma gondii, responsible for toxoplasmosis. Most Apicomplexa possess a relict plastid, the apicoplast, which was acquired by secondary endosymbiosis of a red alga. Despite being nonphotosynthetic, the apicoplast is otherwise metabolically similar to algal and plant plastids and is essential for parasite survival. Previous studies of Toxoplasma gondii identified membrane lipids with some structural features of plastid galactolipids, the major plastid lipid class. However, direct evidence for the plant-like enzymes responsible for galactolipid synthesis in Apicomplexan parasites has not been obtained. Chromera velia is an Apicomplexan relative recently discovered in Australian corals. C. velia retains a photosynthetic plastid, providing a unique model to study the evolution of the apicoplast. Here, we report the unambiguous presence of plant-like monogalactosyldiacylglycerol and digalactosyldiacylglycerol in C. velia and localize digalactosyldiacylglycerol to the plastid. We also provide evidence for a plant-like biosynthesis pathway and identify candidate galactosyltranferases responsible for galactolipid synthesis. Our study provides new insights in the evolution of these important enzymes in plastid-containing eukaryotes and will help reconstruct the evolution of glycerolipid metabolism in important parasites such as Plasmodium and Toxoplasma.
Chloroplast; Evolution; Glycerolipid; Mass Spectrometry (MS); Membrane Lipids; Apicomplexa; Chromera velia; Apicoplast; Galactolipid/Galactosyltransferase; Plastid Evolution
The G protein-coupled receptor (GPCR) signaling pathway plays an essential role in signal transmission and response to external stimuli in mammalian cells. Protein components of this pathway have been characterized in plants and simpler eukaryotes such as yeast, but their presence and role in unicellular photosynthetic eukaryotes have not been determined. We use a comparative genomics approach using whole genome sequences and gene expression libraries of four diatoms (Pseudo-nitzschia multiseries, Thalassiosira pseudonana, Phaeodactylum tricornutum and Fragilariopsis cylindrus) to search for evidence of GPCR signaling pathway proteins that share sequence conservation to known GPCR pathway proteins.
The majority of the core components of GPCR signaling were well conserved in all four diatoms, with protein sequence similarity to GPCRs, human G protein α- and β-subunits and downstream effectors. There was evidence for the Gγ-subunit and thus a full heterotrimeric G protein only in T. pseudonana. Phylogenetic analysis of putative diatom GPCRs indicated similarity but deep divergence to the class C GPCRs, with branches basal to the GABAB receptor subfamily. The extracellular and intracellular regions of these putative diatom GPCR sequences exhibited large variation in sequence length, and seven of these sequences contained the necessary ligand binding domain for class C GPCR activation. Transcriptional data indicated that a number of the putative GPCR sequences are expressed in diatoms under various stress conditions in culture, and that many of the GPCR-activated signaling proteins, including the G protein, are also expressed.
The presence of sequences in all four diatoms that code for the proteins required for a functional mammalian GPCR pathway highlights the highly conserved nature of this pathway and suggests a complex signaling machinery related to environmental perception and response in these unicellular organisms. The lack of evidence for some GPCR pathway proteins in one or more of the diatoms, such as the Gγ-subunit, may be due to differences in genome completeness and genome coverage for the four diatoms. The high divergence of putative diatom GPCR sequences to known class C GPCRs suggests these sequences may represent another, potentially ancestral, subfamily of class C GPCRs.
Cell signaling; Diatom; Environment; G protein-coupled receptor; Human health; Ocean
The establishment of an endosymbiotic relationship typically seems to be driven through complementation of the host's limited metabolic capabilities by the biochemical versatility of the endosymbiont. The most significant examples of endosymbiosis are represented by the endosymbiotic acquisition of plastids and mitochondria, introducing photosynthesis and respiration to eukaryotes. However, there are numerous other endosymbioses that evolved more recently and repeatedly across the tree of life. Recent advances in genome sequencing technology have led to a better understanding of the physiological basis of many endosymbiotic associations. This review focuses on endosymbionts in protists (unicellular eukaryotes). Selected examples illustrate the incorporation of various new biochemical functions, such as photosynthesis, nitrogen fixation and recycling, and methanogenesis, into protist hosts by prokaryotic endosymbionts. Furthermore, photosynthetic eukaryotic endosymbionts display a great diversity of modes of integration into different protist hosts.
In conclusion, endosymbiosis seems to represent a general evolutionary strategy of protists to acquire novel biochemical functions and is thus an important source of genetic innovation.
metabolic complementation; prokaryotic endosymbionts; eukaryotic endosymbionts; evolution; integration
Eukaryotic genes with cyanobacterial ancestry in plastid-lacking protists have been regarded as important evolutionary markers implicating the presence of plastids in the early evolution of eukaryotes. Although recent genomic surveys demonstrated the presence of cyanobacterial and algal ancestry genes in the genomes of plastid-lacking protists, comparative analyses on the origin and distribution of those genes are still limited.
We identified 12 gene families with cyanobacterial ancestry in the genomes of a taxonomically wide range of plastid-lacking eukaryotes (Phytophthora [Chromalveolata], Naegleria [Excavata], Dictyostelium [Amoebozoa], Saccharomyces and Monosiga [Opisthokonta]) using a novel phylogenetic pipeline. The eukaryotic gene clades with cyanobacterial ancestry were mostly composed of genes from bikonts (Archaeplastida, Chromalveolata, Rhizaria and Excavata). We failed to find genes with cyanobacterial ancestry in Saccharomyces and Dictyostelium, except for a photorespiratory enzyme conserved among fungi. Meanwhile, we found several Monosiga genes with cyanobacterial ancestry, which were unrelated to other Opisthokonta genes.
Our data demonstrate that a considerable number of genes with cyanobacterial ancestry have contributed to the genome composition of the plastid-lacking protists, especially bikonts. The origins of those genes might be due to lateral gene transfer events, or an ancient primary or secondary endosymbiosis before the diversification of bikonts. Our data also show that all genes identified in this study constitute multi-gene families with punctate distribution among eukaryotes, suggesting that the transferred genes could have survived through rounds of gene family expansion and differential reduction.
Nucleomorphs are the remnant nuclei of algal endosymbionts that were engulfed by nonphotosynthetic host eukaryotes. These peculiar organelles are found in cryptomonad and chlorarachniophyte algae, where they evolved from red and green algal endosymbionts, respectively. Despite their independent origins, cryptomonad and chlorarachniophyte nucleomorph genomes are similar in size and structure: they are both <1 million base pairs in size (the smallest nuclear genomes known), comprised three chromosomes, and possess subtelomeric ribosomal DNA operons. Here, we report the complete sequence of one of the smallest cryptomonad nucleomorph genomes known, that of the secondarily nonphotosynthetic cryptomonad Cryptomonas paramecium. The genome is 486 kbp in size and contains 518 predicted genes, 466 of which are protein coding. Although C. paramecium lacks photosynthetic ability, its nucleomorph genome still encodes 18 plastid-associated proteins. More than 90% of the “conserved” protein genes in C. paramecium (i.e., those with clear homologs in other eukaryotes) are also present in the nucleomorph genomes of the cryptomonads Guillardia theta and Hemiselmis andersenii. In contrast, 143 of 466 predicted C. paramecium proteins (30.7%) showed no obvious similarity to proteins encoded in any other genome, including G. theta and H. andersenii. Significantly, however, many of these “nucleomorph ORFans” are conserved in position and size between the three genomes, suggesting that they are in fact homologous to one another. Finally, our analyses reveal an unexpected degree of overlap in the genes present in the independently evolved chlorarachniophyte and cryptomonad nucleomorph genomes: ∼80% of a set of 120 conserved nucleomorph genes in the chlorarachniophyte Bigelowiella natans were also present in all three cryptomonad nucleomorph genomes. This result suggests that similar reductive processes have taken place in unrelated lineages of nucleomorph-containing algae.
nucleomorph; cryptomonads; chlorarachniophytes; genome reduction; endosymbiosis
Membrane transporters (MTs) facilitate the movement of molecules between cellular compartments. The evolutionary history of these key components of eukaryote genomes remains unclear. Many photosynthetic microbial eukaryotes (e.g., diatoms, haptophytes, and dinoflagellates) appear to have undergone serial endosymbiosis and thereby recruited foreign genes through endosymbiotic/horizontal gene transfer (E/HGT). Here we used the diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum as models to examine the evolutionary origin of MTs in this important group of marine primary producers. Using phylogenomics, we used 1,014 diatom MTs as query against a broadly sampled protein sequence database that includes novel genome data from the mesophilic red algae Porphyridium cruentum and Calliarthron tuberculosum, and the stramenopile Ectocarpus siliculosus. Our conservative approach resulted in 879 maximum likelihood trees of which 399 genes show a non-lineal history between diatoms and other eukaryotes and prokaryotes (at the bootstrap value ≥70%). Of the eukaryote-derived MTs, 172 (ca. 25% of 697 examined phylogenies) have members of both red/green algae as sister groups, with 103 putatively arising from green algae, 19 from red algae, and 50 have an unresolved affiliation to red and/or green algae. We used topology tests to analyze the most convincing cases of non-lineal gene history in which red and/or green algae were nested within stramenopiles. This analysis showed that ca. 6% of all trees (our most conservative estimate) support an algal origin of MTs in stramenopiles with the majority derived from green algae. Our findings demonstrate the complex evolutionary history of photosynthetic eukaryotes and indicate a reticulate origin of MT genes in diatoms. We postulate that the algal-derived MTs acquired via E/HGT provided diatoms and other related microbial eukaryotes the ability to persist under conditions of fluctuating ocean chemistry, likely contributing to their great success in marine environments.
An important missing piece in the puzzle of how plastids spread across the eukaryotic tree of life is a robust evolutionary framework for the host lineages. Four assemblages are known to harbour plastids derived from red algae and, according to the controversial chromalveolate hypothesis, these all share a common ancestry. Phylogenomic analyses have consistently shown that stramenopiles and alveolates are closely related, but haptophytes and cryptophytes remain contentious; they have been proposed to branch together with several heterotrophic groups in the newly erected Hacrobia. Here, we tested this question by producing a large expressed sequence tag dataset for the katablepharid Roombia truncata, one of the last hacrobian lineages for which genome-level data are unavailable, and combined this dataset with the recently completed genome of the cryptophyte Guillardia theta to build an alignment composed of 258 genes. Our analyses strongly support haptophytes as sister to the SAR group, possibly together with telonemids and centrohelids. We also confirmed the common origin of katablepharids and cryptophytes, but these lineages were not related to other hacrobians; instead, they branch with plants. Our study resolves the evolutionary position of haptophytes, an ecologically critical component of the oceans, and proposes a new hypothesis for the origin of cryptophytes.
phylogenomics; plastid; haptophyte; cryptophyte; katablepharid; tree of life
Euglenophytes are a group of photosynthetic flagellates possessing a plastid derived from a green algal endosymbiont, which was incorporated into an ancestral host cell via secondary endosymbiosis. However, the impact of endosymbiosis on the euglenophyte nuclear genome is not fully understood due to its complex nature as a 'hybrid' of a non-photosynthetic host cell and a secondary endosymbiont.
We analyzed an EST dataset of the model euglenophyte Euglena gracilis using a gene mining program designed to detect laterally transferred genes. We found E. gracilis genes showing affinity not only with green algae, from which the secondary plastid in euglenophytes evolved, but also red algae and/or secondary algae containing red algal-derived plastids. Phylogenetic analyses of these 'red lineage' genes suggest that E. gracilis acquired at least 14 genes via eukaryote-to-eukaryote lateral gene transfer from algal sources other than the green algal endosymbiont that gave rise to its current plastid. We constructed an EST library of the aplastidic euglenid Peranema trichophorum, which is a eukaryovorous relative of euglenophytes, and also identified 'red lineage' genes in its genome.
Our data show genome mosaicism in E. gracilis and P. trichophorum. One possible explanation for the presence of these genes in these organisms is that some or all of them were independently acquired by lateral gene transfer and contributed to the successful integration and functioning of the green algal endosymbiont as a secondary plastid. Alternative hypotheses include the presence of a phagocytosed alga as the single source of those genes, or a cryptic tertiary endosymbiont harboring secondary plastid of red algal origin, which the eukaryovorous ancestor of euglenophytes had acquired prior to the secondary endosymbiosis of a green alga.
The photosynthetic and basal apicomplexan Chromera velia was recently described, expanding the membership of this otherwise nonphotosynthetic group of parasite protists. Apicomplexans are alveolates with secondary plastids of red algal origin, but the evolutionary history of their nuclear genes is still actively discussed. Using deep sequencing of expressed genes, we investigated the phylogenetic affinities of a stringent filtered set of 3,151 expressed sequence tag-contigs by generating clusters with eukaryotic homologs and constructing phylogenetic trees and networks. The phylogenetic positioning of this alveolate alga was determined and sets of phyla-specific proteins extracted. Phylogenetic trees provided conflicting signals, with 444 trees grouping C. velia with the apicomplexans but 354 trees grouping C. velia with the alveolate oyster pathogen Perkinsus marinus, the latter signal being reinforced from the analysis of shared genes and overall sequence similarity. Among the 513 C. velia nuclear genes that reflect a photosynthetic ancestry and for which nuclear homologs were available both from red and green lineages, 263 indicated a red photosynthetic ancestry, whereas 250 indicated a green photosynthetic ancestry. The same 1:1 signal ratio was found among the putative 255 nuclear-encoded plastid proteins identified. This finding of red and green signals for the alveolate mirrors the result observed in the heterokont lineage and supports a common but not necessarily single origin for the plastid in heterokonts and alveolates. The inference of green endosymbiosis preceding red plastid acquisition in these lineages leads to worryingly complicated evolutionary scenarios, prompting the search for other explanations for the green phylogenetic signal and the amount of hosts involved.
Chromera; Apicomplexa; Alveolata; chromalveolata; apicoplast; protist evolution
Genes controlling the cell cycle in two diatoms have been identified and functionally characterized, revealing environmental regulation of the cell cycle.
Despite the enormous importance of diatoms in aquatic ecosystems and their broad industrial potential, little is known about their life cycle control. Diatoms typically inhabit rapidly changing and unstable environments, suggesting that cell cycle regulation in diatoms must have evolved to adequately integrate various environmental signals. The recent genome sequencing of Thalassiosira pseudonana and Phaeodactylum tricornutum allows us to explore the molecular conservation of cell cycle regulation in diatoms.
By profile-based annotation of cell cycle genes, counterparts of conserved as well as new regulators were identified in T. pseudonana and P. tricornutum. In particular, the cyclin gene family was found to be expanded extensively compared to that of other eukaryotes and a novel type of cyclins was discovered, the diatom-specific cyclins. We established a synchronization method for P. tricornutum that enabled assignment of the different annotated genes to specific cell cycle phase transitions. The diatom-specific cyclins are predominantly expressed at the G1-to-S transition and some respond to phosphate availability, hinting at a role in connecting cell division to environmental stimuli.
The discovery of highly conserved and new cell cycle regulators suggests the evolution of unique control mechanisms for diatom cell division, probably contributing to their ability to adapt and survive under highly fluctuating environmental conditions.
Phylogenomic pipelines generate a large collection of phylogenetic trees that require manual inspection to answer questions about gene or genome evolution. A notable application of phylogenomics is to photosynthetic organelle (plastid) endosymbiosis. In the case of primary endosymbiosis, a heterotrophic protist engulfed a cyanobacterium, giving rise to the first photosynthetic eukaryote. Plastid establishment precipitated extensive gene transfer from the endosymbiont to the nuclear genome of the 'host'. Estimating the magnitude of this endosymbiotic gene transfer (EGT) and determining the functions of the prokaryotic genes remain controversial issues. We used phylogenomics to study EGT in the model green alga Chlamydomonas reinhardtii. To facilitate this procedure, we developed PhyloSort to rapidly search large collection of trees for monophyletic relationships. Here we present PhyloSort and its application to estimating EGT in Chlamydomonas.
PhyloSort is an open-source tool to sort phylogenetic trees by searching for user specified subtrees that contain a monophyletic group of interest defined by operational taxonomic units in a phylogenomic context. Using PhyloSort, we identified 897 Chlamydomonas genes of putative cyanobacterial origin, of which 531 had bootstrap support values ≥ 50% for the grouping of the algal and cyanobacterial homologs.
PhyloSort can be applied to quantify the number of genes that support different evolutionary hypotheses such as a taxonomic classification or endosymbiotic or horizontal gene transfer events. In our application, we demonstrate that cyanobacteria account for 3.5–6% of the protein-coding genes in the nuclear genome of Chlamydomonas.
Heterokont algae, together with cryptophytes, haptophytes and some alveolates, possess red-algal derived plastids. The chromalveolate hypothesis proposes that the red-algal derived plastids of all four groups have a monophyletic origin resulting from a single secondary endosymbiotic event. However, due to incongruence between nuclear and plastid phylogenies, this controversial hypothesis remains under debate. Large-scale genomic analyses have shown to be a powerful tool for phylogenetic reconstruction but insufficient sequence data have been available for red-algal derived plastid genomes.
The chloroplast genomes of two brown algae, Ectocarpus siliculosus and Fucus vesiculosus, have been fully sequenced. These species represent two distinct orders of the Phaeophyceae, which is a major group within the heterokont lineage. The sizes of the circular plastid genomes are 139,954 and 124,986 base pairs, respectively, the size difference being due principally to the presence of longer inverted repeat and intergenic regions in E. siliculosus. Gene contents of the two plastids are similar with 139-148 protein-coding genes, 28-31 tRNA genes, and 3 ribosomal RNA genes. The two genomes also exhibit very similar rearrangements compared to other sequenced plastid genomes. The tRNA-Leu gene of E. siliculosus lacks an intron, in contrast to the F. vesiculosus and other heterokont plastid homologues, suggesting its recent loss in the Ectocarpales. Most of the brown algal plastid genes are shared with other red-algal derived plastid genomes, but a few are absent from raphidophyte or diatom plastid genomes. One of these regions is most similar to an apicomplexan nuclear sequence. The phylogenetic relationship between heterokonts, cryptophytes and haptophytes (collectively referred to as chromists) plastids was investigated using several datasets of concatenated proteins from two cyanobacterial genomes and 18 plastid genomes, including most of the available red algal and chromist plastid genomes.
The phylogenetic studies using concatenated plastid proteins still do not resolve the question of the monophyly of all chromist plastids. However, these results support both the monophyly of heterokont plastids and that of cryptophyte and haptophyte plastids, in agreement with nuclear phylogenies.
The transition from endosymbiont to organelle in eukaryotic cells involves the transfer of significant numbers of genes to the host genomes, a process known as endosymbiotic gene transfer (EGT). In the case of plastid organelles, EGTs have been shown to leave a footprint in the nuclear genome that can be indicative of ancient photosynthetic activity in present-day plastid-lacking organisms, or even hint at the existence of cryptic plastids. Here, we evaluated the impact of EGT on eukaryote genomes by reanalyzing the recently published EST dataset for Chromera velia, an interesting test case of a photosynthetic alga closely related to apicomplexan parasites. Previously, 513 genes were reported to originate from red and green algae in a 1:1 ratio. In contrast, by manually inspecting newly generated trees indicating putative algal ancestry, we recovered only 51 genes congruent with EGT, of which 23 and 9 were of red and green algal origin, respectively, whereas 19 were ambiguous regarding the algal provenance. Our approach also uncovered 109 genes that branched within a monocot angiosperm clade, most likely representing a contamination. We emphasize the lack of congruence and the subjectivity resulting from independent phylogenomic screens for EGT, which appear to call for extreme caution when drawing conclusions for major evolutionary events.
Endosymbiotic gene transfer; plastid evolution; protist; algae; chromera
Phylogenetic analyses show the single origin of a plastid metabolite translocator family in the Plantae from a gene encoding an existing endomembrane-derived protein. Red algal secondary endosymbiosis has spread a translocator gene into the ancestor of the “chromalveolate” protists, where it has diversified into a novel clade of proteins.