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Thomas Cavalier-Smith

Publications and source records attributed to Thomas Cavalier-Smith.

At least 19 recordsLinked to original sources
PubMed2006

Global eukaryote phylogeny: Combined small- and large-subunit ribosomal DNA trees support monophyly of Rhizaria, Retaria and Excavata.

Resolution of the phylogenetic relationships among the major eukaryotic groups is one of the most important problems in evolutionary biology that is still only partially solved. This task was initially addressed using a single marker, the small-subunit ribosomal DNA (SSU rDNA), although in recent years it has been shown that it does not contain enough phylogenetic information to robustly resolve global eukaryotic phylogeny. This has prompted the use of multi-gene analyses, especially in the form of long concatenations of numerous conserved protein sequences. However, this approach is severely limited by the small number of taxa for which such a large number of protein sequences is available today. We have explored the alternative approach of using only two markers but a large taxonomic sampling, by analysing a combination of SSU and large-subunit (LSU) rDNA sequences. This strategy allows also the incorporation of sequences from non-cultivated protists, e.g., Radiozoa (=radiolaria minus Phaeodarea). We provide the first LSU rRNA sequences for Heliozoa, Apusozoa (both Apusomonadida and Ancyromonadida), Cercozoa and Radiozoa. Our Bayesian and maximum likelihood analyses for 91 eukaryotic combined SSU+LSU sequences yielded much stronger support than hitherto for the supergroup Rhizaria (Cercozoa plus Radiozoa plus Foraminifera) and several well-recognised groups and also for other problematic clades, such as the Retaria (Radiozoa plus Foraminifera) and, with more moderate support, the Excavata. Within opisthokonts, the combined tree strongly confirms that the filose amoebae Nuclearia are sisters to Fungi whereas other Choanozoa are sisters to animals. The position of some bikont taxa, notably Heliozoa and Apusozoa, remains unresolved. However, our combined trees suggest a more deeply diverging position for Ancyromonas, and perhaps also Apusomonas, than for other bikonts, suggesting that apusozoan zooflagellates may be central for understanding the early evolution of this huge eukaryotic group. Multiple protein sequences will be needed fully to resolve basal bikont phylogeny. Nonetheless, our results suggest that combined SSU+LSU rDNA phylogenies can help to resolve several ambiguous regions of the eukaryotic tree and identify key taxa for subsequent multi-gene analyses.

Animals↗
PubMed2006

Origin of mitochondria by intracellular enslavement of a photosynthetic purple bacterium.

Mitochondria originated by permanent enslavement of purple non-sulphur bacteria. These endosymbionts became organelles through the origin of complex protein-import machinery and insertion into their inner membranes of protein carriers for extracting energy for the host. A chicken-and-egg problem exists: selective advantages for evolving import machinery were absent until inner membrane carriers were present, but this very machinery is now required for carrier insertion. I argue here that this problem was probably circumvented by conversion of the symbiont protein-export machinery into protein-import machinery, in three phases. I suggest that the first carrier entered the periplasmic space via pre-existing beta-barrel proteins in the bacterial outer membrane that later became Tom40, and inserted into the inner membrane probably helped by a pre-existing inner membrane protein, thereby immediately providing the protoeukaryote host with photosynthesate. This would have created a powerful selective advantage for evolving more efficient carrier import by inserting Tom70 receptors. Massive gene transfer to the nucleus inevitably occurred by mutation pressure. Finally, pressure from harmful, non-selected gene transfer to the nucleus probably caused evolution of the presequence mechanism, and photosynthesis was lost.

Bacterial Proteins↗
PubMed2006

Rooting the tree of life by transition analyses.

BACKGROUND: Despite great advances in clarifying the family tree of life, it is still not agreed where its root is or what properties the most ancient cells possessed--the most difficult problems in phylogeny. Protein paralogue trees can theoretically place the root, but are contradictory because of tree-reconstruction artefacts or poor resolution; ribosome-related and DNA-handling enzymes suggested one between neomura (eukaryotes plus archaebacteria) and eubacteria, whereas metabolic enzymes often place it within eubacteria but in contradictory places. Palaeontology shows that eubacteria are much more ancient than eukaryotes, and, together with phylogenetic evidence that archaebacteria are sisters not ancestral to eukaryotes, implies that the root is not within the neomura. Transition analysis, involving comparative/developmental and selective arguments, can polarize major transitions and thereby systematically exclude the root from major clades possessing derived characters and thus locate it; previously the 20 shared neomuran characters were thus argued to be derived, but whether the root was within eubacteria or between them and archaebacteria remained controversial. RESULTS: I analyze 13 major transitions within eubacteria, showing how they can all be congruently polarized. I infer the first fully resolved prokaryote tree, with a basal stem comprising the new infrakingdom Glidobacteria (Chlorobacteria, Hadobacteria, Cyanobacteria), which is entirely non-flagellate and probably ancestrally had gliding motility, and two derived branches (Gracilicutes and Unibacteria/Eurybacteria) that diverged immediately following the origin of flagella. Proteasome evolution shows that the universal root is outside a clade comprising neomura and Actinomycetales (proteates), and thus lies within other eubacteria, contrary to a widespread assumption that it is between eubacteria and neomura. Cell wall and flagellar evolution independently locate the root outside Posibacteria (Actinobacteria and Endobacteria), and thus among negibacteria with two membranes. Posibacteria are derived from Eurybacteria and ancestral to neomura. RNA polymerase and other insertions strongly favour the monophyly of Gracilicutes (Proteobacteria, Planctobacteria, Sphingobacteria, Spirochaetes). Evolution of the negibacterial outer membrane places the root within Eobacteria (Hadobacteria and Chlorobacteria, both primitively without lipopolysaccharide): as all phyla possessing the outer membrane beta-barrel protein Omp85 are highly probably derived, the root lies between them and Chlorobacteria, the only negibacteria without Omp85, or possibly within Chlorobacteria. CONCLUSION: Chlorobacteria are probably the oldest and Archaebacteria the youngest bacteria, with Posibacteria of intermediate age, requiring radical reassessment of dominant views of bacterial evolution. The last ancestor of all life was a eubacterium with acyl-ester membrane lipids, large genome, murein peptidoglycan walls, and fully developed eubacterial molecular biology and cell division. It was a non-flagellate negibacterium with two membranes, probably a photosynthetic green non-sulphur bacterium with relatively primitive secretory machinery, not a heterotrophic posibacterium with one membrane.

Journal Article↗
PubMed2006

Cell evolution and Earth history: stasis and revolution.

This synthesis has three main parts. The first discusses the overall tree of life and nature of the last common ancestor (cenancestor). I emphasize key steps in cellular evolution important for ordering and timing the major evolutionary innovations in the history of the biosphere, explaining especially the origins of the eukaryote cell and of bacterial flagella and cell envelope novelties. Second, I map the tree onto the fossil record and discuss dates of key events and their biogeochemical impact. Finally, I present a broad synthesis, discussing evidence for a three-phase history of life. The first phase began perhaps ca 3.5 Gyr ago, when the origin of cells and anoxic photosynthesis generated the arguably most primitive prokaryote phylum, Chlorobacteria (= Chloroflexi), the first negibacteria with cells bounded by two acyl ester phospholipid membranes. After this 'chlorobacterial age' of benthic anaerobic evolution protected from UV radiation by mineral grains, two momentous quantum evolutionary episodes of cellular innovation and microbial radiation dramatically transformed the Earth's surface: the glycobacterial revolution initiated an oxygenic 'age of cyanobacteria' and, as the ozone layer grew, the rise of plankton; immensely later, probably as recently as ca 0.9 Gyr ago, the neomuran revolution ushered in the 'age of eukaryotes', Archaebacteria (arguably the youngest bacterial phylum), and morphological complexity. Diversification of glycobacteria ca 2.8 Gyr ago, predominantly inhabiting stratified benthic mats, I suggest caused serial depletion of 13C by ribulose 1,5-bis-phosphate caboxylase/oxygenase (Rubisco) to yield ultralight late Archaean organic carbon formerly attributed to methanogenesis plus methanotrophy. The late origin of archaebacterial methanogenesis ca 720 Myr ago perhaps triggered snowball Earth episodes by slight global warming increasing weathering and reducing CO2 levels, to yield runaway cooling; the origin of anaerobic methane oxidation ca 570 Myr ago reduced methane flux at source, stabilizing Phanerozoic climates. I argue that the major cellular innovations exhibit a pattern of quantum evolution followed by very rapid radiation and then substantial stasis, as described by Simpson. They yielded organisms that are a mosaic of extremely conservative and radically novel features, as characterized by De Beer's phrase 'mosaic evolution'. Evolution is not evenly paced and there are no real molecular clocks.

Archaea↗
PubMed2006

Molecular phylogeny of Cercomonadidae and kinetid patterns of Cercomonas and Eocercomonas gen. nov. (Cercomonadida, Cercozoa).

Cercomonads are among the most abundant and widespread zooflagellates in soil and freshwater. We cultured 22 strains and report their complete 18S rRNA sequences and light microscopic morphology. Phylogenetic analysis of 51 Cercomonas rRNA genes shows in each previously identified major clade (A, B) two very robust, highly divergent, multi-species subclades (A1, A2; B1, B2). We studied kinetid ultrastructure of five clade A representatives by serial sections. All have two closely associated left ventral posterior microtubular roots, an anterior dorsal root, a microtubule-nucleating left anterior root, and a cone of microtubules passing to the nucleus. Anterior centrioles (=basal bodies, kinetosomes) of A1 have cartwheels; the posterior centriole does not, suggesting it is older, and implying flagellar transformation similar to other bikonts. Strain C-80 (subclade A2) differs greatly, having a dorsal posterior microtubule band, but lacking the A1-specific fibrillar striated root, nuclear extension to the centrioles, centriolar diaphragm, extrusomes; both mature centrioles lack cartwheels. For clade A2 we establish Eocercomonas gen. n., with type Eocercomonas ramosa sp. n., and for clade B1 Paracercomonas gen. n. (type Paracercomonas marina sp. n.). We establish Paracercomonas ekelundi sp. n. for culture SCCAP C1 and propose a Cercomonas longicauda neotype and Cercomonas (=Neocercomonas) jutlandica comb. n. and Paracercomonas (=Cercomonas) metabolica comb. n.

Animals↗
PubMed2006

Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista).

Heterokonts are evolutionarily important as the most nutritionally diverse eukaryote supergroup and the most species-rich branch of the eukaryotic kingdom Chromista. Ancestrally photosynthetic/phagotrophic algae (mixotrophs), they include several ecologically important purely heterotrophic lineages, all grossly understudied phylogenetically and of uncertain relationships. We sequenced 18S rRNA genes from 14 phagotrophic non-photosynthetic heterokonts and a probable Ochromonas, performed phylogenetic analysis of 210-430 Heterokonta, and revised higher classification of Heterokonta and its three phyla: the predominantly photosynthetic Ochrophyta; the non-photosynthetic Pseudofungi; and Bigyra (now comprising subphyla Opalozoa, Bicoecia, Sagenista). The deepest heterokont divergence is apparently between Bigyra, as revised here, and Ochrophyta/Pseudofungi. We found a third universal heterokont signature sequence, and deduce three independent losses of ciliary hairs, several of 1-2 cilia, 10 of photosynthesis, but perhaps only two plastid losses. In Ochrophyta, heterotrophic Oikomonas is sister to the photosynthetic Chrysamoeba, whilst the abundant freshwater predator Spumella is biphyletic; neither clade is specifically related to Paraphysomonas, indicating four losses of photosynthesis by chrysomonads. Sister to Chrysomonadea (Chrysophyceae) is Picophagea cl. nov. (Picophagus, Chlamydomyxa). The diatom-parasite Pirsonia belongs in Pseudofungi. Heliozoan-like actinophryids (e.g. Actinosphaerium) are Opalozoa, not related to pedinellids within Hypogyristea cl. nov. of Ochrophyta as once thought. The zooflagellate class Bicoecea (perhaps the ancestral phenotype of Bigyra) is unexpectedly diverse and a major focus of our study. We describe four new biciliate bicoecean genera and five new species: Nerada mexicana, Labromonas fenchelii (=Pseudobodo tremulans sensu Fenchel), Boroka karpovii (=P. tremulans sensu Karpov), Anoeca atlantica and Cafeteria mylnikovii; several cultures were previously misidentified as Pseudobodo tremulans. Nerada and the uniciliate Paramonas are related to Siluania and Adriamonas; this clade (Pseudodendromonadales emend.) is probably sister to Bicosoeca. Genetically diverse Caecitellus is probably related to Anoeca, Symbiomonas and Cafeteria (collectively Anoecales emend.). Boroka is sister to Pseudodendromonadales/Bicoecales/Anoecales. Placidiales are probably divergent bicoeceans (the GenBank Placidia sequence is a basidiomycete/heterokont chimaera). Two GenBank 'opalinid' sequences are fungal; Pseudopirsonia is cercozoan; two previous GenBank 'Caecitellus' sequences are Adriamonas.

Cells, Cultured↗
PubMed2005

Myosin domain evolution and the primary divergence of eukaryotes.

Eukaryotic cells have two contrasting cytoskeletal and ciliary organizations. The simplest involves a single cilium-bearing centriole, nucleating a cone of individual microtubules (probably ancestral for unikonts: animals, fungi, Choanozoa and Amoebozoa). In contrast, bikonts (plants, chromists and all other protozoa) were ancestrally biciliate with a younger anterior cilium, converted every cell cycle into a dissimilar posterior cilium and multiple ciliary roots of microtubule bands. Here we show by comparative genomic analysis that this fundamental cellular dichotomy also involves different myosin molecular motors. We found 37 different protein domain combinations, often lineage-specific, and many previously unidentified. The sequence phylogeny and taxonomic distribution of myosin domain combinations identified five innovations that strongly support unikont monophyly and the primary bikont/unikont bifurcation. We conclude that the eukaryotic cenancestor (last common ancestor) had a cilium, mitochondria, pseudopodia, and myosins with three contrasting domain combinations and putative functions.

Animals↗
PubMed2005

Polyubiquitin insertions and the phylogeny of Cercozoa and Rhizaria.

A single or double amino acid insertion at the monomer-monomer junction of the universal eukaryotic protein polyubiquitin is unique to Cercozoa and Foraminifera, closely related 'core' phyla in the protozoan infrakingdom Rhizaria. We screened 11 other candidate rhizarians for this insertion: Radiozoa (polycystine and acantharean radiolaria), a 'microheliozoan', and Apusozoa; all lack it, supporting suggestions that Foraminifera are more closely related to Cercozoa than either is to other eukaryotes. The insertion's size was ascertained for 12 additional Cercozoa to help resolve their basal branching order. The earliest branching Cercozoa generally have a single amino acid insertion, like all Foraminifera, but a large derived clade consisting of all Monadofilosa except Metopion, Helk-esimastix, and Cercobodo agilis has two amino acids, suggesting one doubling event and no reversions to a single amino acid. Metromonas and Sainouron, cercozoans of uncertain position, have a double insertion, suggesting that they belong in Monadofilosa. An alternative interpretation, suggested by the higher positions for Metopion and Cercobodo on Bayesian trees compared with most distance trees, cannot be ruled out, i.e. that the second insertion took place earlier, in the ancestral filosan, and was followed by three independent reversions to a single amino acid in Chlorarachnea, Metopion and Cercobodo.

Amino Acid Sequence↗
PubMed2005

18S ribosomal RNA gene sequences of Cochliopodium (Himatismenida) and the phylogeny of Amoebozoa.

Cochliopodium is a very distinctive genus of discoid amoebae covered by a dorsal tectum of carbohydrate microscales. Its phylogenetic position is unclear, since although sharing many features with naked "gymnamoebae", the tectum sets it apart. We sequenced 18S ribosomal RNA genes from three Cochliopodium species (minus, spiniferum and Cochliopodium sp., a new species resembling C. minutum). Phylogenetic analysis shows Cochliopodium as robustly holophyletic and within Amoebozoa, in full accord with morphological data. Cochliopodium is always one of the basal branches within Amoebozoa but its precise position is unstable. In Bayesian analysis it is sister to holophyletic Glycostylida, but distance trees mostly place it between Dermamoeba and a possibly artifactual long-branch cluster including Thecamoeba. These positions are poorly supported and basal amoebozoan branching ill-resolved, making it unclear whether Discosea (Glycostylida, Himatismenida, Dermamoebida) is holophyletic; however, Thecamoeba seems not specifically related to Dermamoeba. We also sequenced the small-subunit rRNA gene of Vannella persistens, which constantly grouped with other Vannella species, and two Hartmannella strains. Our trees suggest that Vexilliferidae, Variosea and Hartmannella are polyphyletic, confirming the existence of two very distinct Hartmannella clades: that comprising H. cantabrigiensis and another divergent species is sister to Glaeseria, whilst Hartmannella vermiformis branches more deeply.

Animals↗
PubMed2005

Economy, speed and size matter: evolutionary forces driving nuclear genome miniaturization and expansion.

BACKGROUND: Nuclear genome size varies 300 000-fold, whereas transcriptome size varies merely 17-fold. In the largest genomes nearly all DNA is non-genic secondary DNA, mostly intergenic but also within introns. There is now compelling evidence that secondary DNA is functional, i.e. positively selected by organismal selection, not the purely neutral or 'selfish' outcome of mutation pressure. The skeletal DNA theory argued that nuclear volumes are genetically determined primarily by nuclear DNA amounts, modulated somewhat by genes affecting the degree of DNA packing or unfolding; the huge spread of nuclear genome sizes is the necessary consequence of the origin of the nuclear envelope and the nucleation of its assembly by DNA, plus the adaptively significant 300 000-fold range of cell volumes and selection for balanced growth by optimizing karyoplasmic volume ratios (essentially invariant with cell volume in growing/multiplying cells). This simple explanation of the C-value paradox is refined here in the light of new insights into the nature of heterochromatin and the nuclear lamina, the genetic control of cell volume, and large-scale eukaryote phylogeny, placing special emphasis on protist test cases of the basic principles of nuclear genome size evolution. GENOME MINIATURIZATION: and Expansion Intracellular parasites (e.g. Plasmodium, microsporidia) dwarfed their genomes by gene loss and eliminating virtually all secondary DNA. The primary driving forces for genome reduction are metabolic and spatial economy and cell multiplication speed. Most extreme nuclear shrinkage yielded genomes as tiny as 0.38 Mb (making the nuclear genome size range effectively 1.8 million-fold!) in some minute enslaved nuclei (nucleomorphs) of cryptomonads and chlorarachneans, chimaeric cells that also retain a separate normal large nucleus. The latter shows typical correlation between genome size and cell volume, but nucleomorphs do not despite co-existing in the same cell for >500 My. Thus mutation pressure does not inexorably increase genome size; selection can eliminate essentially all non-coding DNA if need be. Nucleomorphs and microsporidia even reduced gene size. Expansion of secondary DNA in the main nucleus, and in large-celled eukaryotes generally, must be positively selected for function. Ciliate nuclear dimorphism provides a key test that refutes the selfish DNA and strongly supports the skeletal DNA/karyoplasmic ratio interpretation of genome size evolution. GENETIC CONTROL OF CELL VOLUME IS MULTIGENIC: The quantitatively proportional correlation between genome size and cell size cannot be explained by purely mutational theories, as eukaryote cell volumes are causally determined by cell cycle control genes, not by DNA amounts.

Animals↗
PubMed2005

Culturing and environmental DNA sequencing uncover hidden kinetoplastid biodiversity and a major marine clade within ancestrally freshwater Neobodo designis.

Bodonid flagellates (class Kinetoplastea) are abundant, free-living protozoa in freshwater, soil and marine habitats, with undersampled global biodiversity. To investigate overall bodonid diversity, kinetoplastid-specific PCR primers were used to amplify and sequence 18S rRNA genes from DNA extracted from 16 diverse environmental samples; of 39 different kinetoplastid sequences, 35 belong to the subclass Metakinetoplastina, where most group with the genus Neobodo or the species Bodo saltans, whilst four group with the subclass Prokinetoplastina (Ichthyobodo). To study divergence between freshwater and marine members of the genus Neobodo, 26 new Neobodo designis strains were cultured and their 18S rRNA genes were sequenced. It is shown that the morphospecies N. designis is a remarkably ancient species complex with a major marine clade nested among older freshwater clades, suggesting that these lineages were constrained physiologically from moving between these environments for most of their long history. Other major bodonid clades show less-deep separation between marine and freshwater strains, but have extensive genetic diversity within all lineages and an apparently biogeographically distinct distribution of B. saltans subclades. Clade-specific 18S rRNA gene primers were used for two N. designis subclades to test their global distribution and genetic diversity. The non-overlap between environmental DNA sequences and those from cultures suggests that there are hundreds, possibly thousands, of different rRNA gene sequences of free-living bodonids globally.

Animals↗
PubMed2004

Only six kingdoms of life.

There are many more phyla of microbes than of macro-organisms, but microbial biodiversity is poorly understood because most microbes are uncultured. Phylogenetic analysis of rDNA sequences cloned after PCR amplification of DNA extracted directly from environmental samples is a powerful way of exploring our degree of ignorance of major groups. As there are only five eukaryotic kingdoms, two claims using such methods for numerous novel 'kingdom-level' lineages among anaerobic eukaryotes would be remarkable, if true. By reanalysing those data with 167 known species (not merely 8-37), I identified relatives for all 8-10 'mysterious' lineages. All probably belong to one of five already recognized phyla (Amoebozoa, Cercozoa, Apusozoa, Myzozoa, Loukozoa) within the basal kingdom Protozoa, mostly in known classes, sometimes even in known orders, families or genera. This strengthens the idea that the ancestral eukaryote was a mitochondrial aerobe. Analogous claims of novel bacterial divisions or kingdoms may reflect the weak resolution and grossly non-clock-like evolution of ribosomal rRNA, not genuine phylum-level biological disparity. Critical interpretation of environmental DNA sequences suggests that our overall picture of microbial biodiversity at phylum or division level is already rather good and comprehensive and that there are no uncharacterized kingdoms of life. However, immense lower-level diversity remains to be mapped, as does the root of the tree of life.

Animals↗
PubMed2004

Phylum-specific environmental DNA analysis reveals remarkably high global biodiversity of Cercozoa (Protozoa).

This study presents the first 18S rRNA multi-library environmental PCR survey of a single protozoan phylum, Cercozoa Cavalier-Smith 1998, from a range of different habitats. Phylogenetic analysis reveals at least nine novel clades within the phylum, several possibly at the level of order or above. Further experiments are described to ascertain the true ecological and geographical distributions of some clades that might be inferred from the tree to be restricted in either or both ways. These results suggest that the diversity of cercozoan taxa may run into thousands of lineages, making it comparable in diversity to the largest better-characterized protozoan phyla, e.g. Ciliophora (ciliates and suctorians) and Foraminifera. New sequences of cultured Spongomonas, Metromonas and Metopion are also presented. In the light of these additions, and the increased taxon sampling from the environmental libraries, some revisions of cercozoan classification are made: the transfer of Spongomonadea from Reticulofilosa to Monadofilosa; the removal of Metopiida from Sarcomonadea; and the creation of the new order Metromonadida, currently containing the single genus Metromonas. Although Metromonas groups with weak to moderate support with Chlorarachnea, it is here placed in superclass Monadofilosa, to which it is morphologically more similar.

Animals↗
PubMed2003

Molecular phylogeny of centrohelid heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss.

Recent molecular and cellular evidence indicates that eukaryotes comprise three major lineages: the probably ancestrally uniciliate protozoan phylum Amoebozoa; the ancestrally posteriorly uniciliate opisthokont clade (animals, Choanozoa, and fungi); and a very diverse ancestrally biciliate clade, the bikonts-plants, chromalveolates, and excavate and rhizarian Protozoa. As Heliozoa are the only eukaryote phylum not yet placed on molecular sequence trees, we have sequenced the 18S rRNA genes of three centrohelid heliozoa, Raphidiophrys ambigua, Heterophrys marina, and Chlamydaster sterni, to investigate their phylogenetic position. Phylogenetic analysis by distance and maximum likelihood methods allowing for intersite rate variation and invariable sites confirms that centrohelid heliozoa are a robust clade that does not fall within any other phyla. In particular, they are decisively very distant from the heterokont pedinellid chromists, at one time thought to be related to heliozoa, and lack the unique heterokont signature sequence. They also appear not to be specifically related to either Amoebozoa or Radiolaria, with which they have sometimes been classified, so it is desirable to retain Heliozoa as a separate protozoan phylum. Even though centrohelids have no cilia or centrioles, the centrohelid clade branches among the bikont eukaryotes, but there is no strong bootstrap support for any particular position. Distance trees usually place centrohelids as sisters to haptophytes, whereas parsimony puts them as sisters to red algae, but there is no reason to think that either position is correct; both have very low bootstrap support. Quartet puzzling places them with fairly low support as sisters to the apusozoan zooflagellate Ancyromonas. As Ancyromonas is the only other eukaryote that shares the character combination of flat plate-like mitochondrial cristae and kinetocyst-type extrusomes with centrohelids, this position is biologically plausible, but because of weak support and conflict between trees it might not be correct. Irrespective of their precise position, our trees (together with previous evidence that Chlamydaster sterni has the derived dihydrofolate reductase/thymidylate synthetase gene fusion unique to bikonts) indicate that centrohelid heliozoa are ancestrally derived from a bikont flagellate by the loss of cilia. The centroplast that nucleates their axonemal microtubules is therefore almost certainly homologous with the centrosome of ciliated eukaryotes and should simply be called a centrosome.

Animals↗
PubMed2003

Phylogeny of choanozoa, apusozoa, and other protozoa and early eukaryote megaevolution.

The primary diversification of eukaryotes involved protozoa, especially zooflagellates-flagellate protozoa without plastids. Understanding the origins of the higher eukaryotic kingdoms (two purely heterotrophic, Animalia and Fungi, and two primarily photosynthetic, Plantae and Chromista) depends on clarifying evolutionary relationships among the phyla of the ancestral kingdom Protozoa. We therefore sequenced 18S rRNA genes from 10 strains from the protozoan phyla Choanozoa and Apusozoa. Eukaryote diversity is encompassed by three early-radiating, arguably monophyletic groups: Amoebozoa, opisthokonts, and bikonts. Our taxon-rich rRNA phylogeny for eukaryotes allowing for intersite rate variation strongly supports the opisthokont clade (animals, Choanozoa, Fungi). It agrees with the view that Choanozoa are sisters of or ancestral to animals and reveals a novel nonflagellate choanozoan lineage, Ministeriida, sister either to choanoflagellates, traditionally considered animal ancestors, or to animals. Maximum likelihood trees suggest that within animals Placozoa are derived from medusozoan Cnidaria (we therefore place Placozoa as a class within subphylum Medusozoa of the Cnidaria) and hexactinellid sponges evolved from demosponges. The bikont and amoebozoan radiations are both very ill resolved. Bikonts comprise the kingdoms Plantae and Chromista and three major protozoan groups: alveolates, excavates, and Rhizaria. Our analysis weakly suggests that Apusozoa, represented by Ancyromonas and the apusomonads ( Apusomonas and the highly diverse and much more ancient genus Amastigomonas, from which it evolved), are not closely related to other Rhizaria and may be the most divergent bikont lineages. Although Ancyromonas and apusomonads appear deeply divergent in 18S rRNA trees, the trees neither refute nor support the monophyly of Apusozoa. The bikont phylum Cercozoa weakly but consistently appears as sister to Retaria (Foraminifera; Radiolaria), together forming a hitherto largely unrecognized major protozoan assemblage (core Rhizaria) in the eukaryote tree. Both 18S rRNA sequence trees and a rare deletion show that nonciliate haplosporidian and paramyxid parasites of shellfish (together comprising the Ascetosporea) are not two separate phyla, as often thought, but part of the Cercozoa, and may be related to the plant-parasitic plasmodiophorids and phagomyxids, which were originally the only parasites included in the Cercozoa. We discuss rRNA trees in relation to other evidence concerning the basal diversification and root of the eukaryotic tree and argue that bikonts and opisthokonts, at least, are holophyletic. Amoebozoa and bikonts may be sisters-jointly called anterokonts, as they ancestrally had an anterior cilium, not a posterior one like opisthokonts; this contrasting ciliary orientation may reflect a primary divergence in feeding mode of the first eukaryotes. Anterokonts also differ from opisthokonts in sterol biosynthesis (cycloartenol versus lanosterol pathway), major exoskeletal polymers (cellulose versus chitin), and mitochondrial cristae (ancestrally tubular not flat), possibly also primary divergences.

Amino Acid Motifs↗