[Show abstract][Hide abstract] ABSTRACT: Coastal marine waters in many regions worldwide support abundant populations of extremely small (1-3 μm diameter) unicellular eukaryotic green algae, dominant taxa including several species in the class Mamiellophyceae. Their diminutive size conceals surprising levels of genetic diversity and defies classical species' descriptions. We present a detailed analysis within the genus Ostreococcus and show that morphological characteristics cannot be used to describe diversity within this group. Karyotypic analyses of the best-characterized species O. tauri show it to carry two chromosomes that vary in size between individual clonal lines, probably an evolutionarily ancient feature that emerged before species' divergences within the Mamiellales. By using a culturing technique specifically adapted to members of the genus Ostreococcus, we purified >30 clonal lines of a new species, Ostreococcus mediterraneus sp. nov., previously known as Ostreococcus clade D, that has been overlooked in several studies based on PCR-amplification of genetic markers from environment-extracted DNA. Phylogenetic analyses of the S-adenosylmethionine synthetase gene, and of the complete small subunit ribosomal RNA gene, including detailed comparisons of predicted ITS2 (internal transcribed spacer 2) secondary structures, clearly support that this is a separate species. In addition, karyotypic analyses reveal that the chromosomal location of its ribosomal RNA gene cluster differs from other Ostreococcus clades.
[Show abstract][Hide abstract] ABSTRACT: The definition of species plays a pivotal role in biology. It has been proposed that Compensatory Base Changes (CBCs) in the fast-evolving Internal Transcribed Spacer 2 (ITS2) correlate with speciation and thus can be used to distinguish species. The applicability of CBC - based species concepts using ITS2, however, rests on the homology of the investigated ITS2 positions. We studied the ITS2 molecule of 147 strains of Chlorophyceae (Chlorophyta, Viridiplantae) including 26 new sequences in the order Chaetophorales, and compared their secondary structures to ITS2 in the sister class Ulvophyceae, represented by the order Ulvales. Using a phylogenetic/comparative approach, it was possible to identify 1) the first consensus structure model of the ITS2 molecule that can be applied to two classes of green algae [Ulvophyceae (Ulvales), Chlorophyceae] and 2) landmarks (the spacer regions separating the ITS2 Helices) for more robust prediction of the secondary structures in green algae. Moreover, we found that CBCs in homologous positions in these 147 strains (representing 115 validly described species) are either completely absent or mostly associated with internal branches representing higher order taxonomic levels (genera, families, orders). As reported for the Ulvales, CBCs are not diagnostic at the species level in the dataset used.
[Show abstract][Hide abstract] ABSTRACT: The great majority of metazoans belong to bilaterian phyla. They diversified during a short interval in Earth's history known as the Cambrian explosion, ∼540 million years ago. However, the genetic basis of these events is poorly understood. Here we argue that the vertebrate genome organizer CTCF (CCCTC-binding factor) played an important role for the evolution of bilaterian animals. We provide evidence that the CTCF protein and a genome-wide abundance of CTCF-specific binding motifs are unique to bilaterian phyla, but absent in other eukaryotes. We demonstrate that CTCF-binding sites within vertebrate and Drosophila Hox gene clusters have been maintained for several hundred million years, suggesting an ancient origin of the previously known interaction between Hox gene regulation and CTCF. In addition, a close correlation between the presence of CTCF and Hox gene clusters throughout the animal kingdom suggests conservation of the Hox-CTCF link across the Bilateria. On the basis of these findings, we propose the existence of a Hox-CTCF kernel as principal organizer of bilaterian body plans. Such a kernel could explain (i) the formation of Hox clusters in Bilateria, (ii) the diversity of bilaterian body plans, and (iii) the uniqueness and time of onset of the Cambrian explosion.
Proceedings of the National Academy of Sciences 10/2012; 109(43):17507-12. DOI:10.1073/pnas.1111941109 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The class Pedinophyceae was established for asymmetric uniflagellate green algae, and was originally considered as an ancestral lineage of viridiplants. However, analyses of 71 concatenated plastid proteins [Turmel et al. (2009): Mol. Biol. Evol. 26: 2317-2331] recovered Pedinomonas within the Chlorellales (Trebouxiophyceae), thereby questioning the Pedinophyceae as an independent class. For the present study, complete nuclear and plastid-encoded rRNA operon sequences have been determined for 37 taxa of green algae including 6 members of the Pedinophyceae, providing 9272 aligned nucleotide positions. Phylogenies using both rRNA operons consistently rejected any relationship between Pedinophyceae and the Chlorellales. Instead, the Pedinophyceae were significantly resolved as sister of all phycoplast-containing 'core' chlorophytes, i.e. Chlorodendrophyceae, Trebouxiophyceae, Ulvophyceae and Chlorophyceae. Reinvestigation of plastid proteins discovered biased phylogenetic signal among protein partitions, indicating the published Pedinomonas + Chlorellales association as likely artificial. Marine pedinophytes (Resultomonas and Marsupiomonas; Marsupiomonadales ord. nov.), formed a sister clade to the order Pedinomonadales, occurring in freshwater and soil habitats. Synapomorphies in rRNA secondary structures were integrated in taxonomic diagnoses of the Pedinophyceae and were also used for BLAST searches targeting environmental sequence databases. The latter approach revealed conserved habitat preferences for the Marsupiomonadales and Pedinomonadales, and identified several novel pedinophyte lineages.
[Show abstract][Hide abstract] ABSTRACT: The second Internal Transcriber Spacer (ITS2) is a fast evolving part of the nuclear-encoded rRNA operon located between the 5.8S and 28S rRNA genes. Based on crossing experiments it has been proposed that even a single Compensatory Base Change (CBC) in helices 2 and 3 of the ITS2 indicates sexual incompatibility and thus separates biological species. Taxa without any CBC in these ITS2 regions were designated as a 'CBC clade'. However, in depth comparative analyses of ITS2 secondary structures, ITS2 phylogeny, the origin of CBCs, and their relationship to biological species have rarely been performed. To gain 'close-up' insights into ITS2 evolution, (1) 86 sequences of ITS2 including secondary structures have been investigated in the green algal order Ulvales (Chlorophyta, Viridiplantae), (2) after recording all existing substitutions, CBCs and hemi-CBCs (hCBCs) were mapped upon the ITS2 phylogeny, rather than merely comparing ITS2 characters among pairs of taxa, and (3) the relation between CBCs, hCBCs, CBC clades, and the taxonomic level of organisms was investigated in detail.
High sequence and length conservation allowed the generation of an ITS2 consensus secondary structure, and introduction of a novel numbering system of ITS2 nucleotides and base pairs. Alignments and analyses were based on this structural information, leading to the following results: (1) in the Ulvales, the presence of a CBC is not linked to any particular taxonomic level, (2) most CBC 'clades' sensu Coleman are paraphyletic, and should rather be termed CBC grades. (3) the phenetic approach of pairwise comparison of sequences can be misleading, and thus, CBCs/hCBCs must be investigated in their evolutionary context, including homoplasy events (4) CBCs and hCBCs in ITS2 helices evolved independently, and we found no evidence for a CBC that originated via a two-fold hCBC substitution.
Our case study revealed several discrepancies between ITS2 evolution in the Ulvales and generally accepted assumptions underlying ITS2 evolution as e.g. the CBC clade concept. Therefore, we developed a suite of methods providing a critical 'close-up' view into ITS2 evolution by directly tracing the evolutionary history of individual positions, and we caution against a non-critical use of the ITS2 CBC clade concept for species delimitation.
[Show abstract][Hide abstract] ABSTRACT: Previously published molecular phylogenetic analyses of the Chaetophorales (Chlorophyceae) suffered from limited taxon sampling (six genera with only a single species per genus). To test the monophyly of species-rich genera, and to analyze the phylogenetic relationships among families and genera in the Chaetophorales, we determined nuclear-encoded SSU rDNA sequences from 30 strains of Chaetophorales, performed phylogenetic analyses using various methods, and screened clades for support by unique molecular synapomorphies in the SSU rRNA secondary structure. The Schizomeridaceae and the weakly supported Aphanochaetaceae were recovered as basal lineages. The derived family Chaetophoraceae diverged into two clades: the “Uronema clade” containing unbranched filaments, and a sister clade designated as “branched Chaetophoraceae” comprising Chaetophora, Stigeoclonium, Draparnaldia, Caespitella, and Fritschiella. Although some terminal clades corresponded to genera described (e.g., Caespitella and Draparnaldia), other clades were in conflict with traditional taxonomic designations. Especially, the genera Stigeoclonium and Chaetophora were shown to be polyphyletic. The globose species Chaetophora elegans was unrelated to lobate Chaetophora spp. (e.g., Chaetophora lobata). Since the original description of Chaetophora referred to a lobate thallus organization, the latter clade represented Chaetophora sensu stricto. In consequence, C. lobata was designated as lectotype of Chaetophora. Two Stigeoclonium species, Stigeoclonium farctum Berthold and Stigeoclonium‘Longipilus’, diverged independently from the type species of Stigeoclonium, Stigeoclonium tenue (C. Agardh) Kütz. These results indicated that some commonly used taxonomic characters are either homoplasious or plesiomorphic and call for a reevaluation of the systematics of the Chaetophorales using novel morphological and molecular approaches.
[Show abstract][Hide abstract] ABSTRACT: Molecular phylogenetic analyses of the Mamiellophyceae classis nova, a ubiquitous group of largely picoplanktonic green algae comprising scaly and non-scaly prasinophyte unicells, were performed using single and concatenated gene sequence comparisons of the nuclear- and plastid-encoded rRNA operons. The study resolved all major clades within the class, identified molecular signature sequences for most clades through an exhaustive search for non-homoplasious synapomorphies [Marin et al. (2003): Protist 154: 99-145] and incorporated these signatures into the diagnoses of two novel orders, Monomastigales ord nov., Dolichomastigales ord. nov., and four novel families, Monomastigaceae fam. nov., Dolichomastigaceae fam. nov., Crustomastigaceae fam. nov., and Bathycoccaceae fam. nov., within a revised classification of the class. A database search for the presence of environmental rDNA sequences in the Monomastigales and Dolichomastigales identified an unexpectedly large genetic diversity of Monomastigales confined to freshwater, a novel clade (Dolicho_B) in the Dolichomastigaceae from deep sea sediments and a novel freshwater clade in the Crustomastigaceae. The Mamiellophyceae represent one of the ecologically most successful groups of eukaryotic, photosynthetic picoplankters in marine and likely also freshwater environments.
[Show abstract][Hide abstract] ABSTRACT: The zinc finger (ZF) protein CTCF (CCCTC-binding factor) is highly conserved in Drosophila and vertebrates where it has been shown to mediate chromatin insulation at a genomewide level. A mode of genetic regulation that involves insulators and insulator binding proteins to establish independent transcriptional units is currently not known in nematodes including Caenorhabditis elegans. We therefore searched in nematodes for orthologs of proteins that are involved in chromatin insulation.
While orthologs for other insulator proteins were absent in all 35 analysed nematode species, we find orthologs of CTCF in a subset of nematodes. As an example for these we cloned the Trichinella spiralis CTCF-like gene and revealed a genomic structure very similar to the Drosophila counterpart. To investigate the pattern of CTCF occurrence in nematodes, we performed phylogenetic analysis with the ZF protein sets of completely sequenced nematodes. We show that three ZF proteins from three basal nematodes cluster together with known CTCF proteins whereas no zinc finger protein of C. elegans and other derived nematodes does so.
Our findings show that CTCF and possibly chromatin insulation are present in basal nematodes. We suggest that the insulator protein CTCF has been secondarily lost in derived nematodes like C. elegans. We propose a switch in the regulation of gene expression during nematode evolution, from the common vertebrate and insect type involving distantly acting regulatory elements and chromatin insulation to a so far poorly characterised mode present in more derived nematodes. Here, all or some of these components are missing. Instead operons, polycistronic transcriptional units common in derived nematodes, seemingly adopted their function.
[Show abstract][Hide abstract] ABSTRACT: WD40/BEACH domain proteins have been implicated in membrane trafficking and membrane composition events in Dictyostelium and Drosophila. In this paper, we show that the Arabidopsis SPIRRIG (SPI) gene encodes a WD40/BEACH domain protein. The cellular analysis revealed fragmented vacuoles in root hairs similar to those found in the corresponding Dictyostelium mutants, suggesting a related cellular function. The phenotypic analysis revealed that spi mutants share all phenotypic aspects of mutants in the actin polymerization-regulating ARP2/3 pathway, including distorted trichomes, less lobing of epidermal pavement cells, disconnected epidermal cells on various organs, and shorter root hairs. This complete phenotypic overlap suggests that this WD40/BEACH domain protein and the actin-regulating ARP2/3 pathway are involved in similar growth processes.
The Plant Journal 05/2009; 59(4):612-21. DOI:10.1111/j.1365-313X.2009.03900.x · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background
Land plants (embryophytes) evolved from streptophyte green algae, a small group of freshwater algae ranging from scaly, unicellular flagellates (Mesostigma) to complex, filamentous thalli with branching, cell differentiation and apical growth (Charales). Streptophyte algae and embryophytes form the division Streptophyta, whereas the remaining green algae are classified as Chlorophyta. The Charales (stoneworts) are often considered to be sister to land plants, suggesting progressive evolution towards cellular complexity within streptophyte green algae. Many cellular (e.g. phragmoplast, plasmodesmata, hexameric cellulose synthase, structure of flagellated cells, oogamous sexual reproduction with zygote retention) and physiological characters (e.g. type of photorespiration, phytochrome system) originated within streptophyte algae.Recent ProgressPhylogenetic studies have demonstrated that Mesostigma (flagellate) and Chlorokybus (sarcinoid) form the earliest divergence within streptophytes, as sister to all other Streptophyta including embryophytes. The question whether Charales, Coleochaetales or Zygnematales are the sister to embryophytes is still (or, again) hotly debated. Projects to study genome evolution within streptophytes including protein families and polyadenylation signals have been initiated. In agreement with morphological and physiological features, many molecular traits believed to be specific for embryophytes have been shown to predate the Chlorophyta/Streptophyta split, or to have originated within streptophyte algae. Molecular phylogenies and the fossil record allow a detailed reconstruction of the early evolutionary events that led to the origin of true land plants, and shaped the current diversity and ecology of streptophyte green algae and their embryophyte descendants. Conclusions The Streptophyta/Chlorophyta divergence correlates with a remarkably conservative preference for freshwater/marine habitats, and the early freshwater adaptation of streptophyte algae was a major advantage for the earliest land plants, even before the origin of the embryo and the sporophyte generation. The complete genomes of a few key streptophyte algae taxa will be required for a better understanding of the colonization of terrestrial habitats by streptophytes.
Annals of Botany 04/2009; 103(7):999-1004. DOI:10.1093/aob/mcp044 · 3.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Paulinella chromatophora is a freshwater filose amoeba with photosynthetic endosymbionts (chromatophores) of cyanobacterial origin that are closely related to free-living Prochlorococcus and Synechococcus species (PS-clade). Members of the PS-clade of cyanobacteria contain a proteobacterial form 1A RubisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) that was acquired by horizontal gene transfer (HGT) of a carboxysomal operon. In rDNA-phylogenies, the Paulinella chromatophore diverged basal to the PS-clade, raising the question whether the HGT occurred before or after the split of the chromatophore ancestor.
Phylogenetic analyses of the almost complete rDNA operon with an improved taxon sampling containing most known cyanobacterial lineages recovered the Paulinella chromatophore as sister to the complete PS-clade. The sequence of the complete carboxysomal operon of Paulinella was determined. Analysis of RubisCO large subunit (rbcL) sequences revealed that Paulinella shares the proteobacterial form 1A RubisCO with the PS-clade. The gamma-proteobacterium Nitrococcus mobilis was identified as sister of the Paulinella chromatophore and the PS-clade in the RubisCO phylogeny. Gene content and order in the carboxysomal operon correlates well with the RubisCO phylogeny demonstrating that the complete carboxysomal operon was acquired by the common ancestor of the Paulinella chromatophore and the PS-clade through HGT. The carboxysomal operon shows a significantly elevated AT content in Paulinella, which in the rbcL gene is confined to third codon positions. Combined phylogenies using rbcL and the rDNA-operon resulted in a nearly fully resolved tree of the PS-clade.
The HGT of the carboxysomal operon predated the divergence of the chromatophore ancestor from the PS-clade. Following HGT and divergence of the chromatophore ancestor, diversification of the PS-clade into at least three subclades occurred. The gamma-proteobacterium Nitrococcus mobilis represents the closest known relative to the donor of the carboxysomal operon. The isolated position of the Paulinella chromatophore in molecular phylogenies as well as its elevated AT content suggests that the Paulinella chromatophore has already undergone typical steps in the reductive evolution of an endosymbiont.
[Show abstract][Hide abstract] ABSTRACT: One of the major steps in the evolution of life was the origin of photosynthesis in nucleated cells underpinning the evolution of plants. It is well accepted that this evolutionary process was initiated when a photosynthetic bacterium (a cyanobacterium) was taken up by a colorless host cell, probably more than a billion years ago, and transformed into a photosynthetic organelle (a plastid) during a process known as primary endosymbiosis. Here, we use sequence comparisons and phylogenetic analyses of the prokaryotic rDNA operon to show that the thecate, filose amoeba Paulinella chromatophora Lauterborn obtained its photosynthetic organelles by a similar but more recent process, which involved a different cyanobacterium, indicating that the evolution of photosynthetic organelles from cyanobacteria was not a unique event, as is commonly believed, but may be an ongoing process.
[Show abstract][Hide abstract] ABSTRACT: The flagellar hair ultrastructure of 16 strains of species of the prasinophycean genera Mantoniella, Mamiella, Pseudoscourfieldia, Nephroselmis, Tetraselmis, Scherffelia, Pterosperma, and Pyraminonas was examined in detail by whole-mount electron microscopy. The flagellar hairs of all genera displayed a high degree of ultrastructural complexity that was completely conserved within each strain. In all strains, flagellar hairs occurred on the sides of the flagella (lateral hairs); in several strains, special flagellar hairs also were found on the flagellar tips (tip hairs; absent in the Chlorodendrales and in Nephroselmis). Two groups of lateral hairs were distinguished: 1) T-hairs (“Tetraselmis-type” flagellar hairs), characterized by a smooth, tubular shaft of ca. 15 nm diameter and an overall length of 0.5–1.3 μm, and 2) Pt-hairs (“Pterosperma-type lateral flagellar hairs”), which were considerably longer (ca. 1.5–5.4 μm), characterized by a thick shaft of ca. 30 nm diameter, which was covered with a layer of regularly spaced small particles of ca. 10 nm diameter. In both groups of flagellar hairs, a strain-specific number of subunits (1–101) in linear arrangement was attached to the distal end of the shaft. Tip hairs were either structurally related to T-hairs (Mamiellales, Pseudoscourfieldia) or represented a separate group, Pt-hairs (“Pterosperma-type flagellar tip hairs”; Pterosperma, Pyramimonas). In four genera (Mantoniella, Mamiella, Pseudoscourfieldia, Nephroselmis), both groups of lateral hairs occurred together on the same cell. Interestingly in these taxa the Pt-hairs were exclusively attached to the shorter immature flagella (no. 2), but, in contrast, in Mantoniella and Pseudoscourfieldia the tip hairs were restricted to the longer mature flagellum (no. 1). Thus, flagella of different developmental status differ in their hair-scale complement. The occurrence, distribution, and ultrastructure of flagellar hairs can be used to identify and classify prasinophytes at all taxonomic levels.
[Show abstract][Hide abstract] ABSTRACT: Although the combination of different genes in phylogenetic analyses is a promising approach, the methodology is not well established and analyses often suffer from inadequate, noncongruent taxon sampling, long-branch attraction, or conflicting evolutionary models of the genes analyzed. Conflicts or congruence between multigene and single-gene phylogenies, as well as the assumed superiority of the multigene approach, are often difficult to assess solely because of incongruent taxon sampling. In the present study, a data set of 43 nuclear-encoded SSU rDNA and plastid-encoded rbcL gene sequences was generated from the same strains of conjugating green algae (Zygnematophyceae, Streptophyta). Phylogenetic analyses used the genes individually and in combination, either as concatenated sequences or with the log-likelihood summation method. Single-gene analyses, although mostly congruent, revealed some conflicting nodes and showed different patterns of statistical support. Combined analyses confidently resolved the conflicts between the single-gene analyses, enhanced phylogenetic resolution, and were better supported by morphological information. Long-branch taxa were not the same for the two genes analyzed, and, thus, their effect on phylogenetic resolution was minimized in the combined analyses.
[Show abstract][Hide abstract] ABSTRACT: Sequence comparisons and a revised classification of the Euglenophyceae were based on 92 new SSU rDNA sequences obtained from strains of Euglena, Astasia, Phacus, Trachelomonas, Colacium, Cryptoglena, Lepocinclis, Eutreptia, Eutreptiella and Tetreutreptia. Sequence data also provided molecular signatures for taxa from genus to class level in the SSU rRNA secondary structure, revealed by a novel approach (search for non-homoplasious synapomorphies) and used for taxonomic diagnoses. Photosynthetic euglenoids and secondary heterotrophs formed a clade, designated as Euglenophyceae (emend.) with two orders: Euglenales and Eutreptiales. The mostly marine Eutreptiales (Eutreptia, Eutreptiella; not Distigma) comprised taxa with two or four emergent flagella (the quadriflagellate Tetreutreptia was integrated within Eutreptiella). The Euglenales (freshwater genera with < or = one emergent flagellum) formed nine clades and two individual branches (single strains); however, only two clades were congruent with traditional genera: Trachelomonas (incl. Strombomonas) and Colacium. Euglena was polyphyletic and diverged into four independent clades (intermixed with Astasia, Khawkinea and Lepocinclis) and two individual branches (e.g. E. polymorpha). Phacus was also subdivided into Phacus s. str. and two combined lineages (mixed with Lepocinclis spp. or Cryptoglena). In consequence, Euglena (s. str.), Phacus and other genera were emended and one lineage (mixed Phacus/Lepocinclis-clade) was recognized as the previously neglected genus Monomorphina Mereschkowsky (1877). The sister clade of Phacus s. str. (mixed Euglena/Lepocinclis-clade) was identified as Lepocinclis Perty (emended).
[Show abstract][Hide abstract] ABSTRACT: Nuclear-encoded SSU rDNA sequences have been obtained from 64 strains of conjugating green algae (Zygnemophyceae, Streptophyta, Viridiplantae). Molecular phylogenetic analyses of 90 SSU rDNA sequences of Viridiplantae (inciuding 78 from the Zygnemophyceae) were performed using complex evolutionary models and maximum likelihood, distance, and maximum parsimony methods. The significance of the results was tested by bootstrap analyses, deletion of long-branch taxa, relative rate tests, and Kishino-Hasegawa tests with user-defined trees. All results support the monophyly of the class Zygnemophyceae and of the order Desmidiales. The second order, Zygnematales, forms a series of early-branching clades in paraphyletic succession, with the two traditional families Mesotaeniaceae and Zygnemataceae not recovered as lineages. Instead, a long-branch Spirogyra/Sirogonium clade and the later-diverging Netrium and Roya clades represent independent clades. Within the order Desmidiales, the families Gonatozygaceae and Closteriaceae are monophyletic, whereas the Peniaceae (represented only by Penium margaritaceum) and the Desmidiaceae represent a single weakly supported lineage. Within the Desmidiaceae short internal branches and varying rates of sequence evolution among taxa reduce the phylogenetic resolution significantly. The SSU rDNA-based phylogeny is largely congruent with a published analysis of the rbcL phylogeny of the Zygnemophyceae (McCourt et al. 2000) and is also in general agreement with classification schemes based on cell wall ultrastructure. The extended taxon sampling at the subgenus level provides solid evidence that many genera in the Zygnemophyceae are not monophyletic and that the genus concept in the group needs to be revised.
[Show abstract][Hide abstract] ABSTRACT: The plastid-bearing members of the Cryptophyta contain two functional eukaryotic genomes of different phylogenetic origin, residing in the nucleus and in the nucleomorph, respectively. These widespread and diverse protists thus offer a unique opportunity to study the coevolution of two different eukaryotic genomes within one group of organisms. In this study, the SSU rRNA genes of both genomes were PCR-amplified with specific primers and phylogenetic analyses were performed on different data sets using different evolutionary models. The results show that the composition of the principal clades obtained from the phylogenetic analyses of both genes was largely congruent, but striking differences in evolutionary rates were observed. These affected the topologies of the nuclear and nucleomorph phylogenies differently, resulting in long-branch attraction artifacts when simple evolutionary models were applied. Deletion of long-branch taxa stabilized the internal branching order in both phylogenies and resulted in a completely resolved topology in the nucleomorph phylogeny. A comparison of the tree topologies derived from SSU rDNA sequences with characters previously used in cryptophyte systematics revealed that the biliprotein type was congruent, but the type of inner periplast component incongruent, with the molecular trees. The latter is indicative of a hidden cellular dimorphism (cells with two periplast types present in a single clonal strain) of presumably widespread occurrence throughout cryptophyte diversity, which, in consequence, has far-reaching implications for cryptophyte systematics as it is practiced today.
[Show abstract][Hide abstract] ABSTRACT: Traditionally, brackish and marine coccoid green algae are placed as isolated species of common trebouxiophycean or chlorophycean genera (e.g. Chlorella, Chlorococcum); exclusively the marine genera Chlorocystis and Halochlorococcum are characterized by quadriflagellated zoospores and a ‘Codiolum-stage’ (characteristic for the Ulvophyceae) in their life histories. In Chlorocystis cohnii, sexual reproduction could only be induced by presence of the tube-dwelling diatom Berkeleya rutilans. Phylogenetic analyses of the SSU rDNA sequences of 32, mostly marine, coccoid or sarcinoid strains confirmed the monophyly of the Ulvophyceae. The class contain 11 independent well-supported lineages; of which eight contain coccoid species. (1) The basal branch of the ulvophytes is the Oltmannsiellopsis clade (in agreement with the order Oltmannsiellopsidales emend.) which contains the quadriflagellated monads (O. viridis and O. unicellularis) and the type species of the coccoid genus Halochlorococcum (H. marinum). (2) The other species of Halochlorococcum (except H. saccatum) formed together with Chlorocystis and Chlorella salina a significant clade corresponding to the order Chlorocystidales in emended form. (3) Gomontia polyrhiza is closely related to Urospora penicelliformis within the Acrosiphonia group (Acrosiphoniales). (4) The sarcinoid Trichosarcina polymorpha and T. mucosa, and the freshwater planctonic Helicodictyon planctonicum are member of the Ulothrix clade together with filamenteous taxa. (5) The type species of Pseudoneochloris (P. marina) form together with three unidentified marine coccoid strains an independent lineage within the Ulvophyceae (called Pseudoneochloris clade). (6–8) Ignatius tetrasporus (isolated from soil), the strain CCMP 250 originating from desert soil and the marine strain CCMP 1293 isolated from Palau Islands represent three single lineages within the ulvophytes. From our results, the class Ulvophyceae can be subdivided taxonomically into at least 11 orders (including the Ulvales, Cladophorales and Dasycladales) with the largest biodiversity at the coccoid level.
†Present address: Department of Molecular and Cell Biology, Brown University, Providence, RI 02912, USA.
[Show abstract][Hide abstract] ABSTRACT: The genus Chlamydomonas (including Chloromonas) is one of the largest green algal genera comprising more than 600 species. To initiate a comprehensive analysis of the phylogeny and systematics of the genus, we determined nuclear-encoded SSU rRNA sequences from 32 strains of Chlamydomonas, Chloromonas and Chlorogonium with emphasis on oogamous taxa and related strains, and incorporated these into global molecular phylogenetic analyses of 132 strains of Chlorophyceae. In addition, we studied the morphology and reproduction of oogamous and related strains by light microscopy. We recognize and designate 18 monophyletic lineages (clades) within the Chlorophyceae, 11 of which are confined to the CW (basal bodies displaced clockwise) subgroup. The majority of clades recognized within the Chlorophyceae do not correspond to any of the traditional classification systems, which are still largely based on the organization level. Strains assigned to Chlamydomonas and Chloromonas were found in seven different clades confirming the polyphyly of the two genera as presently conceived. To initiate the taxonomic revision of Chlamydomonas, C. reinhardtii is proposed as the conserved type of the genus. In consequence, species in clades other than the clade containing C. reinhardtii must be transferred to other genera, a process initiated in this contribution. The oogamous strains studied represent a monophyletic lineage, which is described as Oogamochlamys gen. nov. comprising three species (O. gigantea, O. zimbabwiensis and O. ettlii spec. nov.). The sister clade to Oogamochlamys consists of isogamous strains characterized by chloroplasts with incisions and is described as Lobochlamys gen. nov. with two species (L. culleus and L. segnis). Another clade is characterized by asteroid or perforated, parietal chloroplasts and contains the type species of Chloromonas (C. reticulata). Thus, the polyphyletic Chloromonas (traditionally defined as "Chlamydomonas without pyrenoids") can be legitimized as a monophyletic genus by restriction to this clade and is here emended on the basis of chloroplast characters (the clade contains strains with or without pyrenoids thus rejecting the character "absence of pyrenoids").