Evolutionary Relationships Among the Eukaryotic Crown Taxa Taking into Account Site-to-Site Rate Variation in 18S rRNA

In this study we constructed a bootstrapped distance tree of 500 small subunit ribosomal RNA sequences from organisms belonging to the so-called crown of eukaryote evolution. Taking into account the substitution rate of the individual nucleotides of the rRNA sequence alignment, our results suggest that (1) animals, true fungi, and choanoflagellates share a common origin: The branch joining these taxa is highly supported by bootstrap analysis (bootstrap support [BS] > 90%), (2) stramenopiles and alveolates are sister groups (BS = 75%), (3) within the alveolates, dinoflagellates and apicomplexans share a common ancestor BS > 95%), while in turn they both share a common origin with the ciliates (BS > 80%), and (4) within the stramenopiles, heterokont algae, hyphochytriomycetes, and oomycetes form a monophyletic grouping well supported by bootstrap analysis (BS > 85%), preceded by the well-supported successive divergence of labyrinthulomycetes and bicosoecids. On the other hand, many evolutionary relationships between crown taxa are still obscure on the basis of 18S rRNA. The branching order between the animal-fungal-choanoflagellates clade and the chlorobionts, the alveolates and stramenopiles, red algae, and several smaller groups of organisms remains largely unresolved. When among-site rate variation is not considered, the inferred tree topologies are inferior to those where the substitution rate spectrum for the 18S rRNA is taken into account. This is primarily indicated by the erroneous branching of fast-evolving sequences. Moreover, when different substitution rates among sites are not considered, the animals no longer appear as a monophyletic grouping in most distance trees. Received: 11 June 1997 / Accepted: 21 July 1997     

A NOVEL BIOFOULING ASSAY USING COMPUTER-ASSISTED MOTION ANALYSIS OF HINCKSIA IRREGULARIS SPORE SWIMMING

Chloroplasts in heterokont algae probably originated from a red algal endosymbiont which was engulfed and retained by a eukaryotic host, and are surrounded by four envelope membranes. The outermost of these membranes is called chloroplast ER (CER) and usually connects with the nuclear envelope. This information, however, is based mainly on studies on single-plastid heterokont algae. In multi-plastid heterokont algae, it is still unclear whether CER is continuous with the nuclear envelope. Since nuclear-encoded chloroplast proteins are synthesized by ribosomes on the ER membrane, clarifying the ER-CER structure in the heterokont algae is important in order to know the targeting pathway of those proteins. We did a detailed ultrastructural observation of endomembrane systems in a multi-plastid heterokont alga: Heterosigma akashiwo , and confirmed that the CER membrane was continuous with the ER membrane. However, unlike the CER membranes in other heterokont algae, it seemed to have very few ribosome attached. We also performed experiments for protein targeting into canine microsomes using a precursor for a nuclear-encoded chloroplast protein, a fucoxanthin-chlorophyll protein (FCP), of H. akashiwo , to see if the protein is targeted to the ER. It demonstrated that the precursor has a functional signal sequence for ER targeting, and is co-translationally translocated into the microsomes. Based on these data, we propose a hypothesis that, in H. akashiwo , nuclear-encoded chloroplast protein precursors that have been co-translationally inserted into the ER lumen are sorted in the ER and transported to the chloroplasts through the ER.     

Complete large subunit ribosomal RNA sequences from the heterokont algae ochromonas danica, nannochloropsis salina, and tribonema aequale, and phylogenetic analysis

The large subunit ribosomal RNA sequences from the heterokont algae Ochromonas danica, Nannochloropsis salina, and Tribonema aequale were determined. These sequences were combined with small subunit ribosomal RNA sequences in order to carry out a phylogenetic analysis based on neighbor-joining, maximum parsimony, and maximum likelihood methods. Our results indicate that heterokont fungi and heterokont algae each are monophyletic, and confirm that they together form a monophyletic group called ``stramenopiles.' Within the heterokont algae, the eustigmatophyte Nannochloropsis salina either clusters with the chrysophyte Ochromonas danica or forms a sister group to a cluster comprising the phaeophyte Scytosiphon lomentaria and the xanthophyte Tribonema aequale. The alveolates were identified as the closest relatives of the stramenopiles, but the exact order of divergence between the eukaryotic crown taxa could not be established with confidence. Received: 22 November 1996 / Accepted: 14 February 1997     

ENDOMEMBRANE ULTRASTRUCTURE AND POSSIBLE CHLOROPLAST PROTEIN IMPORT PATHWAY IN HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE)

Chloroplasts in heterokont algae probably originated from a red algal endosymbiont which was engulfed and retained by a eukaryotic host, and are surrounded by four envelope membranes. The outermost of these membranes is called chloroplast ER (CER) and usually connects with the nuclear envelope. This information, however, is based mainly on studies on single‐plastid heterokont algae. In multi‐plastid heterokont algae, it is still unclear whether CER is continuous with the nuclear envelope. Since nuclear‐encoded chloroplast proteins are synthesized by ribosomes on the ER membrane, clarifying the ER‐CER structure in the heterokont algae is important in order to know the targeting pathway of those proteins. We did a detailed ultrastructural observation of endomembrane systems in a multi‐plastid heterokont alga: Heterosigma akashiwo, and confirmed that the CER membrane was continuous with the ER membrane. However, unlike the CER membranes in other heterokont algae, it seemed to have very few ribosome attached. We also performed experiments for protein targeting into canine microsomes using a precursor for a nuclear‐encoded chloroplast protein, a fucoxanthin‐chlorophyll protein (FCP), of H. akashiwo, to see if the protein is targeted to the ER. It demonstrated that the precursor has a functional signal sequence for ER targeting, and is co‐translationally translocated into the microsomes. Based on these data, we propose a hypothesis that, in H. akashiwo, nuclear‐encoded chloroplast protein precursors that have been co‐translationally inserted into the ER lumen are sorted in the ER and transported to the chloroplasts through the ER.     

Evolutionary relationships among heterokont algae (the autotrophic stramenopiles) based on combined analyses of small and large subunit ribosomal RNA

In order to study the phylogenetic relationships within the stramenopiles, and particularly among the heterokont algae, we have determined complete or nearly complete large-subunit ribosomal RNA sequences for different species of raphidophytes, phaeophytes, xanthophytes, chrysophytes, synurophytes and pinguiophytes. With the small- and large-subunit ribosomal RNA sequences of representatives for nearly all known groups of heterokont algae, phylogenetic trees were constructed from a concatenated alignment of both ribosomal RNAs, including more than 5,000 positions. By using different tree construction methods, inferred phylogenies showed phaeophytes and xanthophytes as sister taxa, as well as the pelagophytes and dictyochophytes, and the chrysophytes/synurophytes and eustigmatophytes. All these relationships are highly supported by bootstrap analysis. However, apart from these sister group relationships, very few other internodes are well resolved and most groups of heterokont algae seem to have diverged within a relatively short time frame.     

Evolution of glutamine synthetase in heterokonts: evidence for endosymbiotic gene transfer and the early evolution of photosynthesis

Although the endosymbiotic evolution of chloroplasts through primary and secondary associations is well established, the evolutionary timing and stability of the secondary endosymbiotic events is less well resolved. Heterokonts include both photosynthetic and nonphotosynthetic members and the nonphotosynthetic lineages branch basally in phylogenetic reconstructions. Molecular and morphological data indicate that heterokont chloroplasts evolved via a secondary endosymbiosis, involving a heterotrophic host cell and a photosynthetic ancestor of the red algae and this endosymbiotic event may have preceded the divergence of heterokonts and alveolates. If photosynthesis evolved early in this lineage, nuclear genomes of the nonphotosynthetic groups may contain genes that are not essential to photosynthesis but were derived from the endosymbiont genome through gene transfer. These genes offer the potential to trace the evolutionary history of chloroplast gains and losses within these lineages. Glutamine synthetase (GS) is essential for ammonium assimilation and glutamine biosynthesis in all organisms. Three paralogous gene families (GSI, GSII, and GSIII) have been identified and are broadly distributed among prokaryotic and eukaryotic lineages. In diatoms (Heterokonta), the nuclear-encoded chloroplast and cytosolic-localized GS isoforms are encoded by members of the GSII and GSIII family, respectively. Here, we explore the evolutionary history of GSII in both photosynthetic and nonphotosynthetic heterokonts, red algae, and other eukaryotes. GSII cDNA sequences were obtained from two species of oomycetes by polymerase chain reaction amplification. Additional GSII sequences from eukaryotes and bacteria were obtained from publicly available databases and genome projects. Bayesian inference and maximum likelihood phylogenetic analyses of GSII provided strong support for the monophyly of heterokonts, rhodophytes, chlorophytes, and plants and strong to moderate support for the Opisthokonts. Although the phylogeny is reflective of the unikont/bikont division of eukaryotes, we propose based on the robustness of the phylogenetic analyses that the heterokont GSII gene evolved via endosymbiotic gene transfer from the nucleus of the red-algal endosymbiont to the nucleus of the host. The lack of GSIII sequences in the oomycetes examined here further suggests that the GSIII gene that functions in the cytosol of photosynthetic heterokonts was replaced by the endosymbiont-derived GSII gene.     

Phylogenetic analyses of the rbcL sequences from haptophytes and heterokont algae suggest their chloroplasts are unrelated

Using the large subunit of RuBisCo (rbcL) sequences from cyanobacteria, proteobacteria, and diverse groups of algae and green plants, we evaluated the plastid relationship between haptophytes and heterokont algae. The rbcL sequences were determined from three taxa of heterokont algae (Bumilleriopsis filiformis, Pelagomonas calceolata, and Pseudopedinella elastica) and added to 25 published sequences to obtain a data set comprising 1,434 unambiguously aligned sites (approximately 98% of the total rbcL gene). Higher levels of mutational saturation in third codon positions were observed by plotting the pairwise substitutions with and without corrections for multiple substitutions at the same site for first and second codon positions only and for third positions only. In accordance with this finding phylogeny reconstructions were completed by omitting third codon positions, thus using 956 bp in weighted-parsimony and maximum-likelihood analyses. The midpoint-rooted phylogenies showed two major clusters, one containing cyanobacteria, glaucocystophytes, a phototrophic euglenoid, chlorophytes, and embryophytes (the green lineage), the other containing proteobacteria, haptophytes, red algae, a cryptophyte, and heterokont algae (the non-green lineage). In the nongreen lineage, the haptophytes formed a sister group to the clade containing heterokont algae, red algae, and the cryptophyte Guillardia theta. This branching pattern was well supported in terms of bootstrap values in weighted- parsimony and maximum-likelihood analyses (100% and 92%, respectively). However, the phylogenetic relationship among red algae, heterokonts, and a cryptophyte taxon was not especially well resolved. A four- cluster analysis was performed to further explore the statistical significance of the relationship between proteobacteria, red algae (including and excluding Guillardia theta), haptophytes, and heterokont algae. This test strongly favored the hypothesis that the heterokonts and red algae are more closely related to each other than either is to proteobacteria or haptophytes. Hence, this molecular study based on a plastid-encoded gene provides additional evidence for a distant relationship between haptophytes and the heterokont algae. It suggests an evolutionary scenario in which the ancestor of the haptophyte lineage engulfed a phototrophic eukaryote and, more recently, the heterokont lineage became phototrophic by engulfing a red alga.      

Biology and systematics of heterokont and haptophyte algae

In this paper, I review what is currently known of phylogenetic relationships of heterokont and haptophyte algae. Heterokont algae are a monophyletic group that is classified into 17 classes and represents a diverse group of marine, freshwater, and terrestrial algae. Classes are distinguished by morphology, chloroplast pigments, ultrastructural features, and gene sequence data. Electron microscopy and molecular biology have contributed significantly to our understanding of their evolutionary relationships, but even today class relationships are poorly understood. Haptophyte algae are a second monophyletic group that consists of two classes of predominately marine phytoplankton. The closest relatives of the haptophytes are currently unknown, but recent evidence indicates they may be part of a large assemblage (chromalveolates) that includes heterokont algae and other stramenopiles, alveolates, and cryptophytes. Heterokont and haptophyte algae are important primary producers in aquatic habitats, and they are probably the primary carbon source for petroleum products (crude oil, natural gas).     

A MOLECULAR PHYLOGENY OF THE HETEROKONT ALGAE BASED ON ANALYSES OF CHLOROPLAST-ENCODED rbcL SEQUENCE DATA1

Nearly complete ribulose-1,5-bisphosphate carboxylase/ oxygenase (rbcL)sequences from 27 taxa of heterokont algae were determined and combined with rbcL sequences obtained from GenBank for four other heterokont algae and three red algae. The phylogeny of the morphologically diverse haterokont algae was inferred from an unambiguously aligned data matrix using the red algae as the root, Significantly higher levels of mutational saturation in third codon positions were found when plotting the pair-wise substitutions with and without corrections for multiple substitutions at the same site for first and second codon positions only and for third positions only. In light of this observation, third codon positions were excluded from phylogenetic analyses. Both weighted-parsimony and maximum-likelihood analyses supported with high bootstrap values the monophyly of the nine currently recognized classes of heterokont algae. The Eustigmatophyceae were the most basal group, and the Dictyochophyceae branched off as the second most basal group. The branching pattern for the other classes was well supported in terms of bootstrap values in the weightedparsimony analysis but was weakly supported in the maximum-likelihood analysis (<50%). In the parsimony analysis, the diatoms formed a sister group to the branch containing the Chrysophyceae and Synurophyceae. This clade, charactetized by siliceous structures (frustules, cysts, scales), was the sister group to the Pelagophyceae/Sarcinochrysidales and Phaeo-/Xantho-/ Raphidophyceae clades. In the latter clade, the raphido-phytes were sister to the Phaeophyceae and Xanthophyceae. A relative rate test revealed that the rbcL gene in the Chrysophyceae and Synurophyceae has experienced a significantly different rate of substitutions compared to other classes of heterokont algae. The branch lengths in the maximum-likelihood reconstruction suggest that these two classes have evolved at an accelerated rate. Six major carotenoids were analyzed cladistically to study the usefulness of carotenoid pigmentation as a class-level character in the heterokont algae. In addition, each carotenoid was mapped onto both the rbcL tree and a consensus tree derived from nuclear-encoded small-subunit ribosomal DNA (SSU rDNA) sequences. Carotenoid pigmentation does not provide unambiguous phylogenetic information, whether analyzed cladistically by itself or when mapped onto phylogenetic trees based upon molecular sequence data.     

The evolution of stramenopiles and alveolates as derived by “substitution rate calibration» of small ribosomal subunit RNA

The substitution rate of the individual positions in an alignment of 750 eukaryotic small ribosomal subunit RNA sequences was estimated. From the resulting rate distribution, an equation was derived that gives a more precise relationship between sequence dissimilarity and evolutionary distance than hitherto available. Trees constructed on the basis of evolutionary distances computed by this new equation for small ribosomal subunit RNA sequences from ciliates, apicomplexans, dinoflagellates, oomycetes, hyphochytriomycetes, bicosoecids, labyrinthuloids, and heterokont algae show a more consistent tree topology than trees constructed in the absence of substitution rate calibration. In particular, they do not suffer from anomalies caused by the presence of extremely long branches.     

ENDOMEMBRANE STRUCTURE AND THE CHLOROPLAST PROTEIN TARGETING PATHWAY IN HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE, CHROMISTA)

Chloroplasts in heterokont algae are surrounded by four membranes and probably originated from a red algal endosymbiont that was engulfed and retained by eukaryotic host. Understanding how nuclear-encoded chloroplast proteins are translocated from the cytoplasm into the chloroplast across these membranes could give us some insights about how the endosymbiont was integrated into the host cell in the process of secondary symbiogenesis. In multiplastid heterokont algae such as raphidophytes, it has been unclear if the outermost of the four membranes surrounding the chloroplast (the chloroplast endoplasmic reticulum [CER] membrane) is continuous with the nuclear envelope and rough endoplasmic reticulum (ER). Here, we report detailed ultrastructural observations of the raphidophyte Heterosigma akashiwo (Hada) Hada ex Y. Hara et Chihara that show that the CER membranes were continuous with ER membranes that had attached ribosomes, implying that the chloroplast with three envelope membranes is located within the ER lumen, that is, topologically the same structure as that of monoplastid heterokont algae. However, the CER membrane of H. akashiwo had very few, if any, ribosomes attached, unlike the CER membranes in other heterokont algae. To verify that proteins are first targeted to the ER, we assayed protein import into canine microsomes using a precursor for a nuclear-encoded chloroplast protein, the fucoxanthin-chlorophyll a / c protein of H. akashiwo. This demonstrated that the precursor has a functional signal sequence for ER targeting and is cotranslationally translocated into the ER, where a signal sequence of about 17 amino acids is removed. Based on these data, we hypothesize that in H. akashiwo , nuclear-encoded chloroplast protein precursors that have been cotranslationally transported into the ER lumen are sorted in the ER and transported to the chloroplasts through the ER lumen.     

两性绵霉线粒体中存在丙酮酸激酶?另一种丙酮酸激酶基因(pyk2)的克隆及序列分析

在藻状菌纲两性绵霉 (Achlyabisexualis)的cDNA文库中筛选并克隆了另外一种类型的丙酮酸激酶基因pyk2 ,完成了这种基因的全序列测定 .将这种这种基因编码的丙酮酸激酶PYK2与前文报道的两性绵霉丙酮酸激酶PYK1氨基酸序列进行比较 ,显示二者有 4 0 8%同源性 .用PSORTII计算机程序分析表明 ,与存在于细胞质中的PYK1不同 ,PYK2的N端带有线粒体的导肽序列 ,因而可能位于线粒体中 .结合有关光合生物硅藻一些糖酵解酶位于线粒体的报道 ,认为不论是光合还是非光合的原生生物的线粒体中都可能存在有糖酵解酶 .     

PIGMENT COMPOSITION AND rbcL SEQUENCE DATA FROM THE SILICOFLAGELLATE DICTYOCHA SPECULUM: A HETEROKONT ALGA WITH PIGMENTS SIMILAR TO SOME HAPTOPH Y TES

Monophyly of plastids in the morphologically diverse heterokont algae has rarely been questioned. However, HPLC analysis revealed that the pigment composition of the silicoflagellate Dictyocha speculum Ehrenberg is similar to that observed in a group of haptophytes (“type 4”sensu Jeffrey and Wright 1994 . Dictyocha speculum and type 4 haptophytes possess acylfucoxanthins (19′-butanoyloxy- and 19′-hexanoyloxyfucoxanthin) in addition to fuco-, diadino-, and diatoxanthin and chl a, c, and c₃. The pigment composition of two pedinellids (Apedinella radians[Lohmann] Campbell and Mesopedinella arctica Daugbjerg), a sister group to D. speculum, deviates from D. speculum by lack of chl c ₃ and acylfucoxanthins. The distinct pigment composition suggested that plastid evolution in D. speculum differs from that of other heterokont algae. This prompted determination of the plastid-encoded rbcL gene from D. speculum to gain further insight into the evolutionary history of plastids in heterokont algae and haptophytes. A phylogenetic inference based on parsimony, maximum likelihood, and LogDet transformation methods included 35 heterokonts, 19 haptophytes, 8 red algae, and 1 cryptomonad. Three proteobacteria possessing type I RUBISCO were used to root the tree. In phylogenetic analyses, D. speculum was closely related to Rhizochromulina sp. and pedinellids, despite the latter possessing a different pigment composition. Surprisingly, the Dictyochophyceae clustered outside the lineage of heterokont algae but not within the haptophytes. Hence, analyses deduced from rbcL sequences indicated that the plastids in heterokont algae might have a more complex evolutionary history and that the shared pigment composition in D. speculum and type 4 haptophytes could be explained by convergent evolution or gene transfer. The pigment composition in D. speculum may have implications for pigment-based characterization of phytoplankton community structure in natural samples.     

Plastid genome sequence of the cryptophyte alga Rhodomonas salina CCMP1319: lateral transfer of putative DNA replication machinery and a test of chromist plastid phylogeny

Cryptophytes are a group of unicellular algae with chlorophyll c-containing plastids derived from the uptake of a secondary (i.e., eukaryotic) endosymbiont. Biochemical and molecular data indicate that cryptophyte plastids are derived from red algae, yet the question of whether or not cryptophytes acquired their red algal plastids independent of those in heterokont, haptophyte, and dinoflagellate algae is of long-standing debate. To better understand the origin and evolution of the cryptophyte plastid, we have sequenced the plastid genome of Rhodomonas salina CCMP1319: at 135,854 bp, it is the largest secondary plastid genome characterized thus far. It also possesses interesting features not seen in the distantly related cryptophyte Guillardia theta or in other red secondary plastids, including pseudogenes, introns, and a bacterial-derived gene for the tau/gamma subunit of DNA polymerase III (dnaX), the first time putative DNA replication machinery has been found encoded in any plastid genome. Phylogenetic analyses indicate that dnaX was acquired by lateral gene transfer (LGT) in an ancestor of Rhodomonas, most likely from a firmicute bacterium. A phylogenomic survey revealed no additional cases of LGT, beyond a noncyanobacterial type rpl36 gene similar to that recently characterized in other cryptophytes and haptophytes. Rigorous concatenated analysis of 45 proteins encoded in 15 complete plastid genomes produced trees in which the heterokont, haptophyte, and cryptophyte (i.e., chromist) plastids were monophyletic, and heterokonts and haptophytes were each other's closest relatives. However, statistical support for chromist monophyly disappears when amino acids are recoded according to their chemical properties in order to minimize the impact of composition bias, and a significant fraction of the concatenate appears consistent with a sister-group relationship between cryptophyte and haptophyte plastids.     

The Mammalian Pathogenic Oomycetes

The oomycetes are fungal-like microbes similar to those found within some members of the kingdom Fungi. Although these two groups of microbes share morphological features, there are several contrasting differences: a) phylogenetic analysis placed the oomycetes basal to plants and green algae; b) oomycetes lack ergosterol in their cytoplasmic membrane; c) chitin is not the main compound in the cell wall of oomycetes; and d) asexual reproduction in the oomycetes occurs by the development of sporangia containing numerous biflagellate zoospores. Pythium insidiosum was considered to be the only oomycete pathogenic for mammals. However, in 1999, Grooters reported that several dogs were diagnosed with an unusual oomycete in the genus Lagenidium causing extensive cutaneous and subcutaneous infections. Thereafter, the infection has been also reported in humans and cats, and it could possibly affect other mammalian species as well. This review highlights the epidemiological, clinical and pathological features, as well as the diagnosis and management of the infections caused by this unique group of mammalian pathogenic oomycetes.     

GENOMIC INSIGHTS INTO EVOLUTIONARY RELATIONSHIPS AMONG HETEROKONT LINEAGES EMPHASIZING THE PHAEOPHYCEAE1

Heterokonts comprise a large and diverse group of organisms unified by the heterokont biflagellate condition. Monophyly of many of these lineages is well established, but evolutionary relationships among the various lineages remain elusive. Among these lineages, the brown algae (Phaeophyceae) are a monophyletic, taxonomically diverse, and ecologically critical group common to marine environments. Despite their biological and scientific importance, consensus regarding brown algal phylogeny and taxonomic relationships is missing. Our long‐term research goal is to produce a well‐resolved taxon‐rich phylogeny of the class to assess evolutionary patterns and taxonomic relationships among brown algal lineages and their relationship to other closely related heterokont groups. To accomplish this goal and augment existing loci for phaeophycean‐wide systematic studies, we generated expressed sequence tags (ESTs) from several major brown algal lineages and from the heterokont lineage representing the closest sister group to brown algae. To date, we have successfully constructed cDNA libraries for two lineages (Choristocarpus tenellus Zanardini and Schizocladia ischiensis E. C. Henry, Okuda et H. Kawai) and in the library test phase obtained up to 1,600 ESTs per organism. Annotation results showed a gene discovery rate of 45%–50% for each library revealing 500–700 unique genes from each organism. We have identified several potential genes for phylogenetic inference and used these loci for preliminary molecular clock analyses. Our molecular clock analysis suggests that the basal divergence in brown algae occurred around the time of the pennate‐centric diatom divergence. Here we report this analysis and other uses of ESTs in brown algal phylogenomics and the utility of these data for resolving the phylogeny of this group.     

Evolution of filamentous plant pathogens: gene exchange across eukaryotic kingdoms

Filamentous fungi and oomycetes are eukaryotic microorganisms that grow by producing networks of thread-like hyphae, which secrete enzymes to break down complex nutrients, such as wood and plant material, and recover the resulting simple sugars and amino acids by osmotrophy. These organisms are extremely similar in both appearance and lifestyle and include some of the most economically important plant pathogens . However, the morphological similarity of fungi and oomycetes is misleading because they represent some of the most distantly related eukaryote evolutionary groupings, and their shared osmotrophic growth habit is interpreted as being the result of convergent evolution . The fungi branch with the animals, whereas the oomycetes branch with photosynthetic algae as part of the Chromalveolata . In this report, we provide strong phylogenetic evidence that multiple horizontal gene transfers (HGT) have occurred from filamentous ascomycete fungi to the distantly related oomycetes. We also present evidence that a subset of the associated gene families was initially the product of prokaryote-to-fungi HGT. The predicted functions of the gene products associated with fungi-to-oomycete HGT suggest that this process has played a significant role in the evolution of the osmotrophic, filamentous lifestyle on two separate branches of the eukaryote tree.     

Not in your usual Top 10: protists that infect plants and algae

Fungi, nematodes and oomycetes belong to the most prominent eukaryotic plant pathogenic organisms. Unicellular organisms from other eukaryotic lineages, commonly addressed as protists, also infect plants. This review provides an introduction to plant pathogenic protists, including algae infecting oomycetes, and their current state of research.     

Seaweeds in cold seas: evolution and carbon acquisition

Much evidence suggests that life originated in hydrothermal habitats, and for much of the time since the origin of cyanobacteria (at least 2.5 Ga ago) and of eukaryotic algae (at least 2.1 Ga ago) the average sea surface and land surface temperatures were higher than they are today. However, there have been at least four significant glacial episodes prior to the Pleistocene glaciations. Two of these (approx. 2.1 and 0.7 Ga ago) may have involved a 'Snowball Earth' with a very great impact on the algae (sensu lato) of the time (cyanobacteria, Chlorophyta and Rhodophyta) and especially those that were adapted to warm habitats. By contrast, it is possible that heterokont, dinophyte and haptophyte phototrophs only evolved after the Carboniferous-Permian ice age (approx. 250 Ma ago) and so did not encounter low (相似文献   

Ribosomal RNA Analysis Indicates a Benthic Pennate Diatom Ancestry for the Endosymbionts of the Dinoflagellates Peridinium foliaceum and Peridinium balticum (Pyrrhophyta)

ABSTRACT. The establishment of chloroplasts as cellular organelles in the dinoflagellate, heterokont (stramenopile), haptophyte, and cryptophyte algae is widely accepted to have been the result of secondary endosymbiotic events, that is, the uptake of a photosynthetic eukaryote by a phagotrophic eukaryote. However, the circumstances that promote such associations between two phylogenetically distinct organisms and result in the integration of their genomes to form a single functional photosynthetic cell is unclear. The dinoflagellates Peridinium foliaceum and Peridinium balticum are unusual in that each contains a membrance-bound eukaryotic heterokont endosymbiont. These symbioses have been interpreted, through data derived from ultrastructural and biochemical investigations, to represent an intermediate stage of secondary endosymbiotic chloroplast acquistion. In this study we have examined the phylogenetic origin of the P. foliaceum and P. Balticum heterokont endosymbionts through analaysis of their nuclear small subunit ribosomal RNA genes. Our analyses clearly demonstrate both endosymbionts are pennate diatoms belonging to the family Bacillariaceae. Since members of the Bacillariaceae are usually benthic, living on shallow marine sediments, the manner in which establishment of a symbiosis between a planktonic flagellated dinoflagellate and a botton-dwelling diatom is discussed. In particular, specific environmentally associated life strategy stages of the host and symbiont, coupled with diatom food preferences by the dinoflagellate, may have been vital to the formation of this association.