Acronema sippewissettensis Gen. Nov. Sp. Nov., microbial mat bicosoecid (Bicosoecales = Bicosoecida)

Structure-guided discovery of protein and glycan components in native mastigonemes

Mastigonemes, the hair-like lateral appendages lining cilia or flagella, participate in mechanosensation and cellular motion, but their constituents and structure have remained unclear. Here, we report the cryo-EM structure of native mastigonemes isolated from Chlamydomonas at 3.0 Å resolution. The long stem assembles as a super spiral, with each helical turn comprising four pairs of anti-parallel mastigoneme-like protein 1 (Mst1). A large array of arabinoglycans, which represents a common class of glycosylation in plants and algae, is resolved surrounding the type II poly-hydroxyproline (Hyp) helix in Mst1. The EM map unveils a mastigoneme axial protein (Mstax) that is rich in heavily glycosylated Hyp and contains a PKD2-like transmembrane domain (TMD). Mstax, with nearly 8,000 residues spanning from the intracellular region to the distal end of the mastigoneme, provides the framework for Mst1 assembly. Our study provides insights into the complexity of protein and glycan interactions in native bio-architectures.

Diversity of 'simple' multicellular eukaryotes: 45 independent cases and six types of multicellularity

Multicellularity evolved multiple times in the history of life, with most reviewers agreeing that it appeared at least 20 times in eukaryotes. However, a specific list of multicellular eukaryotes with clear criteria for inclusion has not yet been published. Herein, an updated critical review of eukaryotic multicellularity is presented, based on current understanding of eukaryotic phylogeny and new discoveries in microbiology, phycology and mycology. As a result, 45 independent multicellular lineages are identified that fall into six distinct types. Functional criteria, as distinct from a purely topological definition of a cell, are introduced to bring uniformity and clarity to the existing definitions of terms such as colony, multicellularity, thallus or plasmodium. The category of clonal multicellularity is expanded to include: (i) septated multinucleated thalli found in Pseudofungi and early-branching Fungi such as Chytridiomycota and Blastocladiomycota; and (ii) multicellular reproductive structures formed by plasmotomy in intracellular parasites such as Phytomyxea. Furthermore, (iii) endogeneous budding, as found in Paramyxida, is described as a form of multicellularity. The best-known case of clonal multicellularity, i.e. (iv) non-separation of cells after cell division, as known from Metazoa and Ochrophyta, is also discussed. The category of aggregative multicellularity is expanded to include not only (v) pseudoplasmodial forms, such a sorocarp-forming Acrasida, but also (vi) meroplasmodial organisms, such as members of Variosea or Filoreta. A common set of topological, geometric, genetic and life-cycle criteria are presented that form a coherent, philosophically sound framework for discussing multicellularity. A possibility of a seventh type of multicellularity is discussed, that of multi-species superorganisms formed by protists with obligatory bacterial symbionts, such as some members of Oxymonada or Parabasalia. Its inclusion is dependent on the philosophical stance taken towards the concepts of individuality and organism in biology. Taxa that merit special attention are identified, such as colonial Centrohelea, and a new speculative form of multicellularity, possibly present in some reticulopodial amoebae, is briefly described. Because of insufficient phylogenetic and morphological data, not all lineages could be unequivocally identified, and the true total number of all multicellular eukaryotic lineages is therefore higher, likely close to a hundred.

Ciliary mechanosensation - roles of polycystins and mastigonemes

Cilia are surface-exposed organelles that provide motility and sensory functions for cells, and it is widely believed that mechanosensation can be mediated through cilia. Polycystin-1 and -2 (PC-1 and PC-2, respectively) are transmembrane proteins that can localize to cilia; however, the molecular mechanisms by which polycystins contribute to mechanosensation are still controversial. Studies detail two prevailing models for the molecular roles of polycystins on cilia; one stresses the mechanosensation capabilities and the other unveils their ligand-receptor nature. The discovery that polycystins interact with mastigonemes, the 'hair-like' protrusions of flagella, is a novel finding in identifying the interactors of polycystins in cilia. While the functions of polycystins proposed by both models may coexist in cilia, it is hoped that a precise understanding of the mechanism of action of polycystins can be achieved by uncovering their distribution and interacting factors inside cilia. This will hopefully provide a satisfying answer to the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD), which is caused by mutations in PC-1 and PC-2. In this Review, we discuss the characteristics of polycystins in the context of cilia and summarize the functions of mastigonemes in unicellular ciliates. Finally, we compare flagella and molecular features of PC-2 between unicellular and multicellular organisms, with the aim of providing new insights into the ciliary roles of polycystins in general.

Global comparison of bicosoecid Cafeteria-like flagellates from the deep ocean and surface waters, with reorganization of the family Cafeteriaceae

Cafeteria is one of the most common and ecologically significant genera of heterotrophic nanoflagellates in marine plankton. We could isolate and cultivate 29 strains morphologically similar to Cafeteria obtained from surface waters and the deep sea all over the world's ocean. Morphological characterization obtained by high resolution microscopy revealed only small differences between the strains. Sequencing the type material of the type species C. roenbergensis (CCAP 1900/1) and molecular analyses (18S rDNA, 28S rDNA) of newly isolated strains resulted in a revision and separation of the Cafeteriaceae into two known species (C. roenbergensis, C. mylnikovii) and six new species (C. maldiviensis, C. biegae, C. loberiensis, C. chilensis, C. graefeae, C. burkhardae). Many deposited Cafeteria sequences at GenBank and most of our own sequences clustered within one clade (C. burkhardae) with a p-distance of 5% to strain CCAP 1900/1. Only C. maldiviensis clustered together with the type species C. roenbergensis. While C. burkhardae seems to have a cosmopolitan distribution, the distribution of the other species seems to be more restricted. A strain from the Angola Basin had a p-distance of 10% to Cafeteria species and clustered separately within the Anoecales requiring the erection of a new genus, Bilabrum gen. nov., with B. latius sp. nov. as type species.

Bothrosome Formation in Schizochytrium aggregatum (Labyrinthulomycetes, Stramenopiles) during Zoospore Settlement

Labyrinthulomycetes are characterized by the presence of ectoplasmic nets originating from an organelle known as the bothrosome, whose evolutionary origin is unclear. To address this issue, we investigated the developmental process from a zoospore to a vegetative cell in Schizochytrium aggregatum. After disappearance of the flagellum during zoospore settlement, the bothrosome emerged at the anterior-ventral pole of the cells. A new Golgi body also appeared at this stage, and the bothrosome was positioned close to both the new and the old Golgi bodies. This observation suggested that the Golgi body is related to the formation of the bothrosome. Actin appeared as a spot in the same location as the newly appeared bothrosome, as determined by immunofluorescence labeling. An immunoelectron microscopic analysis revealed that actin was present in the ectoplasmic nets and in the cytoplasm around the bothrosome, indicating that the electron-dense materials of the bothrosome are not the polar center of F-actin. This suggests that actin filaments pull the endoplasmic reticulum to the bothrosome and induce the membrane to become evaginated within ectoplasmic nets.

Phylogeny of Heterokonta: Incisomonas marina, a uniciliate gliding opalozoan related to Solenicola (Nanomonadea), and evidence that Actinophryida evolved from raphidophytes

Environmental rDNA sequencing has revealed many novel heterokont clades of unknown morphology. We describe a new marine heterotrophic heterokont flagellate, Incisomonas marina, which most unusually lacks an anterior cilium. It glides and swims with its cilium trailing behind, but is predominantly sedentary on the substratum, with or without a cilium. 18S rDNA sequence phylogeny groups Incisomonas strongly within clade MAST-3; with others it forms a robust sister clade to Solenicola, here grouped with it as new order Uniciliatida, placed within new class Nanomonadea encompassing MAST-3. Our comprehensive maximum likelihood heterokont phylogeny shows Nanomonadea as sister to MAST-12 plus Opalinata within Opalozoa, and that Actinophryida are not Opalozoa (previously suggested by distance trees), but highly modified raphidomonads, arguably related to Heliorapha (formerly Ciliophrys) azurina gen., comb. n. We discuss evolution of Actinophryida from photosynthetic raphidophytes. Clades MAST-4,6-11 form one early-branching bigyran clade. Olisthodiscus weakly groups with Hypogyristea not Raphidomonadea. Phylogenetic analysis shows that MAST-13 is all Bicosoeca. Some gliding uniciliates similar to Incisomonas marina seem to have been misclassified: therefore we establish Incisomonas devorata comb. n. for Rigidomastix devoratum, revise the genus Rigidomastix, transfer Clautriavia parva to Kiitoksia. We make 17 new familes (13 heterokont (three algal), two cercozoan, two amoebozoan).

A new class of the stramenopiles, Placididea Classis nova: description of Placidia cafeteriopsis gen. et sp. nov

A marine flagellate resembling Cafeteria roenbergensis (bicosoecids, stramenopiles) in cell shape and behavior of the cell while attached to substratum was collected from the coast of Japan. The flagellate was examined by light and electron microscopy, and the 18S rDNA was sequenced to elucidate its taxonomic and phylogenetic position. Ultrastructural features suggested that the flagellate is not a bicosoecid, but a relative of the recently described stramenopile, Wobblia lunata. 18S rDNA phylogenetic trees also revealed that the flagellate forms a monophyletic clade with W. lunata and that it is distantly related to Cafeteria and other bicosoecids. The flagellate differs from W. lunata due to its lack of wobbling motion as well as intracellular features such as the number of mitochondria, flagellar apparatus architecture, the presence of a paranuclear body and cytoplasmic microtubules. The similarity of 18S rDNA sequences was 81% between the flagellate and W. lunata. This new flagellate was described as Placidia cafeteriopsis gen. et sp. nov. Because the phylogenetic lineage comprised of W. lunata and P. cafeteriopsis was one of the major, deep-branching clades of the stramenopiles, the class Placididea (= Placidiophyceae) classis nova was proposed.

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.

Ultrastructure and 18S rDNA sequence analysis of Wobblia lunata gen. et sp. nov., a new heterotrophic flagellate (Stramenopiles, Incertae sedis)

A new heterotrophic flagellate Wobblia lunata gen. et sp. nov. is described. This organism usually attaches to the substratum showing a wobbling motion, and sometimes glides on the substratum or swims freely in the medium. W. lunata has various features characteristic of the stramenopiles. These include a hairy flagellum with tripartite tubular hairs, a mitochondrion with tubular cristae, arrangement of flagellar apparatus components and a double helix in the flagellar transition zone. W. lunata shares a double helix with heterotrophic stramenopiles, including Developayella elegans, oomycetes, hyphochytrids, opalinids and proteromonads, and could be placed in the phylum Bigyra Cavalier-Smith. However, from 18S rDNA tree analysis, these organisms form two distantly-related clades in the stramenopiles, and Wobblia appears at the base of the stramenopiles. Evaluation of morphological features and comparison of 18S rDNA sequences indicate that W. lunata is a member of the stramenopiles, but it is distinct from any other stramenopiles so far described. Its phylogenetic position within the stramenopiles is uncertain and therefore W. lunata is described as a stramenopile incertae sedis.

Symbiomonas scintillans gen. et sp. nov. and Picophagus flagellatus gen. et sp. nov. (Heterokonta): two new heterotrophic flagellates of picoplanktonic size

Two new oceanic free-living heterotrophic Heterokonta species with picoplanktonic size (< 2 microm) are described. Symbiomonas scintillans Guillou et Chrétiennot-Dinet gen. et sp. nov. was isolated from samples collected both in the equatorial Pacific Ocean and the Mediterranean Sea. This new species possesses ultrastructural features of the bicosoecids, such as the absence of a helix in the flagellar transitional region (found in Cafeteria roenbergensis and in a few bicosoecids), and a flagellar root system very similar to that of C. roenbergensis, Acronema sippewissettensis, and Bicosoeca maris. This new species is characterized by a single flagellum with mastigonemes, the presence of endosymbiotic bacteria located close to the nucleus, the absence of a lorica and a R3 root composed of a 6+3+x microtubular structure. Phylogenetical analyses of nuclear-encoded SSU rDNA gene sequences indicate that this species is close to the bicosoecids C. roenbergensis and Siluania monomastiga. Picophagus flagellatus Guillou et Chrétiennot-Dinet gen. et sp. nov. was collected in the equatorial Pacific Ocean. Cells are naked and possess two flagella. This species is characterized by the lack of a transitional helix and lateral filaments on the flagellar tubular hairs, the absence of siliceous scales, two unequal flagella, R1 + R3 roots, and the absence of a rhizoplast. SSU rDNA analyses place this strain at the base of the Chrysophyceae/Synurophyceae lineages.