The Life Cycle of Cryptochlora perforans (Chlorarachniophyta)

Observations on the behaviour of different life cycle stages, gamete fusions, and measurements of nuclear DNA contents in Cryptochlora perforans resulted in a first concept concerning life histories in Chlorarachniophyta: the life cycle of Cr. perforans is diplohaplontic (gamete fusion with karyogamy - mitosis - meiosis - mitosis). In the haploid as well as in the diploid life cycle phases amoeboid and coccoid stages occur. The isomorphic gametes are modified amoebae frequently without filopodia. Only haploid flagellate stages are known representing mito- or meiozoospores. Diploid coccoid stages have a granular cytoplasmic structure and may be somewhat larger than haploid ones. Nevertheless, positive identification of haploid (gametophytic) and diploid (sporophytic) stages is only possible on the basis of nuclear DNA contents.     

Mechanism of Protein Targeting to the Chlorarachniophyte Plastids and the Evolution of Complex Plastids with Four Membranes — A Hypothesis

Chlorarachniophyta are phototrophic amoeboflagellates, with plastids surrounded by four membranes. Contrary to other plastids of this type which occur in chromists, their outermost membrane bears no ribosomes. It is argued that the nuclear-encoded chlorarachniophyte plastid proteins are first transported into the ER, then to the Colgi apparatus, and finally to the plastids. The same import mechanism could be originally present in the chromist ancestor, prior to the fusion of their plastids with the RER membranes. According to the most recent concept, the complex plastids of Chromista and Chlorarachniophyta have evolved through replacement of the cyanobacterial plastids. The assumption that these plastids had an envelope composed not of two, but of three membranes makes it possible to avoid the erlier discerned difficulties with conversion of a eukaryotic alga into a complex plastid. My scenario provides an additional support to the hypothesis on polyphy-letic origin of four-membraned plastids.     

Arrangement of F-actin and microtubules in the pseudopodia ofCryptochlora perforans (Chlorarachniophyta)

Summary The distribution of actin and the arrangement of microtubules within the filopodia of amoeboid stages of Chlorarachniophyta were studied inCryptochlora perforans by indirect immunofluorescence. Actin is located along the whole pseudopodium, but at different concentrations. Microtubules run like coiled cables throughout the length of the pseudopodium. At the leading edges the pseudopodium frequently appears fan-shaped and the microtubules then show a spread-out arrangement, but they do not reach the cytoplasm front. Colchicine inhibited particle motility in the filopodia. The particle transport seems to be insensitive to cytochalasin D, but cells contracted their filopodia.Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement     

The RAB Family GTPase Rab1A from Plasmodium falciparum Defines a Unique Paralog Shared by Chromalveolates and Rhizaria

ABSTRACT. The RAB GTPases, which are involved in regulation of endomembrane trafficking, exhibit a complex but incompletely understood evolutionary history. We elucidated the evolution of the RAB1 subfamily ancestrally implicated in the endoplasmic reticulum-to-Golgi traffic. We found that RAB1 paralogs have been generated over the course of eukaryotic evolution, with some duplications coinciding with the advent of major eukaryotic lineages (e.g. Metazoa, haptophytes). We also identified a unique, derived RAB1 paralog, orthologous to the Plasmodium Rab1A, that occurs in stramenopiles, alveolates, and Rhizaria, represented by the chlorarachniophyte Gymnochlora stellata . This finding is consistent with the recently documented existence of a major eukaryotic clade ("SAR") comprising these three lineages. We further found a Rab1A-like protein in the cryptophyte Guillardia theta , but it exhibits unusual features among RAB proteins: absence of a C-terminal prenylation motif and an N-terminal extension with two MSP domains; and its phylogenetic relationships could not be established convincingly due to its divergent nature. Our results nevertheless point to a unique membrane trafficking pathway shared by at least some lineages of chromalveolates and Rhizaria, an insight that has implications towards interpreting the early evolution of eukaryotes and the endomembrane system.     

Origins of the secondary plastids of Euglenophyta and Chlorarachniophyta as revealed by an analysis of the plastid‐targeting,nuclear‐encoded gene psbO1

Because the secondary plastids of the Euglenophyta and Chlorarachniophyta are very similar to green plant plastids in their pigment composition, it is generally considered that ancestral green algae were engulfed by other eukaryotic host cells to become the plastids of these two algal divisions. Recent molecular phylogenetic studies have attempted to resolve the phylogenetic positions of these plastids; however, almost all of the studies analyzed only plastid‐encoded genes. This limitation may affect the results of comparisons between genes from primary and secondary plastids, because genes in endosymbionts have a higher mutation rate than the genes of their host cells. Thus, the phylogeny of these secondary plastids must be elucidated using other molecular markers. Here, we compared the plastid‐targeting, nuclear‐encoded, oxygen‐evolving enhancer (psbO) genes from various green plants, the Euglenophyta and Chlorarachniophyta. A phylogenetic analysis based on the PsbO amino acid sequences indicated that the chlorarachniophyte plastids are positioned within the Chlorophyta (including Ulvophyceae, Chlorophyceae, and Prasinophyceae, but excluding Mesostigma). In contrast, plastids of the Euglenophyta and Mesostigma are positioned outside the Chlorophyta and Streptophyta. The relationship of these three phylogenetic groups was consistent with the grouping of the primary structures of the thylakoid‐targeting domain and its adjacent amino acids in the PsbO N‐terminal sequences. Furthermore, the serine‐X‐alanine (SXA) motif of PsbO was exactly the same in the Chlorarachniophyta and the prasinophycean Tetraselmis. Therefore, the chlorarachniophyte secondary plastids likely evolved from the ancestral Tetraselmis‐like alga within the Chlorophyta, whereas the Euglenophyte plastids may have originated from the unknown basal lineage of green plants.     

Taxonomic studies on the Chlorarachniophyta. II. Generic delimitation of the chlorarachniophytes and description of Gymnochlora stellata gen. et sp. nov. and Lotharella gen. nov.

A new chlorarachniophytan alga, Gymnochlora stellata Ishida et Y. Hara gen. et sp. nov., has been isolated from Anae Island in Guam. It is a green, star-shaped, unicellular, amoeboid organism with several filopodia that do not form a reticulopodial network. Neither zoospores nor walled coccoid cells have been observed throughout the life cycle. The chloroplast ultrastructure is similar to those of described species; however, the pyrenoid matrix, which is invaded by many tubular structures originating from the inner membrane of the chloroplast envelope, is unique. A classification system is proposed for the Chlorarachniophyta. In this system, the ultrastructural features of the pyrenoid and the location of the nucleomorph in the periplastidial compartment are used as generic criteria, while the morphological features of the vegetative cells and life cycle patterns are used for species criteria. The described species, except for Cryptochlora perforans Calderon-Saenz et Schnetter, are also reassessed under the new system, and consequent nomenclatural requirements for the genus Chlorarachnion are dealt with in this paper. The taxonomic rank of a previously described species, Chlorarachnion globosum Ishida et Y. Hara, is elevated and Lotharella globosa (Ishida et Y. Hara) Ishida et Y. Hara gen. nov. et comb. nov. is proposed.     

CYTOLOGY AND ULTRASTRUCTURE OF CHLORARACHNION REPTANS (CHLORARACHNIOPHYTA DIVISIO NOVA,CHLORARACHNIOPHYCEAE CLASSIS NOVA)1

The green amoeboid cells of Chlorarachnion reptans Geitler are completely naked and each contains a central nucleus, several bilobed chloroplasts each with a central projecting pyrenoid enveloped by a capping vesicle, several Golgi bodies, mitochondria with tubular cristae, extensive rough ER, and a distinct layer of peripheral vesicles. Complex extrusome-like organelles occur rarely in both the amoeboid and flagellate stages. The only organelles entering the reticulopodia are mitochondria, but microtubules are also present. The chloroplasts contain chlorophylls a and b, but histochemical tests suggest that the carbohydrate storage product probably is not a starch. The chloroplast lamellae are composed of one to three thylakoids or form deep stacks. A girdle lamella and interlamellar partitions are absent. Each chloroplast is bounded by either four separate membranes, a pair of membranes with vesicular profiles between them, or three membranes; all three arrangements may occur in the same chloroplast. A periplastidal compartment occurs near the base of the pyrenoid where there are always four surrounding membranes. The compartment has a relatively dense matrix and contains ribosome-like particles and small dense spheres; it extends over and into a deep invagination in the pyrenoid where its contents are enclosed in a double-membraned envelope which is penetrated by wide pores. The zoospores are ovoid and each bears a single laterally inserted flagellum which appears to be wrapped helically around the cell body during swimming. The flagellum lies in a groove in the cell surface and bears fine lateral hairs. Neither a second flagellum or vestige of one, nor an eyespot, is present. A single microtubular root and a larger homogeneous root run from the flagellar base parallel to the emerging flagellum, between the nuclear envelope and the plasmalemma. In the simple flagellar transition region, fine filaments connect adjacent axonemal doublets. A detailed comparison of C. reptans with all other algal taxa results in the conclusion that it must be segregated in the new class Chlorarachniophyceae, the only class in the new division Chlorarachniophyta. The possibility that C. reptans evolved from a symbiosis between a colorless amoeboid cell and a chlorophyll b- containing eukaryote is considered, but the possible affinities of the symbiont remain enigmatic. The implications of the unique chloroplast structure of C. reptans for current hypotheses concerning the origin of chloroplasts are discussed.     

The Phylogenetic Position of Alpha- and Beta-Tubulins from the Chlorarachnion Host and Cercomonas (Cercozoa)

Alpha and beta-tubulin genes from Chlorarachnion and an alpha-tubulin gene from Cercomonas have been characterised. We found the Cercomonas and Chlorarachnion alpha-tubulins to be closely related to one another, confirming the proposed relationship of these genera. In addition, the Chlorarachnion host and Cercomonas also appear to be more distantly related to Heterolobosea. Euglenozoa, chlorophytes. Heterokonts, and alveolates. Chlorarachnion was also found to have two distinctly different types of both alpha- and beta-tubulin. one type being highly-divergent. Chlorarachnion contains a secondary endosymbiont of green algal origin, raising the possibility that one type of Chlorarachnion tubulins comes from the host and the other from the endosymbiont. Probing pulsed field-separated chromosomes showed that the highly-divergent genes are encoded by the host genome, and neither alpha- nor beta-tubulin cDNAs were found to include 5' extensions that might serve as targeting peptides. It appears that Chlorarachnion has distinct and divergent tubulin paralogues that are all derived from the host lineage. One Chlorarachnion beta-tubulin was also found to be a pseudogene, which is still expressed but aberrantly processed. Numerous unspliced introns and deletions resulting from mis-splicing are contained in the mRNAs from this gene.