Ultrastructural analysis of flagellar development in plurilocular sporangia of Ectocarpus siliculosus (Phaeophyceae)

A phylogenetic profiling approach identifies novel ciliogenesis genes in Drosophila and C. elegans

Cilia are cellular projections that perform sensory and motile functions in eukaryotic cells. A defining feature of cilia is that they are evolutionarily ancient, yet not universally conserved. In this study, we have used the resulting presence and absence pattern in the genomes of diverse eukaryotes to identify a set of 386 human genes associated with cilium assembly or motility. Comprehensive tissue-specific RNAi in Drosophila and mutant analysis in C. elegans revealed signature ciliary defects for 70-80% of novel genes, a percentage similar to that for known genes within the cluster. Further characterization identified different phenotypic classes, including a set of genes related to the cartwheel component Bld10/CEP135 and two highly conserved regulators of cilium biogenesis. We propose this dataset defines the core set of genes required for cilium assembly and motility across eukaryotes and presents a valuable resource for future studies of cilium biology and associated disorders.

Ultrastructural observations of mitochondrial morphology through the life cycle of the brown alga, Mutiomo cylindricus (Cutleriaceae, Tilopteridales)

Mitochondrial morphology varies according to development and the physiological conditions of the cell. Here, we performed electron tomography using serial sections to analyze the number, individual volume, and morphological complexity of mitochondria in the cells across two generations in the life cycle of the brown alga Mutimo cylindricus. This species shows a heteromorphic alternation of generations between the macroscopic gametophyte and the crustose sporophyte during its life cycle and displays anisogamous sexual reproduction. We observed the mitochondria in the vegetative cells of gametophytes and sporophytes to mainly show tubular or discoidal shapes with high morphological complexity. The morphology of the mitochondria in the male and female gametes changed to a nearly spherical or oval shape from a tubular or discoidal shape before release. In this species, degradation of the paternal mitochondria was observed in the zygote 2 h after fertilization. Morphological changes in the mitochondria were not observed until 6 h after fertilization. Twenty-four-hour-old zygotes before and after cytokinesis showed a similar number of mitochondria as 6-h-old zygotes; however, the volume and morphological complexity increased. The results indicated that the maternal mitochondria did not undergo fission or fusion until this stage. Based on the analysis results of the number and total volume of mitochondria before and after the release of the gametes, it is possible that the mitochondria in the female gametes fuse immediately before release.

Phototaxis and chemotaxis of brown algal swarmers

Brown algae exhibit three patterns of sexual reproduction: isogamy, anisogamy, and oogamy. Unicellular swarmers including gametes and zoospores bear two heterogenous flagella, an anterior flagellum with mastigonemes (fine tripartite hairs) and a posterior one. In seawater, these flagellates usually receive physico-chemical signals for finding partners and good habitats. It is well known that brown algal swarmers change their swimming direction depending on blue light (phototaxis), and male gametes do so, based on the sex pheromones from female gametes (chemotaxis). In recent years, the comparative analysis of chemotaxis in isogamy, anisogamy, and oogamy has been conducted. In this paper, we focused on the phototaxis and chemotaxis of brown algal gametes comparing the current knowledge with our recent studies.

Distribution of alginate and cellulose and regulatory role of calcium in the cell wall of the brown alga Ectocarpus siliculosus (Ectocarpales, Phaeophyceae)

MAIN CONCLUSION

This work investigated a correlation between the three-dimensional architecture and compound-components of the brown algal cell wall. Calcium greatly contributes to the cell wall integrity. Brown algae have a unique cell wall consisting of alginate, cellulose, and sulfated polysaccharides. However, the relationship between the architecture and the composition of the cell wall is poorly understood. Here, we investigated the architecture of the cell wall and the effect of extracellular calcium in the sporophyte and gametophyte of the model brown alga, Ectocarpus siliculosus (Dillwyn) Lyngbye, using transmission electron microscopy, histochemical, and immunohistochemical studies. The lateral cell wall of vegetative cells of the sporophyte thalli had multilayered architecture containing electron-dense and negatively stained fibrils. Electron tomographic analysis showed that the amount of the electron-dense fibrils and the junctions was different between inner and outer layers, and between the perpendicular and tangential directions of the cell wall. By immersing the gametophyte thalli in the low-calcium (one-eighth of the normal concentration) artificial seawater medium, the fibrous layers of the lateral cell wall of vegetative cells became swollen. Destruction of cell wall integrity was also induced by the addition of sorbitol. The results demonstrated that electron-dense fibrils were composed of alginate-calcium fibrous gels, and electron negatively stained fibrils were crystalline cellulose microfibrils. It was concluded that the spatial arrangement of electron-dense fibrils was different between the layers and between the directions of the cell wall, and calcium was necessary for maintaining the fibrous layers in the cell wall. This study provides insights into the design principle of the brown algal cell wall.

Ubiquitous distribution of helmchrome in phototactic swarmers of the stramenopiles

Most swarmers (swimming cells) of the stramenopile group, ranging from unicellular protist to giant kelps (brown algae), have two heterogeneous flagella: a long anterior flagellum (AF) and a relatively shorter posterior flagellum (PF). These flagellated cells often exhibit phototaxis upon light stimulation, although the mechanism by which how the phototactic response is regulated remains largely unknown. A flavoprotein concentrating at the paraflagellar body (PFB) on the basal part of the PF, which can emit green autofluorescence under blue light irradiance, has been proposed as a possible blue light photoreceptor for brown algal phototaxis although the nature of the flavoprotein still remains elusive. Recently, we identified helmchrome as a PF-specific flavoprotein protein in a LC-MS/MS-based proteomics study of brown algal flagella (Fu et al. 2014). To verify the conservation of helmchrome, in the present study, the absence or presence and the localization of helmchrome in swarmers of various algal species were investigated. The results showed that helmchrome was only detected in phototactic swarmers but not the non-phototactic ones of the stramenopile group. Electron microscopy further revealed that the helmchrome detectable swarmers bear a conserved PFB-eyespot complex, which may serve as structural basis for light sensing. It is speculated that all three conserved properties: helmchrome, the PFB structure, and the eyespot apparatus, will be essential parts for phototaxis of stramenopile swarmers.

Proteomics analysis of heterogeneous flagella in brown algae (stramenopiles)

Flagella are conserved organelles among eukaryotes and they are composed of many proteins, which are necessary for flagellar assembly, maintenance and function. Stramenopiles, which include brown algae, diatoms and oomycetes, possess two laterally inserted flagella. The anterior flagellum (AF) extends forward and bears tripartite mastigonemes, whilst the smooth posterior flagellum (PF) often has a paraflagellar body structure. These heterogeneous flagella have served as crucial structures in algal studies especially from a viewpoint of phylogeny. However, the protein compositions of the flagella are still largely unknown. Here we report a LC-MS/MS based proteomics analysis of brown algal flagella. In total, 495 flagellar proteins were identified. Functional annotation of the proteome data revealed that brown algal flagellar proteins were associated with cell motility, signal transduction and various metabolic activities. We separately isolated AF and PF and analyzed their protein compositions. This analysis led to the identification of several AF- and PF-specific proteins. Among the PF-specific proteins, we found a candidate novel blue light receptor protein involved in phototaxis, and named it HELMCHROME because of the steering function of PF. Immunological analysis revealed that this protein was localized along the whole length of the PF and concentrated in the paraflagellar body.