Volume 52, Issue 2 - Special Issue: Advances in Algal Cell Biology and Genomics

No abstract available

Plants and algae acquired photosynthesis through the assimilation of a prokaryotic endosymbiont related to the ancestors of modern-day cyanobacteria. This landmark event, known as the primary endosymbiotic origin of plastids, is generally thought to have occurred only once during the history of eukaryotes and to have given rise to the plastids of green algae, land plants, red algae and glaucophyte algae through vertical evolution. Plastids have also spread horizontally across the tree of eukaryotes by “secondary” endosymbioses involving heterotrophic host eukaryotes and both green and red algal endosymbionts. Here I provide an overview of current research in the area of plastid evolution, focusing on the latest advances in the field of algal comparative genomics. Recent genome-scale analyses of both photosynthetic and non-photosynthetic eukaryotes have provided fresh new insight into the pattern and process of secondary endosymbiosis, although it is still not possible to discern with confidence the number of endosymbiotic events that gave rise to the known spectrum of eukaryotic phototrophs. In fact, with more genomic data has come the intriguing possibility that the nuclear genomes of some secondary plastid-containing algae are a mosaic of genes derived from multiple endosymbioses, adding yet another layer of complexity to the convoluted evolutionary history of these fascinating organisms.

Phaeodactylum tricornutum is a pleiomorphic diatom that exists in three different forms depending on environmental conditions, viz. oval, fusiform or triradiate. In spite of numerous observations of the three morphotypes, very little information is available about the process of transformation. In this review, we summarize results of previous studies and add new observations on cell wall structure of the three morphotypes and the environmental conditions under which each form appears (for different strains of Phaeodactylum ). New data on the cellular mechanisms of transformation between the three morphotypes and on the differentiation of oval cells are also presented. The ultrastructural morphology of the three morphotypes and strain/genotype-dependent transformations of morphotypes are discussed.

Sorting of soluble vacuolar proteins destined for vacuoles is facilitated by vacuolar sorting receptors (VSRs). To date, three different types of VSRs have been described, namely the mammalian mannose-6-phosphate receptor, the yeast VPS10p receptor, and a plant-type receptor. We searched completed algal genomes for the presence of the plant-type VSRs and examined their structure and phylogeny. We show that green algae and some chromalveolates contain a plant-type vacuolar receptor that seems to be absent in glaucophytes and rhodophytes. All putative plant-type protistan VSRs examined have a similar protein structure. The results of a phylogenetic analysis suggest that plant-type VSRs may have evolved in the last common ancestor of the Viridiplantae and that chromalveolates obtained this type of VSR by horizontal or endocytotic gene transfer. Gene duplications took place in several viridiplant and chromalveolate lineages. We present hypotheses about the evolution of paralogous plant-type VSRs in chlorophytes and chromalveolates.

We studied mitosis and cytokinesis during spermatogenesis in the red alga Porphyra yezoensis Ueda using electron microscopy with the cryofixation-freeze-substitution technique and by fluorescence microscopy using tubulin immunofluorescence and SYBR Gold staining. Spermatogenesis occurred along the blade margin with a series of cell divisions. During interphase, two nucleus-associated organelles (NAOs) were located close to the nuclear envelope and had a ring-on-ring morphology. In prophase, the NAOs migrated to the future mitotic poles. The nuclear envelope of prometaphase nuclei became deeply depressed below each NAO, and the two rings of the NAOs appeared to separate until early telophase. The chromosomes became aligned at the equator in metaphase, although individual chromosomes and their kinetochores were not clearly defined. The two rings of the NAO moved next to each other on the nuclear envelope during mid-telophase, and then NAO replication occurred. In late telophase, the pair of NAOs repositioned on the nuclear envelope. Cytokinesis started with furrowing of the plasma membrane.

Functional (with centrioles) and non-functional (without centrioles) mitotic spindle formation was studied in Fucus distichus zygotes. In normally fertilized zygotes, a pair of centrioles derived from sperm existed at both mitotic poles. Both centrin and γ-tubulin were associated with centrioles. In multipolar spindles of fertilized multinucleate eggs, mitotic poles with centrioles were stained with an anti-γ-tubulin antibody, but the other poles were not, and cytokinesis proceeded abnormally. When karyogamy was blocked with colchicine, centrioles remained around the sperm nucleus. A monastral spindle or a bipolar, barrel-shaped spindle was formed on the egg nucleus; however, there were no centrioles at these mitotic poles. When cytoplasmic fragments containing only an egg or a sperm nucleus were separately prepared and cultured, the egg nucleus appeared to enter, but did not complete, mitosis. In cytoplasm containing only a sperm nucleus, mitosis and cytokinesis did not occur. We concluded that the egg nucleus has the potential to form a non-functional spindle without the sperm centrioles. However, this acentrosomal spindle was non-functional; mitosis remained at the stage of chromosome condensation and did not proceed to cytokinesis. Even though some cells proceeded to cytokinesis, they showed abnormal chromosome condensation.

We studied development of the cytokinetic diaphragm (i.e., the membranous septum formed during cytokinesis) in vegetative cells of the brown algae Sphacelaria rigidula , Halopteris congesta and Dictyota dichotoma after cryofixation-freeze-substitution. Cytokinesis began by the gathering of organelles and large vesicles between daughter nuclei following telophase. Subsequently, a thin cytoplasmic strand was formed along this plane, where endoplasmic reticulum (ER), dictyosome vesicles and particular membranous elements, the flat cisternae, were accumulated. Their fusion formed a patchy diaphragm with irregular gaps. Fine tubular channels perforated the diaphragm during all stages of its formation. Following diaphragm completion, cell wall material was deposited in it. The new walls had ER-free plasmodesmata. In D. dichotoma , diaphragm development did not follow a definite pattern, i.e., centripetal or centrifugal, a phenomenon also confirmed in H. congesta . In contrast, in apical cells of S. rigidula the diaphragm started developing from the periphery, growing to some extent centripetally. In these cells, local cell wall deposition was greatest at the division site. In apical cells in which cytokinesis was experimentally inhibited a ring of wall material was usually deposited at the cytokinetic plane.

The spermatozoid release process from antheridial filaments of Chara contraria was studied using cytoskeletal drugs and ATPase inhibitors. The drugs did not change the onset of spermatozoid release, but affected other factors. The myosin ATPase inhibitor 2,3-butanedione monoxime (BDM) inhibited spermatozoid movement before release, and the microtubule (MT) inhibitor oryzalin and the F-actin inhibitor latrunculin B (LatB) accelerated spermatozoid movement after release. Spermatozoids released into 10 and 20 mM BDM solutions were completely immobile and the nucleus was deformed. None of the three drugs affected spermatozoid release rate, but they synchronized the release between individual antheridial filaments. After release, the size of the spiral-shaped spermatozoid increased over time. This size increase was reduced by oryzalin but increased by LatB. The data suggest that MTs and F-actin are involved in spermatozoid release and mobility in Chara contraria . The spiral shape of the nucleus of the spermatozoid is affected by both MTs and F-actin.

Bioluminescence in Pyrocystis lunula depends on circadian movement of scintillons and chloroplasts between active and inactive locations. The involvement of the cytoskeleton in these transport processes was investigated using cytoskeleton-specific drugs. Chloroplast movements were visualized by chlorophyll autofluorescence and confocal scanning laser microscopy. Latrunculin B and Oryzalin prevented translocation of chloroplasts. Four hours after transition to the night phase, mechanically inducible bioluminescence was inhibited by Latrunculin B, the myosin inhibitor 2,3 butanedione monoxime, and Oryzalin in dose-dependent fashions. The actin stabilizer Jasplakinolide slightly enhanced bioluminescence but eliminated Latrunculin B effects. The microtubule depolymerizer Colchicine had no effect on bioluminescence. These results suggest that F-actin and microtubules are involved in the movements of chloroplasts and scintillons. Although the drugs tested affected bioluminescence, further experiments are needed to determine whether effects on bioluminescence were mediated by drug-induced failed translocations of scintillons or chloroplasts.

A low temperature up-regulated gene was isolated from Spirogyra varians using the differentially expressed gene method. The full cDNA sequence consisted of 1798 bp with an open reading frame of 1560 bp encoding 520 amino acid residues, which had neither a cleavage site nor a signal peptide. The deduced amino acid sequence was highly similar to the bi-functional 3-dehydroquinate dehydratase/shikimate dehydrogenase (DHQ/SDH) gene of higher plants. As the gene was cold regulated, we named it SVCR3 ( Spirogyra varians cold regulated gene). SVCR3 had two conserved domains of DHQase I and SDH (AroE), as well as a shikimate-binding site as found in other DHQ/SDH genes. Northern blot analysis showed that SVCR3 was up-regulated at 4°C. When combined with light (>50 μmol photon m -2 s -1 ) the expression of SVCR3 increased more at 4°C, but the expression was not affected by light intensity alone up to 200 μmol photon m -2 s -1 at temperatures higher than 10°C. Using the zymogram method (which is a visualization method for gel areas containing specific enzymes based on chromogenic reactions), we showed that there were three isozymes of DHQ/SDH in Spirogyra varians . This is the first report of DHQ/SDH-like gene regulation by cold stress.

No abstract available

No abstract available