Terumitsu Hori

A blue-light-activated adenylyl cyclase mediates photoavoidance in Euglena gracilis.

Blue light regulates processes such as the development of plants and fungi and the behaviour of microbes. Two types of blue-light receptor flavoprotein have been identified: cryptochromes, which have partial similarity to photolyases, and phototropins, which are photoregulated protein kinases. The former have also been found in animals with evidence of essential roles in circadian rhythms. Euglena gracilis, a unicellular flagellate, abruptly changes its swimming direction after a sudden increase or decrease in incident blue light intensity, that is, step-up or step-down photophobic responses, resulting in photoavoidance or photoaccumulation, respectively. Although these photobehaviours of Euglena have been studied for a century, the photoreceptor molecules mediating them have remained unknown. Here we report the discovery and biochemical characterization of a new type of blue-light receptor flavoprotein, photoactivated adenylyl cyclase, in the photoreceptor organelle of Euglena gracilis, with molecular genetic evidence that it mediates the step-up photophobic response.

Ultrastructure of the flagellar apparatus inPyramimonas octopus (Prasinophyceae)

SummaryWhile green algae usually lack one of the outer dynein arms in the axoneme, flagella of the octoflagellated prasinophytePyramimonas octopus possess dynein arms on all peripheral doublets. The outer dynein arm on doublet no. 1 is modified, and additional structures are associated with doublets no. 2 and 6. The flagellar scales are asymmetrically arranged. Thus the two rows of thick flagellar hairscales are displaced towards doublet no. 6,i.e., in the direction of the effective stroke of each flagellum. The underlayer of small scales includes two nearly opposite double rows scales, arranged in the longitudinal direction of the flagellum. The hairscales emerge from these rows. The double rows are separated on one side by 9, on the other by 11 rows of helically arranged scales. The central pair of microtubules twists, but the axoneme itself (represented by the 9 peripheral doublets), does not seem to rotate. The flagella are arranged in two groups, showing modified 180° rotational symmetry. The effective strokes of the two central flagella are exactly opposite, while the other flagella beat in six intermediate directions.

High-speed video analysis of the flagellar beat and swimming patterns of algae: possible evolutionary trends in green algae

SummaryThe flagellar beat and swimming patterns of flagellated cells of 22 green plants, including 17 green flagellates (volvocalean and prasinophyte algae), motile cells of three seaweeds,Bryopsis, Caulerpa, andUlva, sperms of a liverwort,Marchantia, and a fern,Athyrium, were examined using a high-speed video system. So-called breast-stroke is widely distributed in green plants, and occurs rarely in prasinophyte flagellates and ulvophycean algae; in these algal groups flagellar beat similar to that found in animal sperm is common, both during forward and backward swimming. Different types of swimming patterns were observed in prasinophytes. The results indicate evolutionary trends of flagellar beat and swimming patterns in green plants such as change from backward to forward swimming, from flagellar to ciliary beating and from uni-directional (parallel) to radial-directional (cruciate) beating. Such trends are shown in two prasinophyte groups, thePyramimonas-lineage andTetraselmis-lineage.

Preferential digestion of male chloroplast nuclei and mitochondrial nuclei during gametogenesis ofBryopsis maxima Okamura

The preferential degradation of male chloroplast nuclei (synonymous with nucleoid, ct-nuclei) occurs in young zygotes of isogamous algae such as Chalmydomonas reinhardtii (KuROIWA etal. 1982, TSUBO and MATSUDA 1984, C. moewusii (COLEMAN and MAGUIRE 1983), Acetabularia calyculus (KuRoIWA et al. !985) and Dictyosphaeria c a v e r n o s a (KuROIWA et al. 1985), and seems to be responsible for maternal inheritance of chloroplast genes (KuRoIWA 1985). However, no information is available to date on the behavior of chloroplast DNA (ct-DNA) during gametogenesis and zygote formation in anisogamous algae. This report demonstrates the preferential destruction of male ctDNA and mitochondrial DNA (mt-DNA) during the late period of gametogenesis in an anisogamous alga, Bryopsis maxima Okamura. The alga was collected at Kimiga-hama Coshi, chiba Pref. in Japan, then placed into aquaria containing seawater. Cultures were continuously aerated, maintained at 14~ and subjected to an alternating 12L: 12D cycle; illumination was with white fluorescent light (3,000 lux). The multinucleate coencytic thalli were dioecious and composed of a long main axis

Revision of the genusTetraselmis (Class Prasinophyceae)

Information available on the structure of species belonging to the generaTetraselmis, Platymonas andPrasinocladus has been reviewed. Detailed comparison of these data has convinced the authors that species in these genera all belong to the same genus. Stalk development by cast-off thecae, a characteristic used to definePrasinocladus, is variable within different species and is not reliable for separation of a genus. Similarly, the penetration of the pyrenoid by a lobe of the nucleus cannot be held reliable in separation of these genera because it occurs in varying degree in different species.Platymunas G.S. West (1916),Prasinocladus Kuckuck (1894) andAulacochlamys Margalef (1946) are considered to be synonyms ofTetraselmis Stein (1878).Tetraselmis is redescribed using characters visible with light and electron microscopy as well as life-history characteristics. The description reviews information on many species ofTetraselmis that have been found in the western and eastern Pacific as well as species from Great Britain. It is determined that variations in life-histories may be explained by different environmental factors, whereas structure of vegetative cells, as viewed by electron microscopy, seems to be quite stable and characteristic for each species.

ABSOLUTE CONFIGURATION ANALYSIS OF THE FLAGELLAR APPARATUS OF PTEROSPERMA CRISTATUM (PRASINOPHYCEAE) AND CONSIDERATION OF ITS PHYLOGENETIC POSITION1

Pterosperma cristatum Schiller, a member of the Pra‐sinophyceae, was examined with light and electron microscopy with special attention on the absolute configuration of flagellar apparatus components and associated structures. This alga is characterized by asymmetrically arranged basal bodies, connecting fibers and microtubular roots. The microtubular root system is homologous with the cruciate root system, the so‐called X‐2‐X‐2 root system typical of non‐charophycean green algae. Two ducts are associated with microtubular roots. A similar flagellar apparatus and duct system was found in two other prasinophyte genera, Pyramimonas and Halosphaera. The close phylogenetic affinity of these three genera is discussed.

Studies on the ultrastructure and taxonomy of the genusTetraselmis (Prasinophyceae)

Comparative ultrastructural investigations on many isolates ofTetraselmis from Japan and the Pacific coast of North America, and on cultures from the Culture Centre of Algae and Protozoa, Cambridge, England, have revealed that the species have characteristic fine structural features of the pyrenoid. Using the pyrenoid structure as a basic character it is proposed that the genus be subdivided into four subgenera,Tetraselmis, Prasinocladia, Tetrathele andParviselmis. In the present paper, species of the subgenusTetraselmis, includingT. cordiformis, T. ascus, T. convolutae andT. astigmatica sp. nov., are described in detail.

Discovery of signaling effect of UV-B/C light in the extended UV-A/blue-type action spectra for step-down and step-up photophobic responses in the unicellular flagellate algaEuglena gracilis

SummaryCultures of unicellular algal flagellateEuglena gracilis grown in different conditions were subjected to action spectroscopy for step-down and step-up photophobic responses, respectively. The spectral region was extended into the UV-B/C as well as in the UV-A and visible regions with the Okazaki Large Spectrograph as the monochromatic light source. The photophobic responses of the cells were measured with an individual-cell assay method with the aid of a computerized video motion analyzer. In the UV-A and visible regions, the shapes of the action spectra were the so-called UV-A/blue type. In the newly studied UV-B/C region, new action peaks were found at 270 nm for the step-down response and at 280 nm for the step-up one. The absorption spectrum of flavin adenine dinucleotide (FAD) appeared to fit the action spectrum for the step-up response, whereas the shape of the step-down action spectrum, which has a UV-A peak (at 370 nm) higher than the blue peak (at 450 nm), appeared to be mimicked by the absorption spectrum of a mixed solution of 6-biopterin and FAD. These observations might also account for the fact that the UV-B/C peak wavelength at 270 nm of the action spectrum for the step-down response is shorter by 10 nm than the action spectrum for the step-up response at 280 nm.

Preferential digestion of chloroplast DNA in male gametangia during the late stage of gametogenesis in the anisogamous alga Bryopsis maxima

SummaryThe fate of chloroplast nuclei (cp-nuclei) and mitochondrial nuclei (mt-nuclei) was followed during gametogenesis in male and female coenocytic thalli in the anisogamous algaBryopsis maxima by epifluorescence microscopy, after staining with 4′,6-diamidino-2-phenylindole (DAPI), by quantification of chloroplast DNA (cp-DNA) by fluorimetry using a video-intensified, photon-counting system (VIMPICS), and by CsCl density gradient centrifugation. The male and female coenocytic thalli, 48 h before the release of gametes, contain a large number of chloroplasts, each of which is larger in size than the cell nucleus and the mitochondria and contains about 150 cp-nuclei. The size of each chloroplast in the female and male gametangia decreases markedly during gametogenesis as a result of continuous divisions till about 10 h before the release of gametes and, eventually, the numbers of cp-nuclei per chloroplast in the male and female gametangia fall to about 20 and 5, respectively. Two hours later, as the preferential digestion of cp-DNA in the male gametangium occurs, the number of cp-nuclei in the chloroplast of each male gamete falls to zero while the number of cp-nuclei in female gamete does not change, even after release of female gametes. Several mt-nuclei are observed in all of the female gametes. By contrast, the mt-nuclei in the bulk of the male gametes disappear but those in a few gametes remain. The profiles after CsCl density gradient centrifugation of DNAs extracted from male and female plants and gametes support the cytological data. The results suggest that the preferential digestion of cp-DNA in male plants occurs about 8 h before the release of gametes and that there is differential digestion of cp-DNA and mitochondrial DNA (mt-DNA).

The ultrastructure of the flagellar root system of Imantonia rotunda (Prymnesiophyceae)

A detailed study of the fine structure of the flagellar root system in Imantonia rotunda Reynolds (Prymnesiophyceae) has been made and a reconstruction is presented. There are two divergent basal bodies connected by a striated distal fibrous connecting band, a pair of median connecting bands and a small proximal connecting band. Associated with the connecting bands and the basal-bodies are a number of microtubular roots, none of which is composed of more than three microtubules, at least in the region of the flagellar apparatus. A haptonema trace was not observed. The root system in I. rotunda is compared with those (often more complex) observed in other members of the class and possible homologies are proposed.