Class Ulvophyceae Research Articles

Ultrastructure and Taxonomy of Sporocladopsis novae-zelandiae (Ulvophyceae, Chlorophyta)

The ultrastructure of vegetative and reproductive cells in Sporocladopsis novae-zelandiae has been studied for the first time to determine the precise taxonomic position of this genus. Vegetative cells contained one parietal chloroplast with 5–8 ulvophycean-type pyrenoids and transverse cell walls with plasmodesmata. The growth of the transverse walls was centripetal by furrow ingrowth. Ovoid zoosporangia initially had one nucleus and a large basal vacuole. Zoospore release occurred by the apical disintegration of the zoosporangial wall and expansion of an apical mucilage plug. Emptied zoosporangia could be occupied by a series of successive new zoosporangia developed from the same mother cell. Zoospores were bi- or quadriflagellate. The flagellar apparatus showed a 180 ° rotational symmetry and overlapped basal bodies with bilobed terminal caps covering their proximal ends. Our study permits us to conclude that Sporocladopsis is an ulvophycean genus. This conclusion is based on the occurrence of overlapping basal bodies, cytokinesis by infurrowing, and ulvophycean type of pyrenoids. Bilobed terminal caps and zoospore release by apical disintegration of the cell wall (which is accompanied by expansion of a mucilage plug) in Sporocladopsis suggest affinities with the order Ulvales. However, the absence of plasmodesmata and the absence of percurrent proliferation of zoosporangia in all members of the order Ulvales challenge this taxonomical position. The plasmodesmata in Sporocladopsis are similar to those in the Trentepohliales and could suggest a relationship with the latter, but the typical features of the Trentepohliales are absent in Sporocladopsis and this absence does not support an affinity between Sporocladopsis and the Trentepohliales. In conclusion, the fine structural characters of Sporocladospsis are paradoxical and it is therefore difficult to relate this genus to an order within the class Ulvophyceae. Molecular data of the genus, which are not available at present, would be necessary before making a final taxonomic decision concerning its ordinal position inside the class Ulvophyceae.

Ultrastructure and taxonomy of the genus Endophyton (Ulvales, Ulvophyceae)

Fine structure of vegetative cells, development and morphology of sporangia and general morphology of zoospores were studied for the first time in the genus Endophyton Gardner. Vegetative cells contained one parietal perforate chloroplast with 2–4 ulvophycean pyrenoids and transverse cell walls without plasmodesmata. Sporangial walls were formed by two distinct electron-dense and fuzzy layers which contained a contiguous electron-translucent, fibrillar material. Zoospores were naked, with an anterior dense aggregation of vesicles and a cup-shaped chloroplast with one eyespot. The flagellar apparatus showed 180° rotational symmetry and counterclockwise absolute orientation of its components. Microtubular roots had a cruciate pattern, in which d = 2 and s = 4 with 3/1 arrangement. Electron-dense bilobed terminal caps covered the proximal end of each basal body. These observations support the suggested close relationship of Endophyton to the allied genus Entocladia and other ulvalean algae, and reinforce the criteria for including the genus in the class Ulvophyceae, order Ulvales, family Ulvellaceae.

Ultrastructure and taxonomy of the genus Endophyton (Ulvales, Ulvophyceae)

Fine structure of vegetative cells, development and morphology of sporangia and general morphology of zoospores were studied for the first time in the genus Endophyton Gardner. Vegetative cells contained one parietal perforate chloroplast with 2–4 ulvophycean pyrenoids and transverse cell walls without plasmodesmata. Sporangial walls were formed by two distinct electron-dense and fuzzy layers which contained a contiguous electron-translucent, fibrillar material. Zoospores were naked, with an anterior dense aggregation of vesicles and a cup-shaped chloroplast with one eyespot. The flagellar apparatus showed 180° rotational symmetry and counterclockwise absolute orientation of its components. Microtubular roots had a cruciate pattern, in which d=2 and s=4 with 3/1 arrangement. Electron-dense bilobed terminal caps covered the proximal end of each basal body. These observations support the suggested close relationship of Endophyton to the allied genus Entocladia and other ulvalean algae, and reinforce the criteria for including the genus in the class Ulvophyceae, order Ulvales, family Ulvellaceae.

Evolution of the Sacoglossa (Mollusca, Opisthobranchia) and the ecological associations with their food plants

Evolution in the opisthobranch order Sacoglossa has been closely linked to their specialized suctorial herbivorous habits. All shelled Sacoglossa (about 20% of the species) feed on one algal genus, Caulerpa. The non-shelled Sacoglossa have 'radiated' to other diets, mainly siphonalean or septate green algae (Class Ulvophyceae). Comparing the phylogeny of sacoglossan genera with the phylogeny of the Ulvophyceae indicates that co-speciation may have taken place at the basal node of the Sacoglossa, and that host switching has taken place several times in the two non-shelled clades. It is suggested that the most important evolutionary process has been speciation by 'resource-tracking'; the resource tracked is most probably cell wall composition of the algal prey. The fossil record of extant sacoglossan genera dates back to the Eocene and, based on the fossil record of siphonalean green algae, the Sacoglossa most likely appeared in the Cretaceous. It is hypothesized that the ancestral sacoglossan was epifaunal, suctorial and herbivorous, and the 'ancestral' food plant was not Caulerpa, but filamentous, calcified, now extinct, Udoteaceae.

Evidence for independent acquisition of group I introns in green algae.

We report the occurrence of a group I intron, 452 nucleotides in length, in the nuclear small-subunit ribosomal gene of the benthic seaweed Urospora penicilliformis, a member of the green algal class Ulvophyceae. Group I introns have been reported in fungi, myxomycetes, the ciliate genus Tetrahymena, and recently in five unicellular chlorophycean algae. The sequence of the conserved core of the U. penicilliformis group I intron was aligned with that of 15 other introns of this type, and the evolutionary relationships among these introns were examined. Comparison of the "intron" tree with phylogenetic hypotheses of the intron-harboring organisms shows that nuclear group I introns in green algae have arisen several times and are not lineage specific.

CELL DIVISION IN GLOEOTILOPSIS PLANCTONICA, A NEWLY IDENTIFIED ULVOPHYCEAN ALGA (CHLOROPHYTA) STUDIED BY FREEZE FIXATION AND FREEZE SUBSTITUTION1

ABSTRACTGloeotilopsis planctonica Iyengar & Philipose, a soil isolate, was cultured for the first time since its description in 1956. Cell division was investigated ultrastructurally by using rapid freeze fixation and freeze substitution. Vegetative interphase cells contain a persisting basal apparatus (centrioles) in which the absolute configuration of the basal body pair is identical to that in ulvophycean flagellated cells. Basal apparatus replication and segregation occurs shortly before prophase. Also before mitosis, a shallow plasma membrane indentation is formed, subtended by 2–3 cortical microtubules, in the forthcoming division plane. Daughter basal body pairs are located lateral to the mitotic spindle poles. A Golgi stack occupies each polar position of the mitotic spindle. During mitosis, which is structurally identical to that described previously for members of the Ulotrichales sensu Sluiman (Ulvophyceae), an extensive network of tubular membranes (trans Golgi reticulum, TGR) develops from each polar Golgi stack to the incipient cleavage furrow. A microtubular system, possibly organized by the basal apparatus, is associated with both the TGR and the cytokinetic septum. The TGR appears to be involved in the further development of the furrow after mitosis is completed. I conclude on the basis of this ultrastructural study that the genus Gloeotilopsis, which includes G. planctonica Iyengar & Philipose (type species) and G. sterilis Deason, should be classified in the Ulotrichales sensu Sluiman, class Ulvophyceae.

Terrestrial rhizophytes and H+ currents circulating over at least a millimetre: an obligate relationship?

summaryCirculating ionic currents are apparently ubiquitous in growing and differentiating eukaryotes. In many the current is carried by H+, in which case the medium around the apex of growing organs is alkalinized while the medium adjacent to subapical portions is acidified. These two zones may help in, respectively, molybdenum acquisition and reduction of aluminium toxicity, and phosphorus and iron acquisition, by rhizophytic plants. An analysis of the taxonomic distribution of H+‐borne circulating current shows that all rhizophytes (terrestrial, freshwater and, probably, marine) except those in the green algal class Ulvophyceae have such currents; the Ulvophyceae have circulating currents carried by Cl−. The Ulvophyceae do not seem to have any alternative means of causing pH zonation in the rhizosphere. Their marine habitat, with the possibility of molybdenum acquisition from the alkaline bulk water phase, may mean that the Ulvophyceae are not disadvantaged by the absence of rhizosphere acid‐alkaline zonation as would be a land plant; similarly, they are less likely to suffer aluminium toxicity to the growing zone of rhizoids. Ulvophyceae do not occur as terrestrial plants; this may relate not only to the absence of rhizosphere pH zonation in these organisms but also to mechanical problems related to their coenocytic habit, which in turn results from their inability to produce plasmodesmata.

PHYLOGENY OF THE ULVOPHYCEAE (CHLOROPHYTA): CLADISTIC ANALYSIS OF NUCLEAR‐ENCODED rRNA SEQUENCE DATA1

ABSTRACTCladistic analysis of nuclear‐encoded rRNA sequence data provided us with the basis for some new hypotheses of relationships within the green algal class Ulvophyceae. The orders Ulotrichales and Ulvales are separated from the clade formed by the remaining orders of siphonous and siphonocladous Ulvophyceae (Caulerpales, Siphonocladales /Cladophorales [S/C] complex, and the Dasycladales), by the Chlorophyceae and Pleurastrophyceae. Our results suggest that the Ulvophyceae is not a monophyletic group. Examination of inter‐ and intra‐ordinal relationships within the siphonous and siphonocladous ulvophycean algae revealed that Cladophora, Chaetomorpha, Anadyomene, Microdictyon, Cladophoropsis and Dictyosphaeria form a clade. Thus the hypothesis, based on ultrastructural features, that the Siphonocladales and Cladophorales are closely related is supported. Also, the Caulerpales is a monophyletic group with two lineages; Caulerpa, Halimeda, and Udotea comprise one, and Bryopsis and Codium comprise the other. The Dasycladales (Cymopolia and Batophora) also forms a clade, but this clade is not inferred to be the sister group to the S/C complex as has been proposed. Instead, it is either the sister taxon to the Caulerpales or basal to the Caulerpales and S/C clade The Trentepohliales is also included at the base of the siphonous and siphonocladous ulvophycean clade. The Pleurastrophyceae, which, like the Ulvophyceae, posses a counter‐clockwise arrangement of flagellar basal bodies, are more closely related to the Chlorophyceae than to the Ulvophyceae based on rRNA sequences. Thus, the arrangement of basal bodies does not diagnose a monophyletic group. Previously reported hypotheses of phylogenetic relationships of ulvophycean algae were tested. In each case, additional evolutionary steps were required to obtain the proposed relationships. Relationships of ulvophycean algae to other classes of green algae are discussed.

Zoospore flagellar apparatus architecture and the taxonomic position of Phaeophila dendroides (Ulvophyceae, Chlorophyta)

Abstract The quadriflagellate zoospores of Phaeophila dendroides (P. Crouan et H. Crouan) Batters (Chlorophyta), a cosmopolitan marine filamentous ‘chaetophoraceous’ microalga, have ultrastructural features that conform to the pattern characteristic of members of the class Ulvophyceae. The flagellar apparatus exhibits 180° rotational symmetry and a counterclockwise absolute configuration of major components. Unique flagellar apparatus features include six microtubular rootlets, and single-lobed terminal caps. The results suggest that the nearest extant relatives of P. dendroides are filamentous members of the order Ulvales. The structural and reproductive features of P. dendroides cannot be accommodated in any of the existing orders and families of the Ulvophyceae, and therefore the Phaeophilales ord. nov. and Phaeophilaceae fam. nov. are proposed. Phaeophila dendroides probably does not represent a phylogenetic intermediate between the Ulvales and other orders of Ulvophyceae.

Ultrastructure and taxonomic position of Chlorosarcina stigmaticaDeason (Chlorophyceae, Chlorophyta)

Using cryofixation methods we studied the ultrastructure of thallus cells and zoospores of a chlorosarcinoid alga present in the Innsbruck culture collection (strain nr. T 105). We confirm an earlier observation by Gärtner et al. (1988) that pyrenoids are absent, and that it represents Chlorosarcina stigmatica Deason 1959. This species was recently transferred to another genus because descendents of the original culture were found to possess pyrenoids. Our observations on T 105 demonstrate however that the original concept of C. stigmatica as a species of Chlorosarcina (a genus characterized by the absence of pyrenoids) is still valid. We found the zoospore flagellar apparatus to be characteristic of the class Chlorophyceae sensu Mattox and Stewart . Other species, i.e. C. brevispinosa and C. longispinosa are known to have an ulvophycean flagellar apparatus. It is proposed to transfer these to a separate new genus, Pleurastrosarcina , in the class Ulvophyceae.

Flagellar apparatus of male gametes and other aspects of gamete and zygote ultrastructure in Prasiola and Rosenvingiella (Chlorophyta, Prasiolales) from British Columbia

We examined gametes, syngamy, and zygote formation in Prasiola meridionalis Setchell & Gardner and Rosenvingiella constricta (S. & G.) Silva with the transmission electron microscope. We compare the structures we observed with those of the Atlantic species P. stipitata Suhr and describe some previously unreported features. The sperm possessed a number of unusual structural elements in the flagellar apparatus and elsewhere, in particular a 9 + 1 microtubule configuration in the flagellar axoneme. The flagellar apparatus exhibited 180° rotational symmetry and counterclockwise absolute orientation of major components. At syngamy, the flagellar apparatus changed little in composition and arrangement, suggesting that it has a minimal, if any, function in syngamy or the planozygote. Karyogamy occurred within a few hours of plasmogamy, after settlement of the planozygote and concurrently with wall formation. The three species, P. meridionalis, P. stipitata, and R. constricta, are very similar in gamete structure, which indicates that the genera Prasiola and Rosenvingiella are close relatives. The flagellar apparatus features provide evidence supporting recognition of the order Prasiolales, which may be assigned to either the class Ulvophyceae or the class Pleurastrophyceae.

THE FLAGELLAR APPARATUS OFENTOCLADIA VIRIDISMOTILE CELLS, AND THE TAXONOMIC POSITION OF THE RESURRECTED FAMILY ULVELLACEAE (ULVALES, CHLOROPHYTA)1

ABSTRACTThe flagellar apparatuses of the quadriflagellate zoo‐spores and biflagellate female gametes of the marine chaetophoracean algaEntocladia viridisReinke are significantly different from those of algae belonging to Chaetophoraceae sensu stricto, but closely resemble those of ulvacean genera. These differences permit the taxonomic reassignment of certain marine chaetophoracean genera and an evaluation of the flagellar apparatus features used to characterize the class Ulvophyceae. Critical features of the zoospore include arrangement of the four basal bodies into an upper and a lower pair with the proximal ends of the upper basal bodies overlapping, terminal caps, proximal sheaths connected to one another by striated bands, and a cruciate microtubular rootlet system having a3‐2–3‐2alternation pattern and striated microtubule‐associated components that accompany the two‐membered rootlets. An indistinct distal fiber occurs just anterior to the basal bodies, and is closely associated with the insertion into the flagellar apparatus of the three‐membered rootlets. The flagellar apparatus demonstrates 180° rotational symmetry, and its components show counterclockwise absolute orientation when viewed from above. Newly described features include the prominently bilobed structure of the terminal caps on the upper basal body pair, and the presence of both a granular zone and an additional single microtubule anterior to each of the four rootlets, an arrangement termed the “stacked rootlet configuration.” Rhizoplasts were not observed and are presumed to be absent. The gamete is identical, except for the absence of the lower basal body pair and the presence of an electron‐dense membrane associated structure that resembles the mating structure found inUlvagametes. These findings, correlated with life history data, sporangial and gametangial structure and developmental patterns, chloroplast pigment arrays, and vegetative cell ultrastructural features, compel the removal ofEntocladia viridisand similar members of the marine Chaetophoraceae to a separate family, the Ulvellaceae. The latter is referred to the order Ulvales of the Ulvophyceae. The counterclockwise absolute orientation of components, and terminal caps, may be the most consistent flagellar apparatus features of ulvophycean green algae, while variations in other features previously considered diagnostic for the Ulvophyceae may serve instead to identify discrete lineages within this class.