AGGREGATION TO COLONIES OF THE GREEN ALGA PEDIASTRUM: EXPERIMENTS AND SIMULATIONS

We simulate the aggregation of zoospores of the green alga Pediastrum simplex into a planar colony — including radially outward cell orientation. The model assumptions are: cells are randomly kicked by flagella, an ellipsoidal boundary repells cells and flagella, overlapping cell surfaces lead to dissipative cell-cell repulsion, cell-cell attractive forces occur around arches close to the cell's surfaces, there is friction of the cells with the medium; the process ends by an extension of the attractive arches followed by the stop of flagellar propulsion and finally a change of cell shapes from spherical to nearly triangular with contact-inhibited horn-like extensions. These assumptions are directly observed or indirectly inferred from the literature, light microscopy (video recordings) and scanning electron microscopy. We perform calculations with cell numbers ranging from 4 to 64. Additional simulations permit to discard alternative models, including planarization via cell-cell attraction only, chemotaxis or cohesive intercellular gliding. Our model yields patterns agreeing well with symmetrical as well as with disordered natural aggregates.

Ultrastructural aspects of basal body associated fibrous structures in green algae: a critical review

Ultrastructural aspects of fibrous structures associated with basal bodies of green algae are critically discussed. It is apparent that variation among these structures is much greater than in microtubular flagellar root systems and it is therefore suggested that fibrous structures may be more useful than microtubular roots in elucidating phylogenetic relationships within the Chlorophyceae sensu Stewart and Mattox and the Prasinophyceae sensu Christensen. Two main types of fibrous structures are distinguished: (1) Connecting fibres (these connect different basal bodies); (2) Fibrous roots (these originate at basal bodies and terminate somewhere else in the cell). Fibrous roots are of two types: (a) microtubular-root associated striated fibres (striation pattern 25-35 nm; system I-fibres); (b) striated fibres composed of a bundle of filaments (filament diameter: 5-10 nm; striation pattern greater than 80 nm; system II fibres). Numbers, disposition and substructure of connecting fibres and fibrious roots are variable in different genera of green algae. In the experimental secretion new observations on fibrous roots in the ulvalean genus Enteromorpha as well as preliminary information on fibrous structures in Carteria obtusa and Bryopsis lyngbyei are included. Functional and evolutionary aspects of fibrous structures associated with the flagellar apparatus of green algae are discussed.

Cellulose microfibril orientation and cell shaping in developing guard cells of Allium: The role of microtubules and ion accumulation

The role of microtubules and ions in cell shaping was investigated in differentiating guard cells of Allium using light and electron microscopy and cytochemistry. Microtubules appear soon after cytokinesis in a discrete zone close to the plasmalemma adjacent to the common wall between guard cells. The microtubules fan out from this zone, which corresponds to the future pore site, towards the other sides of the cell. Soon new cellulose microfibrils are deposited on the wall adjacent to the microtubules and oriented parallel to them. As the wall thickens, the shape of the cell shifts from cylindrical to kidney-like. Studies with polarized light show that guard cells gradually assume a birefringence pattern during development characteristic of wall microfibrils radiating away from the pore site. Retardation increases from 10 Å when cells just begin to take shape, to 80-100 Å at maturity. Both microfibril and microtubule orientation remain constant during development. Observations on aberrant cells including those produced under the influence of drugs such as colchicine, which leads to loss of microtubules, abnormal wall thickenings and disruption of wall birefringence, further support the role of microtubules in cell shaping through their function in the localization of wall deposition and the orientation of cellulose microfibrils in the new wall layer. Potassium first appears in guard mother cells before division and rapidly accumulates afterwards during cell shaping, as judged by the cobaltinitrite reaction. Some chloride and perhaps organic acid anions also accumulate. Thus, these ions, which are known to play a role in the function of mature guard cells, also seem to be important in the early growth and shaping of these cells.

The effect of colchicine on colony formation in the algae Hydrodictyon, Pediastrum and Sorastrum

Uninucleate, biflagellate zoospores of Hydrodictyon, Pediastrum and Sorastrum, derived from multinucleate parental cells, aggregate and adhere to form distinctively patterned colonies; earlier work has shown that microtubules underlie the plasmalemma of these zoospores and are also conspicuous in the developing horns of aggregating cells of Pediastrum and Sorastrum. Colchicine applied to parental cells inhibited cytoplasmic cleavage and production of uninucleate zoospores. When zoospores were treated with colchicine, their peripheral microtubules disappeared; the spores failed to aggregate in ordered arrays and did not develop horns. The microtubules therefore appear to play an important role in determining the arrangement of cells in developing colonies by affecting the shape of the cells at the time of their aggregation.