Plates I-VI, Fig. i)In the Acantharia, two different symbiotic algae are known. One is a dinoflagellate with a typical mesocaryon and the other, studied here, exhibits the characters of the Prymnesiophyceae (Haptophyceae).Detailed descriptions of the non-motile and presumed premotile stages of this symbiont are given, with special reference to the haptonema, the scales and the plastid.Comparisons are made with other members of the Prymnesiophyceae and the systematic position of the symbiotic alga is discussed.

Several motility phenomena displayed by members of the Heliozoa, Radiolaria, and Acantharia (Protozoa, Actinopoda) are reviewed. These phenomena include (1) cytoplasmic streaming, internal saltatory motion, and transport of particles at the cell surface; (2) axopod contraction and extension; and (3) contraction of nonactin filament systems.Cytoplasmic streaming and saltatory motion require energy derived from oxidative phosphorylation. In addition, calcium appears to be involved in the regulation of these movements, and a role for calmodulin is suggested. At present, the molecular basis for these motility phenomena remains obscure.We have focused our attention on the rapid contraction of axopods and stalk in the marine heliozoan Actinocoryne contractilis (Febvre-Chevalier : J. Marine Biol. Assoc. U. K. 60:901-928, 1980). Contraction is accompanied by the cataclysmic breakdown of microtubules. For this species, a Na+ and Ca2'-de endent action potential precedes axopod contraction. A lack of contraction in C$+-free media (lo-' M Ca2') suggests that Ca2' fluxes across the cell membrane are required.Motile phenomena associated with nonactin filaments of fibrous systems in actinopods, especially in the myonemes of the acantharians (Febvre and FebvreChevalier: Biol. Cell. 44:283-304, 1982) are also examined. In vitro contraction of these filaments is CA2+dependent and ATP independent; cycles of contraction and extension are caused by Ca'+-dependent conformational changes in pairs of twisted filaments. In vivo, the Ca2+dependent contraction of these myonemes may be independent of direct mitochondria1 control, but metabolic ATP and calmodulin may be required to regulate the level of free cytoplasmic calcium.

No abstract

Actinocoryne contractilis is a new marine, benthic and stalked member of the Centrohelidia (Heliozoa) whose biology, motile behaviour and cytology are described. Preliminary results concerning the life cycle are given. The stalked form emits small subspherical planktonic heliozoans which settle and give rise to the stalked stage again.

Abstract. In the marine heliozoan Actinocoryne contractilis, uninterrupted rods of microtubules stiffen the axopodia and the stalk. Stimulation in sea water elicits an extremely fast contraction (millisecond range) accompanied by almost complete Mt dissociation. Using high-speed cinematography and light transmittance measurements, we have studied the process of Mt disassembly in real time. In sea water, Mt disassembly follows an exponential decrease (mean half time of 4 ms) or proceeds by short steps. Cell contraction and Mt disassembly have been inhibited or slowed down through the use of artificial media. Although kinetics are slower (mean half time of 3 s), the curves of the length change against time look similar. The rapid as well as the slower process are accompanied by the formation of breakpoints on the stalk, from which disassembly proceeds. In specimens fixed during the slowed contraction, the presence across the Mt rods, of a single or multiple destabilization band that may consist of granular material and polymorphic forms of tubulin supports the hypothesis of "intercalary destabilization and breakdown" of axonemal Mts.S EVERAL schemes have been suggested to explain the way in which microtubule assembly and disassembly can be brought about. One of the first models proposed that subunits may be exchanged along the whole length of the microtubnles (Mts) 1 (Inou~ and Sato, 1967; Inou~ and Ritter, 1975). Later, on the basis of biochemical data, the assembly-disassembly process was interpreted as the result of an equilibrium between mierotubule-polymers and a critical concentration of tubulin subunits at steady state (Johnson and Borisy, 1977). The role of GTP was defined (Carlier and Pantaloni, 1981) showing that the Mrs with terminal bound GTP subunits (GTP caps) are more stable than those uncapped (terminal guanosine diphosphate [GDP] subunits) (Carlier, 1982). These data, and the behavior of individual Mts in a solution of tubulin (catastrophic disassembly), suggested a GTP cap model for dynamic instability (Mitchison and Kirschner, 1984). However, Mt kinetics is sometimes so fast that a breakdown and annealing hypothesis was suggested to account for rapid remodeling of microtubules (Salmon, 1984).The present paper is focused on a remarkably rapid disassembly of Mts in the stalked heliozoan Actinocoryne contractilis. The first ultrastructural evidence for a Mt-based cytoskeleton in the heliozoans was provided by Tilney and Porter (1965). It was then suggested that the microtubule rods in these microorganisms were involved in elongation 1. Abbreviations used in this paper: Am, artificial media; DB, destabilization bands; GDP, guanosine diphosphate; Mr, microtubule; NMG, N-methyl-glucamine; PEGc, polyethylene glycol compound; SW, sea water.and shortening of long stiff processes, the axopodia (Tflney and Porter, 1967). We report experiments which suggest that "intercalary destabilization and breakdown of Mts" may be one of the systems used in vivo by heliozoans for the very rapid shortening of the axopo...