Microtubule dissassembly in vivo: intercalary destabilization and breakdown of microtubules in the heliozoan Actinocoryne contractilis

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.     

Host specificity of Lepeophtheirus crassus (Wilson and Bere) (Copepoda: Caligidae) parasitic on the marlin sucker Remora osteochir (Cuvier) in the Atlantic Ocean

Three species of remoras--Remora brachyptera (Lowe), Remora osteochir (Cuvier), and Remora remora (Linnaeus)--were collected from 4 species of billfishes--Istiophorus platypterus (Shaw), Makaira nigricans Lacepéde, Tetrapturus albidus Poey, and Tetrapturus pfluegeri Robins and de Sylva--on board a Japanese long-liner Shoyo Maru during her cruise in 2002 across the Atlantic. However, only the marlin sucker (R. osteochir) was found to carry a parasitic copepod, Lepeophtheirus crassus (Wilson and Bere, 1936). Although 12 species of parasitic copepods have been reported from billfishes around the world ocean, none of them is L. crassus. Thus, L. crassus is considered a parasite specific to the marlin sucker.     

Motility processes in Acantharia (protozoa) III. Calcium regulation of the contraction - relaxation cycles of in vivo myonemes

The myonemes in the marine pelagic protozoa Acantharia are contractile organelles involved in buoyancy regulation. It was previously shown that they can perform three kinds of movement: rapid contraction, slow undulation and slow relaxation. They consist of a periodically striated bundle of 2–4 nm nonactin filaments that are twisted in pairs and shortened by a coiling mechanism. After permeabilization or demembranation, contraction and relaxation can still be performed by varying Ca²⁺ concentration and ATP is not needed. In the present paper, we have studied the role of Ca²⁺ and inhibitors of energy production in intact cells. Our data suggest that; (i) the in vivo rapid contraction subsequent to mechanical or electrical stimulation is triggered by Ca²⁺ influx across the cell membrane; (ii) the slow contraction that takes place during the undulating movement depends on Ca²⁺ release provided by internal calcium stores; (iii) the rapid contraction as well as the progressive shortening that occurs during the slow undulating movement are caused by Ca²⁺ binding to the myoneme filaments; (iv) ATP is not directly involved in the saturation by Ca²⁺ of Ca²⁺ sensitive sites located along the myoneme microstrands; (v) regulation of the movements of Ca²⁺ within the cytoplasm depends mainly upon the alternative pathway of ATP production; (vi) calmodulin is presumably involved in this regulation. A tentative cytophysiologic interpretation of the mechanism of contractility is proposed.     

Motility processes in Acantharia. II. A Ca2+ dependent system of contractile 2–4 nm filaments isolated from demembranated myonemes

Myonemes of the acantharians are contractile ribbon-like organelles. As previously shown, their motility is based on the coiling mechanism of double-twisted 2–4 nm nonactin filaments [14]. Myonemes have been isolated and manipulated in vitro. After demembranation, the contraction takes place when the Ca²⁺ concentration is above 10^?7 M, whereas relaxation occurs below this threshold concentration. The response to Ca²⁺ ions is an on/off mechanism. Both contraction and relaxation can be induced repeatedly without fatigue phenomena. Other divalent cations such as Sr²⁺, Ba²⁺, Ma²⁺, CO²⁺, and La³⁺ can replace Ca²⁺ in inducing contraction of the demembranated myonemes although with less efficiency. Contraction and relaxation are ATP-independent and calmodulin is not involved in this in vitro motility process. The myoneme is a strongly resistant structure which is capable of contracting and relaxing under various extreme conditions which indicates very stable proteins and resistant functions.     

Axonemal microtubule pattern ofCienkowskya mereschkovskyi and a revision of heliozoan taxonomy

Summary Cienkowskya mereschkovskyi is a spherical heliozoan with a mucous stalk and a cell coat with spicules randomly distributed.The axonemal microtubules arise from a central MTOC with a centroplast. The microtubular pattern displays a triradial symmetry and is characterized in cross-section by one central hexagon with alternate angles of 106° and 134°. From this slightly irregular central pattern, convex branches made of a series of microtubules and linkers diverge. Any three microtubules (Mt's) along the branch form 166° angles. Around the central hexagon links between successive branches always delineate 3 pairs of enneagons and 3 heptagons, while the peripheral polygons are less strictly defined. None of the previously published theories on Mt-pattern formation are sufficient to totally explain such a complex pattern.A comparison between the fine structure ofCienkowskya and other Heliozoa with reference to very recent data leads us to try to amend the existing terminology for the substructure of the MTOC. An attempt at a more coherent classification is also made using electron microscopy criteria with new arguments compared to existing ones (Febvre-Chevalier, 1982, 1983). A taxonomical key comprising several new names is proposed.