Studies on the Cell Fusion of Heliozoans. VI. Enhancement of the Fusion Reaction Induced by Cold and Colchicine Treatment

Abstract

ABSTRACT. Using low temperature and colchicine, we examined how axopodia play a role in the initiation and subsequent process of cell fusion in a large heliozoan Echinosphaerium nucleofilum (strain MA). Although the fusion reaction is naturally temperature-dependent, preliminary treatment with a low temperature (2?) induced the reaction more efficiently than in the control kept at room temperature (2?).During the recovery process from the cold, the reaction was enhanced conspicuously in the presence of colchicine; the fusion indices were 2.05 to 5.68 after a 9 h incubation with 4 to 8 mM colchicine. When treated with 4 to 10 mM colchicine without the preliminary cold treatment, the reaction also was enhanced. This means that the reaction depended upon the colchicine concentration and resulted in partial or complete degradation of axopodia; fusion indices were 1.08 to 3.77 after a 9 h incubation with 4 to 10 mM colchicine. We discuss these results and the electron-microscopical data, in relation to the role of axopodia in the fusion reaction.As indicated by the genus name, the large heliozoan Echinosphaerium is characterized by axopodia radiating from its spherical cell body that contain hundreds of microtubules as cytoskeletal entities. The axopodia have been investigated with respect to their stability and/or lability (1,5,6,8,13), the saltatory movement of intra-axopodial particles such as mitochondria, electron-dense bodies and kinetocysts (2-4, 21), the sliding and rotational locomotion of the organisms (15, 22), and feeding behavior (6). However, it is not yet known how the axopodia play a role in the fusion process in heliozoans.In the present study, therefore, we aimed to clarify the role of axopodia on the fusion process, especially in the very early stage. As tools, anti-mitotic factors such as colchicine and low temperature were used to induce partial or complete degradation of the axopodia and to inhibit their re-extension. We found that the fusion reaction was enhanced by the experimentally-induced axopodial degradation, particularly with the inhibition of axopodial re-extension.