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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 7718

Guy Brugerolle and Jean-Pierre Mignot; [Les Caracteristiques Ultrastructurales de L'helioflagelle Dimorpha mutans Gruber (Sarcodina - Actinopoda) et leur Interet Phyletique]. Protistologica XX(1):97-112, 1984

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Dimorpha mutans bears two long orthogonal kinetosomes which give rise to the anteriorly directed flagellum embedded in mucus like material, and to the lateral directed or recurrent flagellum. These two flagella lack tubular mastigonemes on their surface. The flagella associated cytoskeleton is constituted by a microtubular fibre connected to each kinetosome. These two fibres are linked to a subjacent axopodial microtubular bundle by microfibrillar layers. The axopodial cytoskeleton includes a fibro-granular centroplast without a central dense disc attached to the kinetosome base. From it diverge the axopodial bundles of mts. In each axopod the mts are arranged in quicunx resulting in a quadrangular arrangement or in squared packed arrays which differ from the hexagonal pattern of Centrohelida and from the double spiral of Actinophryida. The centroplast is lodged in a cup-like depression on the nucleus and some axopodial bundles pass through the nucleus forming channels isolated by the normal nuclear envelope. Mitochondria display vermicular cristae and the Golgi bodies are positioned around the nucleus in the anterior region. Kinetocysts show concentric structures with a dense internal cylinder which is reminiscent of those of Tetradimorpha and Clathrulina. By the existence of a centroplast the genera Dimorpha and Tetradimorpha are close to the Centrohelida from which they differ by other characters (arrangement of axopodial mts, kinetocyst structure, shape of mitochondrial cristae). The organization of Dimorpha and Tetradimorpha cells is very similar, which confirms the "centrohelidian type" of these two helioflagellates which differ from Ciliophrys and Actinomonas more related to the "Actinophryidian type". These first ultrastructural studies are in favour of an evolution of Heliozoa from the helioflagellates by regression and elimination of the flagellar system and development of the axopodial system rather than an evolution from the filosial Amoeba.