Ultrastructure of the amoebo-flagellate Protonaegleria westphali

Abstract

Investigation of the ultrastructure of Protonaegleria westphali has been carried out by means of scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM). SEM investigation demonstrated much enlarged trophozoites, flagellates and cysts corresponding to those under light microscopical observation. In situ fixation of moving trophozoites revealed attachment to the substratum by many uroidal and lateral filopodia. The typical flagellate stage has four flagella inserted two by two at the anterior pole of the cell. The smooth wall of cysts had prominent pores sealed by a mucous plug. Apart from their greater size, trophozoites and cysts resemble those of the genus Naegleria. Mitochondria are not as elongated as in the case of Naegleria; rather, they are round. The cyst is surrounded by a thick, layered endocyst (0.2–0.5 Μm) and a delicate ecotcyst loosely apposed to the endocyst. Both walls join at the region of the prominent pores, forming a characteristically thick collar. This, together with the pore structure (up to 1.0 Μm in diameter) places the amoeba in group I of N. gruberi, according to Pussard and Pons (1979). The flagellate state usually has four flagella which are anchored firmly by a prominent flagellar apparatus or mastigont at the anterior pole of the cell, comparable to that of the genus Tetramitus. The flagella show a typical 9+2 arrangement of microtubules (MT) and are surrounded by a sheath which is continuous with the cell membrane. Main elements of the mastigont could be demonstrated as typical kinetosomes of 0.75 Μm length. Each is closely associated with the cross-striated rhizoplast located perpendicular to it. The rhizoplasts, 2.5 Μm long and 70 nm in diameter, are directed towards the nucleus and terminate freely within the cytoplasm. Fibrillar sheaves or spurs, 0.4–0.6 Μm long and consisting of a single row of parallel microtubules with relatively heavy walls, are closely connected, with the kinetosomes as a supporting structure. The microtubules of the spurs are oriented parallel to the axis of the kinetosome. In contrast to the flagellates of Naegleria, those of the genus Protonaegleria are enveloped by parallel subpellicular MT spaced up to 85 nm apart and extending from the leading end of the cell to its rear. The nucleus of flagellates containing a large nucleolus was comparable to that of the trophozoite. Mitotic stages were not seen in the sections, nor was a cytostome as described in Tetramitus flagellates. The relationships of flagellates and of the genera Tetramitus and Naegleria are discussed with respect to their common features. We assume that Protonaegleria is related more closely to Naegleria than to Tetramitus, due to the morphological characteris of the flagellates and cysts.