Structural characterization of Eutreptia (Euglenophyta). III. Flagellar structure and possible function of the paraxial rods

Abstract

The flagellar paraxial rods of the motile unicellular euglenoid flagellate Eutreptia pertyi Pringsheim (Euglenophyta) are highly ordered and differentiated structures. In the dorsal flagellum (F1) the rod (PAR1) is composed of one predominant substructural component, the oblique filaments that are arranged in complementary axes that give the rod a lattice-like appearance. In the ventral flagellum (F2), the rod (PAR2) is composed of three major substructural components: horizontal, longitudinal and oblique filaments organized into trigonal-hexagonal ‘unit cells’ that collectively impart a paracrystalline, plate-like appearance to the rod. The organization of paraxial rod subcomponents appears to be related to the position and organization of flagellar mastigonemes. The relatively small size and whorled shape of PAR1, the complementary axes of the oblique filaments and the helical arrangement of some flagellar mastigonemes are related to the rapid, helical waveform of the dorsal flagellum. Conversely, the relatively large size and pronounced crescent shape of PAR2, the disposition of subcomponents into three unequal axes, the unidirectional axis of the oblique filaments and the overall plate-like substructure are related to the slow, planar waveform of the ventral flagellum. Analytical ultrastructural characterization of the paraxial rods in this species has provided a basis for comparing similar structures in the flagella of dinoflagellates and kinetoplastid zooflagellates. The paraxial rod appears to be an important structural marker for establishing homologies among species in the Euglenophyta and for suggesting taxonomic relationships within the division based upon flagellar criteria. The paraxial rod also seems to be a reliable structural character for suggesting phylogenetic affiliations among other protists that manifest this type of flagellar accessory component.