Structural organization of a dinokont : microarchitecture and evolutionary implications of the flagellar paraxial rod in Peridinium inconspicuum

This ultrastructural investigation characterizes the structural organization of the vegetative cell of the freshwater, photosynthetic dinokont Peridinium inconspicuum (Lemmermann), with major emphasis on the longitudinal flagellum (LF) and the flagellar paraxial rod (PAR). Particular attention is also given to (1) the organization of the transverse flagellum, its mastigonemes and paraxonemal fiber complex, (2) the flagellar roots and surrounding organelles, i. e. stigma and peduncle, (3) the protruded peduncle, and its possible role as a phagotrophic apparatus, (4) the apical pore complex, which is unique to the peridinoid and gonyaulacoids, and (5) the trichocyst systems, and their discharging mechanism. The longitudinal (LF) and transverse flagella (TF) of dinokonts differ significantly in their structural organization, motility behaviors, and functions. The skeleton of the transverse flagellum is composed of a helical axoneme, a three-membered paraxonemal fiber complex (PAF), and a mastigoneme-anchoring fibril (MaF). The three PAF fibrils originate from the flagellar collar and are attached laterally to each other by fibrous connectives, with PAF2 connected to an axonemal doublet. The skeleton of the longitudinal flagellum is basically made up of a fairly straight axoneme, a hemicylindrical PAR, and flagellar dense bodies. The paraxial rod is made up of 500-600 hemicylindrical 10-nm-wide filaments, originating from the transitional zone of the axoneme, and bound to axonemal doublets 7 and 8 along the length of the flagellum. The PAR has an amorphous center, and a paraxial rod fibril (PARF) that lies parallel to the axoneme and within the PAR "wall". The surface of the transverse flagellum bears three different types of mastigonemes that are anchored to the MaF, whereas the longitudinal flagellum contains a fourth type of mastigoneme attached to the axoneme. The transverse flagellum beats in a helicoid pattern and appears to be the primary propulsive unit, whereas the longitudinal flagellum propagates planar waves and serves more as a steering unit. The PAR microarchitecture of Peridinium inconspicuum is compared with those reported in other PAR-bearing heterokont protists. i.e. euglenoids and kinetoplastids. The data suggest that the PAR of these diverse flagellates is most likely the same structure that has modified during the evolution of individual groups.