Ultrastructure of the Amoeboflagellate Tetramitus rostratus1

Abstract

The life-cycle of the amoeboflagellate T. rostratus includes amoeboid, cyst and flagellate stages. The ultrastructure of these 3 stages is illustrated, with particular emphasis on flagellate morphology. Amoeba morphology is typical of that of limax amoebas. Cysts, forming from trophic amoebas, are enclosed by a wall made up of 2 layers: ectocyst (circa 70 nm) and endocyst (200 nm). The wall apparently forms from precursor material present in vesicles in the pre-cyst stage cytoplasm. Flagellate morphology is characterized by a well-defined top-shaped profile, maintained by microtubules under the plasma membrane. The flagellar apparatus or mastigont consists of 4 flagella, their basal bodies, sheaves of microtubules associated with 2 of the basal bodies, and several rhizoplasts (periodicity 20 nm). A deep, microtubule-supported, ventral invagination appears to function as a gullet. A small number of mitotic stages observed in amoeboid and flagellate individuals suggests similarity in the division process in both stages: intranuclear mitotic apparatus, nucleolus persisting through mitosis, no centrioles or basal bodies functioning as centrioles and difficulty in resolving chromosomes. Ultrastructures of several amoeboflagellates are compared and the phylogenetic significance of those features common to different species is evaluated. Tetramitus most closely resembles Naegleria spp.