A cytostome/cytopharynx in green euglenoid flagellates (Euglenales) and its phylogenetic implications.

The green, phototrophic euglenoid, Colacium libellae, has a vestigial cytostome and cytopharynx. The membrane forming the simple pocket is decorated with a dense, microfilamentous mesh. The mesh binds the pocket to a band of reinforcing microtubules which is homologous with the bodonid MTR. The opening of the cytostome at the reservoir-canal transition zone implies that the phototrophic euglenoid canal is formed by the invagination of the vestibulum of the phagotrophic euglenoids. Our observations support the hypothesis that the phagotrophic euglenoids arose from a bondonid ancestor and gave rise to the phototrophs by chloroplast acquisition.