In the polymorphic ciliate Tetrahymena vorax, the non-selective phagocytosis seen in microstomes changes to a highly selective process in macrostomes.

Ciliates use phagocytosis to acquire edible particles. The polymorphic ciliate Tetrahymena vorax appears in two forms ('microstomes' and 'macrostomes'). Transformation of microstomes into macrostomes takes place in the presence of the ciliate Tetrahymena thermophila and enables the macrostome to phagocytose the latter species. The non-specific, constitutive phagocytosis in microstomes thereby changes into a specific inducible process in macrostomes. The purpose of this study was to determine whether the phagocytotic process in macrostomes is specifically aimed at catching T. thermophila. The two forms of phagocytosis represent an interesting model system for studying the mechanism whereby phagosomes are formed. The macrostomal form capture deciliated and ciliated Tetrahymena thermophila, latex beads with diameters of 20.3 and 30.0 microm and small microstomal cells. However, the macrostomes select T. thermophila as a prey when they have the opportunity to choose between deciliated T. thermophila and latex beads and between T. thermophila and microstomes. The non-selective formation of phagosomes seen in microstomes changes to a highly selective process during the transformation to macrostomes. Unlike microstomes, macrostomes do not form a closed vacuole after capturing a latex bead, indicating that mechanical stimulation by the prey does not in itself trigger phagocytosis in the macrostomal form of T. vorax. Although macrostomes captured T. thermophila in preference to microstomes, phagocytosis of microstomes started immediately following capture, indicating that the substance/molecule that triggers the formation of the phagosome is not specific for T. thermophila cells. After capturing a T. thermophila cell, the macrostomal cell, which normally swims in a forward direction, reverses direction and swims backwards for a short time before starting to rotate. Macrostomal cells did not change their swimming pattern after capturing a latex bead. We believe, therefore, that backward swimming is more likely to be related to signals resulting from phagocytosis than from mechanical stimulation of the pouch. Cytochalasin B (10 microg ml(-1)) inhibits phagocytosis in both microstomes and macrostomes, indicating that actin filaments play an active role in phagocytosis in both cell types. The antitubulin drug nocodazole (0.3-30 micromol l(-1)) inhibits the formation of more than one phagosome in the macrostome, indicating that membrane transport to the oral apparatus in macrostomes is guided by microtubules. Nocodazole has no effect on the process of phagocytosis in microstomes.