Morphological study of cysts of Pelomyxa palustris Greeff, 1874

Abstract

Pelomyxa palustris Greeff, 1874, is the only representative of pelomyxoid amoebae with rest cysts in its life cycle. The morphology of the P. palustris cysts was studied using light and electron microscopy. The encystation of P. palustris under the climatic conditions of northwestern Russia occurs in August through September. The rest of the cysts have complex, trilaminar walls. The most developed are the two inner layers, i.e., the electron-dense structureless endocyst and the laminated mesocyst; the thickness of each layer can reach 0.6–0.7 μm. The thickness of the superficial electron-dense ectocyst lamina usually does not exceed 0.1–0.2 μm. The encysted cell of P. palustris has a unique structure. About 60% of its cell volume is occupied by a giant central vacuole filled with prokaryotic cytobionts. This vacuole has also been found to contain vacuoles and vesicles of different natures, restricted by vacuole membranes, autophagosomes, and lipid droplets. The amoeba cytoplasm occupies the space between the endocyst inner surface and the central vacuole. It contains no inclusions, prokaryotic cytobionts and most of cell organelles. In the cytoplasm there are 4 large nuclei filled with relatively homogeneous karyoplasm. The nuclear envelope forms numerous long tubular outgrowths, piercing the cytoplasm and underlining the central vacuole membrane. In this state the encysted pelomyxoids survive until the beginning of excystation. The excystation of P. palustris in the studied region occurs in spring, during the second half of April through the beginning of May. The cysts undergo complex morphofunctional changes due to reorganization of the wall and formation of young multinucleate amoebae. Out of the three initial lamina of the wall, only one persists until the moment of encystation. The central vacuole is destroyed and its content penetrates into the cytoplasm. Pelomyxoid nuclei divide twice. Prokaryotic cytobionts are localized in the cytoplasm and in the perinuclear area. Young multinuclear P. palustris individuals exiting cysts do not differ from the adult forms by their organization.