THE MORPHOLOGICAL STUDY OF THE CYSTS PELOMYXA PALUSTRIS GREEFF, 1874
A. O. Frolov,1, * L. V. Chystjakova,2 A. V. Goodkov,3
M. N. Malysheva 1
1 Zoological Institute RAS, St. Petersburg,
2 Biological Research Institute of St. Petersburg State University, and
3 Institute of Cytology RAS, St. Petersburg;
*e-mail: frolal@online.ru
Pelomyxa palustris Greeff, 1874, is the only species of pelomixoid amoebas with the rest cysts in its life
cycle. The morphology of the P. palustris has been studied by the light and electronic microscopy. Encystation
of P. palustris under climatic conditions of North-West of Russia occurs within August-September. Rest cysts
have a complex, trilaminar wall. Two inner lamina are the dense endocyst and the laminated mesocyst, thickness of each
layer runs up to 0.6-0.7 µm. Thickness of the electron-dense ectocyst usually does not exceed 0.1-0.2 µm. The
encystated cell of P. palustris has the unique structure. About 60 % of the cell volume are occupied by a huge
vacuole placed in the center and filled up with the prokaryotic cytobionts. Different vacuoles, small vesicles of
various nature, autophagosomes and lipid drops could be found inside that huge vacuole. The amoebae cytoplasm
occupies the space in between endocyst's inner surface and the central vacuole. No any inclusions, prokaryotic
cytobionts and most of cell organelles are absent in the cytoplasm. There are 4 large nuclei filled with relatively
homogeneous karyoplasm lying in the cytoplasm. Nuclear envelope forms a lot of long tubular channels, running through
the cytoplasm and lining the membrane of the central vacuole. Encysted pelomixoid stay in this state up until the
beginning of excystation. Excystation of P. palustris in the studied region occurs in spring, during the latter
half of April and the beginning of May. Cysts undergo complex morphofunctional changes, related to the reorganization
of the wall and formation of young multinucleate amoebas. Only one wall lamina of the 3 initial ones is left up to the
moment of excystation. The central vacuole endures ruination and its content penetrates into the cytoplasm. Pelomixoid
nuclei divide twice. Prokaryotic cytobionts are localized in cytoplasm and in the perinuclear area. Young multinuclear
species of P. palustris coming out of the cysts do not differ in their structure from the adult forms.
Key words: pelobionts, Pelomyxa palustris, life cycle, cysts, ultrastructure
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