ИСТИНА

The blastogregarines (the taxon including only one genus Siedleckia) are the problem Sporozoa group. Certain superficial resemblance allowed some experts to place them among gregarines and even in the basis of gregarine phylogenetic tree. However, their life cycle has been poorly studied so far. No stages typical for gregarines (e.g., syzygy) have been found in them and, moreover, their life cycle cannot be compared to the Sporozoan one in general. No ultrastructural and molecular data on blastogregarines have been available so far. The facts mentioned above allowed some specialists to place blastogregarines in other taxa than Sporozoans, e.g., in parasitic Dinoflagellates. I have studied the fine structure of the trophozoites of blastogregarine Siedleckia sp. (probably Siedleckia nematoides), an intestinal parasite of Scoloplos armiger polychaete worm inhabiting the White Sea. Trophozoites have smooth surface lacking any grooves or folds. Integument is represented by the three-membranous pellicle similar to that of the gregarines. Pellicle is underlain by a single layer of numerous longitudinal microtubules arising from the polar ring in mucron. The mucron of Siedleckia contains a large vacuole with loose fibrous inclusions, which is surrounded by lamellar structures (probably, smooth EPR) and rhoptry-like organelles with forward-directed ducts. Poor preservation of the mucronal vacuole wall and surrounding structures allows us to suppose the presence of the lytic enzymes active at the early stages of fixation. Anterior part of the vacuole is encircled by microtubular structure similar to the conoid of the others Sporozoa. However, this structure seems to consist of the straight microtubules rather than the coiled ones. Contact zone between the parasite and host plasmalemmas is formed only by the semi-desmosoma-like structure at the host cell side. Well-developed rough EPR, single grains of amylopectin and numerous nuclei containing partly condensed chromatin are located in the anterior half of the blastogregarine body. Posterior half of the body is tightly stuffed with amylopectin and contains almost no nuclei. Thus, the fine structure of Siedleckia trophozoites represents many plesiomorph traits of Sporozoa indicating blastogregarines’ affiliation to this group. Preliminary molecular data confirm plesiomorph status of blastogregarines in Apicomplexa clade, though supporting their position as a sister-group to coccidians rather than gregarines.