Ultrastructure of oocysts and sporozoites of urosporid gregarine from marine polychaete
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| Year of publication | 2016 |
| Type | Conference abstract |
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| Description | Apicomplexans represent a very successful group of unicellular parasites that consists entirely of parasitic species infecting a wide range of vertebrates and invertebrates. Gregarines represent a basal group of the phylum Apicomplexa. The life cycle of eugregarines is in general characterised by a lack of merogony (asexual reproduction for autoinfection), but a presence of syzygy (pregamic paired association of gamonts) and gametocyst stage. Within gametocyst, gametogony (sexual reproduction), fusion of gametes and sporogony (formation of invasive stages) take place. The crucial step for parasite dissemination and effective infection of new hosts is formation of sporozoites within a protective envelope during the sporogony. The knowledge of the morphology of these stages can provide a possible evolutionary pathways particularly within gregarines and apicomplexans in general. In the body cavity of polychaete Travisia forbesii brown bodies comprising white spheres were found. Most of these spheres were identified as cysts of gregarines, that inhabit the host`s body cavity (Urospora spp.). Each cyst was surrounded by several closely adjacent layers of flattened host coelomocytes, forming a brownish capsule around them. The wall of the cyst consisted from three distinct layers: (i) multi-lamellar innermost, (ii) hyaline middle and (iii) outermost layer consisting of short fibrils. Under this wall numerous spindle-shaped and heteropolar oocysts, typical of the urosporids [1], were observed under this wall. At the one pole of oocyst, a transparent funnel was situated, while the opposite end was elongated into a narrowing, transparent tail. Inside the oocysts, a refringent residuum and eight sporozoites were observed. The latter were localised by fours at the each oocyst pole. The envelope consisted of two-layers: (a) outermost layer (= exospore), consisted of one membranous lamellae, (b) innermost (= endospore), was made of two membranous lamellae. The distance between exo-, endospore and sporozoite pellicle varied considerably, however, in most cases endospore was closely appressed to the sporozoites. The banana-shaped sporozoites were covered with a three-layered apicomplexan pellicle. The pellicle was underlain by 20 longitudinal subpellicular microtubules, a large nucleus was often localised in the cell middle region along long axis of sporozoite. The structures resembling conoid and polar ring placed at apical end of sporozoites, the anterior third of the cell was packed with rhoptries and numerous micronemes. Among the oocysts, zygotes with envelope were found. They were round, covered by pellicle and contained different inclusions and vacuoles. We also observed dead cells and solitary “naked” sporozoites. Our observations are in agreement to the previous ones performed on spores of Urospora spp. and other urosporids [1]. General morphology of cysts and oocyst walls, as well as sporozoite morphology are comparable to species investigated to date [2]. The presence of dead cells and function of solitary sporozoites remain undiscovered. |
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