Trematode reproduction in the molluscan host: an ultrastructural study of the germinal mass in the rediae of Himasthla elongata (Mehlis, 1831) (Digenea: Echinostomatidae)

Histological and electron microscopic analysis of germinal material in miracidia of Schistosoma mansoni

We performed histological and electron microscopic analysis of miracidia of Schistosoma mansoni in order to examine their germinal elements. In total, about 20 germinal cells at different stages of maturation were found. We described their ultrastructure and proposed a scheme of reproduction of mother sporocysts of S. mansoni based on our data and literature information. According to this scheme, the only germinal elements present in the miracidia are germinal cells (undifferentiated cells were not found). Regardless of their size and localisation, none of the germinal cells in the miracidia has undergone full differentiation. This process is completed after the metamorphosis of the larva into the sporocyst.

Germinal development in embryonic rediae of the hemiuroid digenean Bunocotyle progenetica: an ultrastructural study

Embryonic development of reproductive organs in rediae of the digenean Bunocotyle progenetica was studied using transmission electron microscopy. The germinal primordium becomes morphologically distinct in early embryos as a weakly separated cell mass with a forming cavity. It consists of undifferentiated, differentiating, and supporting cells. As embryos develop, the supporting cells form a wall around the enlarging cavity. Other cells of the germinal primordium are incorporated into the wall as solitary cells or as small cell aggregations. Those situated posteriorly give rise to an incipient germinal mass functioning during postembryonic development. Undifferentiated and differentiating cells in the middle and the anterior part of the primordium ensure a considerable growth of the cavity wall, which incorporates solitary germinal cells. In advanced embryonic rediae, these cells mature, cleave, and give rise to germinal balls, which enter the forming brood cavity. In the most mature embryonic rediae, all these early cercarial embryos reside in a brood cavity, which is lined by that time with a syncytium continuous with the supporting tissue of the incipient germinal mass. Based on our results and the literature data, we suggest that the morphogenesis of the reproductive apparatus of the daughter parthenitae in hemiuroid digeneans may be characterized by (1) emergence of an incipient brood cavity within the germinal primordium, (2) formation of the cavity lining from the cells of the germinal primordium, (3) fragmentation and uneven distribution of the germinal material of the germinal primordium around the cavity, and (4) an anticipatory development of some of this germinal material.

Elucidation of Himasthla leptosoma (Creplin, 1829) Dietz, 1909 (Digenea, Himasthlidae) life cycle with insights into species composition of the north Atlantic Himasthla associated with periwinkles Littorina spp

Trematodes of the genus Himasthla are usual parasites of coastal birds in nearshore ecosystems of northern European seas and the Atlantic coast of North America. Their first intermediate hosts are marine and brackish-water gastropods, while second intermediate hosts are various invertebrates. We analysed sequences of partial 28S rRNA and nad1 genes and the morphology of intramolluscan stages, particularly cercariae of Himasthla spp. parasitizing intertidal molluscs Littorina spp. in the White Sea, the Barents Sea and coasts of North Norway and Iceland. We showed that only three Himasthla spp. are associated with periwinkles in these regions. Intramolluscan stages of H. elongata were found in Littorina littorea, of H. littorinae, in both L. saxatilis and L. obtusata, and of Cercaria littorinae obtusatae, predominantly, in L. obtusata. Other Himasthla spp. previously reported from Littorina spp. in North Atlantic are either synonymous with one of these species or described erroneously. Based on a comparison of newly generated 28S rDNA sequences with GenBank data, rediae and cercariae of C. littorinae obtusatae were identified as belonging to H. leptosoma. Some previously unknown morphological features of young and mature rediae and cercariae of the three Himasthla spp. are described. We provide a key to the rediae and highlight characters important for identification of cercariae. Genetic diversity within the studied species was only partially determined by their specificity to the molluscan host. The nad1 network constructed for H. leptosoma lacked geographical structure, which is explained by a high gene flow owing to highly vagile definitive hosts, shorebirds.

The fine structure of the germinal mass, brood cavity and birth canal of the rediae of the monoxenous hemiuroid digenean Bunocotyle progenetica Chabaud & Buttner, 1959

Bunocotyle progenetica is a hemiuroid digenean whose sexual adults become fully developed and lay their eggs inside the rediae in the molluscan host. In this study, the fine structure of the germinal mass, brood cavity and birth canal in the B. progenetica rediae was examined using transmission electron and confocal microscopy. The large germinal mass attached to the body wall has a cellular composition typical for this organ. The characteristic traits of this germinal mass are weakly developed supporting tissue and the presence of deep lacunae opening into the brood cavity. These lacunae presumably participate in feeding the deeply lying embryos and facilitate their release into the brood cavity. The germinal mass is also characterized by intensive degeneration of cellular elements, which may represent a mechanism controlling the offspring number, limited in this species by the size of the redial brood cavity. The brood-cavity lining consists of flattened cells bearing lamellar projections and is connected anteriorly with the epithelium of the birth canal. The brood-cavity musculature, which is well developed in other hemiuroid digeneans, is significantly reduced in B. progenetica, most likely because their cystophorous cercariae remain inside the rediae, removing the need for muscle contractions pushing them through the brood cavity. The birth canal comprises three regions distinguished by the structure of the lining and muscle arrangement. The comparison of rediae of B. progenetica with parthenitae of other digeneans has shown that the organization of the redial reproductive apparatus in this species may have been influenced by life-cycle modification.

Reproduction of trematodes in the molluscan host: an ultrastructural study of the germinal mass and brood cavity in daughter rediae of Tristriata anatis Belopolskaia, 1953 (Digenea: Notocotylidae)

This study describes the fine structure of the germinal mass in daughter rediae of Tristriata anatis. The germinal mass consists of undifferentiated cells, germinal cells and supporting cells and contains numerous cercarial embryos up to tail bud stage. Supporting cells and their outgrowths form a tight meshwork of the germinal mass. In its basal part, this meshwork serves as scaffolding for undifferentiated and germinal cells, naked cell aggregates and early germinal balls. More mature embryos are located apically. The hypertrophied supporting tissue appears to be involved in an intensive transport of substances, as indicated by abundant gap junctions between cell outgrowths and numerous pinocytotic vesicles and microtubules in their cytoplasm. Germinal cells contain annulate lamellae and the nuage, typical organelles of animal oocytes. In young rediae containing embryonic cercariae at the tail bud stage, the supporting tissue starts to degenerate in the apical part of the germinal mass, and a primordial brood cavity emerges though it develops fully only in mature rediae containing late embryonic cercariae. An unusual feature of the germinal mass in T. anatis rediae is an enhancement of the embryo brooding function. At the same time, the performance of this function by the brood cavity is reduced. This is the first time such a redistribution of the embryo brooding function between the germinal mass and the brood cavity has been reported.

Reproduction of Echinostoma caproni mother sporocysts (Trematoda)

The localisation and the composition of germinal material in miracidia and mother sporocysts of Echinostoma caproni were studied with the use of histological and electron microscopic methods. Germinal material in miracidia was localised in the posterior body half and was represented by 3-4 undifferentiated cells and 5-7 germinal cells. Taken together, these cells are referred to as the primordium of the germinal mass. In the mother sporocyst, germinal elements also form and develop in the germinal mass, which is located caudally. It comprises undifferentiated cells and germinal cells as well as embryos of various ages (up to the stage of 30-50 blastomeres). Germinal cells divide only by cleavage. New germinal cells are formed only from undifferentiated cells, which can proliferate in the germinal mass and nowhere else. This indicates that the germinal mass is the reproductive organ of E. caproni mother sporocyst.

Genomic diversity of cercarial clones of Himasthla elongata (Trematoda, Echinostomatidae) determined with AFLP technique

The aim of this study was to reveal genomic diversity formed during parthenogenetic reproduction of rediae of the trematode Himasthla elongata in its molluskan host Littorina littorea. We applied amplification fragment length polymorphism (AFLP) to determine the genomic diversity of individual cercariae within the clone, that is, the infrapopulation of parthenogenetic progeny in a single molluskan host. The level of genomic diversity of particular cercariae isolates from a single clone, detected with EcoR1/Mse1 AFLP reaction, was significantly lower than the variability of cercariae from different clones. The presence of intraclonal genomic diversity indicates a nonsexual shuffle of alleles during parthenogenesis in the rediae of H. elongata. The obtained polymorphic AFLP fragments were long enough to detect the sequences that may be responsible for clonal genomic variability. Based on this, AFLP can be recommended as a tool for the study of genetic mechanisms of this variability.

Growing an Embryo from a Single Cell: A Hurdle in Animal Life

A requirement that an animal be able to feed to grow constrains how a cell can grow into an animal, and it forces an alternation between growth (increase in mass) and proliferation (increase in cell number). A growth-only phase that transforms a stem cell of ordinary proportions into a huge cell, the oocyte, requires dramatic adaptations to help a nucleus direct a 10(5)-fold expansion of cytoplasmic volume. Proliferation without growth transforms the huge egg into an embryo while still accommodating an impotent nucleus overwhelmed by the voluminous cytoplasm. This growth program characterizes animals that deposit their eggs externally, but it is changed in mammals and in endoparasites. In these organisms, development in a nutritive environment releases the growth constraint, but growth of cells before gastrulation requires a new program to sustain pluripotency during this growth.