Fine structure of the uteri in two hymenolepidid tapeworm Skrjabinacanthus diplocoronatus and Urocystis prolifer (Cestoda: Cyclophyllidea) parasitic in shrews that display different fecundity of the strobilae

Distribution of the cestode Urocystis prolifer Villot, 1880 (Cyclophyllidea: Hymenolepididae) in the Palaearctic and new data on its postembryonic development

Urocystis prolifer Villot, 1880 is an intestinal parasite of Sorex spp. In the Palaearctic. There are significant differences in the descriptions of both adults and stages of ontogenesis of U. prolifer as described by various authors. The experimental infection of intermediate hosts with cestodes has been conducted. An overview of the geographical distribution, infestation of the definitive hosts and the development of the metacestode stages of U. prolifer are presented. The cestode is characterized by an extensive geographic area in the Palaearctic, wide host specificity and very high rates of infection of its definitive host. Urocystis prolifer has been recorded mostly in the taiga and forest zones of Palaearctic. Fourteen species of Sorex were registered as the definitive host. Redescription of U. prolifer and an amended generic diagnosis are provided. A complete description of the ontogeny from oncosphere to fully developed metacestode is given. Features of development of the metacestode are an asexual larval reproduction, the absence of the anterior and posterior obturator valve in the cyst of the fully developed urocyst, as well as excretory bodies.

Three in one: evolution of viviparity, coenocytic placenta and polyembryony in cyclostome bryozoans

BACKGROUND

Placentation has evolved multiple times among both chordates and invertebrates. Although they are structurally less complex, invertebrate placentae are much more diverse in their origin, development and position. Aquatic colonial suspension-feeders from the phylum Bryozoa acquired placental analogues multiple times, representing an outstanding example of their structural diversity and evolution. Among them, the clade Cyclostomata is the only one in which placentation is associated with viviparity and polyembryony-a unique combination not present in any other invertebrate group.

RESULTS

The histological and ultrastructural study of the sexual polymorphic zooids (gonozooids) in two cyclostome species, Crisia eburnea and Crisiella producta, revealed embryos embedded in a placental analogue (nutritive tissue) with a unique structure-comprising coenocytes and solitary cells-previously unknown in animals. Coenocytes originate via nuclear multiplication and cytoplasmic growth among the cells surrounding the early embryo. This process also affects cells of the membranous sac, which initially serves as a hydrostatic system but later becomes main part of the placenta. The nutritive tissue is both highly dynamic, permanently rearranging its structure, and highly integrated with its coenocytic 'elements' being interconnected via cytoplasmic bridges and various cell contacts. This tissue shows evidence of both nutrient synthesis and transport (bidirectional transcytosis), supporting the enclosed multiple progeny. Growing primary embryo produces secondary embryos (via fission) that develop into larvae; both the secondary embyos and larvae show signs of endocytosis. Interzooidal communication pores are occupied by 1‒2 specialized pore-cells probably involved in the transport of nutrients between zooids.

CONCLUSIONS

Cyclostome nutritive tissue is currently the only known example of a coenocytic placental analogue, although syncytial 'elements' could potentially be formed in them too. Structurally and functionally (but not developmentally) the nutritive tissue can be compared with the syncytial placental analogues of certain invertebrates and chordates. Evolution of the cyclostome placenta, involving transformation of the hydrostatic apparatus (membranous sac) and change of its function to embryonic nourishment, is an example of exaptation that is rather widespread among matrotrophic bryozoans. We speculate that the acquisition of a highly advanced placenta providing massive nourishment might support the evolution of polyembryony in cyclostomes. In turn, massive and continuous embryonic production led to the evolution of enlarged incubating polymorphic gonozooids hosting multiple progeny.

The first data on the vitellogenesis of paruterinid tapeworms: an ultrastructural study of Dictyterina cholodkowskii (Cestoda: Cyclophyllidea)

The present study provides the first ultrastructural data of the vitellogenesis in a cestode species of the cyclophyllidean family Paruterinidae, aiming to expand the limited data on the vitellogenesis in cyclophyllidean cestodes and to explore the potential of ultrastructural characters associated with vitellogenesis for phylogenetic and taxonomic studies of this order. The process of vitellocyte formation in Dictyterina cholodkowskii follows the general pattern observed in other tapeworms but exhibits several specific differences in the ultrastructure of vitelline cells. The vitellarium contains vitellocytes at various stages of maturation. The periphery of the vitellarium and the space between maturing vitellocytes are occupied by interstitial cells. Differentiation into mature vitellocytes is characterized by high secretory activity, which involves the development of granular endoplasmic reticulum, Golgi complexes, mitochondria and vitelline globules of various sizes. During vitellogenesis, the progressive fusion of these globules results in the formation of two large membrane-limited vitelline vesicles that eventually fuse into a single large vesicle. Mature vitellocytes are composed of a single vitelline vesicle, a high content of cytoplasmic organelles and have no nucleus. No traces of lipid droplets and glycogen granules are detected in the cytoplasm of mature vitellocytes, which might be related to biological peculiarities of this family, i.e. the release of eggs into environment within the tissues of the paruterine organ, which may serve as a source of nutrients for embryos.

Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.

Fine structure of the uterus in tapeworm Tetrabothrius erostris (Cestoda: Tetrabothriidea)

The uterine organization in Tetrabothrius erostris (Tetrabothriidea) was investigated by the methods of transmission and scanning electron microscopy. In sexually mature proglottids, the uterine wall consists of a syncytial epithelium (1.4-2.5 μm thick, except in regions containing nuclei). The ribosomes, mitochondria and numerous cisternae of granular endoplasmic reticulum with concentric or parallel profiles with electron lucent material are observed in the epithelium. The uterine wall is characterized by the abundance of lipid droplets that are localized inside the long protrusions of the uterine epithelium (called fungiform papillae) up to 15-17 μm and in the surrounding medullary parenchyma. The protrusions with lipid droplets in the proximal ends of the uterus are located closely to each other. A basal matrix (up to 0.6 μm thick) supports the uterine epithelium. The musculature consisting of 1-2 muscle layers is well developed; large myocytons are connected with the myofibrils and have a nucleus that reaches 4 μm in size. In gravid proglottids, the epithelium without nuclei is reduced to 0.2-1.6 μm thick. The number of protrusions of the uterine epithelium and lipid droplets in the epithelial layer decreases. Sparse small muscle bundles underlay the uterine wall at this stage; the basal matrix is feebly marked. The matrotrophy or the support by nutrition from the parent organism to embryos is discussed for T. erostris which belongs to oligolecital cestodes and possesses numerous lipid droplets in the uterine wall during the development of embryos.

Relationships between uterus and eggs in cestodes from different taxa, as revealed by scanning electron microscopy

Uterine organization and interaction with developing eggs in Tetrabothrius erostris (Tetrabothriidea), Nippotaenia mogurndae (Nippotaeniidea), Arostrilepis tenuicirrosa, and Monocercus arionis (Cyclophyllidea), cestodes belonging to three different orders, were investigated by scanning electron microscopy. The interactions were traced from sexually mature to gravid proglottids for all species. Pieces of evidence of interactions among these species include specific tight contacts between microlamellae of the uterine epithelium and the egg capsule, networks of fibrils between eggs and uterus, or numerous branched diverticula of the uterine wall that surround eggs or combinations of these. The contacts between uterine epithelium and eggs take place in mature and post-mature proglottids, at a period of development when eggs are newly formed and the embryos are rapidly developing. The eggs grow and develop actively in tight contact with the uterine wall. The maximum diameter of eggs increases 1.5-2 times (or 3.5-4 times in M. arionis) during development. In all species, the intimate contacts between uterus and eggs have weakened or disappeared by the time the proglottids have become gravid. The association between uterus and eggs thus appears as strong evidence of active trophic interaction (or matrotrophy) between the parent organism and developing eggs.