Seasonal dynamics of a complex cheilostome bryozoan symbiosis: vertical transfer challenged.

Symbiotic associations are dynamic systems influenced by both intrinsic and extrinsic factors. Here we describe for the first time the developmental and seasonal changes of the funicular bodies in the bryozoan Dendrobeania fruticosa, which are unique temporary organs of cheilostome bryozoans containing prokaryotic symbionts. Histological and ultrastructural studies showed that these organs undergo strong seasonal modification in the White Sea during the ice-free period. Initially (in June) they play a trophic function and support the development of a large population of bacteria. From June to September, both funicular bodies and bacteria show signs of degradation accompanied by development of presumed virus-like particles (VLPs); these self-organize to hollow spheres inside bacteria and are also detected outside of them. Although the destruction of bacteria coincides with the development of VLPs and spheres, the general picture differs considerably from the known instances of bacteriophagy in bryozoans. We broadly discuss potential routes of bacterial infection in Bryozoa and question the hypothesis of vertical transfer, which, although widely accepted in the literature, is contradicted by molecular, morphological and ecological evidence.

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.

First evidence of virus-like particles in the bacterial symbionts of Bryozoa.

Bacteriophage communities associated with humans and vertebrate animals have been extensively studied, but the data on phages living in invertebrates remain scarce. In fact, they have never been reported for most animal phyla. Our ultrastructural study showed for the first time a variety of virus-like particles (VLPs) and supposed virus-related structures inside symbiotic bacteria in two marine species from the phylum Bryozoa, the cheilostomes Bugula neritina and Paralicornia sinuosa. We also documented the effect of VLPs on bacterial hosts: we explain different bacterial 'ultrastructural types' detected in bryozoan tissues as stages in the gradual destruction of prokaryotic cells caused by viral multiplication during the lytic cycle. We speculate that viruses destroying bacteria regulate symbiont numbers in the bryozoan hosts, a phenomenon known in some insects. We develop two hypotheses explaining exo- and endogenous circulation of the viruses during the life-cycle of B. neritina. Finally, we compare unusual 'sea-urchin'-like structures found in the collapsed bacteria in P. sinuosa with so-called metamorphosis associated contractile structures (MACs) formed in the cells of the marine bacterium Pseudoalteromonas luteoviolacea which are known to trigger larval metamorphosis in a polychaete worm.

Ultrastructural evidence for nutritional relationships between a marine colonial invertebrate (Bryozoa) and its bacterial symbionts.

Autozooids of the cheilostome bryozoan Aquiloniella scabra contain rod-like bacteria in the funicular bodies - the complex swellings of the funicular strands. Each funicular body contains symbionts in the central cavity surrounded by a large, synthetically active internal "sheath-cell" (bacteriocyte) and a group of the flat external cells. The tightly interdigitating lobes of these cells form a capsule well-isolated from the body cavity. Slit-like spaces between bacteria are filled with electron-dense matrix and cytoplasmic processes of various sizes and shapes (often branching) produced by the "sheath-cell". The cell ultrastructure and complex construction of the funicular bodies as well as multiplication of the bacteria in them suggest metabolic exchange between host and symbiont, involving the nourishment of bacteria. We suggest that the bacteria, in turn, influence the bryozoan mesothelial tissue to form the funicular bodies as capsules for bacterial incubation. We present ultrastructural data, discuss possible variants in the development of the funicular bodies in Bryozoa, and propose the possible role of bacteria in the life of their bryozoan host.

Comparative analysis of the nervous system structure of polymorphic zooids in marine bryozoans.

The nervous system structure was compared for the first time in avicularia and vibracula in seven species of the cheilostome bryozoans from six families by immunohistochemical methods and confocal scanning microscopy. Regardless of significant differences in heterozooid shape, size, and position in a colony, their muscular and nervous systems have a common structure, which suggests their parallel evolution.

[FUNCTIONAL DIFFERENTIATION IN BRYOZOAN COLONY: A PROTEOMIC ANALYSIS].

Bryozoans are typical modular organisms. They consist of repetitive structural units, the zooids. Bryozoan colonies grow by zooidal budding, with the distribution pattern of the budding loci underlying the diversity of colony forms. Budding is usually restricted to the zooids at the periphery of the colony, which form a "growing edge" or local terminal growth zones. Non-budding parts of the colony can be functionally subdivided, too. In many species colonies consists of regular, often repetitive zones of feeding and non-feeding modules, associated with a periodical degeneration and regeneration of the polypide, retractile tentacle crown with a gut and the accompanying musculature. So, there is functional differentiation in bryozoan colonies but its mechanisms are unknown. Presumably, budding and/or polypide recycling in different colony parts are induced or inhibited by certain determinants of functional specialization. An effective tool of their identification is the comparison of proteomes of functionally different zones. Here we report the results of proteomic analysis of three bryozoan species from the White Sea, which have a different colony form: Flustrellidra hispida, Terminoflustra membranaceotruncata and Securiflustra securifrons. Using differential two-dimensional electrophoresis (2D-DIGE), we compared proteomes of the growing edge and the zones consisting of feeding and non-feeding zooids in these species. We estimated the overall proteome variability, revealed proteins whose relative abundance gradually changed along the proximal-distal colony axis and suggested that they might be involved in the functional differentiation of the colony.

Distribution of serotonin and FMRF-amide in the nervous system of different zooidal types of cheilostome bryozoa: A case study of Arctonula arctica.

In the White Sea bryozoans Arctonula arctica, the structure of the nervous system and distribution of 5-hydroxytryptamine (5-HT) and FMRF-amide were studied for the first time using immunohistochemical methods and confocal scanning microscopy. The neurotransmitters studied have been actively involved into the integrative processes, gut functioning, and regulation of motion activity. In avicularia, 5-HT and FMRF-amide receptors are capable of performing the same functions, except for participation in the gut functioning, because they have no digestive system.