Development of the hyaline layer around the planktonic embryos and larvae of the asteroidPatiriella calcarand the presence of associated bacteria

Symbiont transmission in marine sponges: reproduction, development, and metamorphosis

Marine sponges (phylum Porifera) form symbioses with diverse microbial communities that can be transmitted between generations through their developmental stages. Here, we integrate embryology and microbiology to review how symbiotic microorganisms are transmitted in this early-diverging lineage. We describe that vertical transmission is widespread but not universal, that microbes are vertically transmitted during a select developmental window, and that properties of the developmental microbiome depends on whether a species is a high or low microbial abundance sponge. Reproduction, development, and symbiosis are thus deeply rooted, but why these partnerships form remains the central and elusive tenet of these developmental symbioses.

Bacterial Exposure Mediates Developmental Plasticity and Resistance to Lethal Vibrio lentus Infection in Purple Sea Urchin (Strongylocentrotus purpuratus) Larvae

Exposure to and colonization by bacteria during development have wide-ranging beneficial effects on animal biology but can also inhibit growth or cause disease. The immune system is the prime mediator of these microbial interactions and is itself shaped by them. Studies using diverse animal taxa have begun to elucidate the mechanisms underlying the acquisition and transmission of bacterial symbionts and their interactions with developing immune systems. Moreover, the contexts of these associations are often confounded by stark differences between "wild type" microbiota and the bacterial communities associated with animals raised in conventional or germ-free laboratories. In this study, we investigate the spatio-temporal kinetics of bacterial colonization and associated effects on growth and immune function in larvae of the purple sea urchin (Strongylocentrotus purpuratus) as a model for host-microbe interactions and immune system development. We also compare the host-associated microbiota of developing embryos and larvae raised in natural seawater or exposed to adult-associated bacteria in the laboratory. Bacteria associated with zygotes, embryos, and early larvae are detectable with 16S amplicon sequencing, but 16S-FISH indicates that the vast majority of larval bacterial load is acquired after feeding begins and is localized to the gut lumen. The bacterial communities of laboratory-cultured embryos are significantly less diverse than the natural microbiota but recapitulate its major components (Alphaproteobacteria, Gammaproteobacteria, and Bacteroidetes), suggesting that biologically relevant host-microbe interactions can be studied in the laboratory. We also demonstrate that bacterial exposure in early development induces changes in morphology and in the immune system. In the absence of bacteria, larvae grow larger at the 4-arm stage. Additionally, bacteria-exposed larvae are significantly more resistant to lethal infection with the larva-associated pathogen Vibrio lentus suggesting that early exposure to high levels of microbes, as would be expected in natural conditions, affects the immune state in later larvae. These results expand our knowledge of microbial influences on early sea urchin development and establish a model in which to study the interactions between the developing larval immune system and the acquisition of larval microbiota.

The Microbial Landscape of Sea Stars and the Anatomical and Interspecies Variability of Their Microbiome

Sea stars are among the most important predators in benthic ecosystems worldwide which is partly attributed to their unique gastrointestinal features and feeding behaviors. Despite their ecological importance, the microbiome of these animals and its influence on adult host health and development largely remains unknown. To begin to understand such interactions we sought to understand what bacteria are associated with these animals, how the microbiome is partitioned across regions of the body and how seawater influences their microbiome. We analyzed the microbiome composition of a geographically and taxonomically diverse set of sea star taxa by using 16S rRNA gene amplicon sequencing and compared microorganisms associated with different regions of their body and to their local environment. In addition, we estimated the bacterial and coelomocyte abundance in the sea star coelomic fluid and bacterioplankton abundance in the surrounding seawater via epifluorescence microscopy. The average bacterial cell abundance observed in the coelomic fluid was one to two orders of magnitude lower than the bacterioplankton abundance in the surrounding seawater suggesting a selection against the presence of microorganisms in the coelomic fluid. The sea star microbiome was also significantly different from seawater with relatively few shared microbial taxa. Microbial communities were found to be significantly different between the pyloric caeca, gonads, coelomic fluid, and body wall of the animals. The most noticeable difference between anatomical sites was the greater relative abundance of Spirochaetae and Tenericutes found in hard tissues (gonads, pyloric caeca, and body wall) than in the coelomic fluid. The microbiome of sea stars thus appears to be anatomically partitioned, distinct from the microbial community of seawater and contains a relatively low abundance of bacteria within the coelomic cavity.

Sea Urchin Larvae as a Model for Postembryonic Development

Larvae are a diverse set of postembryonic life forms distinct from juveniles or adults that have evolved in many animal phyla. Echinoids (sea urchins and sand dollars) generate rapidly developing, morphologically simple, and optically transparent larvae and are a well-established model system supported by a broad array of genomic resources, experimental approaches, and imaging techniques. As such, they provide a unique opportunity to study postembryonic processes such as endocrine signaling, immunity, host-microbe interactions, and regeneration. Here we review a broad array of literature focusing on these important processes in sea urchin larvae, providing support for the claim that they represent excellent experimental study systems. Specifically, there is strong evidence emerging that endocrine signaling, immunity, and host-microbe interactions play major roles in larval development and physiology. Future research should take advantage of sea urchin larvae as a model to study these processes in more detail.

Synthesis and secretion of molecules exhibiting the HL1 epitope during development of the hyaline layer of the asteroid Pisaster ochraceus

A complex ECM layer called the hyaline layer (HL) surrounds embryos and larvae of the starfish Pisaster ochraceus. When preserved by freeze substitution, the HL of a bipinnaria larva consists of six sublayers. From the plasmalemma outwards these are the intervillous layer (iv), the H3, H2, H1 sublayers that make up the supporting layer, a boundary layer (b) and the coarse outer meshwork (cm). HL development begins at fertilization when exocytosis of the cortical granules releases ECM into the perivitelline space and elevates the fertilization membrane. Over the course of early development the layers are added in a sequential manner and by hatching the embryo is surrounded by a thin HL containing most if not all of the layers. The layers thicken over the next few days. By the bipinnaria stage the larvae are surrounded by a thick six-layered HL. HL1 is a monoclonal antibody that reacts against an epitope found in all regions of the HL of the bipinnaria larva except the H2 sublayer. Western blots show that it is present on several molecules during HL development. The number and pattern of the HL1-labeled molecules change during development, suggesting that either new molecules are being produced or that some molecules are precursors of others. Light (immunofluorescence) and TEM (immunogold) studies using HL1 in the early stages of development show that HL1-positive material is not present in the corticle granules and that it only begins to be manufactured and secreted in quantity in the blastula stage at 18-20 h. Following this it continues to be secreted at least as far as the bipinnaria stage. Molecules containing the HL1 antigen therefore do not appear to play a major role in early development of the HL but are necessary for later events. The results suggest that, like the sea urchin HL, the starfish HL undergoes a sequential organization of the different HL layers from ECM components, which are released into the perivitelline space.

Ultrastructural aspects of the development of the hyaline layer and extracellular matrix lining the gastrointestinal tract in embryos and larvae of the starfish Pisaster ochraceus preserved by freeze substitution

Embryos and larvae of the starfish Pisaster ochraceus are surrounded by a complex extracellular matrix (ECM) layer called the hyaline layer (HL). A similar but less well-organized ECM layer lines some regions of the larval gut. Examination of material preserved by freeze substitution shows that the HL consists of a coarse outer meshwork, a boundary layer, a supporting layer, which is divided into three sublayers, H1, H2, and H3, and an intervillus layer. The development of the HL has been studied in material preserved by freeze substitution. Development begins at fertilization when exocytosis of the cortical granules releases ECM into the perivitelline space and elevates the fertilization membrane. Shortly after, plaques of dense material with attached fibers are present on the outer surface of the egg plasmalemma. Following this, these plaques and fibers are associated with the tips of short microvilli, suggesting that they may induce microvillus formation. Next, the tips of some of the microvilli are joined by short regions of the H1 sublayer. Some of these H1 regions have short segments of boundary layer material associated with their outer surfaces while others are naked. Just prior to hatching, the H1 and boundary layers completely surround the embryo, separating the developing coarse meshwork and intervillus layers. Short segments of the H2 and H3 sublayers are also present. Posthatching, the microvilli and all HL layers increase in thickness and density, particularly the H2, boundary, and coarse outer meshwork layers. The results suggest a sequential organization of HL components from ECM that is secreted into the perivitelline space.

Lectin histochemistry of the hyaline layer around the larvae of Patiriella species (Asteroidea) with different developmental modes

Larvae of sea stars are surrounded by an extracellular matrix called the hyaline layer. The lectin-binding properties of this matrix were investigated in an ultrastructural study of Patiriella species having different modes of development. The planktonic bipinnaria and brachiolaria of P. regularis and the planktonic brachiolaria of P. calcar demonstrated the same labeling of the hyaline layer for three lectins: Con A, SBA, and WGA. In both species the outer coarse meshwork stained for all three lectins, whereas the intervillous layer displayed patchy labeling. In the benthic brachiolaria of P. exigua, the outer coarse meshwork displayed heavy labeling for all three lectins. The heavy labeling of the outer coarse meshwork of P. exigua compared with that of the other species suggests an increased number of lectin binding sites in the hyaline layer of this species. The similar ultrastructure and histochemistry of the hyaline layer of P. regularis and P. calcar may reflect similar requirements of their extracellular cover in their planktonic environment. Lectin labeling shows that hypertrophy of the hyaline layer of P. exigua, in particular the outer coarse meshwork, involves elaboration of the carbohydrate composition of the matrix. Modifications seen in the ultrastructure and histochemistry of the hyaline layer of P. exigua appear to be associated with the evolution of benthic development.