Metamorphic processes in the soft coralsHeteroxenia fuscescensandXenia umbellata: The effect of protein kinase C activators and inhibitors

Two Distinct Bacterial Biofilm Components Trigger Metamorphosis in the Colonial Hydrozoan Hydractinia echinata

In marine environments, the bacterially induced metamorphosis of larvae is a widespread cross-kingdom communication phenomenon that is critical for the persistence of many marine invertebrates. However, the majority of inducing bacterial signals and underlying cellular mechanisms remain enigmatic. The marine hydroid Hydractinia echinata is a well-known model system for investigating bacterially stimulated larval metamorphosis, as larvae transform into the colonial adult stage within 24 h of signal detection. Although H. echinata has served as a cell biological model system for decades, the identity and influence of bacterial signals on the morphogenic transition remained largely unexplored. Using a bioassay-guided analysis, we first determined that specific bacterial (lyso)phospholipids, naturally present in bacterial membranes and vesicles, elicit metamorphosis in Hydractinia larvae in a dose-response manner. Lysophospholipids, as single compounds or in combination (50 μM), induced metamorphosis in up to 50% of all larvae within 48 h. Using fluorescence-labeled bacterial phospholipids, we demonstrated that phospholipids are incorporated into the larval membranes, where interactions with internal signaling cascades are proposed to occur. Second, we identified two structurally distinct exopolysaccharides of bacterial biofilms, the new Rha-Man polysaccharide from Pseudoalteromonas sp. strain P1-9 and curdlan from Alcaligenes faecalis, to induce metamorphosis in up to 75% of tested larvae. We also found that combinations of (lyso)phospholipids and curdlan induced transformation within 24 h, thereby exceeding the morphogenic activity observed for single compounds and bacterial biofilms. Our results demonstrate that two structurally distinct, bacterium-derived metabolites converge to induce high transformation rates of Hydractinia larvae and thus may help ensure optimal habitat selection. IMPORTANCE Bacterial biofilms profoundly influence the recruitment and settlement of marine invertebrates, critical steps for diverse marine processes such as the formation of coral reefs, the maintenance of marine fisheries, and the fouling of submerged surfaces. However, the complex composition of biofilms often makes the characterization of individual signals and regulatory mechanisms challenging. Developing tractable model systems to characterize these coevolved interactions is the key to understanding fundamental processes in evolutionary biology. Here, we characterized two types of bacterial signaling molecules, phospholipids and polysaccharides, that induce the morphogenic transition. We then analyzed their abundance and combinatorial activity. This study highlights the general importance of multiple bacterial signal converging activity in development-related cross-kingdom signaling and poses the question of whether complex lipids and polysaccharides are general metamorphic cues for cnidarian larvae.

Are G-protein-coupled receptors involved in mediating larval settlement and metamorphosis of coral planulae?

Larvae of the scleractinian coral Pocillopora damicornis are induced to settle and metamorphose by the presence of marine bacterial biofilms, and the larvae of Montipora capitata respond to a combination of filamentous and crustose coralline algae. The primary goal of this study was to better understand metamorphosis of cnidarian larvae by determining what types of receptors and signal-transduction pathways are involved during stimulation of metamorphosis of P. damicornis and M. capitata. Evidence from studies on larvae of hydrozoans suggests that G-protein-coupled receptors (GPCRs) are good candidates. Settlement experiments were conducted in which competent larvae were exposed to neuropharmacological agents that affect GPCRs and their associated signal-transduction pathways, AC/cAMP and PI/DAG/PKC. On the basis of the results of these experiments, we conclude that GPCRs and these pathways do not mediate settlement and metamorphosis in either coral species. Two compounds that had an effect on both species, forskolin and phorbol-12-myristate-13-acetate (TPA), may be acting on other cellular processes not related to GPCRs. This study strengthens our understanding of the underlying physiological mechanisms that regulate metamorphosis in coral larvae.