AbstractBacterial-induced metamorphosis of larvae is a widespread cross-kingdom communication phenomenon within the marine environment and critical for the persistence of many invertebrate populations. However, the chemical structures of the majority of inducing bacterial signals and the underlying cellular mechanisms remain enigmatic. Hydractinia echinata larvae transform upon detection of bacterial biofilm components into the colonial adult stage. Despite serving as cell biological model system for decades, the inducing bacterial signals remained undiscovered. Using a chemical-ecology driven analysis, we herein identified that specific bacterial (lyso)phospholipids and polysaccharides, naturally present in bacterial biofilms, elicit metamorphosis in Hydractinia larvae. While (lyso)phospholipids (e.g. 16:0LPG/18:1LPE, 16:0 LPA/18:1LPE) as single compounds or in combinations induced up to 50% of all larvae to transform within 48 h, two structurally distinct polysaccharides, the newly identified Rha-Man polysaccharide from Pseudoalteromonas sp. P1-9 and curdlan from Alcaligenes faecalis caused up to 75% of all larvae to transform within 24 h. We also found combinations of (lyso)phospholipids and curdlan induced the transformation in almost all larvae within 24 h, thereby exceeding the morphogenic activity observed for single compounds and axenic bacterial biofilms. By using fluorescence-labeled bacterial phospholipids, we demonstrated their incorporation into the larval membranes, where interactions with internal signaling cascades could occur. Our results demonstrate that multiple and structurally distinct bacterial-derived metabolites converge to induce high transformation rates of Hydractinia larvae, which might ensure optimal habitat selection despite the general widespread occurrence of both compound classes.Significance StatementBacterial biofilms profoundly influence the recruitment and settlement of marine invertebrates, critical steps for diverse marine processes such as coral reef formation, marine fisheries and the fouling of submerged surfaces. Yet, the complex composition of biofilms often makes it challenging to characterize the individual signals and regulatory mechanisms. Developing tractable model systems to characterize these ancient co-evolved interactions is the key to understand fundamental processes in evolutionary biology. Here, we characterized for the first time two types of bacterial signaling molecules that induce the morphogenic transition and analyzed their abundance and combinatorial activity. This study highlights the crucial role of the converging activity of multiple bacterial signals in development-related cross-kingdom signaling.
Two Distinct Bacterial Biofilm Components Trigger Metamorphosis in the Colonial Hydrozoan Hydractinia echinata
