Metamorphic-Signal Transduction in Hydroides elegans (Polychaeta: Serpulidae) Is Not Mediated by a G Protein

Evidence from larvae of hydrozoans, gastropods, and barnacles suggests that G protein-coupled receptors mediate induction of settlement and metamorphosis in response to environmental cues. We examined responses of larvae of the serpulid polychaete Hydroides elegans to neuropharmacological agents to determine if G protein-coupled receptors or their associated signal-transduction pathways regulated induction of metamorphosis by bacterial cues. Larvae of Hydroides elegans metamorphose rapidly and in high proportions when exposed to bacterial biofilms. Neither the G-protein activator Gpp[NH]p nor the inhibitor GDP-{beta}-S affected metamorphosis. Although the nonspecific phosphodiesterase inhibitors IBMX, theophylline, and papaverine induced larvae to metamorphose, RO-20-1724 (an inhibitor selective for cAMP-specific phosphodiesterase IV) and the cyclic nucleotide analogs db-cAMP and db-cGMP had no effect on metamorphosis. The adenylate cyclase activator forskolin inhibited responses of larvae to inductive bacterial biofilms. These apparently conflicting results may be due to side effects of IBMX, theophylline, papaverine, and forskolin on ion transport. The phorbol ester TPA, an activator of protein kinase C, also had no effect on larval metamorphosis. These experiments indicate that G protein-coupled receptors and signal transduction by the adenylate cyclase/cyclic AMP or phosphatidyl-inositol/ diacylglycerol/protein kinase C pathways are not components of the morphogenetic pathway that is directly responsible for processing metamorphic cues in H. elegans.