Lumichrome - A larval metamorphosis-inducing substance in the ascidian Halocynthia roretzi

It has long been known that metamorphosis of ascidian larvae is induced by exposure to adult tunic extract or larval-conditioned seawater. However, su ch a natural 'inducer' has not been identified, probably due to its very lo w concentration in organisms. Here we have succeeded in isolating the same metamorphosis-inducing substance from the larvae, the larval-conditioned se awater, and the adult tunic of the ascidian Halocynthia roretzi. Structural analysis revealed that this substance was identical to lumichrome. Lumichr ome was active toward H. roretzi larvae, but inactive toward another ascidi an larvae, suggesting that lumichrome is species-specific. Riboflavin (vita min B-2), from which lumichrome might be derived from, was found to be inac tive in induction of larval metamorphosis. In addition, it was demonstrated that lumichrome is localized predominantly in the basal region of the adhe sive organ and the posterior part of the larval trunk. Thus, we propose tha t lumichrome functions as a natural inducer for larval metamorphosis in H. roretzi. This is the first natural metamorphosis-inducing substance to be i dentified in ascidians.