Lumichrome

Abstract

It has long been known that metamorphosis of ascidian larvae is induced by exposure to adult tunic extract or larval-conditioned seawater. However, such a natural`inducer' has not been identified, probably due to its very low concentration in organisms. Here we have succeeded in isolating the same metamorphosis-inducing substance from the larvae, the larval-conditioned seawater, and the adult tunic of the ascidian Halocynthia roretzi. Structural analysis revealed that this substance was identical to lumichrome. Lumichrome was active toward H. roretzi larvae, but inactive toward another ascidian larvae, suggesting that lumichrome is species-specific. Riboflavin (vitamin B 2 ), from which lumichrome might be derived from, was found to be inactive in induction of larval metamorphosis. In addition, it was demonstrated that lumichrome is localized predominantly in the basal region of the adhesive organ and the posterior part of the larval trunk. Thus, we propose that lumichrome functions as a natural inducer for larval metamorphosis in H. roretzi. This is the first natural metamorphosisinducing substance to be identified in ascidians.Keywords: larva; lumichrome; metamorphosis; ascidian.Most sessile organisms, e.g. ascidians, sponges, corals, bryozoans, barnacles, and mussels, have a planktonic larval phase in their life cycles, lasting from minutes to months, following which they then settle and metamorphose into their adult forms. The selection of settlement sites is a critical event for these organisms because settling on unsuitable places affects their survival severely. Larval settlement and the successive metamorphosis are thought to be influenced by a variety of environmental factors, e.g. light, temperature, and water flow, but chemical substances that are mainly derived from conspecific adults (those of the same species) or prey organisms are believed to have particularly important roles in induction of larval settlement and metamorphosis [1]. Although many attempts have been made to identify such chemical substances for various marine sessile organisms, only a few are known to date; d-tocopherol epoxides from the brown alga Sargassum tortile for the hydroid Coryne uchidai [2,3] and a settlementinducing protein complex for the barnacle Balanus amphitrite [4]. Perhaps this is due to difficulties in culturing synchronous competent larvae as well as the low endogenous levels of chemical substances.Ascidians produce tadpole larvae, whose settlement is initiated by attachment to substrata with their papillae followed by metamorphosis into juveniles [5]. Metamorphosis is associated with tail resorption which has been reported to be induced by a variety of factors including copper ion [6], calcium ion, some amino acids, iodine [7], dimethylsulfoxide, acetylcholine, dicapryloylglycerol, ammonium ion [8], and vital dyes [9]. These divergent reagents coincidently accelerated the onset of larval metamorphosis in the same manner, but apparently they are unnatural metamorphosis-inducers for larvae. Many ascidians live gre...

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