Qualitative variation of alkaloids in color morphs of Cystodytes (Ascidiacea)

Abstract

Despite their great variability, particularly of color, Mediterranean specimens of the genus Cystodytes are usually attributed to the cosmopolitan species Cystodytes dellechiajei. In this study, we used MALDI-TOF and HPLC techniques to assess alkaloid distribution in the four most abundant color morphs of Cystodytes in the western Mediterranean (green, purple, brown and blue). The intraspecimen location of these compounds (either in tunic or zooids) was also analyzed. Two major chemotypes were found: (1) that of the purple morph was based on the sulfur-containing pyridoacridines, shermilamine B, kuanoniamine D and their deacetylated forms; (2) the chemotype of the blue and green morphs was based on the C 9-unsubstituted pyridoacridines, ascididemin and 11-hydroxyascididemin. In the brown morph, ascididemin was only detected by MALDI-TOF. All of these alkaloids were present in both the tunic and zooids, with the exception of the purple morph, in which shermilamine B and kuanoniamine D were found in the tunic, whilst their deacetylated forms were found in both tunic and zooids. Whereas a clear link between these pigments (pyridoacridines) and color was found in the purple morph, color in the other morphs may depend on other unknown molecules. MALDI-TOF proved to be a rapid and reliable tool with which to detect targeted compounds of low molecular mass at both colony and intraspecimen levels, making it effective for the rapid assessment of chemotypes. The chemical differences found raised questions about the taxonomic status of the color morphs attributed to the nominal species C. dellechiajei in the Mediterranean. Our results stress the importance of a detailed morphotype description when working with marine natural products, especially for taxa whose taxonomy is not well resolved, in order to understand fully the variation in secondary chemistry within and between species.