Storage of retinal in the eggs of the ascidian, Halocynthia roretzi

Retinoids in the eggs of the solitary ascidian, Halocynthia roretzi, were analyzed by high performance liquid chromatography. Retinal was the almost exclusive retinoid (>>99%), and the concentration of retinal was 25.9-40.1 (30.6 on average) ng/mg of protein. The egg retinal consisted of four isomers: all-trans (50.9%), 9-cis (6.8%), 11-cis (20.4%) and 13-cis (21.9%). The presence of retinal in the eggs of this ascidian is a characteristic shared with the wide range of oviparous vertebrates, although the isomer composition differs between ascidian eggs and vertebrate eggs; in vertebrate eggs, almost all the retinal is in the all-trans form. The egg retinal was bound to a protein complex via a Schiff base linkage. The electrophoretic characteristics of the protein complex were similar to that of egg yolk proteins of oviparous vertebrates. The results presented in this study strongly suggest that, as is found with oviparous vertebrates, retinal in the ascidian eggs is the essential mode of retinoid storage, and is the precursor of photoreceptive pigment chromophores and retinoic acid during development.