Meeting Abstract

S10-1.5  Monday, Jan. 7  Watch your steps! Opsins and photoreceptors in sea urchin tube feet ULLRICH-LUTER, Esther; ARNONE, Maria Ina*; Univ. of Bonn and Natural History Museum, Berlin; Stazione Zoologica Anton Dohrn, Napoli miarnone@szn.it

Sea urchins, due to their derived morphological body plan, have long been considered to be of limited value regarding the reconstruction of ancestral deuterostome character states. In contrast, recent molecular findings show that the animals express a huge variety of vertebrate and even mammalian gene orthologs, including such essential for function and development of photoreceptors. We recently demonstrated that one of the six sea urchin opsin (photopigment) proteins is expressed within microvillar, r-opsin expressing photoreceptors cells ( PRCs). These PRCs are located in the animal’s numerous tube feet and, surprisingly, lack any associated screening pigment. Indeed, one of the tube foot PRC clusters may account for directional vision by being shaded through the opaque calcite skeleton. Since juveniles display no phototaxis until skeleton completion, we suggest a model in which the entire sea urchin, deploying its skeleton as PRC screening device, functions as a huge compound eye. Moreover, we are currently investigating on another sea urchin photoreceptor system, expressing a c-type opsin, phylogenetically clustering with chordate and protostome ciliary opsins. Specific antibodies and mRNA detection revealed expression in the sea urchin dermis and internal nervous system as well as in spines of other echinoderms. Analysis of the observed expression patterns does not indicate involvement of c-opsin in sea urchin directed vision. However, the c-opsin expressing cells might comprise the corresponding receptor for the long proposed “dermal light sense” and might have a function in “shadow responses” of echinoderms. Investigating the echinoderm c-opsin system is promising regarding information about c-opsin function at the base of deuterostomes.