Molecular and structural characterization of photoreceptor cells in Annelida in order to utilise the photosensitive system as comparative character for inferring metazoan interrelationships

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Comparative ultrastructural investigations revealed a high structural diversity of light sensitive organs, and of their photoreceptor cells. Conflicting views on photoreceptor cell type homology have since then prevented the usage of eyes for inferring phylogenies of higher rank taxa. A new comparative cell biological approach combining the ultrastructural and the molecular characterisation of cell types, however, now allows to trace the evolution of photoreceptor cells across Metazoa. This approach has so far revealed that the two morphologically different photoreceptor cell types, rhabdomeric and ciliary, are distinctly conserved in Bilateria. These types differ significantly in their ¿molecular fingerprint¿, which is the cell-type-specific deployment of opsins and other key molecules of the phototransduction cascade. The current project follows this new approach. Adding to their ultrastructural characterisation, we will determine the molecular fingerprint of diverse photoreceptor cells in a broad range of species, by cloning and in situ-localisation of opsins and G-alpha subunits of the phototransductory, heterotrimeric G-proteins. This will allow establishing robust homology hypotheses on light sensitive organs. In the first application period Annelida, with their great diversity in photoreceptors, are in focus. As a remote outgroup, we will include Branchiostoma, representing a suitable reference system for the identification of plesiomorphic characters. For the next application periods this approach will be extended to molluscs, plathelminthes, Tentaculata, and basal Deuterostomia to convey eyes into valuable characters for metazoan systematics.