Meeting Abstract

10.4  Jan. 4  New Insights on the Evolution of Metazoan Photoreceptors PLACHETZKI, David C.*; OAKLEY, Todd H.; Univ. of California, Santa Barbara plachetzki@umail.ucsb.edu

Duplication and divergence models are commonly invoked as a means of explaining evolutionary processes. While these concepts are usually applied to the gene and species levels, here we examine their role in the evolution of cell types. Metazoan photoreceptor cells are grouped into either ciliary (C) or rhabdomeric (R) classes. One hypothesis for the evolution of these cell types is that they originated by duplication from a common ancestor prior to the evolution of Bilateria. Support for this hypothesis comes from reports of both C and R photoreceptors in various deuterostome and protostome phyla; however, molecular data from non-bilaterian animals have not been applied to the problem. The phylogeny of opsin proteins, G-protein coupled receptors (GPCRs), can be used to infer the evolutionary history of photoreceptors. Screens of genome trace data for opsin loci from the sponge Reniera and the cnidarians Hydra magnipapillata and Nematostella vectensis yielded three significant results. First, we uncovered multiple GPCRs in the sponge, but did not obtain any opsins. Although opsin gene loss in sponges is a distinct possibility, this absence is consistent with an origin of opsins after the origin of animals, but before the cnidarian/bilaterian split. Second, we obtained an opsin sequence from Nematostella that falls within the C-/R-opsin clade previously known only from bilaterian animals. This result suggests an emergence of both cell types at or before the ancestor of Cnidaria + Bilateria. Finally, we obtained clear support for a new class of opsin present in both Hydra and Nematostella. Expression appears confined to neuronal cell types in Hydra. These results suggest the possibility of a new type of animal photoreceptor that can be considered paralogous to C and R types, and may be unique to Cnidaria.