26.2 Monday, Jan. 5 The Origins and Evolution of Metazoan Phototransduction Pathways:A History of Paraphyly and Derivation PLACHETZKI, D/C*; OAKLEY, T/H; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara email@example.com
How does evolution proceed from common starting points and go on to generate the biodiversity we see today? The evolutionary origin of novel and complex forms has puzzled biologists since before Darwin's time and persists as one of the central questions in biology today. Here, we address this "novelty" question using the biochemical evolution of animal photosensitivity as a model. By integrating studies of comparative genomics, phylogenetics, gene expression and gene function, we have traced the history of the opsin-mediated phototransduction cascade from its origins early in metazoan evolution. This study has yielded three major findings: 1. A new major clade of animal opsins is described from the Cnidaria. 2. Reconstructing the character history and ancestral composition of the animal phototransduction pathway strongly suggests that the ciliary mode of phototransduction, common to vertebrate visual photoreceptor cells, represents the ancestral state of animal opsin-based photosensitivity. A corollary of this finding is that the rhabdomeric pathway, common to protostome visual photoreceptor cells, represents a derived condition that evolved prior to bilaterian animals, but was absent in Eumetazoa. 3. Specific amino acid substitutions that were likely to play a role in this diversification were identified allowing hypotheses on the molecular basis for the evolution of disparate animal phototransduction pathways. The implications of these findings for animal photoreceptor evolution and for eye evolution are discussed.