Animal phototransduction cascades are mediated by the sensory receptor protein opsin. Using a comparative phylogenomics approach, we investigate a global phylogeny for opsin by including sequence data from the genomes of early branching animal lineages. We find that opsin proteins originated prior to the last common ancestor of cnidarians and bilaterians, but were not present in earlier branching animal lineages. Our phylogeny also provides an scaffold for investigations into the molecular basis for phototransduction diversification. Using various statistical measures, we provide compelling evidence that changes in two amino acid residues in particular have underlined the diversification of the various classes of opsin, a finding supported by previous biochemical studies. We further explore the macroevolutionary dynamics of gene duplication and functional co-option that have each flavored the evolutionary history of animal phototransduction.

In order to polarize the wealth of existing data on the composition of bilaterian phototransduction cascades we require an understanding of the ancestral state of this pathway. Focusing on specific ion channel components of this cascade, our studies of gene expression and photobehavior in the cnidarian Hydra magnipapillata provide strong evidence that cnidarian phototransduction relies on Cyclic Nucleotide-Gated ion channels. This finding renders cnidarian phototransduction as similar to the ciliary pathway used by vertebrates and other taxa for vision, in contrast to the rhabdomeric pathway common to insects and other invertebrates. We use these data to investigate ancestral composition of the animal phototransduction. Our results force recognition that the ciliary pathway represents the ancestral state of animal phototransduction, whereas the rhabdomeric pathway is a derived condition.

Our studies have rested heavily on genomic data from the cnidarians H. magnipapillata and Nematostella vectensis. Both of these animals lack eyes but display a rich set of photosensitive phenotypes. In what capacity do these animals integrate photosensitivity into their behavior? We show that opsin and other phototransduction components are expressed in the cnidarian-specific stinging cells known as nematocytes. We further show that light intensity acts to attenuate nematocyte discharge. This finding adds a new dimension to our understanding of basic nematocyte biology and offers a novel context for exploring nematocyte function.