NAKANISHI, N; CAMARA, A/C; JACOBS, D/K; UCLA; UCLA; UCLA: Evolution of sensory genes in basal Metazoa

Homeodomain genes involved in sensory and neural differentiation form basal branches of homeodomain gene trees, indicating that these genes evolved early and are present in basal Metazoa. Substantiating this claim, we previously reported the recovery of sensory-specific homeodomains sine oculis and Brain 3 from all cnidarian and sponge classes. Here we report the recovery of an Optix homolog, a sense organ related homeodomain gene, from the scyphozoan Aurelia. Its presence in Aurelia is consistent with recent observations of the gene in hydrozoans, suggesting that the diversification of the sine oculis related homeobox (Six) gene subfamilies preceded the evolution of the Cnidaria. Across the Bilateria sine oculis functions in a gene network involving homologues of eyeless, eyes absent, and dachshund in the development of sensory structures and the nervous system. We also report recovery of a homolog of the eyes absent gene from Aurelia. Further study using cnidarian model systems is now expected to reveal additional components of the gene network as well as its evolutionary origins. The presence of sensory genes in sponges and cnidarians argues for the evolution of functional groups of organized ciliated cells with sensory properties early in the evolution of the Metazoa.