JACOBS, David/K*; BEBENEK, Ilona/G; GATES, Ruth/D; LEE, Shannon/E; UCLA: Recovery of sine oculis in cnidarians and sponges: Evidence for antecedents of bilaterian sensory systems

Homeodomain gene family members involved in sensory and neural differentiation form basal branches of homeodomain gene trees leading to the prediction that these genes evolved early and should be found in basal Metazoa. In confirmation of this perspective we report the recovery of homologs of sine oculis from all cnidarian and sponge classes. RTPCR results suggest that these genes are preferentially expressed in the sense organ bearing tissues of the Aurelia medusa. These results are consistent with observations of other “basal” homeodomain containing genes including members of the POU homeodoman. The presence of these genes in sponges and cnidarians suggests that organization of sets of ciliated cells with sensory properties into functional groups was an early event in the evolution of metazoan development. It appears that gene familes involved in bilaterian axial organization and differentiation, such as the engrailed gene and many of the Hox genes, evolved later than the genes involved in sense organ development discussed here. This suggests selection of model systems in basal metazoa should be given careful consideration. Some taxa such as Hydra and Nematostella that have proved useful in the assessment of axial organization in Cnidaria lack sense organs. Similarly, fresh water sponges, currently the focus of much research, may be inappropriate for studies of larval development more typical of marine sponges. In terms of genomics, multiple models within basal groups will likely prove useful, as no single taxon can represent all the organs and life history stages of potential interest. Furthermore, recovery from multiple taxa provides a richness of data suitable for phylogenetic analyses and will provide opportunities for controlled comparison between systems.