ABSTRACT
Cnidaria have traditionally been viewed as the most basal animals with complex, multicellular structures dedicated to sensory perception. However, sponges also have a surprising range of the genes required for sensory and neural functions in Bilateria. We develop arguments explaining the shared aspects of developmental regulation across sense organs and between sense organs and other structures, focusing on explanations that involve divergent evolution from a common ancestral condition. In Bilateria, distinct sense-organ types share components of developmental-gene regulation. These regulators are also present in basal metazoans, suggesting evolution of multiple bilaterian organs from a smaller number of antecedent sensory structures in a metazoan ancestor. More specifi cally, we hypothesize that developmental genetic similarities between sense organs and appendages may refl ect descent from closely associated structures, or a composite organ, in the common ancestor of Cnidaria and Bilateria, and we argue that such similarities between
*Department of Ecology and Evolutionary Biology, UCLA, 621 Young Drive South, Los Angeles CA 90095-1606. †Department of Molecular, Cellular and Developmental Biology, UCLA, 621 Young Drive South, Los Angeles CA 90095-1606. ¥Department of Molecular and Cell Biology, 142 Life Sciences Addition, University of California, Berkeley, CA 94720. 1E-mail: djacobs@ucla.edu
bilaterian sense organs and kidneys may derive from a multifunctional aggregations of choanocyte-like cells in a metazoan ancestor. We hope the speculative arguments presented here will stimulate further discussion of these and related questions.
