Integrative and Comparative Biology Advance Access published online on September 27, 2007
Integrative and Comparative Biology, doi:10.1093/icb/icm094
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Evolution of sensory structures in basal metazoa
*Department of Ecology and Evolutionary Biology, UCLA, 621 Young Drive South, Los Angeles, CA 90095-1606, USA; Department of Molecular, Cellular and Developmental Biology, UCLA, 621 Young Drive South, Los Angeles, CA 90095-1606, USA; Department of Molecular and Cell Biology, 142 Life Sciences Addition, University of California, Berkeley, CA 94720, USA
Correspondence: 1E-mail: djacobs{at}ucla.edu
Cnidaria have traditionally been viewed as the most basal animals with complex, organ-like multicellular structures dedicated to sensory perception. However, sponges also have a surprising range of the genes required for sensory and neural functions in Bilateria. Here, we: (1) discuss "sense organ" regulatory genes, including; sine oculis, Brain 3, and eyes absent, that are expressed in cnidarian sense organs; (2) assess the sensory features of the planula, polyp, and medusa life-history stages of Cnidaria; and (3) discuss physiological and molecular data that suggest sensory and "neural" processes in sponges. We then develop arguments explaining the shared aspects of developmental regulation across sense organs and between sense organs and other structures. We focus on explanations involving 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 fewer antecedent sensory structures in a metazoan ancestor. More broadly, we hypothesize that developmental genetic similarities between sense organs and appendages may reflect descent from closely associated structures, or a composite organ, in the common ancestor of Cnidaria and Bilateria, and we argue that such similarities between bilaterian sense organs and kidneys may derive from a multifunctional aggregations of choanocyte-like cells in a metazoan ancestor. We hope these speculative arguments presented here will stimulate further discussion of these and related questions.
From the symposium "Key Transitions in Animal Evolution" presented at the annual meeting of the Society for Integrative and Comparative Biology, January 3–7 2007, at Phoenix, Arizona.