Radical alterations in the roles of homeobox genes during echinoderm evolution

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Abstract
Echinoderms possess one of the most highly derived body architectures of all metazoan phyla, with radial symmetry, a calcitic endoskeleton, and a water vascular system1,2. How these dramatic morphological changes evolved has been the subject of extensive speculation and debate3,4,5, but remains unresolved. Because echinoderms are closely related to chordates and postdate the protostome/deuterostome divergence2,3,6,7, they must have evolved from bilaterally symmetrical ancestors1,2,3,4,5,6. Here we report the expression domains in echinoderms of three important developmental regulatory genes ( distal-less, engrailed and orthodenticle ), all of which encode transcription factors that contain a homeodomain8. Our findings show that the reorganization of body architecture involved extensive changes in the deployment and roles of homeobox genes. These changes include modifications in the symmetry of expression domains and the evolution of several new developmental roles, as well as the loss of roles conserved between arthropods and chordates. Some of these modifications seem to have evolved very early in the history of echinoderms, whereas others probably evolved during the subsequent diversification of adult and larval morphology. These results demonstrate the evolutionary lability of regulatory genes that are widely viewed as conservative.

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