MOOI, R.*; DAVID, B.; WRAY, G.; California Academy of Sciences, San Francisco; University of Burgundy, France; Duke University, Durham: Arrays in rays: terminal addition in echinoderms by the “Ocular Plate Rule”

The echinoderms are deuterostomes that superimpose radial symmetry upon a bilateral larval morphology. Consequently, they are not the first animals that come to mind when the concepts of segmentation and terminal addition are being discussed. However, it has long been recognized that echinoderms have serial elements along their radii formed in accordance with the Ocular Plate Rule (OPR). The OPR is a special case of terminal growth, forming elements of the ambulacra that define the rays in echinoderms. New elements are added at the terminus of the ray, which may or may not be marked by a calcified element called the terminal plate (the “ocular” of sea urchins). The oldest elements, such as tube feet and plates, are found at the mouth. The OPR operates in every echinoderm, from the occasionally bizarre fossils of the Cambrian to the most familiar extant taxa. Using the OPR and other criteria of recognition, echinoderm body wall can be divided into two main regions: extraxial components are associated with the somatocoels; axial components (formed in accordance with the OPR) with the hydrocoel. Although axial and extraxial skeleton appear to be composed of the same biomineral matrix, the genes involved in patterning these two skeletal components are distinct. During development of axial skeleton, for instance, the regulatory protein engrailed is expressed in a spatial and temporal pattern that conforms to the OPR. Using these results, we compare patterns of development in axial regions of echinoderms to those found in the anterior-posterior axes of the earliest echinoderms as well as other invertebrates.