Meeting Abstract

122.5  Monday, Jan. 7  The canonical echinoid apical organ evolved from within the euechinoids: evidence from the cidaroid Eucidaris tribuloides MACNEIL, K.E.A.*; PATEL, D; TAYLOR, V; BISHOP, C.D.; BURKE, R.D.; St. Francis-Xavier Univ., Antigonish, Nova Scotia; Univ. of Victoria, Victoria, B.C. cbishop@stfx.ca

Descriptions of the structure and development of larval nervous systems of all five classes of echinoderms have recently been reported. Among these taxa, several differences in neural development and neuroanatomy support the hypothesis that echinoid larvae are the most derived. The cidaroids, a major clade of echinoids, are considered to have several primitive features that more closely represent the common ancestor to all extant echinoids. To test whether cidaroid larvae also present features that are ancestral to the euechinoids, and to clarify the timing and nature of changes from a dipleurula-like condition to the echinopluteus condition we have investigated the development and anatomy of the larval nervous system of Eucidaris tribuloides. Using markers for neurons (SynB, serotonin), ciliary band (Hnf6), oral ectoderm (Chd) and anterior ectoderm (Nk2.1), we describe the development and organization of the larval nervous system. In most respects the larval nervous system of E. tribuloides more closely resembles that of non-echinoid larvae. We also used LiCl and 1-azakenpaullone (GSK-3β inhibitors) and ZnSO4 (an animalizing agent) to test for the presence of known echinoid anterior-posterior axial signaling mechanisms. Collectively these neuroanatomical and experimental data have allowed us to conclude that the evolution of the canonical sea urchin apical organ was derived within the euechinoids about 250 million years ago.