On the origin of the chordate central nervous system: expression of onecut in the sea urchin embryo Albert J. Poustka, a,à Alexander Ku ¨ hn, a Vesna Radosavljevic, a Ruth Wellenreuther, b Hans Lehrach, a and Georgia Panopoulou a a Max Planck Institute for Molecular Genetics, Department of Vertebrate Genomics, Evolution and Development Group, Ihnestrasse 73, 14195 Berlin, Germany b Deutsches Krebsforschungszentrum, Abteilung Molekulare Genomanalyse (B050), Im Neuenheimer Feld 580, 69120 Heidelberg, Germany à Author for correspondence (email: poustka@molgen.mpg.de) SUMMARY We identified a transcription factor of the onecut class in the sea urchin Strongylocentrotus purpuratus that represents an ortholog of the mammalian gene HNF6, the founding member of the onecut class of proteins. The isolated sea urchin gene, named SpOnecut, encodes a protein of 483 amino acids with one cut domain and a homeodomain. Phylogenetic analysis clearly places the sea urchin gene into this family, most closely related to the ascidian onecut gene HNF-6. Nevertheless, phylogenetic analysis reveals a difficult phylogeny indicating that certain members of the family evolve more rapidly than others and also that the cut domain and homeodomain evolve at a different pace. In fly, worm, as- cidian, and teleost fish, the onecut genes isolated so far are exclusively expressed in cells of the central nervous system (CNS), whereas in mammals the two copies of the gene have acquired additional functions in liver and pancreas devel- opment. In the sea urchin embryo, expression is first detected in the emerging ciliary band at the late blastula stage. During the gastrula stage, expression is limited to the ciliary band. In the early pluteus stage, SpOnecut is expressed at the apical organ and the elongating arms but continues most prom- inently in the ciliary band. This is the first gene known that exclusively marks the ciliary band and therein the apical organ in a pluteus larva, whereas chordate orthologs execute essen- tial functions in dorsal CNS development. The significance of this finding for the hypothesis that the ciliary bands and apical organs of the hypothetical ‘‘dipleurula’’-like chordate ancestor and the chordate/vertebrate CNS are of common origin is discussed. INTRODUCTION The three main groups of deuterostomes, the chordates, the echinoderms, and the hemichordates, have been shown to be monophyletic (Turbeville et al. 1994; Wada and Satoh 1994; Bromham and Degnan 1999; Cameron et al. 2000; Peterson and Eernisse 2001; Winchell et al. 2002). Although a wide variety of developmental modes (i.e., direct and indirect de- velopment) and life forms (i.e., sessile and free living) exists within deuterostomes, the monophyly indicates that they arose from a common ancestor. Insights into the possible common ancestor of deuterostomes can be gained by studying organisms that are as close as possible to the deuterostome origin and to the divergence of protostomes and deutero- stomes. Echinoderms represent such a life form, and the sea urchin pluteus larva is one of the indirect developing forms that are similar to the hemichordate tornaria, dipleurula type larvae, the proposed type of larval form that could be ances- tral to all deuterostomes (Nielsen 1999). The evolutionary origin of the chordate/vertebrate central nervous system (CNS) is one of the mysteries that challenges comparative evolutionary developmental biology. A century ago, Garstang (1894, 1928) formulated the paedomorphosis theory that pro- posed that the ciliary band of a dipleurula type larva could have moved dorsally and fused during evolution to form the neural tube of the ancestral chordate. Recent studies indicate that the genes that are essential for patterning the chordate CNS are expressed in the ciliary band and/or the apical organ of nonchordate deuterostome larvae. These were studies that showed otx expression in the ciliary bands in the larvae of the acorn worm Ptychodera flava (Harada et al. 2000) and the sea cucumber Stichopus japonicus (Shoguchi et al. 2000) and T-brain expression in the apical organ of Ptychodera flava larvae (Tagawa et al. 2000). Furthermore, group B sox genes and NK2.1 were found in the ciliary band and apical organ of Ptychodera flava larvae (Taguchi et al. 2002; Takacs et al. 2002). Here we present further data that could indicate a com- mon evolutionary origin of cells of the dipleurula ciliary band and the apical organ and certain cell types of the chordate EVOLUTION & DEVELOPMENT 6:4, 227–236 (2004) & BLACKWELL PUBLISHING, INC. 227