PLENARY TALK ABSTRACTS Evening Opening Session: Gene Networks and Functional Genomics Hemichordate neurulation and the origin of the chordate body plan Hiroshi Wada University of Tsukuba, Tsukuba, Japan The origin of the body plan of our own phylum, Chordata, is one of the most fascinating questions in evolutionary biology. Yet, after more than a century debate, the evolutionary origins of the neural tube and notochord, the defining features of chordates, remain unclear. The collar cord of hemichordates have been proposed as predecessor of the neural tube, but this has not been supported by molecular evidence. Here, we examined the development of the hemichordate collar cord and found the shared gene expression patterns between hemichordate and chordate neurulations. Moreover, we found that the nearby endoderm of the collar cord secreted Hedgehog molecules and the collar cord cells could receive the signal. Our data suggest that the anterior endoderm functions as an organizer to pattern the overlying collar cord, similar to the relationship between the notochord and neural tube in chordates. We propose that the gene regulatory networks underling the development of the hemichordate anterior endoderm and collar cord were co-opted by chordates for development of the notochord and the neural tube. Development and evolution of the cardiogenic mesoderm in chordates Lionel Christiaen New York University, New York, NY Craniofacial and cardiac muscles play fundamental roles in animal physiology and have diversified and adapted substantially in higher vertebrates. Recent studies have uncovered a common clonal origin of the heart and subsets of head muscles in amniotes [1, 2]. We recently showed that the cardiogenic mesoderm of the ascidian Ciona intestinalis gives birth to both the heart and atrial siphon muscles (ASM), which express Islet and Tbx1, two markers of the second heart field and branchiomeric muscle precursors in amniotes [3]. We proposed that the existence of a population of cardiopharyngeal mesoderm precursors is an innovation of the monophyletic group comprising tunicates and vertebrates [4]. We found that, in Ciona larvae, the ASM-specific transcription factor COE (Collier/Olf/Ebf) is both necessary and sufficient to inhibit heart fate specification and promote ASM development within the cardiogenic lineage. Using Fluorescence Activated Cell Sorting (FACS) and whole genome transcription profiling by microarray analysis, we investigated the transcriptional changes that underlie heart vs ASM fate specification. Our results suggest that the common heart and ASM progenitors (i.e. the trunk ventral cells) activate an heart-like regulatory program, while COE triggers a skeletal muscle program at the expense of the pre-existing cardiac profiles. Finally, we present evidence that key cardiac regulators inhibit COE expression in the heart precursors, thus uncovering mutually exclusive regulatory inputs that contribute to a dual heart vs ASM fate specification within the cardiogenic lineage. 1. Lescroart, F., et al., Development, 2010. 137(19): p. 3269-79. 2. Nathan, E., et al., Development, 2008. 135(4): p. 647-57. 3. Stolfi, A., et al., Science, 2010. 329(5991): p. 565-8. 4. Tolkin, T. and L. Christiaen, Curr Top Dev Biol, 2012. 100: p. 107- 42. 1 Plenary Session I Genomic Logic and Cell Specification Integration of canonical and non-canonical Wnt signaling pathways patterns the neuroectoderm along the anterior-posterior axis of sea urchin embryos Ryan Range, Robert Angerer and Lynne Angerer National Institute of Dental and Craniofacial Research, National Institutes of Health Bethesda, MD Patterning the neuroectoderm along the anterior-posterior (AP) axis is a critical event in the early development of deuterostome embryos. However, the mechanisms that regulate the specification and patterning of the neuroectoderm are incompletely understood. Remarkably, the anterior neuroectoderm (ANE) of the deuterostome sea urchin embryo expresses many of the same transcription factors and secreted modulators of Wnt signaling as the early vertebrate ANE (forebrain/eye field). Moreover, as is the case in vertebrate embryos, confining the ANE to the anterior end of the embryo requires a Wnt signaling-dependent mechanism. Here we use morpholino- or dominant negative-mediated interference to knock down the activities of the different Wnt signaling pathway branches. Our results show that the early sea urchin embryo integrates information from the Wnt/β-catenin, Wnt/Fzl5/8-JNK, and Fzl1/2/7-PKC pathways to provide precise spatiotemporal control of neuroectoderm patterning along its anterior-posterior axis. Together, through the Wnt1 and Wnt8 ligands, they orchestrate a progressive posterior-to-anterior wave of re- specification that restricts the initial, ubiquitous, maternally specified, ANE regulatory state to the most anterior blastomeres. In these anterior cells, the Wnt receptor antagonist, Dkk1, protects the ANE fate through a negative feedback mechanism. Because these different Wnt pathways converge on the same cell fate specification process, our data suggest they may function as integrated components of an interactive Wnt signaling network. Our findings also provide strong support for the idea that the sea urchin anterior neuroectoderm regulatory state and the patterning mechanisms that position and define its borders represent an ancient regulatory mechanism that was present in the common echinoderm/vertebrate ancestor. A regulatory fate map for the developing gut Isabelle S. Peter California Institute of Technology, Pasadena CA Cells which form the sea urchin embryonic gut are progressively specified during development according to their position in the mature organ. Morphologically, the gut consists of three compartments, the fore-, mid- and hindgut, which are separated by two contractile sphincters. The cells which compose these compartments derive from two cell lineages which just prior to gastrulation express different regulatory states. Over the past few years we have solved the regulatory processes which determine the early specification of endodermal cells. The sufficiency of these mechanisms was recently demonstrated using a Boolean computational model of the early endomesoderm GRN. Based on these insights, we are currently identifying the regulatory apparatus which determines spatial complexity during later gut development. Our results indicate not only the different regulatory domains which subdivide the cells of the mature gut, but also the temporal and spatial order in which these different cell fates become established and distinguished during gut development. 2 Conservation and Divergence of a Gene Regulatory Network that Controls Gut Patterning in Deuterostomes M. Ina Arnone and Rosella Annunziata Stazione Zoologica Anton Dohrn, Napoli, Italy How to make a functional gut from a primitive archenteron? How the mechanisms involved in gut patterning evolved? To answer these questions, we used a system approach aimed to elucidate the GRN involved in patterning and regionalization of the endoderm in the sea urchin embryo, to assess the degree of conservation of this network amongst the deuterostome lineage, and to gain insight into the origin and evolution of the Parahox genes leading to the chordate lineage. We in fact demonstrated that two of the three sea urchin ParaHox genes – discovered through the S. purpuratus genome project–, SpLox and SpCdx, are expressed in a spatial and temporally collinear fashion within the developing digestive tube and play a key role in partitioning of the mid- and hind- gut. The two ParaHox genes are involved in mutual regulation: the posterior SpCdx gene is not expressed in the absence of the anterior SpLox gene and the expression domain of the anterior SpLox gene is not restricted posteriorly when the posterior SpCdx gene is silenced. Moreover, several signaling events are involved in patterning of the larval gut, as, e.g., SpWnt10 mediates SpCdx repressive functions in the most posterior hindgut, while retinoic acid signaling appears to control gut identity along the anterior/posterior axis. A comparison with vertebrates showed a striking conservation of topology of gene expression and signaling events between sea urchin and mouse, thus suggesting the existence of an ancient “kernel” of genes involved in gut patterning processes among deuterostomes. However, when we extended the analysis of this GRN to the starfish P. miniata, despite the high conservation of topology of gene expression, we surprisingly found considerable divergence at the level of gene interactions. Properties of gene regulatory network governing the specification of sea urchin oral ectoderm. Enhu Li and Eric Davidson California Institute of Technology, Pasadena CA The Nodal signaling pathway is known from earlier work to be an essential mediator of oral ectoderm specification in the sea urchin embryo, and indirectly, of aboral ectoderm specification as well. Following expression of the Nodal ligand in the future oral ectoderm during cleavage, a sequence of regulatory gene activations occurs within this territory which depends directly or indirectly on nodal gene expression. Here we describe additional regulatory genes that contribute to the oral ectoderm regulatory state during specification in Strongylocentrotus purpuratus, and show how their spatial expression changes dynamically during development. By means of system wide perturbation analyses we have significantly improved current knowledge of the epistatic relations