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Abstract Background The ParaHox gene cluster is the evolutionary sister to the Hox cluster. Whilst the role of the Hox cluster in patterning the anterior-posterior axis of bilaterian animals is well established, and the organisation of vertebrate Hox clusters is intimately linked to gene regulation, much less is known about the more recently discovered ParaHox cluster. ParaHox gene clustering, and its relationship to expression, has only been described in deuterostomes. Conventional protostome models (Drosophila melanogaster and Caenorhabditis elegans) are secondarily derived with respect to ParaHox genes, suffering gene loss and cluster break-up. Results We provide the first evidence for ParaHox gene clustering from a less-derived protostome animal, the annelid Platynereis dumerilii. Clustering of these genes is thus not a sole preserve of the deuterostome lineage within Bilateria. This protostome ParaHox cluster is not entirely intact however, with Pdu-Cdx being on the opposite end of the same chromosome arm from Pdu-Gsx and Pdu-Xlox. From the genomic sequence around the P. dumerilii ParaHox genes the neighbouring genes are identified, compared with other taxa, and the ancestral arrangement deduced. Conclusion We relate the organisation of the ParaHox genes to their expression, and from comparisons with other taxa hypothesise that a relatively complex pattern of ParaHox gene expression existed in the protostome-deuterostome ancestor, which was secondarily simplified along several invertebrate lineages. Detailed comparisons of the gene content around the ParaHox genes enables the reconstruction of the genome surrounding the ParaHox cluster of the protostome-deuterostome ancestor, which existed over 550 million years ago.

The spectacular escalation in complexity in early bilaterian evolution correlates with a strong increase in the number of microRNAs (1,2). To explore the link between the birth of ancient microRNAs and body plan evolution, we set out to determine the ancient sites of activity of conserved bilaterian microRNA families in a comparative approach. We reason that any specific localization shared between protostomes and deuterostomes (the two major superphyla of bilaterian animals) should probably reflect an ancient specificity of that microRNA in their last common ancestor. Here, we investigate the expression of conserved bilaterian microRNAs in Platynereis dumerilii, a protostome retaining ancestral bilaterian features (3,4), in Capitella, another marine annelid, in the sea urchin Strongylocentrotus, a deuterostome, and in sea anemone Nematostella, representing an out group to the bilaterians. Our comparative data indicate that the oldest known animal microRNA, miR-100, and the related miR-125 and let-7 were initially active in neurosecretory cells located around the mouth. Other sets of ancient microRNAs were first present in locomotor ciliated cells, specific brain centres, or, more broadly, one of four major organ systems: central nervous system, sensory tissue, musculature and gut. These findings reveal that microRNA evolution and the establishment of tissue identities were closely coupled in bilaterian evolution. Also, they outline a minimum set of cell types and tissues that existed in the protostome-deuterostome ancestor., Deep sequencing of Platynereis small RNAs identified 34 microRNA families common to protostomes and deuterostomes (Supplementary Fig. 1, Supplementary Table 1), in accordance with recent studies (2). To investigate the [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2007.04.041 Byline: Kristin Tessmar-Raible (1), Florian Raible (1)(2), Foteini Christodoulou (1), Keren Guy (1), Martina Rembold (1), Harald Hausen (3), Detlev Arendt (1) Keywords: DEVBIO; EVO_ECOL; RNA Abstract: Neurosecretory control centers form part of the forebrain in many animal phyla, including vertebrates, insects, and annelids. The evolutionary origin of these centers is largely unknown. To identify conserved, and thus phylogenetically ancient, components of neurosecretory brain centers, we characterize and compare neurons that express the prohormone vasotocin (vasopressin/oxytocin)-neurophysin in the developing forebrain of the annelid Platynereis dumerilii and of the zebrafish. These neurons express the same tissue-restricted microRNA, miR-7, and conserved, cell-type-specific combinations of transcription factors (nk2.1, rx, and otp) that specify their identity, as evidenced by the specific requirement of zebrafish rx3 for vasotocin-neurophysin expression. MiR-7 also labels another shared population of neurons containing RFamides. Since the vasotocinergic and RFamidergic neurons appear to be directly sensory in annelid and fish, we propose that cell types with dual sensory-neurosecretory properties were the starting point for the evolution of neurosecretory brain centers in Bilateria. Author Affiliation: (1) Developmental Biology Unit, European Molecular Biology Laboratory, MeyerhofstraAe 1, D-69012 Heidelberg, Germany (2) Computational Biology Unit, European Molecular Biology Laboratory, MeyerhofstraAe 1, D-69012 Heidelberg, Germany (3) Institut fur Biologie, FU Berlin, Konigin-Luisen-StraAe 1-3, D-14195 Berlin, Germany Article History: Received 3 August 2006; Revised 12 January 2007; Accepted 26 April 2007 Article Note: (miscellaneous) Published: June 28, 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2006.08.070 Byline: Florian Raible (a)(b), Kristin Tessmar-Raible (b), Enrique Arboleda (c), Tobias Kaller (b)(d), Peer Bork (a), Detlev Arendt (b), Maria I. Arnone (c) Keywords: Sea urchin; Rhodopsin-type G-protein-coupled receptor (GPCR); Opsin; Surreal-GPCR; Gene cluster; Chemosensation; Olfaction; Pedicellariae, Tube feet Abstract: Rhodopsin-type G-protein-coupled receptors (GPCRs) contribute the majority of sensory receptors in vertebrates. With 979 members, they form the largest GPCR family in the sequenced sea urchin genome, constituting more than 3% of all predicted genes. The sea urchin genome encodes at least six Opsin proteins. Of these, one rhabdomeric, one ciliary and two G.sub.o-type Opsins can be assigned to ancient bilaterian Opsin subfamilies. Moreover, we identified four greatly expanded subfamilies of rhodopsin-type GPCRs that we call sea urchin specific rapidly expanded lineages of GPCRs (surreal-GPCRs). Our analysis of two of these groups revealed genomic clustering and single-exon gene structures similar to the most expanded group of vertebrate rhodopsin-type GPCRs, the olfactory receptors. We hypothesize that these genes arose by rapid duplication in the echinoid lineage and act as chemosensory receptors of the animal. In support of this, group B surreal-GPCRs are most prominently expressed in distinct classes of pedicellariae and tube feet of the adult sea urchin, structures that have previously been shown to react to chemical stimuli and to harbor sensory neurons in echinoderms. Notably, these structures also express different opsins, indicating that sea urchins possess an intricate molecular set-up to sense their environment. Author Affiliation: (a) Computational Unit, EMBL, 69117 Heidelberg, Germany (b) Developmental Unit, EMBL, 69117 Heidelberg, Germany (c) Stazione Zoologica Anton Dohrn, 80121 Napoli, Italy (d) FU Berlin, Institut fur Biologie, Chemie, Pharmazie, Zoologie, Evolution und Systematik der Tiere, 14195 Berlin, Germany Article History: Received 7 June 2006; Revised 28 August 2006; Accepted 30 August 2006

The midbrain-hindbrain organizer (MHO) is a signalling centre that orchestrates development of the mesencephalic and anterior metencephalic primordia. In recent years, details have been revealed about the molecular nature of these signals, their transmission and reception, and the regulatory processes associated with MHO function. This article reviews recent progress in understanding the genetic and molecular components of the MHO, and how they synergize to control brain development.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/S0092-8674(00)80604-2 Byline: Zhaohui Shao (1)(3), Florian Raible (1)(3), Ramin Mollaaghababa (2), Jeffrey R. Guyon (1)(3), Chao-ting Wu (3), Welcome Bender (2), Robert E. Kingston (1)(3) Abstract: The Polycomb group (PcG) genes are required for maintenance of homeotic gene repression during development. Mutations in these genes can be suppressed by mutations in genes of the SWI/SNF family. We have purified a complex, termed PRC1 (Polycomb repressive complex 1), that contains the products of the PcG genes Polycomb, Posterior sex combs, polyhomeotic, Sex combs on midleg, and several other proteins. Preincubation of PRC1 with nucleosomal arrays blocked the ability of these arrays to be remodeled by SWI/SNF. Addition of PRC1 to arrays at the same time as SWI/SNF did not block remodeling. Thus, PRC1 and SWI/SNF might compete with each other for the nucleosomal template. Several different types of repressive complexes, including deacetylases, interact with histone tails. In contrast, PRC1 was active on nucleosomal arrays formed with tailless histones. Author Affiliation: (1) Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts 02114, USA (2) Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA (3) Department of Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA Article History: Received 14 April 1999; Revised 1 June 1999