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Chapter IV Submitted manuscript 63 First expression study of homeobox genes in Brachiopoda: the role of Not and Cdx in bodyplan patterning, neurogenesis, and germ layer specification Andreas Altenburger 1 , Pedro Martinez 2, 3, Andreas Wanninger 1* 1 University of Copenhagen, Department of Biology, Research Group for Comparative Zoology, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark 2 Universitat de Barcelona, Facultat de Biología, Departament de Genètica, Av. Diagonal 645, ES-08028 Barcelona, Spain 3 Institució Catalana de Recerca i Estudis Avançats (ICREA) *corresponding author. E-mail: awanninger@bio.ku.dk ABSTRACT Not is a homeobox containing gene that regulates the formation of the notochord in chordates, while Caudal (Cdx) is a ParaHox gene involved in the formation of posterior tissues of various animal phyla. Here, we present the first expression data of a Not and a Cdx homolog in the articulate brachiopod Terebratalia transversa. The T. transversa homolog, TtrNot, is expressed in the ectoderm from the beginning of gastrulation until completion of larval development, which is marked by a three-lobed body with larval setae. Expression starts at gastrulation in two areas lateral to the blastopore and subsequently extends over the animal pole of the gastrula. With elongation of the gastrula, expression at the animal pole narrows to a small band, whereas the areas lateral to the blastopore shift slightly towards the future anterior region of the larva. Upon formation of the three larval body lobes, TtrNot expressing cells are present only in the posterior part of the apical lobe. Expression ceases entirely at the onset of larval setae formation. TtrNot expression is absent in unfertilized eggs, in INTRODUCTION Homeobox genes are characterized by the presence of a short, wellconserved DNA fragment, which encodes for the homeodomain. The latter is a protein motif of 60 to 63 amino acids, which was first described embryos prior to gastrulation, and in settled individuals during and after metamorphosis. Comparison with the expression patterns of Not genes in other metazoan phyla suggests an ancestral role in gastrulation and germ layer (ectoderm) specification with co-opted functions in notochord formation in chordates and left/right determination in ambulacrarians and vertebrates. TtrCdx is first expressed after gastrulation in the ectoderm of the gastrula in the posterior region of the blastopore. Its expression stays stable in the ectoderm at the posterior pole of the blastopore until the blastopore is closed. Thereafter, the expression remains in the ventral portion of the mantle lobe of the fully developed larva. No TtrCdx expression is detectable in the juvenile after metamorphosis. The expression of TtrCdx is congruent with findings in other metazoans, were genes belonging to the Cdx/caudal family are predominantly localized posteriorly during gastrulation and subsequently play a role in the formation of posterior tissues. for Drosophila melanogaster homeotic genes, and subsequently was found in all animal phyla studied to date (McGinnis et al. 1984, Scott and Weiner 1984, Lanfear and Bromham 2008). Homeobox genes function as developmental control genes that
64 Submitted manuscript Chapter IV encode transcription factors which activate gene cascades (Hueber and Lohmann 2008). In the case of the Not gene, which plays an important role during notochord formation in vertebrates (Stein and Kessel 1995, Talbot et al. 1995, Gont et al. 1996, Stein et al. 1996, Abdelkhalek et al. 2004), the downstream genes are known to regulate mesoderm formation in sea urchins as well as left/right patterning, notochord, mesoderm, and somite formation in vertebrates (Peterson et al. 1999, Yasuo and Lemaire 2001, Beckers et al. 2007). Homologs of the homeobox gene Not have been, among others, identified in Xenopus (Xnot), chick (Gnot1, Gnot2), zebrafish (flh), mouse (noto), Hydra (HvuNot), Drosophila (90Bre), and the basal eumetazoan Trichoplax adhaerens (TadNot), but the developmental role of Not in invertebrates without a notochord is largely unknown (Dessain and McGinnis 1993, von Dassow et al. 1993, Knezevic et al. 1995, Odenthal et al. 1996, Gauchat et al. 2000, Martinelli and Spring 2004, Hoskins et al. 2007). A Not homolog seems to be lacking in the model sponge Amphimedon queenslandica (Bernard Degnan, personal communication). However, the presence of a Not gene in cnidarians and Trichoplax indicates that it was present prior to the evolution of the mesoderm, and thus long before the evolution of the notochord. Accordingly, the ancestral role of Not remains elusive. Given its confirmed absence in the poriferan genome would make it a good candidate for a eumetazoan apomorphy. Cdx is a member of the ParaHox gene cluster which probably originated by duplication from an ancestral ProtoHox gene cluster which led to the Hox and ParaHox clusters, respectively (Brooke et al. 1998). Cdx has been found to be involved in the development of posterior tissues of almost all animal phyla in which it has been investigated and is thus often termed “caudal” (Epstein et al. 1997, Copf et al. 2004). In addition to the posterior tissues, it was found to be expressed in the mesoderm of taxa as diverse as Artemia, Capitella, Patella, Branchiostoma, and Mus; in the gut of Drosophila, Capitella, Branchiostoma, and Mus; and in the central nervous system of Capitella, Branchiostoma, and Mus (Macdonald and Struhl 1986, Duprey et al. 1988, Le Gouar et al. 2003, Copf et al. 2004, Fröbius and Seaver 2006). Cdx is absent in the recently sequenced poriferan Amphimedon queenslandica (Larroux et al. 2008, Srivastava et al. 2010), but a gene related to Cdx is present in Nematostella vectensis, a representative of Cnidaria, the proposed sister group to Bilateria (Chourrout et al. 2006, Quiquand et al. 2009). The phylogenetic position of Brachiopoda within Bilateria is still controversial (Williams and Carlson 2007, Hejnol et al. 2009, Paps et al. 2009). Most authors include them within Lophotrochozoa, but their position within this clade remains unresolved, and some authors consider them a sister group to Deuterostomia (Nielsen 2002). Within the phylum, Brachiopoda comprises three clades: Linguliformea, Craniiformea, and Rhynchonelliformea. Linguliformea and Craniiformea are often considered sister groups and were traditionally termed “inarticulate”,
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Comparative neurogenesis, muscle de
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2 Principal supervisor Assoc. P
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4 Preface This thesis presents
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6 Abstract Brachiopods are a sm
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8 Acknowledgements The endeavou
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10 Introduction Nervous system Micr
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Page 17 and 18: 16 Results and discussion Results a
Page 19 and 20: 18 Results and discussion posterior
Page 21 and 22: 20 Results and discussion Myogenesi
Page 23 and 24: 22 Results and discussion Wanninger
Page 25 and 26: 24 Results and discussion Growth pa
Page 27 and 28: 26 Results and discussion argument
Page 29 and 30: 28 References References Abdelkhale
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Page 33 and 34: 32 References James, M. A., Ansell,
Page 35 and 36: 34 References regionalization, prol
Page 37 and 38: 36 References 211. Williams, A., Ca
Page 39 and 40: 38 Chapter II Frontiers in Zoology
Page 53 and 54: 52 Chapter III Chapter III Altenbur
Page 55 and 56: 54 Chapter III Altenburger and Wann
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