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Chapter IV Submitted manuscript 75 expression ceasing after gastrulation (Peterson et al. 1999). In ascidians, Not expression starts at the eight cell stage in all blastomeres and is thereafter expressed in the posterior part of the larval tail, the notochord, and a small part of the anterior neural tube at the tailbud stage (Utsumi et al. 2004). Interestingly, we found TtrNot being solely expressed in the ectoderm of T. transversa, while their homologs are expressed in all three germ layers during Xenopus and ascidian embryogenesis (von Dassow et al. 1993, Utsumi et al. 2004). Apart from the development of the nervous system, the notochord, and various germ layers, Not is also responsible for left/right patterning in the mouse, where it is expressed in the “node”, i.e., the organizer of gastrulation (Beckers et al. 2007). In the sea urchins Hemicentrotus pulcherimus and Strongylocentrotus purpuratus, Not is expressed in the archenteron of the gastrula and in the mesoderm of the right coelomic pouch of two-armed pluteus larvae, were it is likewise involved in left/right determination (Peterson et al. 1999, Hibino et al. 2006). The current data suggest an overall role of Not in gastrulation as well as germ layer and nervous system patterning. Whether Not was used in specification of all three germ layers in Urbilateria (as exemplified in the ascidians and Xenopus) or whether its ancestral role was in ectoderm patterning alone (as in Terebratalia) remains to be revealed by future comparative studies. In any case, it appears that the Not gene has been co-opted into several other functions during evolution of respective metazoan (deuterostome) lineages, such as notochord formation in chordates and left/right patterning in ambulacrarians (sea urchin) and vertebrates (mouse). The role of Cdx in metazoan development Cdx is a member of the ParaHox cluster in which three genes are linked in a manner reminiscent of the Hox genes, with the gene order 3’-Gsx-Xlox-Cdx-5’ (Brooke et al. 1998). Compared to Hox genes, ParaHox genes seem to be much more evolutionary labile, since they do not appear together in all species investigated and sometimes they are not clustered (Ferrier and Holland 2002). Cdx expression patterns are known from several animal phyla and there is a wide range of tissues in which Cdx is expressed (Fröbius and Seaver 2006). A gene related to Cdx is present in the proposed bilaterian sister group, the cnidarian Nematostella vectensis (Chourrout et al. 2006, Ryan et al. 2006, 2007). Cdx was first characterized as a posterior patterning gene in Drosophila melanogaster (Mlodzik et al. 1985) and it appears to serve a similar role in a number of other taxa including various arthropods, the nematode Caenorhabditis elegans, and the basal gastropod mollusk Patella vulgata (Waring and Kenyon 1991, Xu et al. 1994, Schulz et al. 1998, Abzhanov and Kaufman 2000, Dearden and Akam 2001, Rabet et al. 2001, Copf et al. 2003, Le Gouar et al. 2003, 2004, Shinmyo et al. 2005, Olesnicky et al. 2006). In the annelids Platynereis dumerilii, Nereis virens, Tubifex tubifex, and Capitella sp., Cdx has an
76 Submitted manuscript Chapter IV anterior and a posterior expression domain (Fröbius and Seaver 2006, Matsuo and Shimizu 2006, Kulakova et al. 2008, Hui et al. 2009). The expression pattern of Cdx in Terebratalia transversa shows some similarity to that of Platynereis dumerilii. In both species Cdx is expressed in the ectoderm at the onset of gastrulation. In P. dumerilii, the ectodermal expression of PduCdx encircles the posterior portion of the slit-like blastopore and extends from there anteriorly along its edges (de Rosa et al. 2005). PduCdx continues to be expressed in the posterior part of the trochophore larva in the posterior midgut and hindgut (Hui et al. 2009). Expression of Cdx in the gut is also found in the sea urchin Strongylocentrotus purpuratus, the lancelet Brachiostoma floridae, and the mouse Mus musculus (Duprey et al. 1988, Brooke et al. 1998, Arnone et al. 2006). We did not find expression of TtrCdx in the larval gut of Terebratalia transversa, which might be due to the fact that those larvae are lecithotrophic and that metamorphosis is catastrophic, i.e., that all major larval tissues degenerate after settlement (Stricker and Reed 1985a, 1985b). Since we did not investigate feeding juveniles with a functional gut, the role of TtrCdx in gut formation remains elusive. Concerning the role of Cdx in the protostome-deuterostome ancestor (PDA), two major hypotheses are currently discussed. Either, expression in the PDA might have been in an anterior and a posterior domain of the nervous system, as in recent acoels as well as the lophotrochozoans Platynereis dumerilii, Capitella sp., Tubifex tubifex, and Patella vulgata (Le Gouar et al. 2003, de Rosa et al. 2005, Matsuo et al. 2005, Fröbius and Seaver 2006, Hejnol and Martindale 2008). Expression in the hindgut and posterior tissues of recent animals would thus have been co-opted. Or, Cdx expression in the PDA was in posterior tissues and a dissociation of Cdx from the ParaHox cluster in Lophotrochozoa allowed for its cooption into the anterior domain of the nervous system, as is the case in acoels and some lophotrochozoans (Hui et al. 2009). In this respect it would be interesting to focus future investigations on the genomic arrangement and the expression of the respective Gsx and Xlox genes of the ParaHox cluster in brachiopods. ACKNOWLEDGEMENTS We thank the Friday Harbor Laboratories and especially Billie Swalla (University of Washington) for providing lab space and assistance in rearing Terebratalia transversa. Olga Lévai (Leica Microsystems, Mannheim, Germany) is thanked for providing the SP5 confocal system that was used for the scans upon which Fig. 6 is based. We are grateful to Marta Chiodin (University of Barcelona) for guidance in lab procedures. Bernard M. Degnan (University of Queensland) is thanked for sharing previously unpublished sequence data of the demosponge Amphimedon queenslandica. This study was funded by the Danish Agency for Science, Technology and Innovation (grant no. 645-06-0294 to AW). Research in the lab of AW and PM was further supported by the EU-funded Marie Curie Network
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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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12 Material and methods Material an
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14 Material and methods purpuratus,
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16 Results and discussion Results a
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18 Results and discussion posterior
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20 Results and discussion Myogenesi
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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
Page 31 and 32: 30 References priapulids and protos
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
Page 63 and 64: 62 Chapter IV Chapter IV Altenburge
Page 65 and 66: 64 Submitted manuscript Chapter IV
Page 75: 74 Submitted manuscript Chapter IV
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