PhD thesis
PhD thesis
PhD thesis
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64 Submitted manuscript<br />
Chapter IV<br />
encode transcription factors which<br />
activate gene cascades (Hueber and<br />
Lohmann 2008). In the case of the<br />
Not gene, which plays an important<br />
role during notochord formation in<br />
vertebrates (Stein and Kessel 1995,<br />
Talbot et al. 1995, Gont et al. 1996, Stein<br />
et al. 1996, Abdelkhalek et al. 2004),<br />
the downstream genes are known to<br />
regulate mesoderm formation in sea<br />
urchins as well as left/right patterning,<br />
notochord, mesoderm, and somite<br />
formation in vertebrates (Peterson et<br />
al. 1999, Yasuo and Lemaire 2001,<br />
Beckers et al. 2007). Homologs of<br />
the homeobox gene Not have been,<br />
among others, identified in Xenopus<br />
(Xnot), chick (Gnot1, Gnot2), zebrafish<br />
(flh), mouse (noto), Hydra (HvuNot),<br />
Drosophila (90Bre), and the basal<br />
eumetazoan Trichoplax adhaerens<br />
(TadNot), but the developmental<br />
role of Not in invertebrates without a<br />
notochord is largely unknown (Dessain<br />
and McGinnis 1993, von Dassow et al.<br />
1993, Knezevic et al. 1995, Odenthal<br />
et al. 1996, Gauchat et al. 2000,<br />
Martinelli and Spring 2004, Hoskins<br />
et al. 2007). A Not homolog seems<br />
to be lacking in the model sponge<br />
Amphimedon queenslandica (Bernard<br />
Degnan, personal communication).<br />
However, the presence of a Not gene<br />
in cnidarians and Trichoplax indicates<br />
that it was present prior to the evolution<br />
of the mesoderm, and thus long<br />
before the evolution of the notochord.<br />
Accordingly, the ancestral role of Not<br />
remains elusive. Given its confirmed<br />
absence in the poriferan genome<br />
would make it a good candidate for a<br />
eumetazoan apomorphy.<br />
Cdx is a member of the ParaHox gene<br />
cluster which probably originated by<br />
duplication from an ancestral ProtoHox<br />
gene cluster which led to the Hox and<br />
ParaHox clusters, respectively (Brooke<br />
et al. 1998). Cdx has been found to be<br />
involved in the development of posterior<br />
tissues of almost all animal phyla in<br />
which it has been investigated and is<br />
thus often termed “caudal” (Epstein et<br />
al. 1997, Copf et al. 2004). In addition<br />
to the posterior tissues, it was found<br />
to be expressed in the mesoderm of<br />
taxa as diverse as Artemia, Capitella,<br />
Patella, Branchiostoma, and Mus;<br />
in the gut of Drosophila, Capitella,<br />
Branchiostoma, and Mus; and in the<br />
central nervous system of Capitella,<br />
Branchiostoma, and Mus (Macdonald<br />
and Struhl 1986, Duprey et al. 1988,<br />
Le Gouar et al. 2003, Copf et al.<br />
2004, Fröbius and Seaver 2006). Cdx<br />
is absent in the recently sequenced<br />
poriferan Amphimedon queenslandica<br />
(Larroux et al. 2008, Srivastava et<br />
al. 2010), but a gene related to Cdx<br />
is present in Nematostella vectensis,<br />
a representative of Cnidaria, the<br />
proposed sister group to Bilateria<br />
(Chourrout et al. 2006, Quiquand et<br />
al. 2009).<br />
The phylogenetic position of<br />
Brachiopoda within Bilateria is still<br />
controversial (Williams and Carlson<br />
2007, Hejnol et al. 2009, Paps et al.<br />
2009). Most authors include them within<br />
Lophotrochozoa, but their position<br />
within this clade remains unresolved,<br />
and some authors consider them a<br />
sister group to Deuterostomia (Nielsen<br />
2002). Within the phylum, Brachiopoda<br />
comprises three clades: Linguliformea,<br />
Craniiformea, and Rhynchonelliformea.<br />
Linguliformea and Craniiformea are<br />
often considered sister groups and<br />
were traditionally termed “inarticulate”,