PhD thesis

Chapter IV
Submitted manuscript
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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