PhD thesis

74 Submitted manuscript
Chapter IV
2004). In addition, Not expressing cells
in this species, which lacks a nervous
system and even neurons, overlaps
with the site of expression of the
neurotransmitter RFamide (Schuchert
1993). In Drosophila, the Not homolog
90Bre is present in the nervous
system, where it is expressed in the
ventral nerve cord and the posterior
brain anlage of germ band retracted
embryos, as well as in the lateral/
posterior region of the eye/antennal
disc (Dessain and McGinnis 1993).
In the ascidians Halocynthia roretzi
and Ciona intestinalis, Hr-Not and Ci-
Not, respectively, are expressed in the
posterior end of the tail, as well as in
the notochord and a small part of the
anterior neural tube in the larval tailbud
stage (Utsumi et al. 2004). These data
hint towards an ancestral role of Not in
metazoan neurogenesis.
The ring-like expression of TtrNot in
the ciliated region of the apical lobe
in Terebratalia larvae is intriguing.
Noto, the Not ortholog in the mouse,
is known to function in ciliogenesis,
and TtrNot might thus serve a similar
role in our study species. In this
context,it should also be considered
that the putative spiralian homolog
of this larval swimming device, the
prototroch, is underlain, and probably
innervated, by a ring nerve (Wanninger
2009). Accordingly, it is tempting
to speculate that Not may also be
expressed in the prototroch ring nerve
of spiralian trochophore larvae and/
or the prototroch itself. In Terebratalia
transversa, however, we did not find
any corresponding neural structure in
the region of Not expression (Fig. 6).
The larval nervous system of T.
transversa differs in several details
from that of the craniiform brachiopod
Novocrania anomola. In N. anomala,
the apical organ comprises four,
centrally positioned serotonergic flaskshaped
cells that are connected to two
ventral neurites which elongate laterally
along the larval body (Altenburger
and Wanninger 2010). By contrast,
the apical organ of T. transversa has
two sets of flask-shaped cells. Each
set contains four cells and each cell is
connected to the larval anterior neuropil
by a single serotonergic neurite. Due to
these differences, the morphology of
the ancestral brachiopod larval apical
organ remains elusive. However, the
data currently available indicate that an
apical organ comprising serotonergic
flask-shaped cells was part of the
brachiopod ground pattern and most
likely constitutes a morphological
apomorphy of Lophotrochzoa, since
such cells are also found in larval
Entoprocta, Mollusca, Nemertea,
Annelida, and Ectoprocta (Pires and
Woollacott 1997, Shimizu et al. 2000,
Friedrich et al. 2002, Voronezhskaya
et al. 2002, 2003, McDougall et al.
2006, Wanninger et al. 2007, Fuchs
and Wanninger 2008, Chernyshev
and Magarlamov 2010, Nielsen and
Worsaae 2010).
Not expression during gastrulation
and germ layer formation
In all species studied so far, Not
expression starts prior to or at the onset
of gastrulation. This is also the case
in the brachiopod investigated herein,
Terebratalia transversa. In the sea
urchin Strongylocentrotus purpuratus,
Not is expressed in the vegetal plate at
the mesenchyme-blastula stage and
in the secondary mesenchyme, with