PhD thesis

22 Results and discussion
Wanninger 2008; 2009). In Nemertea, the lecithotrophic, non-pilidium like larva
of Quasitetrastemma stimpsoni shows a pair of serotonergic flask-shaped cells
in the apical organ plus a pair of subapical cells and two posterior neurons
that are located ventrolaterally (Chernyshev and Magarlamov 2010). Annelid
larvae show a serotonergic apical organ comprising up to four cells. The apical
organ is associated with the prototrochal nerve ring which in turn is connected
to two ventral nerve cords (Voronezhskaya et al. 2003; McDougall et al. 2006;
Brinkmann and Wanninger 2008). The apical organ of ectoproct cyphonautes
larvae comprises two pairs of serotonergic cell bodies from which lateral nerves
project towards the corona (Hay-Schmidt 2000; Gruhl 2009). One of the two cell
clusters in the apical organ contains flask-shaped cells (Nielsen and Worsaae
2010). In the apical organ of the ectoproct coronate larva of Bugula neritina
two flask-shaped serotonergic cells are present (Pires and Woollacott 1997;
Shimizu et al. 2000). In the actinotroch larva of Phoronida, the apical organ
contains numerous serotonergic cells, but these are probably not flask-shaped
(Santagata 2002; Santagata and Zimmer 2002; Wanninger 2008).
Taken together, the data that have recently become available on lophotrochozoan
larval neuroanatomy suggest that an apical organ comprising serotonergic
flask-shaped cells was present in larvae of the last common lophotrochozoan
ancestor (Wanninger 2008). Accordingly, an apical organ containing such cells
might be a morphological apomorphy of Lophotrochozoa.
Distribution of Pax3/7 proteins in larvae of Terebratalia transversa
A sister group relationship of Brachiopoda with Annelida has been hypothesized
based on molecular data as well as on paleontological data and is supported by
the notion that annelids and brachiopods share similarities in the ultrastructure
of their setae (Gustus and Cloney 1972; Orrhage 1973; Field et al. 1988; Lake
1990; Conway Morris and Peel 1995; Lüter 2000b). Several developmental
genes that are involved in the establishment of segments and segmentation in
animals have been characterized, some of which belong to the Pax3/7 group.
Pax3 and Pax7 genes probably arose by duplication from unique ancestral Pax3/7
genes and have similarities in their protein sequence and expression (Hayashi et
al. 2010). Pax3/7 genes are also known as Pax group III genes and include the
pair-rule gene paired (prd), the segment polarity genes gooseberry (gsb), and
gooseberry-neuro (gsbn), a gene that is expressed in the developing nervous
system and, together with engrailed, establishes the posterior commissures in
the fruit fly Drosophila melanogaster (Noll 1993; Colomb et al. 2008). Together
with their vertebrate homologs (Pax-3 and Pax-7) the Pax3/7 group forms one